Category: link to climate change

blogs that relate P&P or invasive plants to climate change; or discuss assisted migration; CO2 storage; etc.

New Publication on Threats to World’s Forests – including Invasives

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Russian Taiga forest

In a new paper, “Forest Resources of the World: Present Status and Future Prospects,” Singh et al. affirm the importance of forests for terrestrial biodiversity, provision of multiple ecosystem services, and supporting the economic well-being of approximately 1.6 billion people directly. This equals about a quarter of Earth’s population. The authors conclude that achieving global Sustainable Development Goals (SDGs), including poverty reduction, food security, and mitigating and adapting to climate change — all depend on sustaining forests.

According to the 2020 Global Forest Resource Assessment, Earth’s forested area comprises ~4.06 billion hectares, or 31% of the total land surface.More than half (54%) of all global forest area is found in five countries: the Russian Federation, Brazil, Canada, the United States, and China. Tropical forests constitute 45% of this total; boreal forests, 27%; temperate forests, 16%; and subtropical forests, 11%. An estimated 93% (3.75 billion ha) regenerate through natural processes; 7% (290 million ha) is planted forest.

The extent of global forest area has been declining for decades but the rate of loss slowed significantly between 1990 and 2020. This reflects decreased deforestation in some countries and an increase in forest area in others. The latter is due to both afforestation and also natural forest growth. However, conversion of tropical forests to agriculture continues apace. From 2010 to 2020, the net loss of forest area was highest in Africa (3.9 million ha) and South America (2.6 million ha). Increases in net forest area occurred in Asia, Oceania and Europe. The status of the top 10 countries or territories in global forest resources as of 2020 is given in Table 1.2 of the chapter. [News sources document that rapid deforestation continues in Brazil, at least.]

Several trends are concerning to those of us who value primary or undisturbed forests. First, the area of naturally regenerating forest has decreased, while the area of planted forest has expanded – but only by 123 million ha. In the last decade, the rate of increase in the area of planted forests has also slowed.

Second, total carbon stock in forests declined from 668 gigatons to 662 gt in 1990–2020. This is only 6%, but it is trending in the wrong direction. As we know, forest conservation counters climate change in two ways: conserved forests are a carbon sink, while degraded or destroyed forests are a significant source of atmospheric CO2. In fact, forests are the 2nd largest storehouses of carbon, after oceans. Global forests sequester about one-third of total CO2 emission from the combustion of fossil fuels. Almost all forest carbon is found in living biomass (44%) and soil organic matter (45%).

Costa Rican rainforest; photo by eflon via Flickr

Third, primary forests are already severely reduced and continue to shrink. Primary forests are those composed of native species, and supporting relatively undisturbed ecological processes. They are irreplaceable for sustaining biological diversity. These forests are already severely reduced – they cover only ~ 1 billion ha. Since 1990, the extent of primary forest has decreased by 81 million ha. More than half are in Brazil, Canada, and Russia.

Singh et al. report that only about 10% of the world’s forests are set aside for biodiversity conservation. Again, trends are in the wrong direction. The rate of increase in the area of forest designated largely for biodiversity conservation has slowed. On the other hand, forest areas designated for other non-extractive purposes have increased: soil and water conservation, recreation, tourism, education, research, and the protection of cultural and spiritual sites.

Singh et al. are cheered by the fact that more than 2 billion hectares are under management with well-defined management plans. The extent of forests under management plans has increased by 233 million ha since 2000.

Singh et al. say that continuously increasing anthropogenic pressure is the main cause of deforestation and forest degradation in unmanaged forests. Citing projections that the world’s population will reach almost 10 billion by 2050, they say this growth will make reconciling the need for forest conservation with the basic requirements of humans for food, shelter, and fuel more difficult than ever.

I appreciate this honesty. Too many experts interviewed on the day that the global population was estimated at 8 billion made optimistic statements about the consequences. They mentioned Earth’s carrying capacity only in reference to First World people demanding excessive resources. There was minimal discussion of humanity’s carbon footprint and no reference to ever-increasing threats to biological diversity. Nor to the fact that people in developing countries want to raise their standards of living – which entails higher demand for resources, including energy. For an example, see The Washington Post editorial, here.

On the other hand, Ruby Mellen in the Post on 15 November mentioned that, according to the World Wildlife Fund, 75% of Earth’s ice-free land has been significantly altered by people, and two-thirds of mammal, fish, reptile, and amphibian species have become endangered in the last ~50 years. Unfortunately, the on-line version of the paper doesn’t have this specific article!

fires in Siberian forest in 2016; European Space Agency

Threats to Forests: Fire

Singh et al. rank fire as the most disastrous threat, affecting biodiversity and carbon sequestration potential.   According to the U.N. Food and Agriculture Organization, about 29% of the total geographical area in the world was affected by forest fires during 2001–2018; more than two-thirds of these fires occurred in Africa. U.S. media, however, focused on fires in the Amazon, temperate areas (U.S., Europe), and, sometimes, boreal forests or Australia. Singh et al. say that areas that are frequently affected by fire are prone to other types of disturbances like drought and outbreaks of insect pests.

tanoaks killed by Phytophthora ramorum in Oregon; photo by Oregon Department of Forestry

Threats to Forests: Diseases and Pests

I am glad that Singh et al. recognize the damage to forest productivity caused by disease and pest infestations. In doing so, they cite familiar sources – Clive Brasier, Peter Vitousek, Juliann Aukema, Gary Lovett, Sandy Liebhold, Kerry Britton, Bitty Roy, Hanno Seebens – regarding surges in pest attacks; the growing diversity of damaging pests; resulting changes in forest species composition and structure that impede ecosystem functions and productivity. Singh et al. follow these sources in calling for improved hygiene in nurseries, adoption of scientific silvicultural practices reducing physical damage to the vegetation, selection of genotypes that are resistant, and reinforcing national and international policies on quarantine and biosecurity measures to minimize pest impacts in the future. They also mention adoption of remote sensing technologies to detect the trees under stress and use of sentinel plantings. They list the 10 most important international agreements dealing with invasive species issues as the International Plant Protection Convention, Ramsar Convention, Convention on International Trade in Endangered Species of Wild Fauna and Flora, Convention on Migratory Species, Convention on Biological Diversity and its Cartagena Protocol on Biosafety,  IUCN Invasive Species Specialist Group, World Trade Organization Agreement on Sanitary and Phytosanitary Measures, Global Invasive Species Program, and International Civil Aviation Organization, and Cartagena.

slash and burn agriculture in Bolivia; photo Neil Palmer

Threats to forests: Development Projects

Singh et al. consider development projects to be the third threat to forest conservation.  Their roads, powerlines, and other linear developments cause habitat loss and fragment landscapes. In their view, environmental impact assessments and other similar requirements are not yet sufficient to safeguard sustainable use of forest resources.

Policy Responses

Singh et al. call for more inclusive forest management structures to respond to the threat climate change poses to forests, industries, and forest-dependent communities. They all for partnerships that bring together researchers from several disciplines with forest managers and local stakeholders. Geoffrey M. Williams and others (including me) advocate for similar conservation approaches. (See pre-print here.)

In this context, Singh et al. mention several reports, plans, and agreements aimed at global forest conservation.  Participants in global fora have recognized the importance of forests in contributing to food security and sustainable development. Among agreements mentioned are the UN’s Strategic Plan for Forests 2030 and recommendations of the International Institute for Sustainable Development (IISD) published in 1994. The former tries to generate greater coherence, collaboration, and synergy across UN programs aimed at encouraging volunteer forest conservation by countries, international, regional, and local organizations, partners, and stakeholders. Unfortunately, they do not discuss the extent to which the 30-year old IISD recommendations have – or have not – been implemented.

They also describe Forest Landscape Restoration as an effective strategy to restore the functionality of forests.Again, the focus is on a collaborative approach aimed at integrating efforts by all forestry-related stakeholders, e.g., scientific and academic organizations, local communities, indigenous peoples, and private sectors, including forest-based enterprises and NGOs.

Also praised is rising attention to trees outside forest. This includes fostering use of trees in agroforestry systems ranging from home gardens to farm forestry systems, shelterbelts, and woodlots. This approach helps to sustain the livelihoods of rural communities and maintain a stable and secure food supply. Meanwhile, it reduces dependence on natural forests

Singh et al. say community forest management and decentralized governance have gained acceptance. They describe examples from Gambia and Rwanda. They concede that such decentralization has its own risks and challenges. For example, e the most marginalized sections of the community must be ensured adequate capacity for robust conflict resolution.

Singh et al. advocate that all nations seek to increase their forest cover; affluent countries that are hampered by physical and climatic conditions should aid poorer nations in increasing and upgrading their forest cover. They suggest “recognition” and encouragement of countries that maintain forest cover above 30% of territory.

See also about loss of floral diversity and blog about IUCN’s global forest assessment.

SOURCE

Singh, M., N.N. Shahina, S. Das, A. Arshad, S. Siril, D. Barman, U. Mog, P. Panwar, G. Shukla, and S. Chakravarty. 2022. Forest Resources of the World: Present Status and Future Prospects. In Panwar, P., G. Shukla, J.­A. ­Bhat, S. ­Chakravarty­. 2022. Editors. Land Degradation Neutrality: Achieving SDG 15 by ­Forest Management; ISBN 978-981-19-5477-1 ISBN 978-981-19-5478-8 (eBook)

https://doi.org/10.1007/978-981-19-5478-8

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Restoring American chestnut: importance of Phytophthora root rot

TACF back-crossed chestnut growing in southern Fairfax County, Virginia

same chestnut, dying in 2021; probably from Phytopthora root rot

In the first half of the 20th Century, American chestnut (Castanea dentata) was functionally extirpated from US forests east of the Mississippi River by chestnut blight, caused by a fungus from Asia, Cryphonectria parasitica. Today, only 10% of the pre-blight chestnut population remains, most as root sprouts less than 2.5 cm dbh (Dalgleish et al. 2015; full citation at the end of the blog).

Volunteer organizations — with recent help from federal and state agencies – have worked for more than a century to develop chestnut trees resistant to the blight. Their aim is to restore the species to the forest.  Their decades of hybridization efforts now appear unlikely to produce a highly blight-resistant chestnut with a genome that is predominantly American, so TACF now plans to incorporate the use of transgenic techniques to enhance resistance to the blight fungus.

However, restoration of chestnut requires addressing a second Asian pathogen: Phytophthora cinnamomi, which causes a fatal root disease. Several studies indicate that up to 80% of seedlings are killed. The pathogen is widespread in soils south of 40o North Latitude, which falls just north of the Maryland-Pennsylvania line. Thus, P. cinnamomi occupies the southern half of American chestnut’s former range. Scientists expect this pathogen to move north in response to the warming climate; indeed, some project that the root disease could reach throughout the entire current chestnut range by 2080.

historic range of American chestnut

Gustafson et al. 2022 modelled chestnut’s vulnerability to P. cinnamomi to current and expected environmental conditions in two state forests in the Appalachians of western Maryland to evaluate the probable impact of the root disease on efforts to restore the tree species.

They found that root rot greatly reduced chestnut biomass on the landscape, even when resistance to root rot was at the target level for selection of root rot-resistant chestnut families using traditional breeding methods.

Gustafson et al. 2022 recommend that chestnut restoration apply the following strategies:

  • Locate restoration plantings at latitudes, elevations, and sites where root rot is not expected to be present well into the future. This probably means sites in the Northeastern US and Canada (Burgess et al. 2017)
  • Enhance the planting stock’s resistance to P. cinnamomi through breeding.
  • Identify soil conditions, including soil microbes, that suppress the pathogen or protect tree roots.
  • Since planting stock – both bareroot and containerized – can transmit P. cinnamomi, either raise seedlings in nurseries located outside the pathogen’s current range or rely on direct seeding. These strategies have their own downsides. Restricting locations of nurseries might complicate efforts to ensure seedlings are adapted to local conditions in the restoration area and seeds would need to be protected from seed predators.

The authors specify these additional important conditions:

  • Planting locations: while Canada is currently outside the range of American chestnut, the same climatic warming that will facilitate northward spread of P. cinnamomi will probably allow the tree to thrive farther north (Barnes and Delborne 2019). Perhaps the tree’s range will shift farther north than the pathogen’s.
  • Breeding: some resistance to Phytophthora root rot has been found in families providing blight resistance used in The American Chestnut Foundation (TACF) breeding program. TACF now plans to cross individuals from those families with transgenic blight-resistant chestnut to combine both resistances.
  • Soils: P. cinnamomi is favored by compacted soils with poor aeration or that tend to remain saturated. These include heavy clay soils and those highly disturbed by agriculture or mining. Restoration sites should be non-disturbed, well-drained sites. (This recommendation contradicts others’ proposals that chestnuts be planted on reclaimed mining sites.) Silvicultural management should also minimize environmental stresses.
that dying chestnut trying to reproduce!

Restoring chestnut will be challenging in any case: successful restoration requires chestnut trees that can compete successfully in the forest and adapt to conditions which are now quite different from those a century ago when the species was dominant. These include abiotic factors, e.g., climate and atmospheric CO2 levels; and biotic factors, e.g., different forest pests and invasive plant species.

In an earlier publication, Gustafson and colleagues (Gustafson et al. 2018) modelled the effects of warmer temperature and elevated atmospheric CO2 levels on chestnut’s growth and competition and the tree’s adaptation to natural and anthropogenic disturbances. They concluded that aggressive restoration programs – involving clearcutting, then planting chestnuts – could restore chestnut as an important component of forested ecosystems in the Appalachian Mountains.  

However, this earlier study did not consider the effects of Phytophthora root rot. The 2022 study demonstrates that these recommendations are probably applicable only to the northernmost portion of former chestnut range, outside the areas infested by Phytophthora root rot, unless breeding is successful in substantially increasing resistance to root rot.

Several studies indicate American chestnut is highly susceptible to P. cinnamomi; rates of root rot induce mortality of 80% or higher have been documented. TACF has found that hybrid chestnut families selected for root rot resistance have a mortality rate of about 45%. Even with this level of tolerance, the model shows that chestnut could not regain anything approaching its former abundance on the landscape. Since the threat of P. cinnamomi to chestnut restoration has become evident, TACF is assessing how to integrate increased tolerance to root rot into their larger blight resistance breeding program (Westbrook et al. 2019).

Soil properties – texture, land use, drainage, waterlogging, drought, temperature, and water-holding capacity – influence infection. So does weather: a single heavy rain event might saturate soil sufficiently to facilitate a P. cinnamomi infection. For these reasons, climate change is expected to exacerbate its geographic spread and pathogenicity.

The sites used in both studies are at the center of chestnut’s former range, which is also at the northern edge of the root rot pathogen’s range. However, the two sites differ in important ways, especially in rainfall and soils. The researchers considered one a mesic site and the other, xeric.

Their 2022 model showed that root rot caused a dramatic reduction in chestnut biomass on both the mesic and xeric sites. Apparently temperature and wetness levels offset each other. That is, higher soil temperatures intensified P. cinnamomi virulence at the xeric site sufficiently to overcome its relative soil dryness. At the mesic site, soil temperature sometimes dropped to levels that are lethal to Phytophthora. On the whole, then, climate change is expected to intensify P. cinnamomi infection rates on both sites and reduce the number of sites where the pathogen is absent.

Gustafson et al. (2022) discuss several assumptions and data gaps that require further study.

SOURCES

Barnes, J.C. and Delborne, J.A., 2019. Rethinking restoration targets for American chestnut using species distribution modeling. Biodiversity and Conservation, 28(12), pp.3199-3220.

Burgess, T.I., Scott, J.K., Mcdougall, K.L., Stukely, M.J., Crane, C., Dunstan, W.A., Brigg, F., Andjic, V., White, D., Rudman, T. and Arentz, F., 2017. Current and projected global distribution of Phytophthora cinnamomi, one of the world’s worst plant pathogens. Global Change Biology, 23(4), pp.1661-1674.

Dalgleish, H.J., Nelson, C.D., Scrivani, J.A. and Jacobs, D.F., 2015. Consequences of shifts in abundance and distribution of American chestnut for restoration of a foundation forest tree. Forests, 7(1), p.4.

Gustafson, E.J., B.R. Miranda, T.J. Dreaden, C.C. Pinchot, D.F. Jacobs. 2022. Beyond blight: Phytophthora root rot under climate change limits populations of reintro Am chestnut Ecosphere. 2022;13:e3917.

Gustafson, E.J., A.M.G. De Bruijn, N. Lichti, D.F. Jacobs, B.R. Sturtevant, D.M. Kashian, B.R. Miranda, and P.A. Townsend. 2018. “Forecasting Effects of Tree Species Reintroduction Strategies on Carbon Stocks in a Future without Historical Analog.” Global Change Biology 24: 5500–17. https://doi.org/10.1111/gcb.14397

Westbrook, Jared W., et al. “Resistance to Phytophthora cinnamomi in American chestnut (Castanea dentata) backcross populations that descended from two Chinese chestnut (Castanea mollissima) sources of resistance.” Plant disease 103.7 (2019): 1631-1641.

Posted by Faith Campbell

[An earlier version of this blog has now been corrected, with additional sources added. I think Cornelia Pinchot, USFS, for the corrections.]

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Hundreds of U.S. Tree Species Endangered, Most due to Non-Native Pests

Horton House on Jekyll Island, Georgia before laurel wilt killed the giant redbay trees; photo by F.T. Campbell

Close to four hundred tree species native to the United States are at risk of extinction. The threats come mainly from non-native insects and diseases – a threat we know gets far too little funding, policy attention, and research.

As Murphy Westwood, Vice President of Science and Conservation at the Morton Arboretum, which led the U.S. portion of a major new study, said to Gabriel Popkin, writing for Science: “We have the technology and resources to shift the needle,” she says. “We can make a difference. We have to try.”

Staggering Numbers

More than 100 tree species native to the “lower 48” states are endangered (Carrero et al. 2022; full citation at the end of this blog). These data come from a global effort to evaluate tree species’ conservation status around the world. I reported on the global project and its U.S. component in September 2021. This month Christina Carrero and colleagues (full citation at the end of this blog) published a summary of the overall picture for the 881 “tree” species (including palms and some cacti and yuccas) native to the contiguous U.S. (the “lower 48”).

This study did not address tree species in Hawai`i or the U.S. Pacific and Caribbean territories. However, we know that another 241 Hawaiian tree species are imperiled (Megan Barstow, cited here).

Assessing Threats: IUCN, NatureServe, and CAPTURE

Carrero and colleagues assessed trees’ status by applying methods developed by IUCN and NatureServe. (See the article for descriptions of these methods.) These two systems consider all types of threats. Meanwhile, three years ago Forest Service scientists assessed the specific impacts of non-native insects and pathogens on tree species in the “lower 48” states and Alaska in “Project CAPTURE” (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation). All three systems propose priorities for conservation efforts. For CAPTURE’s, go here.

Analyses carried out under all three systems (IUCN, NatureServe, and CAPTURE) concur that large numbers of tree species are imperiled. Both IUCN and CAPTURE agree that non-native insects and pathogens are a major cause of that endangerment. While the overall number of threatened species remained about the same for all three systems, NatureServe rated threats much lower for many of the tree species that IUCN and CAPTURE considered most imperiled.

This difference arises from the criteria used to rate a species as at risk. IUCN’s Criterion A is reduction in population size. Under this criterion, even extremely widespread and abundant species can qualify as threatened if the population declines by at least 30% over three generations in the past, present, and/or projected future. NatureServe’s assessment takes into account rapid population decline, but also considers other factors, for example, range size, number of occurrences, and total population size. As a result, widespread taxa are less likely to be placed in “at risk” categories in NatureServe’s system.

In my view, the IUCN criteria better reflect our experience with expanding threats from introduced pests. Chestnut blight, white pine blister rust, dogwood anthracnose, emerald ash borer, laurel wilt disease, beech leaf disease, and other examples all show how rapidly introduced pathogens and insects can spread throughout their hosts’ ranges. (All these pests are profiled here . ) They can change a species’ conservation status within decades whether that host is widespread or not.  

Which Species Are at Risk: IUCN

Carrero and colleagues found that under both IUCN and NatureServe criteria, 11% to 16% of the 881 species native to the “lower 48” states are endangered. Another five species are possibly extinct in the wild. Four of the extinct species are hawthorns (Crataegus); the fifth is the Franklin tree (Franklinia alatamaha) from Georgia. A single specimen of a sixth species, an oak native to Texas (Quercus tardifolia),was recently re-discovered in Big Bend National Park.

Franklinia (with Bachman’s warbler); both are extinct in the wild; painting by John Jacob Audubon

The oak and hawthorn genera each has more than 80 species. Relying on the IUCN process, Carrero and colleagues found that a significant number of these are at risk: 17 oaks (20% of all species in the genus); 29 hawthorns (34.5% percent). A similar proportion of species in the fir (Abies), birch (Betula), and walnut (Juglans) genera are also threatened.

Other genera have an even higher proportion of their species under threat, per the IUCN process:

  • all species in five tree genera, including Persea (redbay, swampbay) and Torreya (yews);
  • two-thirds of chestnuts and chinkapins (Castanea), and cypress (Cupressus);
  • almost half (46.7%) of ash trees (Fraxinus).                                                    

Pines are less threatened as a group, with 15% of species under threat. However, some of these pines are keystone species in their ecosystems, for example the whitebark pine of high western mountains.

Carrero et al. conclude that the principal threats to these tree species are problematic and invasive species; climate change and severe weather; modifications of natural systems; and overharvest (especially logging). Non-native insects and pathogens threaten about 40 species already ranked by the IUCN criteria as being at risk and another 100 species that are not so ranked. Climate change is threatening about 90 species overall.

range of black ash

Considering the invasive species threat, Carrero and colleagues cite specifically ash trees and the bays (Persea spp.). In only 30 years, the emerald ash borer has put five of 14 ash species at risk. All these species are widespread, so they are unlikely to be threatened by other, more localized, causes. In about 20 years, laurel wilt disease threatens to cause extinction of all U.S. tree species in the Persea genus.

Carrero and colleagues note that conservation and restoration of a country’s trees and native forests are extremely important in achieving other conservation goals, including mitigating climate change, regulating water cycles, removing pollutants from the air, and supporting human well-being. They note also forests’ economic importance.

As I noted above, USFS scientists’ “Project CAPTURE” also identified species that deserve immediate conservation efforts.

Where Risk Assessments Diverge

All three systems for assessing risks agree about the severe threat to narrowly endemic Florida torreya and Carolina hemlock.

With three risk ranking systems, all can agree (as above), all can disagree, or pairs can agree in four different ways. Groups of trees fall into each pair, with various degrees of divergence.  Generally, only two of the three systems agree on more widespread species:

  • black ash: IUCN and Project CAPTURE prioritize this species. NatureServe ranked it as “secure” (G5) as recently as 2016.
  • whitebark pine: considered endangered by IUCN, “vulnerable” (G3) by NatureServe. The US Fish and Wildlife Service has proposed listing the species as “threatened” under the Endangered Species Act. https://www.fws.gov/species-publication-action/endangered-and-threatened-wildlife-and-plants-threatened-species-18 However, Project CAPTURE does not include it among its highest priorities for conservation. Perhaps this is because there are significant resistance breeding and restoration projects already under way.
  • tanoak: considered secure by both IUCN and NatureServe, but prioritized by Project CAPTURE for protection.
dead tanoak in Curry County, Oregon; photo by Oregon Department of Forestry

Carrero notes the divergence between IUCN and NatureServe regarding ashes. Four species ranked “apparently secure” (G4) by NatureServe (Carolina, pumpkin, white, and green ash) are all considered vulnerable by IUCN. They are also prioritized by Project CAPTURE. I have described the impact of the emerald ash borer on black ash. Deborah McCullough, noted expert on ash status after invasion by the emerald ash borer, also objects to designating this species as “secure” (pers. comm.).

This same divergence appears for eastern hemlock.

Port-Orford cedar is currently ranked as at risk by IUCN and Project CAPTURE, but not NatureServe. Growing success of the restoration breeding project has prompted IUCN to change the species’ rank from “vulnerable” to “near threatened”. IUCN is expected to reclassify it as of “least concern” in about a decade if breeding efforts continue to be successful (Sniezko presentation to POC restoration webinar February 2022).

While these differing detailed assessments are puzzling, the main points are clear: several hundred of America’s tree species (including many in Hawai`i, which – after all – is our 50th state!) are endangered and current conservation and restoration efforts are inadequate.

Furthermore, a tree species loses its function in the ecosystem long before it becomes extinct. It might still be quite numerous throughout its range – but if each individual has shrunken in size it cannot provide the same ecosystem services. Think of thickets of beech root sprouts – they cannot provide the bounteous nut crops and nesting cavities so important to wildlife. Extinction is the extreme. We should act to conserve species much earlier.

YOU CAN HELP!

Congress is considering the next Farm Bill – which is due to be adopted in 2023. Despite its title, this legislation has often provided authorization and funding for forest conservation (for example, the US Forest Service’ Landscape Scale Restoration Program).

There is already a bill in the House of Representatives aimed at improving the US Department of Agriculture’s prevention and early detection/rapid response programs for invasive pests. Also, it would greatly enhance efforts to restore decimated tree species via resistance breeding, biocontrol, and other strategies. This bill is H.R. 1389.

The bill was introduced by Rep. Peter Welch of Vermont, who has been a solid ally and led on this issue for several years. As of August 2022, the bill has seven cosponsors, most from the Northeast: Rep. Mike Thompson [CA], Rep. Chellie Pingree [ME], Reps. Ann M. Kuster and Chris Pappas [NH], Rep. Elise Stefanik [NY], Rep. Deborah K. Ross [NC], Rep. Brian Fitzpatrick [PA].

Please write your Representative and Senators. Urge them to seek incorporation of H.R. 1389 in the 2023 Farm Bill. Also, ask them to become co-sponsors for the House or Senate bills. (Members of the key House and Senate Committees are listed below, along with supporting organizations and other details.)

Details of the Proposed Legislation

The Invasive Species Prevention and Forest Restoration Act [H.R. 1389]

  • Expands USDA APHIS’ access to emergency funding to combat invasive species when existing federal funds are insufficient and broadens the range of actives that these funds can support.
  • Establishes a grant program to support research on resistance breeding, biocontrol, and other methods to counter tree-killing introduced insects and pathogens.
  • Establishes a second grant program to support application of promising research findings from the first grant program, that is, entities that will grow large numbers of pest-resistant propagules, plant them in forests – and care for them so they survive and thrive.
  • [A successful restoration program requires both early-stage research to identify strategies and other scientists and institutions who can apply that learning; see how the fit together here.]
  • Mandates a study to identify actions needed to overcome the lack of centralization and prioritization of non-native insect and pathogen research and response within the federal government, and develop national strategies for saving tree species.

Incorporating the provisions of H.R. 1389 into the 2023 Farm Bill would boost USDA’s efforts to counter bioinvasion. As Carrera and colleagues and the Morton Arboretum study on which their paper is based demonstrate, our tree species desperately need stronger policies and more generous funding. Federal and state measures to prevent more non-native pathogen and insect pest introductions – and the funding to support this work – have been insufficient for years. New tree-killing pests continue to enter the country and make that deficit larger –see beech leaf disease here. Those here, spread – see emerald ash borer to Oregon.

For example, funding for the USDA Forest Service Forest Health Protection program has been cut by about 50%; funding for USFS Research projects that target 10 high-profile non-native pests has been cut by about 70%.

H.R. 1389 is endorsed by several organizations in the Northeast: Audubon Vermont, the Maine Woodland Owners Association, Massachusetts Forest Alliance, The Nature Conservancy Vermont, the New Hampshire Timberland Owners Association, Vermont Woodlands Association, and the Pennsylvania Forestry Association.

Also, major forest-related national organizations support the bill: The American Chestnut Foundation (TACF), American Forest Foundation, The Association of Consulting Foresters (ACF), Center for Invasive Species Prevention, Ecological Society of America, Entomological Society of America, National Alliance of Forest Owners (NAFO), National Association of State Foresters (NASF), National Woodland Owners Association (NWOA), North American Invasive Species Management Association (NAISMA), Reduce Risk from Invasive Species Coalition, The Society of American Foresters (SAF).

HOUSE AND SENATE AGRICULTURE COMMITTEE MEMBERS – BY STATE

STATEMember, House CommitteeMember, Senate CommitteeKey members * committee leadership # forestry subcommittee leadership @ cosponsor of H.R. 1389
AlabamaBarry Moore  
ArizonaTom O’Halleran  
ArkansasRick CrawfordJohn Boozman* 
CaliforniaJim Costa Salud Carbajal Ro Khanna Lou Correa Josh Harder Jimmie Panetta Doug LaMalfa  
Colorado Michael Bennet # 
ConnecticutJahana Hayes  
FloridaAl Lawson Kat Cammack  
GeorgiaDavid Scott * Sanford Bishop Austin Scott Rick AllenRaphael Warnock Tommy Tuberville 
IllinoisBobby Rush Cheri Bustos Rodney Davis Mary MillerRichard DurbinNote that the report was led by scientists at the Morton Arboretum – in Illinois!
IndianaJim BairdMike Braun 
IowaCindy Axne Randy FeenstraJoni Ernst Charles Grassley 
KansasSharice Davids Tracey MannRoger Marshall# 
Kentucky Mitch McConnell 
MaineChellie Pingree @  
MassachusettsJim McGovern  
Michigan Debbie Stabenow * 
MinnesotaAngie Craig Michelle FischbachAmy Klobuchar Tina Smith 
MississippiTrent KellyCindy Hyde-Smith 
MissouriVicky Hartzler  
NebraskaDon BaconDeb Fischer 
New HampshireAnn McLane Kuster @  
New Jersey Cory Booker 
New Mexico Ben Ray Lujan 
New YorkSean Patrick Maloney Chris JacobsKristen Gillibrand 
North CarolinaAlma Adams David Rouzer  
North Dakota John Hoeven 
OhioShontel Brown Marcy Kaptur Troy BaldersonSherrod Brown 
PennsylvaniaGlenn Thompson  
South DakotaDusty JohnsonJohn Thune 
TennesseeScott DesJarlais  
TexasMichael Cloud Mayra Flores  
Vermont Patrick Leahy 
VirginiaAbigail Spanberger #  
WashingtonKim Schreir  

SOURCES

Christina Carrero, et al. Data sharing for conservation: A standardized checklist of US native tree species and threat assessments to prioritize and coordinate action. Plants People Planet. 2022;1–17. wileyonlinelibrary.com/journal/ppp3

Washington Post: Sarah Kaplan, “As many as one in six U.S. tree species is threatened with extinction” https://www.washingtonpost.com/climate-environment/2022/08/23/extinct-tree-species-sequoias/

Popkin, G. “Up to 135 tree species face extinction—and just eight enjoy federal protection”, Science August 25, 2022. https://www.science.org/content/article/135-u-s-tree-species-face-extinction-and-just-eight-enjoy-federal-protection

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Tree Planting – Warning from New Zealand

Pinus radiata plantation in New Zealand; photo by Jon Sullivan

As countries and conservation organizations ramp up tree planting as one solution to climate change, I worry that many of the plantings will use species not native to the region – with the risk of promoting more bioinvasions. My second fear is that inadequate attention will be paid to ensuring that the propagules thrive.

Warning from New Zealand

New Zealand has adopted a major afforestation initiative (“One Billion Trees”). This program is ostensibly governed by a policy of “right tree, right place, right purpose”. However, Bellingham et al. (2022) [full citation at end of blog] say the program will probably increase the already extensive area of radiata pine plantations and thus the likelihood of exacerbated invasion. They say the species’ potential invasiveness and its effects in natural ecosystems have not been considered.

Bellingham et al. set out to raise the alarm by evaluating the current status of radiata, or Monterrey, pine  (Pinus radiata) in the country. They note that the species already occupies ~1.6 M ha; the species makes up 90% of the country’s planted forests. Despite the species having been detected as spreading outside plantations in 1904, it is generally thought not to have invaded widely.

The authors contend that, to the contrary, radiata pine has already invaded several grasslands and shrublands, including three classes of ecosystems that are naturally uncommon. These are geothermal ecosystems, gumlands (infertile soils that formerly supported forests dominated by the endemic and threatened kauri tree Agathis australis), and inland cliffs. Invasions by pines – including radiata pine – are also affecting primary succession on volcanic substrates, landslides on New Zealand’s steep, erosion-prone terrain, and coastal sand dunes. Finally, pine invasions are overtopping native Myrtaceae shrubs during secondary succession. Bellingham et al. describe the situation as a pervasive and ongoing invasion resulting primarily from spread from plantations to relatively nearby areas.

kauri; photo by Natalia Volna, iTravelNZ

The New Zealanders cite data from South America and South Africa on the damaging effects of invasions by various pine species, especially with respect to fire regimes.

Furthermore, their modelling indicates that up to 76% of New Zealand’s land area is climatically capable of supporting radiata pine — most of the country except areas above 1000 m in elevation or receiving more than 2000 mm of rainfall per year. That is, all but the center and west of the South Island. This model is based on current climate; a warmer/drier climate would probably increase the area suitable to radiata pine.

These invasions by radiata pine have probably been overlooked because the focus has been on montane grasslands (which are invaded by other species of North American conifers). [See below — surveys of knowledge of invasive plants’ impacts.]

Bellingham et al. recognize the economic importance of radiata pine. They believe that early detection of spread from plantations and rapid deployment of containment programs would be the most effective management strategy. They therefore recommend

1) taxing new plantations of non-indigenous conifers to offset the costs of managing invasions, and

2) regulating these plantations more strictly to protect vulnerable ecosystems.

They also note several areas where additional research on the species’ invasiveness, dispersal, and impacts is needed.

Survey of Awareness of Invasive Plants

A few months later a separate group of New Zealand scientists published a study examining tourists’ understanding of invasive plant impacts and willingness to support eradication programs (Lovelock et al.; full citation at end of the blog). One of the invasive plant groups included in the study are conifers introduced from North America and Europe. These conifers are invading montane grasslands, so they are not the specific topic of the earlier article. The other is a beautiful flowering plant, Russell lupine.  These authors say that both plant groups have profound ecological, economic, and environmental impacts. However, the conifers and lupines are also highly visible at places valued by tourists. Lovelock et al. explored whether the plants’ familiarity – and beauty – might affect how people reacted to descriptions of their ecosystem impacts.

Visitors from elsewhere in New Zealand were more aware of invasive plants’ impacts and more willing to support eradication programs for these species specifically. Asian visitors had lower awareness and willingness to support eradication of the invasives than tourists from the United Kingdom, Europe, or North America. This pattern remained after the tourists were informed about the plants’ ecological impacts. All groups were less willing to support eradication of the attractive Russell lupine than the conifers.

Conifers invading montane grasslands are perhaps the most publicized invasive plants in New Zealand [as noted above]. Lovelock et al. report that New Zealand authorities have spent an estimated $NZ166 million to eradicate non-native conifers over large tracts of land on the South Island. Still, only about half the New Zealand visitors surveyed were aware of the ecological problems caused by wild conifers.

invasive lupines in New Zealand; photo by Michael Button via Flickr

Russell lupine (Lupinus × russellii) is invading braided river systems, modifying river flows, reducing nesting site availability for several endangered birds, and provides cover for invasive predators. While initially planted in gardens, the lupines were soon being deliberately spread along the roads to ‘beautify’ the landscape. Foreign tourists often specifically seek river valley invaded by the lupine because pictures of the floral display appear in both official tourism promotional material & tourist-related social media. It is not surprising, then, that even among New Zealanders, only a third were aware of the lupines’ environmental impacts.

The oldest participants (those over 60) had the lowest acceptance of wild conifers. Participants 50–59 years old were most aware of ecological problems caused by wild conifers. Participants 30–39 years old showed the highest acceptance of wild conifers and lowest awareness of ecological issues.

Female participants showed a higher preference for the landscape with wild conifers (45.90%) than males (36.89%). Female participants were also half as aware of ecological problems (25.62% v. 46.12% among male participants).

Nearly all survey participants (96.1%) preferred the landscape with flowering lupine; only 19.4% were aware of associated ecological problems. New Zealand domestic visitors were more aware. After the impacts of lupines were explained, half decided to support eradication. However, the same proportion of all survey participants (42.5%) still wanted to see lupines in the landscape.

Once again, participants older than 50 were more aware of ecological problems arising from lupine invasions.  Both men and women greatly preferred the landscape with Russell lupins.

While the authors do not explore the ramifications of the finding that younger people are less aware of invasive species impacts, I think they bode ill for future protection of the country’s unique flora and fauna. They did note that respondents had a high level of acceptance overall for these species on the New Zealand landscapes.

While the study supported use of simple environmental messaging to influence attitudes about invasive species, also showed that need to consider such social attributes as nationality and ethnicity. So Lovelock et al. call for investigation of how and why place of origin and ethnicity are important in shaping attitudes towards invasives. Conveying conservation messages will be more difficult because tourist materials often contain photographs of the lupines. Much of this information comes from informal media such as social media, which are beyond the control of invasive species managers.

SOURCES

Bellingham, P.J., E.A. Arnst, B.D. Clarkson, T.R. Etherington, L.J. Forester, W.B. Shaw,  R. Sprague, S.K. Wiser, and D.A. Peltzer. 2022. The right tree in the right place? A major economic tree species poses major ecological threats. Biol Invasions Vol.: (0123456789) https://doi.org/10.1007/s10530-022-02892-6  

Lovelock B., Y. Ji, A. Carr, and C-J. Blye. 2022.  Should tourists care more about invasive species? International and domestic visitors’ perceptions of invasive plants and their control in New Zealand.  Biological Invasions (2022) 24:3905–3918 https://doi.org/10.1007/s10530-022-02890-8

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org

Trees’ Value – High Although Major Benefits Not Addressed

loblolly pine – tree species showing highest value in this study; via Flickr

More scientists are examining the importance of American forests in providing ecosystem services – and the threat to those values raised by non-native pests and other factors. This is a broader perspective than used in the past – and it includes climate change.   also here  

Jeannine Cavender-Bareu and colleagues (full citation at the end of this blog) found that changes in the abundance and composition of US trees have the potential to undermine the benefits and societal values derived from those forests now. They examined threats associated with increasing invasive pests and pathogens, greater frequency of major fires, and climate change. Together, these constitute a complex set of global change drivers – and the impact of each is accelerating.

The authors tried to measure the impact of these forces on forests’ ability to provide five key ecosystem services. Two are “regulating” services—regulation of climate and air quality. The other three are “provisioning” services—production of wood products, food crops, and Christmas trees.

Unfortunately, they could not find sufficient data to analyze five other ecosystem services, which are equally or more important. They include both regulatory and provisioning services: water management, such as erosion control, flood and storm surge regulation; urban heat island regulation and energy savings; providing habitats for species (biodiversity); recreation; or ornamental, spiritual, and aesthetic values.

Cavender-Bareu and colleagues concluded that the value of the five analyzed services provided by 400 tree species across the contiguous United States over the years 2010-2012 is $114 billion per year. The non-market “regulatory” values far exceeds their current commercial value. 

  • Climate regulation via carbon storage in tree biomass provides 51% of this net annual value;
  • Human health improvements linked to trees’ filtering of air pollution provide an additional 37% of the annual net value.
  • Provisioning services, such as wood products, fruit and nut crops, and Christmas trees, provide only 12% of the net annual value. (By my calculation, wood products constituate almost three-quarters of this sum.)

The authors then tried to identify which tree lineages, e.g., taxonomic families, genera, or species, provide the greatest proportion of each of these ecosystem services. They also identified threats to these lineages. Together, this knowledge allows managers to target forestry management practices to the specific lineages within a landscape where ecosystem service are most at risk.

Table 1 in the article ranks 10 tree genera by the aggregate net value they provide: pine, oak, maple, Douglas-fir, hemlock, cherry/almond, spruce, hickories, yellow or tulip poplar, and ash. The table also provides separate dollar values for each of the five benefits.

Two lineages—pines and oaks — provide 42% of the value of these services (annually, pines = $25.4 billion; oaks = $22.3 billion). They note that these high values result from the large number of pine and oak species occupying diverse ecological niches. Oaks have the highest annual values for climate moderation or carbon storage ($10.7 billion) and air quality regulation ($11 billion). Oaks’ air quality regulation value reflects three factors: the genus’ abundance, the trees’ size, and the large numbers planted in cities and suburbs, that is, near human populations affected by pollution. Other than this issue of location, closely related tree species tend to have similar air quality regulation values.

Many lineages provide wood products, but the amounts vary widely among related species. Pines dominate annual net revenues from wood products at $7.4 billion, due in part to their high volume and higher than average price. The most valuable species in the context of this study’s set of ecosystem services are loblolly pine (Pinus taeda) and Douglas-fir (Pseudotsuga menziesii).

Edible fruits are concentrated in two lineages — family Rosaceae, especially genera Prunus and Malus; and family Rutaceae, genus Citrus. This category demonstrates the impact of disease: annual net returns from citrus products were actually negative during the 2010 – 2012 period due to abnormally low market prices and the prevalence of citrus greening disease in Florida, Arizona and California.

northern red oak – high value for timber & carbon sequestration; photo by dcrjsr via Wikimedia

Trees at Risk

As climate change progresses, the mix of tree species that provide critical ecosystem services will be altered—with unknown consequences. There could be increases in some services but also widely-expected losses in ecosystem benefits and human well-being.

An estimated 81% of tree species are projected to have at least 10% of their biomass exposed to climates outside their current climate envelope, impacting nearly 40% of total tree biomass in the contiguous U.S. An estimated 40% of species are projected to face increasing fire frequency. In both cases, individual species’ vulnerability depends more on where that species grows than on its genetic lineage. This analysis demonstrates a threatening interaction between these two disturbance agents: the species most valuable for carbon storage are also the most at risk from the increasing fire threat.

Known (established) pests threaten 16% of tree species and potentially affect up to 40% of total tree biomass. At greatest risk are the oak and pine genera (due to mountain pine beetle and oak wilt) plus most of the crop species. The authors cite chestnut blight and Dutch elm disease as examples of pests decimating once-dominant tree species — ones provided many services. In contrast to climate and fire risks, genetic relationships explain much of the risk of pest damage because most pests attack individual species, genera, or families.  (There are exceptions – sudden oak death and the Fusarium fungi vectored by invasive shot hole borers attack species across a wide range of families.)

Cavender-Bareu and colleagues conclude that major losses to pest attack of dominant species and lineages that currently provide high-value ecosystem services would undermine forest capacity to provide important benefits—at least for decades. They note that pest threats appear to be increasing partially as a consequence of climate change, demonstrating that multiple threats can interact and exacerbate outcomes. They say policy interventions aimed at slowing pests’ spread will probably be necessary to preserve the ecosystem service of climate and air quality regulation.

The high diversity of tree taxa in U.S. forests might buffer losses of ecosystem service if the most valuable lineages (oaks and pines) are compromised. However, other species will be needed to fill the voids their loss creates. Ensuring this possibility will require intentional management of forests and trees in the face of myriad and simultaneous threats.

The authors also show how tree-provided ecosystem services are distributed across the U.S. depending largely on the locations of forests, tree plantations, and orchards. Climate and air quality regulation occurs everywhere forests grow. Timber production is concentrated in a subset of the regions that also produce high climate regulation and air pollution removal, including the Southeast, Pacific Northwest, Northeast, and Upper Midwest.

The most valuable tree crops are grown on the coasts, often where forests do not grow—e.g., California; and in several Southwestern, Southern, and Eastern states.

Cavender-Bareu and colleagues found that climate change threatens species in all parts of the continent. Wildfires are expected to increase especially in California and the Intermountain West, which they say coincides with high annual storage of carbon. (This finding is opposite from those of Quirion et al. (2021) which pointed to the slow growth of pines in this region as reducing carbon storage potential.)

Cavender-Bareu and colleagues found that pest threats are highest in the Southwest and Southeast. These pests (native and non-native) are predicted to disproportionally affect species that generate high annual net values for climate regulation, air quality regulation, and wood products – e.g., pines and oaks. As noted above, these values are driven by their abundance. They note that mountain pine beetle and oak wilt have not yet reached areas with high wood product production in Northeast and Southeast.

Other studies (see Aukema et al. 2010) and here & here show that the greatest threats from non-native pests are to the Northeast/Midwest, and the Pacific coast – and Hawai`i & here.

Rock Creek Park, Washington, D.C. – an urban forest! photo by Bonnachaven

Cavender-Bareu and colleagues’ analysis advances our understanding of the threat several change drivers pose to benefits Americans receive from our forests. However, we must remember that some of the most important ecosystem services were not included because of insufficient data. Missing services:

1) most urban ecosystems. Inclusion of urban trees in the analysis would significantly increase the value of avoided health damage due to tree-based removal of air pollution. Urban trees also help regulate climate change (Nowak et al. estimate 643 M Mg of carbon are stored in urban areas, at a value of $2.31 billion annually).

2) many other regulating ecosystem services, such as erosion control, flood regulation, storm surge regulation, urban heat island regulation, energy savings due to shade, and species habitat / biodiversity.

3) recreation, ornamental, spiritual, and aesthetic values.

A complete accounting would also require estimates of the damage trees cause and the cost of their maintenance. For example, the full cost of irrigating almond trees; allergies and irritations due to tree pollen and sap; injuries to people and property caused by falling trees and limbs; trees’ role in spreading fires; trees’ contribution to volatile organic compounds (a pollutant).

The estimated annual values of the climate and air quality regulation have large uncertainty. These arise from uncertainty re: the social cost of carbon, the value of a statistical life, and uncertainty in the air pollution dose–mortality response function. The estimated annual values of the provisioning services are more precise because they are calculated from the market price for the per unit value of tree crops, wood products, and Christmas trees, as well as reliable data on production volume.

SOURCES

Aukema, J.E., D.G. McCullough, B. Von Holle, A.M. Liebhold, K. Britton, & S.J. Frankel. 2010. Historical Accumulation of Nonindigenous Forest Pests in the Continental United States. Bioscience. December 2010 / Vol. 60 No. 11

Cavender-Bareu, J.M., E. Nelson, J.E. Meireles, J.R. Lasky, D.A. Miteva, D.J.Nowak, W.D. Pearse, M.R. Helmus, A.E. Zanne, W.F. Fagan, C. Mihiar, N.Z. Muller, N.J.B. Kraft, S. Polasky. 2022. The hidden value of trees — Quantifying the ecosystem services of tree lineages and their major threats across the contiguous. PLOS Sustainability and Transformation April 5, 2022.  

Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) Insect and Disease Disturbances Correlate With Reduced Carbon Sequestration in Forests of the Contiguous United States. Front. For. Glob. Change 4:716582.  Volume 4 Article 716582  doi: 10.3389/ffgc.2021.716582

Two Teams with a New Take: Insect Losses Due to Invasive Plants

monarch butterfly on swamp milkweed; photo by Jim Hudgins, USFWS

I have been impressed recently by two groups of scientists who are trying to broaden understanding of the impacts of invasive plants by examining the interactions of those plants with insects. As they note, herbivorous insects are key players in terrestrial food webs; they transfer energy captured by plants through photosynthesis to other trophic levels. This importance has been recognized since Elton first established the basic premises of food webs (1927) [Burghardt et al.; full citation at end of blog] Arthropods comprise significant members of nearly every trophic level and are especially important as pollinators. If introduced plants cause changes to herbivore communities, there will probably be effects on predators, parasites, and other wildlife through multitrophic interactions [Lalk et al.; Tallamy, Narango and Mitchell].

[I briefly summarize the findings of a third group of scientists at the end of this blog. The third group looks at the interaction between agriculture – that is, planting of non-native plants! – and climate change.]

One approach to studying this issue, taken by Douglas Tallamy of the University of Delaware and colleagues, is to look at the response of herbivorous insects to NIS woody plants fairly generally. They integrate their studies with growing concern about the global decline in insect populations and diversity. They note that scientists have focused on light pollution, development, industrial agriculture, and pesticides as causes of these declines. They decry the lack of attention to disruption of specialized evolutionary relationships between insect herbivores and their native host plants due to widespread domination by non-indigenous plants [Richard, Tallamy and Mitchell].

In their studies, Tallamy and colleagues consider not just invasive plants, but also non-native plants deliberately planted as crops or ornamentals, or in forestry. They point out that such introduced plants have completely transformed the composition of plant communities in both natural and human-dominated ecosystems around the globe. At least 25% of the world’s planted forests are composed of tree species not native to their locale. At least one-sixth of the globe is highly vulnerable to plant invasions, including biodiversity hotspots [Richard, Tallamy and Mitchell].

A different approach, taken by Lalk and colleagues, is more closely linked to concern about impacts of the plants themselves. They have chosen to pursue knowledge about relationships between individual species of invasive woody plants and the full range of arthropod feeding guilds – pollinators, herbivores, twig and stem borers, leaf litter and soil organisms. In so doing, they decry the general absence of data.

Both teams agree that:

  • Invasive plants are altering ecosystems across broad swaths of North America and the impacts are insufficiently understood.
  • The invasive plant problem will get worse because non-native species continue to be imported and planted. (Reminder: the Tallamy team considers impacts of deliberate planting as well as bioinvasion.)
  • Plant-insect interactions are the foundation of food webs, so changes to them will have repercussions throughout ecosystems.

Tallamy team

Non-native plants have replaced native plant communities to a greater or lesser extent in every North American biome – including anthropogenic landscapes [Burghardt]. The first trophic level in suburban and urban ecosystems throughout the U.S. is dominated by plant species that evolved in Southeast Asia, Europe, and South America [Tallamy and Shropshire]. Abundant non-native plants not only dominate plant biomass; they also reduce native plant taxonomic, functional and phylogenetic diversity and heterogeneity. Thus, they indirectly alter the abundance of native insects  [Burghardt; Richard, Tallamy and Mitchell].

I think these articles might actually underestimate the extent of these impacts. While Richard, Tallamy and Mitchell report that more than 3,300 species of non-native plants are established in continental U.S., years ago Rod Randall said that more than 9,700 non-native plant species were naturalized in the U.S. (probably includes Hawai` i.   The Tallamy team cites USDA Forest Service data showing 9% of forests in the southeast are invaded by just 33 common invasive plant species [Richard, Tallamy and Mitchell], I have cited that and other sources showing even greater extents of plant invasion in the east and here; other regions and here. 

The Tallamy team has conducted several field experiments that demonstrate that the presence of non-native plants suppress numbers and diversity of native lepidopteran caterpillars. These non-native woody plants have not replaced the ecological functions of the native plants that used to support insect populations. This is true whether or not the non-native plants are deliberately planted or are invading various ecosystems on their own. [Richard, Tallamy and Mitchell]. (Of course, they expect plant invasions to grow; they note that some of the many ornamental species that are not yet invasive will become so.)

The result is disruption of the ecological services delivered by native plant communities, including the focus of their studies: plants’ most fundamental contribution to ecosystem function: generation of food for other organisms [Burghardt].

They note that plants’ relationship to insects differs depending on the insects’ feeding guilds — folivores, wood eaters, detritivores, pollinators, frugivores, and seed-eaters; and among herbivores with different mouthparts — chewing or sucking; and as host plant specialists or generalists. They decry studies that fail to recognize these differences [Tallamy, Narango, and Mitchell].

The Tallamy team explores why insect populations decline among non-native plants. That is,  

1) Do insects directly requiring plant resources have lower fitness when using non-native plants; do they not recognize them as viable host plants; or do they avoid them altogether? 

2) Are reductions in numbers of specialist herbivores mitigated by generalists? A decade of research shows that both specialists and generalists decline.

The team’s studies focus on lepidopteran larvae (caterpillars). Insect herbivores are both the largest taxon of primary consumers and extremely important in transferring energy captured by plants through photosynthesis to other trophic levels [Burghardt]. In addition, insects with chewing mouthparts are typically more susceptible to defensive secondary metabolites contained in leaves than are insects with sucking mouthparts that tap into poorly defended xylem or phloem fluids [Tallamy, Narango and Mitchell].

A study by Burghardt et al. found that 75% of all lepidopteran species and 93% of specialist species were found exclusively on native plant species. Non-native plants that were in the same genus as a native plant often supports a lepidopteran community that is a similar but depauperate subset of the community found on its native congener. In fact, the insect abundance and species richness supported by non-native congeners of native species was reduced by 68%.

A meta-analysis of 76 studies by other scientists found that, with few exceptions, caterpillars had higher survival and were larger when reared on native host plants. Plant communities invaded by non-native species had significantly fewer Lepidoptera and less species richness. In three of eight cases examined, non-native plants functioned as ecological traps, inducing females to lay eggs on plants that did not support successful larval development. Richard, Tallamy and Mitchell cite as an example the target of many conservation efforts, monarch butterflies (Danaus plexxipus), which fail to reproduce when they use nonnative swallowworts (Vincetoxicum species.) instead of related milkweeds (Asclepias species.).

Tallamy and Shropshire ranked 1,385 plant genera that occur in the mid-Atlantic region by their ability to support lepidopteran species richness. They found that introduced ornamentals are not the ecological equivalents of native ornamentals. This means that solar energy harnessed by introduced plants is largely unavailable to native specialist insect herbivores.

Tallamy, Narango, and Mitchell describe the following patterns:

1) Insects with chewing mouthparts are typically more susceptible to defensive secondary metabolites contained in leaves than are insects with sucking mouthparts that tap into poorly defended xylem or phloem fluids. As a result, sucking insects find novel non-indigenous plants to be acceptable hosts more often. However, there are more than 4.5 times as many chewing (mandibulate) insect herbivores than sucking (haustellate) species. It follows that the largest guild of insect herbivores is also the most vulnerable to non-native plants as well as being the most valuable to insectivores.

native azalea Rhododendron periclymenoides; photo by F.T. Campbell

2) Woody native species, on average, support more species of phytophagous insects than herbaceous species.

3) Although insects are more likely to accept non-native congeners or con-familial species as novel hosts, non-native congeners still reduced insect abundance and species richness by 68%.

4) Host plant specialists are less likely to develop on evolutionarily novel non-indigenous plants than are insects with a broader diet. There are far more specialist species than generalists, so generalists will not prevent serious declines in species richness and abundance when native plants are replaced by non-indigenous plants. In addition, non-native plants cause significant reductions in species richness and abundance even of generalists. In fact, generalists are often locally specialized on particular plant lineages and thus may function more like specialists than expected.

5) Any reduction in the abundance and diversity of insect herbivores will probably cause a concomitant reduction in the insect predators and parasitoids of those herbivores – although few studies have attempted to measure this impact beyond spiders, which are abundant generalists. The vast majority of parasitoids are highly specialized on particular host lineages.

6) Studies comparing native to non-native plants must avoid using native species that support very few phytophagous insects as their baseline, e.g., in the mid-Atlantic region tulip poplar trees (Liriodendron tulipifera) and Yellowwood (Cladrastus kentuckea).

7) Insects that feed on well-defended living tissues such as leaves, buds, and seeds are less likely to be able to include non-native plants in their diets than are insects that develop on undefended tissues like wood, fruits, and nectar. Although this hypothesis has never been formally tested, they note the ease with which introduced wood borers – emerald ash borer, Asian longhorned beetle, polyphagous and Kuroshio shot-hole borers, redbay ambrosia beetle, Sirex woodwasp (all described in profiles posted here — have become established in the US.

palamedes swallowtail Papilio palamedes; photo by Vincent P. Lucas; this butterfly depends on redbay, a tree decimated by laurel wilt disease vectored by the redbay ambrosia beetle

Lalk and Colleagues

As noted, Lalk and colleagues have a different frame; they focus on individual introduced plant species rather than starting from insects. They also limit their study to invasive plants. The authors say there is considerable knowledge about interactions between invasive herbaceous plants and arthropod communities, but less re: complex interactions between invasive woody plants and arthropod communities, including mutualists (e.g., pollinators), herbivores, twig- and stem-borers, leaf-litter and soil-dwelling arthropods, and other arthropod groups.

They ask why this knowledge gap persists when invasive shrubs and trees are so widespread and causing considerable ecological damage. They suggest the answer is that woody invaders rarely encroach on high-value agricultural systems and some are perceived as contributing ecosystem services, including supporting some pollinators and wildlife.

Lalk and colleagues seek to jump-start additional research by summarizing what is currently known about invasive woody plants’ interactions with insects. They found sufficient data about 11 species – although even these data are minimal. They note that all have been cultivated and sold in the U.S. for more than 100 years. All but one (mimosa) are listed as a noxious weed by at least one state; two states (Rhode Island and Georgia) don’t have a noxious weed list. None of the 11 is listed under the federal noxious weed statute.

Ailanthus altissima

Illustrations of how minimal the existing information is:

  • Tree-of-heaven (Ailanthus altissima) is noted to be supporting expanded populations of the Ailanthus webworm moth (Atteva aurea), which is native to Central America; and to be the principal reproductive host for SLF (Lycorma delicatua)  https://www.dontmovefirewood.org/pest_pathogen/spotted-lanternfly-html/
  • Chinese tallow (Triadica sebifera) is thought to benefit both native generalist bee species and non-indigenous European honeybees (Apis mellifera).
  • Chinese privet (Ligustrum sinense) appears to suppress populations of butterflies, bees, and beetles.

Lalk and colleagues then review what is known about interactions between individual invasive plant species in various feeding guilds. They point out that existing data on these relationships are scarce and sometimes contradictory.

They believe this is because interactions vary depending on phylogenetic relationships, trophic guild, and behavior (e.g., specialized v. generalist pollinator). Arthropods can be “passengers” of a plant invasion. That is, they can be affected by that invasion, with follow-on effects to other arthropods in the community. Also, arthropods can be “drivers” of invasion, increasing the success of the invasive plants.

They then summarize the available information on various interactions. For example, they note that introduced plants can compete with native plants in attracting pollinators, causing cascading effects. Or they can increase pollination services to native plants by attracting additional pollinators.

They note that herbivore pressure on invasive plants can have important impacts on growth, spread, and placement within food webs. They note that these cases support the “enemy release hypothesis”, although they think there are probably additional driving mechanisms.

Lalk and colleagues note that most native twig- and stem-borers (Coleoptera: Buprestidae, Curculionidae, Cerambycidae; Hymenoptera: Siricidae) are not considered primary pests but that some of our most damaging insect species are wood borers (see above).

Some of these borers are decomposers; in that role, they are critical in nutrient cycling.

Arthropods in leaf litter and soil also serve important roles in the decomposition and cycling of nutrients, which affects soil biota, pH, soil nutrients, and soil moisture. They act as a trophic base in many ecosystems. Lalk and colleagues suggest these arthropod communities probably change with plant species due to differences in leaf phytochemistry. They cite one study that found litter community composition differed significantly between litter beneath tree-of-heaven, honeysuckle (Lonicera maackii), and buckthorn (Rhamnus cathartica) compared to litter underneath surrounding native trees.

Recommendations

Both the Tallamy and Lalk teams call for ending widespread planting of non-native plants. Lalk and colleagues discuss briefly the roles of

  • The nursery industry (including retailers); they produce what sells.
  • Scientists and educators have not sufficiently informed home and land owners about which species are invasive or about native alternatives.
  • Private citizens buy and plant what their neighbors have, what they consider aesthetically pleasing, or what is being promoted.
  • States have not prohibited sale of most invasive woody plants. Regulatory actions are not a straightforward matter; they require considerable time, supporting information, and compromise.

Tallamy team calls for restoration ecologists in the eastern U.S. to consider the number of Lepidopterans hosted by a plant species when deciding what to plant. For example, oaks (Quercus), willows (Salix), native cherries (Prunus)and birches (Betula) host orders of magnitude more lepidopteran species in the mid-Atlantic region than tulip poplar.(Those lepidopteran in turn support breeding birds and other insectivorous organisms.) [Tallamy & Shropshire]

Lalk and colleagues focused on identifying several key knowledge gaps:

  • How invasive woody plants affect biodiversity and ecosystem functioning
  • How they themselves function in different habitats.
  • Do non-native plants drive shifts in insect community composition, and if so, what is that shift, and how does it affect other trophic levels?
  • How do IAS woody plants affect pollinators?

The authors do not minimize the difficulty of separating such possible plant impacts from other factors, including climate change and urbanization.

Global Perspective

oil palm plantation in Malaysia; © CEphoto, Uwe Aranas

Outhwaite et al. (full citation at end of this blog) note that past studies have shown that insect biodiversity changes are driven primarily by land-use change (which is another way of saying planting of non-native species – as Dr. Tallamy and colleagues describe it) and increasingly by climate change. They south to examine whether these drivers interact. They found that the combination of climate warming and intensive agriculture is associated with reductions of almost 50% in the abundance and 27% in the number of species within insect assemblages relative to levels in less-disturbed habitats with lower rates of historical climate warming. These patterns were particularly clear in the tropics (perhaps partially because of the longer history of intensive agriculture in temperate zones). They found that high availability of nearby natural habitat (that is, native plants) can mitigate these reductions — but only in low-intensity agricultural systems.

Outhwaite et al. reiterate the importance of insect species in ecosystem functioning, citing pollination, pest control, soil quality regulation & decomposition. To prevent loss of these important ecosystem services, they call for strong efforts to mitigate climate change and implementation of land-management strategies that increase the availability of natural habitats.

SOURCES

Burghardt, K. T., D. W. Tallamy, C. Philips, and K. J. Shropshire. 2010. Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities. Ecosphere 1(5):art11. doi:10.1890/ES10-00032.

Lalk, S. J. Hartshorn, and D.R. Coyle. 2021. IAS Woody Plants and Their Effects on Arthropods in the US: Challenges and Opportunities. Annals of the Entomological Society of America, 114(2), 2021, 192–205 doi: 10.1093/aesa/saaa054

Outhwaite, C.L., P. McCann, and T. Newbold. 2022.  Agriculture and climate change are shaping insect biodiversity worldwide. Nature 605 97-192 (2022)  https://www.nature.com/articles/s41586-022-04644-x

Richard, M. D.W. Tallamy and A.B. Mitchell. 2019. Introduced plants reduce species interactions. Biol Invasions

Tallamy, D.W., D.L. Narango and A.B. Mitchell. 2020. Ecological Entomology (2020), DOI: 10.1111/een.12973 Do Non-native plants contribute to insect declines?

Tallamy, D.W. and K.J. Shropshire. 2009. Ranking Lepidopteran Use of Native Versus Introduced Plants Conservation Biology, Volume 23, No. 4, 941–947 2009 Society for Conservation Biology DOI: 10.1111/j.1523-1739.2009.01202.x

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Two More Key Studies: Forest Pests and Carbon

hemlock woolly adelgid – a pest that has spread north as result of warmer winters; photo from bugwood.org

I recently posted a blog reviewing impacts of insects and pathogens on efforts to sequester atmospheric carbon in forests. I want to add two other studies. The first, by Weed, Ayers, and Hicke (2013; full citation at end of this blog), delved more deeply into three mechanisms by which climate and atmospheric changes associated with increasing greenhouse gases influence biotic disturbances: (1) effects on the physiology of insects and pathogens that cause changes in their abundance and distribution; (2) effects on tree defenses and tolerance; and (3) effects on interactions between disturbance agents and their own enemies, competitors, and mutualists. They also looked at interaction of tree-killing pests with other sources of forest disturbances – e.g., wildfires, drought, bioinvasions by organisms other than insects and pathogens, and human conversion of forested land to other uses. Tree-killing pests can promote destabilizing positive feedbacks with these other sources of forest disturbances. Weed, Ayers, and Hicke (2013) express the concern that recurrent forest disturbances caused by insects and pathogens might counteract carbon mitigation strategies. [This concern is similar to findings by Quirion et al. (2021) cited in the previous blog and by USDA Forest Service scientists studying disturbance agents in western forests (Barrett et al. 2021).  

A second study by Clark and D’Amato (2021) looks intensively at forest growth and change in four types of secondary forests in New England to discover climate change dynamics and their resulting relative ability to sequester atmospheric carbon.

A 2013 Study by Weed, Ayers, and Hicke

Weed, Ayers, and Hicke (2013) begin from the premise that epidemics of forest insects and diseases (native and introduced) are the dominant sources of disturbance to North American forests. They note that, on a global scale, bioinvasions might be at least as important as climate change as threats to the sustainability of forest ecosystems. As agreed by most authorities, they find that the underlying cause of bioinvasions is propagule pressure from global transport, not climate change. However, climate change is strongly connected to management of continuing invasions.

Weed, Ayers, and Hicke (2013) review 79 studies published 1950 – 2012 which addressed a total of 27 insects and 22 diseases. Despite their opening focus on introduced pests, and the fact that six of the insects and nine of the diseases are nonindigenous, most of the research they were able to review has been on native organisms, principally on two species: the mountain pine beetle and southern pine beetle. Less is known about pathogens’ interaction with changes to climate than about insects’. A further complicating factor is the need to study both the insect and the pathogen when considering diseases vectored by insects (e.g., beech bark disease, oak wilt, Dutch elm disease, black stain root diseases, laurel wilt, thousand cankers disease, and pitch canker). [Profiles of most of these diseases are posted here; click on “invasive species”.] It is no surprise, then, that Weed, Ayers, and Hicke (2013) identify several areas where there is insufficient research. They state that despite scientists’ broad knowledge of climate effects on insect and pathogen demography, we still lack capacity to predict pest outbreaks under climate change.

Changing climatic conditions can exacerbate pest-caused disturbances by reducing winter mortality of insects and by increasing the development rate of insects and pathogens during the growing season. The changing conditions can also alter leaf maturation (which affects insect feeding) or synchrony of the life cycles of bark beetles. Contrary to the authors’ expectations, drought does not appear to cause a universal reduction in trees’ creation of defensive chemicals.

Due to pests’ host preferences, these disturbance agents typically alter the composition of tree species within stands – which can change forest types. For example, Weed, Ayers, and Hicke (2013) mention how mountain pine beetles shifted western forests from five needle pines toward subalpine firs. They do not mention balsam woolly adelgid or other fir pests.

The authors expect warming and increases in atmospheric CO2 to promote faster forest maturation in many US regions. Drought, however, will probably slow maturation rates in arid areas such as the southwest and intermountain regions. Climate change increases the likelihood that forest stands will be exposed to different and less suitable climates than those under which the current stands matured, making more stands susceptible to pests.  (The USFS report on western forests said the same — Barrett et al. 2021).  These changes tend to reduce the extent of mature forests and can adversely affect ecosystem services. They note the need for increased capacity to predict future patterns of biotic disturbances and integrate this knowledge with forest ecosystem science and the socioeconomics of human land use.

Weed, Ayers, and Hicke (2013) raise an interesting point regarding the impact of disturbance factors on trees’ ages and sizes. They mention specifically reduction in numbers of large-diameter beech trees due to beech bark disease and elms due to Dutch elm disease. Several large-growing trees, e.g., American chestnut and white pines, have been virtually eliminated from much of their historical ranges. They express the fear that emerald ash borer, sudden oak death, butternut canker, and laurel wilt are in the early stages of having a similar effect on their hosts. [Profiles of most of these pests are posted here; click on “invasive species”.] Weed, Ayers, and Hicke (2013) note the importance to wildlife of this shift – the loss of mature forest habitat changes availability of food supplies, nest cavities, etc. The authors do not relate these specific pest-mediated changes to the climate change-caused alterations. However, they do note that pest impacts exacerbate a situation already arising from loss of mature forests due to human land use patterns.

Weed, Ayers, and Hicke (2013) mention changes in elemental cycling and hydrologic processes resulting from pest-caused mortality; they refer to several studies by Lovett, especially Lovett et al. (2006). These changes can have long-lasting effects on productivity, biodiversity, and elemental cycling. Among them are effects on water transpiration and increased soil moisture and runoff. I had blogged earlier about these impacts as they pertain to black ash swamps. At high elevations, snow accumulates more deeply on the ground while snowmelt is more rapid because loss of canopy will decrease interception of snow by the canopy (leading to reduced sublimation and redistribution of snow) and increase solar radiation to the forest floor.

Weed, Ayers, and Hicke (2013) anticipate that pest outbreaks under climate change will commonly produce persistent changes in the feedbacks that connect biotic communities and elemental cycling.  

Weed, Ayers, and Hicke (2013) summarize their findings as follows:

1) Epidemics of forest pests (native and introduced) exceed other sources of disturbance to North American forests.

2) Insect populations are highly responsive to climate change due to their physiological sensitivity to temperature, high mobility, short generation times, and explosive reproductive potential. Pathogens and declines are also strongly influenced by climate change due to their sensitivity to temperature and moisture. These effects have proven to be more dramatic than expected in the case of pine bark beetles. There is no discussion of whether other insect-host relationships might differ substantially.

3) Changes in biotic disturbance regimes have broad consequences for forest ecosystems and the services they provide to society.

4) Climatic effects on forest pest outbreaks might beget further changes in climate by influencing the exchange of carbon, water, and energy between forests and the atmosphere.

5) In some areas, climate-induced changes might result in increased or decreased disturbance risk.   

eastern white pine; photo by F.T. Campbell

A 2021 Study by Clark and D’Amato

Clark and D’Amato (2021) focused on a research site in New England which provided 69 years of data on succession dynamics. The site has four types of secondary forests. Clark and D’Amato (2021) found that mixed hardwood (oak)-pine systems dominated by large diameter eastern white pine (Pinus strobus) exhibited the greatest increase in biomass over the 69-year period and thus performed best as carbon sinks. These forests also had the greatest structural complexity.

However, these “mixedwood” systems are largely an artifact of past clearing for agriculture and are naturally trending toward greater domination by hardwoods. In fact, new trees growing in all four forest types were predominantly shade-tolerant beech (Fagus grandifolia) and hemlock (Tsuga canadensis). Clark and D’Amato (2021) note that these species are both less compatible with predicted future climatic conditions and are under attack by non-native pests — beech bark disease and hemlock woolly adelgid, respectively. The article makes no mention of possible complications from two other pests of beech, beech leaf disease and beech leaf weevil. [All three pests have profiles here.]

They conclude that if the goal is to maximize carbon sequestration in forests – while maintaining structural complexity – managers must adopt silvicultural strategies intended to maintain the pine component. This strategy is not without risk. Mature white pine constitutes 68% of the biomass in the mixedwood stands. Clark and D’Amato (2021) note that a strategy relying so heavily on one species exposes that strategy to a high risk of catastrophic losses due to stochastic disturbance-related mortality, emerging forest health issues, and/or selective timber harvests targeting the largest trees.  Of course, eastern white pine has already survived one pest, white pine blister rust.

SOURCES

Barrett, T.M. and G.C. Robertson, Editors. 2021. Disturbance and Sustainability in Forests of the Western United States. USDA Forest Service Pacific Northwest Research Station. General Technical Report PNW-GTR-992. March 2021

Clark, P.W. and A.W. D’Amato. 2021. Long-term development of transition hardwood and Pinus strobus – Quercus mixedwood forests with implications for future adaptation and mitigation potential. Forest Ecology and Management 501 (2021) 119654

Lovett, G.M., C.D. Canham, M.A. Arthur, K.C. Weathers, and R.D. Fitzhugh. 2006. Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America. BioScience Vol. 56 No. 5 May 2006)

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. Mclimate changeullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016.  Nonnative forest insects and pathogens in the United States: Impacts and policy options.  Ecological Applications, 26(5), 2016, pp. 1437-1455

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector.  Springer Verlag.

Weed, A.S., M.P. Ayers, J.A. Hicke. 2013. Consequences of climate change for biotic disturbances in North American forests. Ecological Monographs, 83(4), 2013, pp. 441–470

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

New Study on Forest Carbon and Pests: The Picture is Ugly

lodgepole pines killed by mountain pine beetle in British Columbia; photo courtesy of Wikipedia

Natural systems, especially forests, could provide as much as 37% of the near-term mitigation necessary to meet Paris global climate goals. In the US, conservation, restoration, and improved land management could provide carbon sequestration equivalent to an estimated 21% of current net annual emissions.

However, the current U.S. forest carbon sink, which includes soils and standing and downed wood as well as live trees, might be in jeopardy due to increasing levels of disturbance, conversion, and/or declining sequestration rates in old growth stands.

Insects and plant diseases are one such disturbance agent. Acting alone or in combination with other forest stressors, they can damage or kill large numbers of trees in short periods of time, thereby reducing carbon sequestration and increasing emissions of stored carbon through decomposition of wood in dead or injured trees.

Historically, native and introduced insects and diseases have impacted an estimated 15% of the total U.S. forest cover annually. This impact is likely to increase. One study (Fei et al., 2019) found that an estimated 41% of the live forest biomass in the contiguous U.S. could be impacted by the 15 most damaging introduced pests already established in the U.S. Continuing introductions of new pests and exacerbated effects of native pests associated with climate change portend worsening losses of live trees. These rising impact of pests, combined with more frequent and severe fires and other forest disturbances, are likely to negate efforts to improve forests’ carbon sequestration capacity.

Sources of information about introduced pests’ impacts is available from, inter alia Campbell and Schlarbaum Fading Forests  II and III, Lovett et al 2016, Poland et al. 2021, many  blogs on this site, and pests’ profiles posed here under “invasive species” tab. Chapter 4 of Poland et al. (2021) provides a summary of what is known about interactions between invasive species and climate change – both climate impacts on bioinvaders and bioinvaders’ effect on carbon sequestration.

The United States and other major polluting countries have certain advantages. Their strong economies have the scientific and financial resources needed to implement effective invasive species prevention and forest management strategies. At the same time, many of them receive the most new forest pests – because they are major importers. These introduced pests pose the most serious and urgent near-term ecological threat to their forests and all the ecosystem services forests provide.

So, reducing insect and disease impacts to forests can simultaneously serve several goals—carbon sequestration, biodiversity conservation, and protecting the myriad economic and societal benefits of forests. See the recent IUCN report on threatened tree species.

A Major New Study

A new study by Quirion et al. (2021) takes another step in quantifying the threat to U.S. forests’ ability to sequester carbon by analyzing data from National Forest Inventory plots. Unfortunately, the re-measurement data for the period 2001 – 2019 are not available in the NFI for the Rocky Mountain states, which represents a critical data gap in the NFI program. This gap might not have had a significant impact on the national findings, however, because while the insect damage level (measured by an earlier inventory round) was quite severe in the Rocky Mountain States, the relatively slow growth of trees in that region means carbon sequestration rates are low.

Forest stand productivity – and carbon sequestration — will typically decline immediately after pest outbreaks, then recover or even increase beyond pre-outbreak levels depending on the productivity and maximum achieved biomass of replacement plant species and related soil characteristics. However, when prevalence of the disturbance increases, by, for example, more frequent pest outbreaks, carbon stocks in standing trees and sequestration rates can be reduced for extended periods.

Findings

  • Nationally, insects and diseases have decreased carbon sequestration by live trees on forest land by 12.83 teragrams carbon per year. This equals ~ 9% of the contiguous states’ total annual forest carbon sequestration and equivalent to the CO2 emissions from over 10 million passenger vehicles driven for one year.
  • This estimate includes the impacts of both native and introduced insects and diseases, because the NFI database does not distinguish between them.
  • Insect-caused mortality had a larger impact than disease-caused mortality (see below). Forest plots recently impacted by insect disturbance sequestered on average 69% less carbon in live trees than plots with no recent disturbance. Plots recently impacted by disease disturbance sequestered on average 28% less carbon in live trees than plots with no recent disturbance.
  • Ecoprovinces in which the greatest annual reductions in live tree carbon sequestration due to pests were the Southern Rocky Mountain Steppe, Cascade Mixed Forest, Midwest Broadleaf Forest, and Laurentian Mixed Forest. (Ecoprovinces are outlined – but not named – in Quirion et al. 2021; more complete information is provided in the supplementary material.)

If this study had been carried out in the 1920’s, when chestnut blight and white pine blister rust were spreading across vast areas and killing large trees, the impact of diseases would have been much higher. Today, the most widespread impacts of diseases are on either small trees (e.g., redbay succumbing to laurel wilt) or slow-growing, high-elevation trees (e.g., whitebark and limber pine to white pine blister rust). As long as no equivalents of those earlier diseases are introduced, insects will probably continue to have the larger impacts.

western white pine killed by blister rust; photo from National Archives

Quirion et al. 2021 note that their estimates should be considered conservative. The USFS’s inventory records only major disturbances. That is, when mortality or damage is equal to or exceeds 25% of trees or 50% of an individual tree species’ count on an area of at least 0.4 ha. This criterion largely excludes less severe pest disturbances, including those from which trees recover but which might have temporary negative effects on carbon sequestration.

The study’s authors note that their work has important limitations. The dearth of data from the Rocky Mountain states is one. Other factors not considered include transfers of carbon from live biomass to dead organic matter, soils, and salvaged or preemptively harvested wood products.  As trees die from pests or diseases, their carbon becomes dead wood and decays slowly, producing a lag in the carbon emissions to the atmosphere.  A small fraction of the carbon in dead wood might be incorporated into soil organic matter, further delaying the emissions.  A full accounting of the carbon consequences of pests and diseases would require assessment of these lags, probably through a modeling study.

affects of mountan pine beetle on lodgepole pine in Rocky Mountain National Park, Colorado photo from Wikimedia

Actions to Maintain Carbon Sequestration

Quirion et al. (2021) outline several actions that would help protect the ability of America’s forests to sequester carbon. These suggestions address both native and introduced pests, since both contribute to the threatened reduction in capacity.

Concerning native pests, the authors call for improved forest management, but warn that measures must be tailored to species and environmental context.

Concerning introduced insects and pathogens, Quirion et al. (2021) call for strengthening international trade policies and phytosanitary standards, as well as their enforcement. The focus should be on the principal pathways: wood packaging (click on “wood packaging” category for on this blog site) and imported plants (click on “plants as vectors” category for on this blog site). Specific steps to reduce the rate of introduction of wood-boring insects include enforcement to increase compliance with the international treatment standard (ISPM#15), requiring trade partners – especially those which have repeatedly shipped infested packaging – to switch to packaging made from alternative materials. Introductions via the plant trade could be reduced by requiring foreign shippers to employ integrated management and critical control point systems (per criteria set by the U.S.) and using emergency powers (e.g., NAPPRA) to further restrict imports of the plants associated with the highest pest risk, especially plant species that are congeneric with native woody plants in North America. See Lovett et al 2016; Fading Forests II & III

As backup, since even the most stringent prevention and enforcement will not eliminate all risk, the authors urge increased funding for and research into improved inspection, early detection of new outbreaks, and strategic rapid response to newly detected incursions.

To reduce impacts of pests established on the continent – both recently and years ago – they recommend increasing and stabilizing dedicated funding for classical biocontrol, research into technologies such as sterile-insect release and gene drive, and host resistance breeding.

Thinning is useful in reducing damage by native bark beetles to conifers. However, it has not been successful in controlling introduced pests for which trees do not have an evolved resistance. Indeed, preemptive harvesting of susceptible species can harm forest ecosystems directly through impacts of the harvesting operation and indirectly as individual trees that may exhibit resistance are removed, reducing the species’ ability to develop resistance over time.

Further research is needed to clarify several more issues, including whether introduced pests’ impacts are additive to, or interact with, those of native species and/or other forest stressors.

SOURCE

Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) P&P Disturbances Correlate With Reduced Carbon Sequestration in Forests of the Contiguous US. Front. For. Glob. Change 4:716582.  [Volume 4 | Article 716582] doi: 10.3389/ffgc.2021.716582

SOURCES of additional information

Campbell, F.T. and S.E. Schlarbaum. Fading Forest reports at http://treeimprovement.utk.edu/FadingForests.htm

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. McCullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016.  Nonnative forest insects and pathogens in the United States: Impacts and policy options.  Ecological Applications, 26(5), 2016, pp. 1437-1455

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector.  Springer Verlag. Available for download at no cost at https://www.fs.usda.gov/treesearch/pubs/61982

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

“Global Tree Assessment”: #s at Risk, Threats, & Carbon Sequestration Planting

Kew Gardens U.K., home to Botanic Gardens Conservation International; Wikipedia

A massive international effort has completed a “Global Tree Assessment: State of Earth’s Trees”. This is the result of five years’ effort; it aims at a comprehensive assessment of the conservation status of all the Earth’s trees. As a result of their work, the authors issue a call to action and include specific recommendations. 

The leads were the Botanic Gardens Conservation International (BGCI) and International Union for Conservation of Nature’s (IUCN) Species Survival Commission (SSC) Global Tree Specialist Group. They were assisted by about 60 cooperating institutions and more than 500 individual experts. The Morton Arboretum was a major U.S. contributor. Here, my focus is on the global assessment. An accompanying blog contains my analysis of reports on the Morton Arboretum report for the U.S.

The Global Tree Assessment is the largest initiative in the history of the IUCN Red List process.  (This process is described in Box 3 of the report, on p. 12; and on p. 40.) As of the end of 2020, IUCN Red List assessments evaluated 28,463 tree species, representing half of all known tree species. Organizers hope to complete comprehensive conservation assessments of all tree species for inclusion on the IUCN Red List by 2023. Other sources utilized included draft Red List profiles and national-level assessments of those species that are found in only one country.

SUMMARY OF FINDINGS

Using these sources, the Global Tree Assessment evaluated 58,497 tree species worldwide. The study determined that 30% are threatened with extinction. This number could change significantly if a large proportion of the 7,700 species (13.2%) recorded as “Data Deficient” turn out to be at risk. At least 142 species are recorded as already extinct in the wild. Two-fifths (41.5%) are considered to be not at risk. Detailed species’ evaluations are provided at GlobalTreeSearch or GlobalTree Portal.

Brazilian forest converted to cattle pasture

The principal threats to trees globally are forest clearance and other forms of habitat loss (at least 65% of species) and direct exploitation for timber and other products (27% or more). The spread of non-native pests is said to affect 5% of the species. Climate change is having a measurable impact on 4% of the species and is expected to increase. (The situation in the United States differs significantly. Overexploitation plays almost no role and on-going habitat loss is important for only a few of the at-risk species.)

The authors decry the lack of attention, historically, to tree endangerment given trees’ ecological, cultural and economic importance. They hope that increased attention to the biodiversity crisis — an estimated 1 million animal and plant species threatened with extinction — and trees’ importance as carbon sinks will lead to increased conservation of trees and forests.  They warn, however, that tree-planting programs must put the right species in the right place, including utilizing species that are under threat. In other words, tree planting practices need to change. They note that a community of botanists and conservationists is ready to assist.

Centers of tree species diversity – and of species under threat – are in Central and South America, followed by the other tropical regions of Southeast Asia and Africa. Fifty-eight percent of tree species are single country endemics. The highest proportion of endemism is found in New Zealand, Madagascar and New Caledonia. The region with the highest proportion of native tree species under threat is tropical Africa, especially Madagascar. The highest numbers of species “Not Evaluated” or “Data Deficient” are in IndoMalaya (tropical Asia) and Oceania. In those regions, about a third of species fall in one of those categories.

forest in Central America

The assessment authors fear ecosystem collapse caused by major, large-scale disturbance events. Examples are recent unprecedented fires in California, southern Australia, Indonesia, and the Amazon (although they don’t mention Siberia). They also note mass mortality events over large areas of forest caused by other factors, including drought and heat stress and the increased incidence of pests. These events have led to a worrying decline of dominant tree species currently evaluated as “Least Concern.” Citing a 2010 report, they list as examples spruce in Alaska, lodgepole pine in British Columbia, aspen in Saskatchewan and Alberta, and Colorado pinon pine (Pinus edulis) in the American southwest.

The authors emphasize the importance of preventing extinction of monotypic tree families. Such events would represent a disproportionate loss of unique evolutionary history, biological diversity, and potential for future evolution. Of the 257 plant families that include trees, 12 are monotypic. They are scattered around the tropics and former Gondwanaland; none is found in the Neo- or Paleoarctic regions. While extinctions to date have rarely affected plants above the rank of genus, the global assessment authors worry that the on-going sixth extinction wave might result in extinctions at the genus or family level.

In this context, the assessment made a particular effort to evaluate the status of species representing the survival of Gondwanian Rainforest lineages. They found that 29% of these tree species are threatened with extinction. Two case studies focus on Australia. They mention habitat conversion but not two non-native pathogens widespread in Australia, Phytophthora cinnamomi and Austropuccinia psidii.  

formerly common, now endangered, Australian tree Rhodamnia rubescens, infected by Austropuccinia psidii; photo courtesy of Flickr

The proportion of total tree diversity designated as threatened is highest on island nations, e.g., 69% of the trees on St. Helena, 59% of the trees on Madagascar, 57% of the trees on Mauritius. Hawai`i is not treated separately from the United States as a whole. According to Megan Barstow of BGCI (pers. comm.), the just updated IUCN Red List includes 214 threatened tree species in Hawai`i.

[For the U.S. overall, the IUCN reports 1,424 tree species, of which 342 (24%) are considered threatened. In the companion U.S. assessment, the Morton Arboretum and collaborators found that 11% of 841 continental U.S. tree species are threatened.]

MAIN THREATS TO TREES

Habitat loss

Over the past 300 years, global forest area has decreased by about 40%. Conversion of land for crops and pasture continues to threaten more tree species than any other known threat. Additional losses are caused by conversion for urban and industrial development and transport corridors, and by changes in fire regimes. In total, these factors cumulatively threaten 78% of all tree species, 84% if one includes conversion to wood plantations.

Caribbean mahogany (Swietenia mahogani); photo by Miguel Vieria

Forest Exploitation

Exploitation, especially for timber, is the second greatest threat globally, affecting 27% of tree species (more than 7,400 tree species). The report focuses on centuries of harvest of valuable tropical timbers and exploitation for fuelwood, with an emphasis on Madagascar, where nearly half of all tree species (117 out of 244 tree species) are threatened.

Pests and diseases

Tree species are impacted by a wide range of pests and diseases that are spread by natural and artificial causes. Invasive and other problematic species are recorded as threats for 1,356 tree species (5%) recorded on the IUCN Red List. This figure might be low because some of the information is outdated (see my discussion of American beech in the companion blog about the North American report, here.)  Also, climate change is altering the survival opportunities for many pests and diseases in new environments. The example given is the ash genus (Fraxinus), under attack by not only the emerald ash borer in North America and now Russia and Eastern Europe but also the disease Ash Dieback across Europe.  The report refers readers to the International Plant Sentinel Network for early warning system of new and emerging pest and pathogen risks, as well as help in coordinating responses.

black ash (Fraxinus nigra) swamp; Flickr

Climate Change

Climate change is impacting all forest ecosystems and is emerging as a significant recorded threat to individual tree species. In the IUCN Red List assessments, climate change and severe weather is recorded as a threat in 1,080 (4%) cases. Trees of coastal, boreal and montane ecosystems are disproportionately impacted. The authors note that the actual impact of climate change is probably more widespread, as it is also impacting fire regimes and the survival, spread, and virulence of pests.

CURRENT CONSERVATION EFFORTS

In Protected areas

Currently, 15.4% of the global terrestrial surface has formal protection status. The IUCN study authors recognize in situ conservation of trees through protection of existing natural habitats as the best method for conserving tree diversity. It is therefore encouraging that at least 64% of all tree species are included in at least one protected area. However, representation is higher for species that are not threatened – 85% are represented in a conservation area while only 56% of threatened trees species are. Nor does the report assess the effectiveness of protection afforded by the various in situ sites. The authors express hope that the parallel IUCN Red List of Ecosystems will contribute to understanding of the efficacy of conservation efforts targetting forests.

The Global Trees Campaign is a joint initiative of Fauna & Flora International (FFI) and BGCI. Since 1999 the campaign has worked to conserve more than 400 threatened tree species in more than 50 countries. The current focus is on six priority taxa = Acer, Dipterocarps, Magnolia, Nothofagus, Oak, and Rhododendron.

Rhododendron in Cook Forest State Park, PA; photo by F.T. Campbell

In Botanic gardens and seed banks

Especially for species under threat, conservation outside their native habitat – ex situ conservation – is an essential additional component. Currently 30% of tree species are recorded as present in at least one botanic garden or seed bank. Again, representation is higher for species that are not threatened – 45% are represented compared to only 21% of threatened tree species. For 41 species, ex situ conservation provides the only hope of survival, since they are extinct in the wild.

AN URGENT CALL FOR ACTION

The authors and collaborators who prepared the Global Tree Assessment hope that this report will help prompt action and better coordination of priorities and resources to better ensure that all tree species are supported by in situ conservation sites and by appropriate management plans. They state several times the importance of restoration plantings relying on native species. The purpose of plantings needs to include conservation of biological diversity, not just accumulation of carbon credits. The Ecological Restoration Alliance of Botanic Gardens (https://www.erabg.org/) is demonstrating that forest restoration can benefit biodiversity conservation. In many cases, propagation methods need to be developed. Also, projects must include aftercare and monitoring to ensure the survival of planted seedlings.

The IUCN assessment notes that ex situ conservation is an important backup. Education, capacity-building and awareness-raising are needed to equip, support, and empower local communities and other partners with the knowledge and skills to help conserve threatened trees.

Policy

The report say it does not address policy and legislation – a gap that fortunately is not quite true. The report both summarizes pertinent international agreements but also provides specific recommendations.

The international agreements that pertain to tree and forest conservation include:

  • Convention on Biological Diversity (CBD) and several specific programs: the Forestry Programme, Protected Area Programme and Sustainable Use Programme.
  • Global Strategy for Plant Conservation (GSPC), which is now developing post-2020 targets.
  • United Nations Framework Convention on Climate Change (UNFCCC) and countries’ implementing pledges to conserve carbon sinks, e.g., REDD+ (Reducing Emissions from Deforestation and Forest Degradation)
  • United Nations Strategic Plan for Forests 2017-2030
  • Global Plan of Action for the Conservation and Sustainable Use of Forest Genetic Resources
  • Convention on International Trade in Endangered Species, which currently protects 560 tree species, including 308 of the most threatened timbers

The report also mentions the voluntary New York Declaration on Forests, under which more than 200 entities – including governments, businesses, and Indigenous communities — have committed to eliminating deforestation from their supply chains. The supply chains touched on include those for major agricultural commodities, production of which is one of the greatest threat to trees.

SPECIFIC RECOMMENDATIONS

1. Strengthen tree conservation action globally through the formation of a new coalition that brings together existing resources and expertise, and applies lessons from the Global Trees Campaign to radically scale up tree conservation.

2. Use information in the GlobalTree Portal on the conservation status of individual tree species and current conservation action to plan additional action at local, national, and international levels, and for priority taxonomic groups. Build on the Portal by strengthening research on “Data Deficient” tree species, and collating additional information threatened species to avoid duplication of efforts and ensure conservation action is directed where it is needed most.

3. Ensure effective conservation of threatened trees within the protected area network by strengthening local knowledge, monitoring populations of threatened species and, where necessary, increasing enforcement of controls on illegal or non-sustainable harvesting of valuable species. Extend protected area coverage for threatened tree species and species assemblages that are currently not well-represented in protected areas.

4. Ensure that all globally threatened tree species are conserved in well-managed and genetically representative ex situ living and seed bank collections, with associated education and restoration programs.

5. Align work with the UN Decade on Ecosystem Restoration 2021–2030, engaging local communities, government forestry agencies, the business community, and other interested parties to ensure that the most appropriate tree species, including those that are threatened, are used in tree planting and restoration programs.

6. Improve data collection for national inventory and monitoring systems and use this information to reduce deforestation in areas of high tree diversity in association with REDD+ and Nationally Determined Contributions (NDCs).

7. Increase the availability of government, private and corporate funding for threatened tree species, and ensure that funding is directed to species and sites that are in greatest need of conservation.

SOURCE

Global Tree Assessment State of Earth’s Trees September 2021 Botanic Gardens Conservation International available here

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

The Morton Arboretum Assesses U.S. Tree Genera at Risk

 

habitat of the Florida torreya tree; photo via Creative Commons

In August, the Morton Arboretum announced completion of a series of reports on the conservation status of major tree genera native to the continental United States. It is available here.  The series of reports provides individual studies on Carya, Fagus, Gymnocladus, Juglans, Pinus, Taxus, and selected Lauraceae (Lindera, Persea, Sassafras). (Links to the individual reports are provided at the principal link above.)  

The project was funded by the USDA Forest Service and the Institute of Museum and Library Services, The Morton Arboretum and Botanic Gardens Conservation International U.S.

Each report provides a summary of the ecology, distribution, and threats to species in the genus, plus levels of ex situ conservation efforts. The authors hope that the data in these reports will aid in setting conservation priorities and coordinating activities among stakeholders. The aim is to further conservation of U.S. keystone trees.

These reports are part of the overall “Global Tree Assessment: State of Earth’s Trees”  compiled under the auspices of Botanic Gardens Conservation International (BGCI) and IUCN SSC Global Tree Specialist Group. I discuss the global assessment in a separate blog to which I will link. The global report evaluates species’ status according to both the International Union of Conservation of Nature’s (IUCN) Red List and NatureServe. The process used is explained in each both the international and U.S. reports. For the U.S. overall, the global assessment identifies 1,424 species of tree, of which 342 (24%) are considered threatened. Hawai`i specifically is home to 241 endangered tree species (Megan Barstow, BGCI Conservation Officer, pers. comm.). See my blogs about threats to Hawaiian trees.

Quercus lobata (valley oak) at Jack London State Park, California

Like the global assessment, these individual studies of nine genera–carried out by the Morton Arboretum–are a monumental accomplishment. They vary in size and format. The report on oaks was completed first and is the most comprehensive. It is 220 pages, incorporating individual reports on 28 species of concern. The report on pines is 40 pages. It contains summary information and tables on all 37 pine species native to the United States, but lacks write-ups on individual species. The report on Lauracae is 25 pages; it evaluates the threat to five species in three genera from laurel wilt disease. The report on walnuts is 23 pages. It includes brief descriptions of six individual species, including butternut. The report on hickories (Carya spp.) is 20 pages.  It provides brief description of 11 species. The report on yews is 18 pages. It covers three species. The report on Fagus addresses the single species in the genus, American beech. It is 17 pages. The shortest report is on another single species, Kentucky coffeetree; it is 15 pages.

Coverage of Threats from Non-Native Insects and Diseases in the Morton Arboretum Reports

In keeping with my focus, I concentrated my review of these nine reports on their handling of threats from non-native insects and pathogens. Six of the reports make some reference to pests – although the discussion is not always adequate, in my view. There are puzzling failures to mention some pathogens.

Genera subject to minimal threats from pests (native or non-native) include the monotypic Kentucky coffeetree (Gymnocladus dioicus), whichis considered by the IUCN to be Vulnerable due habitat fragmentation, rarity on the landscape, and population decline.

A second such genus is Carya spp., the hickories. The entire genus is assessed by the IUCN as of Least Concern. The Morton study ranked two species, C. floridana and C. myristiciformis, as of conservation concern. 

Three evaluators – the IUCN, the Morton Arboretum, and Potter et al. (2019) – agree that one of the three U.S. yew species, Florida torreya (Taxus floridana or Torreya taxifolia), is Critically Endangered because of its extremely small range, low population, and deer predation. Indeed, Potter et al. (2019) ranked Florida torreya as first priority of all forest trees in the continental United States for conservation efforts. However, the Morton Arboretum analysis makes no mention of the canker disease reported by, among others, the U.S. Forest Service.

A third of the 28 oak (Quercus spp.) species considered to be of conservation concern per the Morton study criteria are reported to be threatened by non-native pests. Pest threats to oak species not considered to be of conservation concerned were not evaluated in the report.

The Morton report records 37 pine species (Pinus spp.) as native to the U.S. Native and introduced insects and pathogens are a threat to many, especially in the West.

Two reports – those on the Lauraceae and beech – focus almost exclusively on threats from non-native pests. The report on walnuts (Juglans spp.) divides its attention between non-native pests and habitat conversion issues. This approach comes into some question as a result of the recent decision by state plant health officials to that thousand cankers disease does not threaten black walnut (J. nigra) in its native range.

black walnut (J. nigra) photo by F.T. Campbell

Here I examine five of the individual genus reports in greater detail.

Oaks

The Morton report says that more than 200 oak species are known across North America, of which 91 are native in the United States. The study concludes that 28 of these native oaks are of conservation concern based on extinction risk, vulnerability to climate change, and low representation in ex situ collections. [The IUCN Red List recognizes 16 U.S. oak species as globally threatened with extinction.] Nearly all of the Morton’s report 28 species are confined to small ranges. In the U.S., regional conservation hotspots are in coastal southern California, including the Channel Islands; southwest Texas; and the southeastern states.

The summary opening section of the Morton report says 10 (36%) of the threatened oaks face a threat by a non-native pathogen. It admits that lack of information probably results in an underestimation of the pest risk. I found it difficult to confirm this overall figure by studying the detailed species reports because in some cases the threatening pathogen is not currently extant near the specific tree species’ habitat. I appreciate the evaluators’ concern about the potential for the pathogen, e.g., Phytophthora ramorum or oak wilt, to spread from its current range to vulnerable species growing on the other side of the continent. However, I wish the overview summary at the beginning of the report were clearer as to which species are currently being infected, which face a potential threat.

The report emphasizes the sudden oak death pathogen (SOD; Phytophthora ramorum), stating that it which currently poses a significant risk to wild populations of Q. parvula. However, the situation is more complex. As I note in my blog on threats to oaks, Q. parvula is divided into two subspecies. In the view of California officials, one, Q. p. var. shrevei, is currently threatened by SOD but the other, Q. p. var. parvula, (Santa Cruz Island oak) is currently outside the area infested by the pathogen. Perhaps the Morton Arboretum evaluators consider the potential risk to the second subspecies to be sufficient to justify stating that the pathogen poses a significant threat to the entire species; but I would appreciate greater clarity on this matter.

The report also mentions the potential threat to several rare oak species in the Southeast if SOD spreads there. While the Morton report rarely discusses species that have not been assessed as under threat, it does note that two species ranked as being of Least concern – coast live oak (Q. agrifolia) and California black oak (Q. kelloggii) – have been highly affected by SOD. 

The Fusarium disease vectored by the polyphagous and Kuroshio shot hole borers is mentioned as a threat to Engelmann (Q. engelmannii)and valley (Q. lobata) oaks. The latter, in particular, is considered by the Morton Arboretum assessors to be already much diminished by habitat conversion. 

In the East, hydrological changes have facilitated serious damage to Ogelthorpe oak (Q. oglethorpensis) by the fungus that causes chestnut blight–Cryphonectria parasitica. 

The Morton study mentions oak wilt (Ceratocystis or Bretziella fagacearum) as an actual or potential factor in decline of oaks in the red oak clade (Sect. Lobatae). Only one of the oak species discussed – Q. arkansana – is in the East, were oak wilt is established. The rest are red oaks in California, where oak wilt is not yet established. Again, there is no discussion of the impact of oak wilt on widespread species not now considered to be of conservation concern.

In the individual species profiles making up the bulk of the Morton report on oaks, but not in the summary, the Morton report also mentions the goldspotted oak borer (Agrilus auroguttatus) as an actual or potential factor in decline of the same oaks in the red oak group. The following species – Q. engelmanni, Q. agrifolia, Q. parvula, Q. pumila — are in California and at most immediate threat.

The Morton study also mentions several native insects that are attacking oaks, and oak decline. It calls for further research to determine their impacts on oak species of concern.

For analyses of the various pests’ impacts on oaks broadly, not focused on at-risk tree species, see my recent blog updating threats to oaks, posted here, and the pest profiles posted at www.dontmovefirewood.org

Pines

The Morton report lists 12 pine species as priorities out of the total of 37 species native to the United States. The report notes that the majority of the at-risk species in the West are threatened primarily by high mortality from one or more pests, in particular native bark beetles.

 Six of the 12 priority species are five-needle pines affected by white pine blister rust (WPBR; Cronartium ribicola). The report contains maps showing the distribution of WPBR. In some cases, the native mountain pine beetle (Dendroctonus ponderosae) contributes to immediate mortality. Presentation of recommendations is scattered and sometimes seems contradictory. Thus, P. longaeva (bristlecone pine) is said by the IUCN to be stable and is not listed among the 12 threatened species, but the Morton Arboretum assessors called for its receiving high conservation priority. P. albicaulis (whitebark pine) is a candidate for listing as Threatened under the Endangered Species Act, but the Morton Arboretum authors did not single it out for priority action beyond listing it among the dozen at-risk species.

P. albicaulis (whitebark pine) at Crater Lake National Park; photo courtesy of Richard Sniezko, USFS

The report also notes impacts by Phytopthora cinnamomi on pines; a maps shows the distribution of this non-native pathogen. A third non-native pathogen — pitch canker (Fusarium circinatum) — is mentioned as affecting Monterrey pine (P. radiata). Torrey pine (Pinus torreyana) is also affected by pitch canker, but this pathogen is ranked by the Morton study as causing only moderate mortality in association with other factors. Torrey pine is ranked as critically endangered and decreasing in populations.

The report also publishes the rankings developed by Potter et al. (2019).  P. torreyana was the top-ranked pine, ranked at 18 (less urgent than, eastern hemlock).

The Morton study authors concluded that native U.S. pines are under serious threat. However, their economic, ecological, and cultural importance makes them obvious targets for continued conservation priority.

For my analysis of the various pests’ impacts on pines broadly, see the pest profiles posted at www.dontmovefirewood.org

Lauraecae

The Morton group analyzed five of the 13 species native to the United States, chosen based on three factors – tree-like habit, susceptibility to laurel wilt disease, and distribution in areas currently affected by the disease. They note the importance of Sassafras as a monotypic genus.

Horton House before death of the redbay trees; photo by F.T. Campbell

The Morton study notes the conservation status of several species needs changing due to the rapid spread of laurel wilt disease. I applaud this willingness to adjust, although I would be inclined to assign a higher ranking based on the most recent data from Olatinwo et al. (2021), cited here.

  • Redbay (Persea borbonia) was assessed in 2018 as IUCN Least Concern; it is now being re-assessed, with a probable upgrade to Vulnerable. The Morton study says that recent evidence points towards the ecological extinction of P. borbonia from coastal forest ecosystems. Potter et al. (2019) ranked redbay as fifth most deserving of conservation effort overall.
  • Silk bay (Persea humilis), endemic to Florida, is currently being assessed for the IUCN; it is recommended that it be designated as Near Threatened.
  • Swamp bay (Persea palustris) is widespread. It is being assessed for the IUCN; it is recommended for the Vulnerable category.
  • Sassafras (Sassafras albidum) is widely distributed. Sassafras had been assessed as of Least Concern as recently as the 2020 edition of the IUCN Red List. The Morton study notes that the current distribution of laurel wilt disease spans only a small percent of its range, so it does not pose an imminent threat to sassafras. However, cold-tolerance tests for the disease’s vector indicate the possibility of northward spread into more of the sassafras’ distribution. I note that laurel wilt is currently present in northern Kentucky and Tennessee.  

American Beech  

The Morton report notes that beech (Fagus grandifolia) is very widespread and a dominant tree in forests throughout the Northeastern United States and Canada. It is the only species in the genus native to North America, so presumably of high conservation interest. The report also notes its ecological importance (see also Lovett et al. 2006).

Beech bark disease is reported by the Morton Arboretum to have devastated Northeastern populations. The disease is well established in all beech-dominated forests in the United States, though it occurs on less than 30% of American beech’s full distribution. After mature beech die, thickets of young, shade-tolerant root sprouts and seedlings grow up, preventing regeneration of other tree species. Nevertheless, American beech was listed as of Least Concern by the IUCN in 2017.

The report makes no mention of beech leaf disease, which came to attention after the Morton assessment project had been almost completed. I think this is a serious gap that undermines the assessment not just of the species’ status in the wild but also of the efficacy of conservation efforts.

healthy American beech; photo by F.T. Campbell

Walnuts

The Morton team evaluated five species of walnut (Juglans californica, J. hindsii, J. major, J. microcarpa, and J. nigra); and butternut (J. cinerea). Thousand cankers disease – caused by the fungus Geosmithia morbida, which is vectored by the walnut twig beetle (Pityophthorus juglandis) – is reported by the Morton team as second in importance to butternut canker. However, as I noted in a recent blog, the states that formerly considered the disease to pose a serious threat no longer think so and are terminating their quarantine regulations. This decision too recent for consideration by the Morton team.

One of the walnuts — Juglans californica (Southern Calif walnut) — is considered threatened by habitat loss. The rest of the walnuts are categorized by the IUCN as of Least Concern.

cankered butternut in New England; photo by F.T. Campbell

Butternut (Juglans cinerea), however, is considered by the IUCN to be Endangered. Although present across much of the Eastern deciduous forest, it is uncommon. It has suffered an estimated 80% population decline as a result of the disease caused by the butternut canker fungus Ophiognomonia clavigignenti-juglandacearum. 

SOURCES

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Am beech. Lisle, IL: The Morton Arboretum. August 2021

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Hickories. Lisle, IL: The Morton Arboretum.

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Kentucky Coffeetree. Lisle, IL: The Morton Arboretum.

Beckman, E., Meyer, A., Denvir, A., Gill, D., Man, G., Pivorunas, D., Shaw, K., & Westwood, M. (2019). Conservation Gap Analysis of Native U.S. Oaks. Lisle, IL: The Morton Arboretum.

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Pines. Lisle, IL: The Morton Arboretum.

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Laurels. Lisle, IL: The Morton Arboretum. August 2021

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Walnuts. Lisle, IL: The Morton Arboretum. August 2021

Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Yews. Lisle, IL: The Morton Arboretum.

Lovett, G.M., C.D. Canham, M.A. Arthur, K.C., Weathers, and R.D. Fitzhugh. 2006. Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America. BioScience Vol. 56 No. 5 May 2006)

Olatinwo, R.O., S.W. Fraedrich & A.E. Mayfield III. 2021. Laurel Wilt: Current and Potential Impacts and Possibilities for Prevention and Management. Forests 2021, 12, 181. 

Potter, K.M., M.E. Escanferla, R.M. Jetton, G. Man, B.S. Crane.  2019. Prioritizing the conservation needs of United States tree species: Evaluating vulnerability to forest insect and disease threats. Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm