drivers of extinction – Page 5 – Center for Invasive Species Prevention

International Phytosanitary System: More Evidence of Failure

Rome: home of the International Plant Protection Convention

I often assert that the international phytosanitary system has proven to be a failure in preventing introductions.

Some of the recent publications support my conclusion – although most don’t say so explicitly. For example, the Fenn-Moltu et al. (2023) study of insect transport and establishment around the world found that the number of invasive species-related treaties, regulations and legislation a country has adopted had no significant effect on either the number of insect species detected at that country’s border or the number of insect species that established in that country’s ecosystems..

Weber et al. also found considerable evidence that international and U.S. phytosanitary systems are not curtailing introduction of insects and entomophagic pathogens. In my earlier blog I review their study of unintentional “self-introductions” of natural enemies of arthropod pests and invasive plants. They conclude that these “self-introductions” might exceed the number of species introduced intentionally. These introductions have been facilitated by the usual factors: the general surge in international trade; lack of surveillance for species that are not associated with live plants or animals; inability to detect or intercept microorganisms; huge invasive host populations that allow rapid establishment of their accidentally introduced natural enemies; and lack of aggressive screening for pests already established. Examples cited include species introduced to the United States’ mainland and Hawai`i specifically.

The U.S. Capitol – one of the entities that can reflect **our** priorities in setting phytosanitary policy

As I point out often, altering human activities that facilitate invasion is a political process. So is amending international agreements that are not effective. We need to determine the cause of the failures of the existing institutions and act to rectify them. See my critiques of both the American and international phytosanitary system Fading Forests II and Fading Forests III (see links at the end of this blog) and my earlier blogs, especially this and this.

SOURCES

Fenn-Moltu, G., S. Ollier, O.K. Bates, A.M. Liebhold, H.F. Nahrung, D.S. Pureswaran, T. Yamanaka, C. Bertelsmeier. 2023. Global flows of insect transport and establishment: The role of biogeography, trade and regulations. Diversity and Distributions DOI: 10.1111/ddi.13772

Weber, D.C., A.E. Hajek, K.A. Hoelmer, U. Schaffner, P.G. Mason, R. Stouthamer, E.J. Talamas, M. Buffington, M.S. Hoddle and T. Haye. 2020. Unintentional Biological Control. Chapter for USDA Agriculture ResearchService. Invasive Insect biocontrol and Behavior Laboratory. https://www.ars.usda.gov/research/publications/?seqNo115=362852

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

www.fadingforests.org

Predicting Impacts – Can We Do It?

Clive Braser and others study *Phytophthora* species in their native habitats of Vietnam; which will become aggressive invaders in North America?

For years, one focus of this blog has been on scientists’ efforts to improve prevention of new introductions of forest pests. In earlier blogs, I summarized and commented on efforts by Mech et al. (2019) and Schultz et al. (2021), who extrapolate from insect-host relationships of pests already established in North America. [Full citations are presented at the end of this blog.] Both limited their analysis to insects; Mech et al. focused on those that attack conifers, Schultz et al. on those that attack single genera of angiosperms (hardwoods).

However, many of the most damaging agents are pathogens; for an indication, review the list under “invasive species” here. Indeed, Beckman et al. (2021) reported that only three non-native organisms pose serious threats to one or more of the 37 species of Pinus native to the U.S. All are pathogens: white pine blister rust (WPBR), pitch canker, and Phytophthora root rot (Phytophthora cinnamomi).

For this reason I welcome a study by Li et al. (2023), who used laboratory tests to evaluate the threat posed by more than 100 fungi associated with bark beetles. Since there are more than 6,000 species of bark and ambrosia beetles and they are commonly intercepted at the U.S. border, determining which should be priorities is important. Li et al. point out that the vast majority of such introductions have had minimal impacts. Two, however, have caused disastrous levels of damage: Dutch elm disease and laurel wilt disease.

Li et al. tested 111 fungi associated with 55 scolytine beetles from areas of Eurasia with latitudes and ecosystems analagous to those in the southeastern U.S. The beetles assessed included beetle species responsible for recent major tree mortality events in Eurasia: Dendroctonus species, Platypus koryoensis (Korean oak wilt), Platypus quercivorus (Japanese oak wilt) and Tomicus species.

The authors tested the fungi’s virulence on four species of trees native to the Southeast – two pines (Pinus taeda and P. elliottii var. elliottii), and two oaks(Quercus shumardii and Q. virginiana).

Li et al. found that none of 111 fungal associates caused a level of damage on these four hosts equal to Dutch elm disease on elms or laurel wilt disease on trees in the Lauraceae. Twenty-two of the fungi were minor pathogens – meaning they might cause damage under certain conditions or when loads of inoculum are large enough.

redbay trees killed in coastal Georgia by laurel wilt; photo by Scott Cameron

I think Li et al. set an extremely high bar for “serious” damage. Surely we wish to prevent introduction of pathogens that cause damage at a lower level than the catastrophes to which these two diseases have exposed a genus (elms) and a family (Lauraceae)! Still, the scientific approach used here is a step toward addressing pathogens. These agents of tree mortality are addressed much less frequently than insects. I hope that scientists will continue to test the virulence of these fungi on some of the thousands of other species that make up the forests of the United States, or at least the dominant species in each ecosystem.

It is discouraging that Raffa et al. (2023) found none of four approaches to predicting a new pest’s impact to be adequate by itself. Instead, they outlined the relative strengths and weaknesses of each approach and the circumstances in which they might offer useful information. I am particularly glad that they have included pathogens, not just insects. The four approaches they review are:

(1) pest status of the organism in its native or previously invaded regions;

(2) statistical patterns of traits and gene sequences associated with high-impact pests;

(3) sentinel plantings to expose trees to novel pests; and

(4) laboratory tests of detached plant parts or seedlings under controlled conditions.

They emphasize that too little information exists regarding pathogens to predict which microbes will become damaging pathogens when introduced to naïve hosts in new ecosystems. See the article, especially Figure 4, for their assessment of the strengths each of the several approaches.

Raffa et al. raise important questions about both the science and equity issues surrounding invasive species. As regards scientific issues, they ask, first, whether it will ever be possible to predict how each unique biotic system will respond to introduction of a new species. Second, they ask how assessors should interpret negative data? In the context of equity and political power, they ask who should make decisions about whether to act?

In my blog I expressed concern about finding that most introduced forest insects are first detected in urban areas whereas introduced pathogens are more commonly detected in forests. I hope scientists will redouble efforts to improve methods for earlier detection of pathogens. Enrico Bonello at Ohio State and others report that spectral-based tools can detect pathogen-infected plants, including trees.

Japanese cherry trees burned on the Washington D.C. mall because infested by scale; on order of Charles Marlatt

Identifying Key Pathways

International trade is considered the single most important pathway for unintentional introductions of insects. Updated figures remind us about the stupendous amounts of goods being moved internationally. According to Weber et al., international shipping moves ~133 million TEU containers per year between countries, the majority between continents. Four times this number move within regions via coastal shipping. On top of that, four billion passenger trips take place by air every year. Air freight carries another ~220 million tons of goods; while this is a tiny fraction of the weight shipped by boat, the packages are delivered in less than a day – greatly increasing the likelihood that any unwanted living organisms will survive the trip. The U.S. also imports large numbers of live plants – although getting accurate numbers is a challenge. MacLachlan et al. (2022) report 5 billion plants imported in 2021, but the USDA APHIS annual report for FY22 puts the number at less than half that figure: 2.2 billion plant units.

Given the high volume of incoming goods, Weber et al. advocate improved surveillance (including analysis of corresponding interceptions) of those pathways that are particularly likely to result in non-native species’ invasions, e.g. live plants, raw lumber(including wood packaging), and bulk commodities e.g. quarried rock. Isitt et al. and Fenn-Moltu et al. concur that investigators should focus on the trade volumes of goods that are likely to transport plant pests – in their cases, plant imports.

The importance of the plant trade as a pathway of introduction for has been understood for at least a century – as witnessed by the introductions of chestnut blight DMF and white pine blister rust, DMF and articles by Charles Marlatt. A decade ago, Liebhold et al. (2012) calculated that the approach rate of pests on imported plants was 12% — more than 100 times higher than the 0.1% approach rate found by Haack et al. (2014) for wood packaging.

Since plant-insect interactions are the foundation of food webs, changes to a region’s flora will have repercussions throughout ecosystems, including insect fauna. See findings by teams led by Doug Tallamy and Sara Lalk; and a chapter in the new forest entomology text written by Bohlmann, and Krokene (citation at end of blog under Allison, Paine, Slippers, and Wingfield). Sandy Liebhold and Aymeric Bonnamour also addressed explicitly links between introductions of non-native plant and insect species. Weber et al. call this phenomenon the “receptive bridgehead effect”: a non-native plant growing prolifically in a new ecosystem provides a suitable host for an organism that feeds on that host, raising the chance for its establishment.

Recent studies confirm the importance of the “receptive bridgehead effect”. Isitt and colleagues found that the large numbers of introduced European insect species – all taxa, not just phytophagous insects – established in North America and Australia/New Zealand were best explained by the numbers of European plants introduced to these regions – in other words, the most important driver appears to be the diversity of non-native plants.

The presence of European plants in North America and Australia/New Zealand promoted establishment of European insects in two ways. First, these high-volume imports increased the propagule pressure of insects associated with this trade. Live plant imports might have facilitated the establishment of ~70% of damaging non-native forest insects in North America. Second, naturalization of introduced European plants provided a landscape replete with suitable hosts. This is especially obvious in Australia/New Zealand, which have unique floras. In Australia, nearly 90% of non-native pest insects are associated with non-native plants. Those non-native insects that do feed on native plants are more likely to be polyphagous.

Amur honeysuckle – one of the hundreds of Asian plants invading North American ecosystems; via Flickr

I hope U.S. phytosanitary officials apply these lessons. Temperate Asia is the source of more non-native plants established in both North America and Australia/New Zealand than is Europe. Already, many insects from Asia have invaded the U.S. The logicof the “receptive bridgehead effect” points to prioritizing efforts to prevent even more Asian insects from reaching our shores!

Fenn-Moltu et al. sought to elucidate which mechanisms facilitate species’ success during the transport and introduction/establishment stages of bioinvasion. They studied the transport stage by analyzing border interceptions of insects from 227 countries by Canada, mainland U.S., Hawai`i, Japan, New Zealand, Great Britain, and South Africa over the 60 year period 1960 – 2019. They studied establishment by analyzing attributes of 2,076 insect species recorded as established after 1960 in the above areas plus Australia (North America was treated as a single unit comprised of the continental U.S. and Canada).

The number of species transported increased with higher Gross National Income in the source country. The number of species transported decreased with geographic distance. They suggest that fewer insects survive longer journeys, but say additional information is needed to verify this as the cause. The number of species transported was not affected by species richness in the native region.

More species established when introduced to a country in the same biogeographic region. They were not surprised that environmental similarity between source and destination apparently strongly affected establishment success. The number of species established was not affected by species richness in the native region. For example, the greatest number of established species originated from the Western and Eastern Palearctic regions, which together comprise only the fifth-largest pool of native insect species.

Gaps Despite Above Studies

As I noted at the beginning, most of the studies examining current levels of pests transported on imported plants have been limited to insects. This is unfortunate given the impact of introduced pathogens (again, review the list damaging organisms under “invasive species” here).

In addition, most studies analyzing the pest risk associated with plant imports use port inspection data – which are not reliable indicators of the pest approach rate. The unsuitability of port inspection data was explained by Liebhold et al. in 2012 and Fenn-Moltu et al. a decade later – as well as Haack et al. 2014 (as the data pertain to wood packaging). Fenn-Moltu et al. note that inspection agencies often (and rightly!) target high-risk sources/commodities, so the records are biased. Other problems might arise from differences in import volume, production practices, and differences in records that identify organism only to genus level rather than species. Fenn-Moltu et al. call for relying on randomized, statistically sound inspection systems; one such example is USDA’s Agriculture Quarantine Inspection System (AQIM). Under AQIM, incoming shipments are randomly selected and put through more thorough inspections to produce statistically based estimates of approach rates, defined as the percent of inspected shipments found to be infested with potential pests (Liebhold et al. 2012). I ask why scientists who are aware of this issue have not obtained AQIM data for pests associated with plant imports. Plant imports have been included in the AQIM system since 2008. Have they not been able to persuade APHIS to provide these data? Or are these data available for only limited types of imported plants? Too narrow a focus would create a different source of potential bias.

Both Isitt et al. and Fenn-Moltu et al. list factors not addressed and other caveats of which we should be aware when extrapolating from their findings.

SOURCES

Allison, J. T.D. Paine, B. Slippers, and M.J. Wingfield, Editors. 2023. Forest Entomology and Pathology Volume 1: Entomology. Springer available gratis at https://link.springer.com/book/10.1007/978-3-031-11553-0

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

Hoddle. M.S. 2023. A new paradigm: proactive biological control of invasive insect pests. BioControl https://doi.org/10.1007/s10526-023-10206-5

Isitt, R., A.M. Liebhold, R.M. Turner, A. Battisti, C. Bertelsmeier, R. Blake, E.G. Brockerhoff, S.B. Heard, P. Krokene, B. Økland, H. Nahrung, D. Rassati, A. Roques, T. Yamanaka, D.S. Pureswaran. 2023. Drivers of asymmetrical insect invasions between three world regions. bioRxiv preprint doi: https://doi.org/q0.1101/2023.01.13.523858

Li, Y., C. Bateman, J. Skelton, B. Wang, A. Black, Y-T Huang, A. Gonzalez, M.A. Jusino, Z.J. Nolen, S. Freemen, Z. Mendel, C-Y Chen, H-F Li, M. Kolarik, M. Knizek, J-H. Park, W. Sittichaya, T-H Pham, S. Ito, M. Torii, L. Gao, A.J. Johnson, M. Lu, J. Sun, Z. Zhang, D.C. Adams, J. Hulcr. 2022. Pre-invasion assessment of exotic bark beetle-vectored fungi to detect tree-killing pathogens. Phytopathology Vol 112 No. 2 February 2022

Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live Plant Imports: the Major Pathway for Forest Insect and Pathogen Invasions of the US. www.frontiersinecology.org

Liebhold, A.M., T. Yamanaka, A. Roques, S. August, S.L. Chown, E.G. Brockerhoff and P. Pyšek. 2018. Plant diversity drives global patterns of insect invasions. Sci Rep 8, 12095 (2018). https://doi.org/10.1038/s41598-018-30605-4

MacLachlan, M.J., A. M. Liebhold, T. Yamanaka, M. R. Springborn. 2022. Hidden patterns of insect establishment risk revealed from two centuries of alien species discoveries. Sci. Adv. 7, eabj1012 (2021).

Mech, A.M., K.A. Thomas, T.D. Marsico, D.A. Herms, C.R. Allen, M.P. Ayres, K.J. K. Gandhi, J. Gurevitch, N.P. Havill, R.A. Hufbauer, A.M. Liebhold, K.F. Raffa, A.N. Schulz, D.R. Uden, and P.C. Tobin. 2019. Evolutionary history predicts high-impact invasions by herbivorous insects. Ecol Evol. 2019 Nov; 9(21): 12216–12230.

Raffa, K.F., E.G. Brockerhoff, J-C. Gregoirem R.C. Hamelin, A.M. Liebhold, A. Santini, R.C. Venette, and M.J. Wingfield. 2023. Approaches to Forecasting Damage by Invasive Forest Insects and Pathogens: A Cross-Assessment. Bioscience Vol. 73, No. 2. February 2023.

Schulz, A.N., A.M. Mech, M.P. Ayres, K. J. K. Gandhi, N.P. Havill, D.A. Herms, A.M. Hoover, R.A. Hufbauer, A.M. Liebhold, T.D. Marsico, K.F. Raffa, P.C. Tobin, D.R. Uden, K.A. Thomas. 2021. Predicting non-native insect impact: focusing on the trees to see the forest. Biological Invasions.

Posted by Faith Campbell

www.fadingforests.org

Ash – Science Support Protection Efforts

As we know, survival of North American species of ash (Fraxinus spp.) is threatened by the emerald ash borer (EAB). DMF Sadof, McCullough, and Ginzel (full citation at end of the blog) hope to prevent demise of another ~ 135 million urban ash trees by 2050 bycountering persistent myths that have hindered adoption of effective protective measures. As they note, USDA APHIS has dropped regulations that had been intended to slow the EAB’s spread – which I concede were not very effective.

Protecting urban ash trees now falls to municipalities, states, their leaders and citizens, non-governmental organizations, and tree care professionals. If they apply knowledge gained since the detection of EAB 20 years ago – and are not paralyzed by myths – they can successfully manage EAB populations and protect their town’s ash trees. [I have also blogged about efforts to breed ash trees resistant to EAB.]

Since some studies have found that “myth-busting” is not effective, perhaps people advocating for EAB control should avoid mentioning the myths per se and instead emphasize the science supporting the proposed actions.

Sadof, McCullough, and Ginzel first review aspects of the biology of ash and EAB that are relevant to arborists and pest management specialists:

Adult EAB beetles feed on tree leaves for a couple of weeks from mid-May through June. This maturation period provides a 2–3 week opportunity to kill the leaf-feeding beetles with systemic insecticides before any eggs are laid.
Once eggs hatch, the first stage larvae immediately move into the phloem (inner bark) and cambium tissue, where they begin feeding. Systemic insecticides rarely enter the phloem, so they kill few larvae during this stage.
Detection of early stages of invasion is hampered by several factors, including beetles’ initial colonization of branches in the upper canopy; initially minimal effect on healthy ash trees; and the frequency of two-year life cycles when beetle densities are low. However, it is important to detect and treat these early infestations because EAB populations increase, tree health declines to eventual death.
Detection efforts should target the ash trees most likely to be infested early in the invasion: stressed trees, preferred species (especially green ash), trees growing in the open in parks, along roadsides or surrounded by impervious surfaces. Authorities can take advantage of the attractiveness of stressed trees by establishing “trap trees” to attract EAB adults. Beetles that feed on the “trap trees” can be killed by systemic insecticides. Or the trees can be removed and chipped to kill eggs and larvae before they can emerge. Sadof, McCullough, and Ginzel say trap trees are effective in slowing spread of new infestations when most ash trees remain healthy. Once EAB densities build and many trees are stressed by larval feeding, volatile (airborne) compounds released by girdled trees no longer attract the beetles.
Woodpecker holes in branches of the upper canopy are often the first evidence of EAB invasion in an area.
Even in late stages of the invasion, when most ash trees that were not protected with systemic insecticides are dead, EAB populations persist and continue to colonize and kill available ash trees, including some as small as >2.5 cm in diameter.

Myth: There Is No Point in Trying to Protect Ash Trees—

EAB Will Eventually Kill Them Anyway

Answer:

When the EAB was first detected in 2002, control measures were limited in number and efficacy. In the 20 intervening years, scientist have learned much about EAB biology and ash physiology. Insecticide chemistry and application methods have improved. Currently recommended strategies are based on long-term field studies. More effective insecticides have been developed. Emamectin benzoate is particularly efficient, including the fact that it needs to be applied only every third year. Managers must pay attention to the application protocols, including appropriate dose (i.e., the amount of insecticide product applied); spacing injection ports around the trunk to ensure that the xylem will transport the chemical to leaves throughout the canopy; and conduct injections in spring after bud break.

Myth: Wounds From Drilling Trees to Inject Systemic Insecticides Injure Trees

Answer:

In the early years, trunk injections sometimes caused substantial injury to trees. Refinement of delivery devices and reductions in the pressure at which insecticides are injected have virtually eliminated these issues. Staff must be properly trained in use of the equipment.

demonstration of injecting pesticide into ash tree; photo by F.T. Campbell

Myth: Using Systemic Insecticides to Protect Ash Trees Harms

Non-target Species and the Environment

Answer:

Sadof, McCullough, and Ginzel point out that continent-wide loss of a tree genus is likely to adversely affect the more than 200 species of native arthropods that are specialists on ash. On the other hand, systemic insecticides are unlikely to harm beneficial natural enemies of EAB, including parasitoid wasps, predatory insects, or woodpeckers. First, the insecticides are contained within the tree’s tissues; they do not kill insects on contact. Second, parasitoids and predators avoid dead beetles. Honeydew excreted by sucking insects might contain sufficient insecticide residue to harm parasitoids — if the tree is heavily infested. However, these insects are rapidly killed by these insecticides if they are applied at the optimal time (early to mid-spring). Proper timing of application greatly reduces the potential for tainted honeydew to accumulate on infested trees. Furthermore, in cities there are few populations of natural enemies of sucking insects.

Most concern is focused on pollinators. Ash trees flower early, before leaves expand. It is reassuring that protocols instruct that the systemic insecticides be applied after bud break — typically after pollen has been shed. I do find it disturbing that apparently there have been no published studies of insecticide concentration in ash pollen.

Myth: It Costs Too Much to Protect Ash Trees

Answer:

Sadof, McCullough, and Ginzel review the several studies and methods developed to estimate the value of urban ash trees – both individually and over a wider area. The value is based on the individual tree’s location, health, and structural condition. These economic studies have consistently shown that it costs less to protect ash trees from EAB with insecticide treatments than to remove ash trees — either proactively or when they decline and die.

Even delaying tree mortality – short of preventing it completely – is worthwhile because it allows municipalities to incorporate tree removal into the budget, rather than be suddenly confronted by large expense that they had not planned for.

Sadof, McCullough, and Ginzel recommend treating ash within a significant area as being most efficient. This approach reduces overall costs and slows rates of ash mortality locally – even for trees that are not treated. In some cases, treating as few as 11% of ash trees slowed the overall rate of ash decline.

An important in comparing costs of treatment to costs of replacement is the high mortality rate of newly planted urban trees: up to two-thirds die shortly after planting. This means that it takes decades to replace a mature tree canopy and the environmental benefits the canopy provides. Sadof, McCullough, and Ginzel conclude that protecting ash trees from EAB has clear positive effects for both the urban forest canopy – and its environmental services – and municipal forestry budgets.

Sadof, McCullough, and Ginzel then outline a viable Integrated Pest Management (IPM) framework that incorporates use of systemic insecticides to protect ash trees from EAB.

1. Define the problem and identify management objectives

Inventory urban trees before EAB is detected. The inventories should identify priority trees based on size (diameter at breast height), tree condition, and suitability of the site where the tree is growing. Focus detection surveillance on green ash trees, especially those in parks, parking lots, and along roads — sites that are sunlit (open) and likely to cause stress to the trees.

2. Monitor and assess the local EAB population to determine when a treatment program should be initiated. Treatment must wait until there is evidence that EAB is present but should not then be delayed, since it should begin while the trees’ vascular systems are still sufficiently healthy to carry the insecticide to branches and leaves. This requires regular inspections of ash trees for visible signs of EAB infestation. Efficiency is improved by focusing on high-risk trees (see above) and noticing woodpecker holes on upper portions of the trunk. Consider debarking symptomatic trees or establishing “trap tree” networks.

3. Identify and gather resources needed to implement an insecticide treatment program. Web-based calculators guide budget decisions based on the municipality’s tree inventory and local costs of treatments. Treating one-third of trees annually with emamectin benzoate can save money while maximizing the number of trees protected. Training city forestry staff in trunk injection methods is cheaper than hiring contractors and ensures better treatment quality and efficiency.

downy woodpecker; photo by Steven Bellovin, Columbia University

4. Incorporate multiple tactics to protect tree health and control EAB.

Ensure trees are actively transpiring when injecting the systemic insecticides; this might require irrigation. Encourage parasitoids and woodpecker foraging on untreated trees. In areas where ash trees are closely spaced, consider an area-wide urban SLAM program. In this strategy, treating a proportion of ash trees at two-year intervals reduces EAB eggs and overall EAB populations. Non-treated trees with EAB larvae might support parasitoid biocontrol populations whose offspring can attack EAB larvae on previously treated ash trees as the emamectin benzoate concentration wanes.

Sadof, McCullough, and Ginzel also suggest establishing a citizen monitoring program to both reduce costs and build community support for ash management. Community participation has been particularly effective when professionals take appropriate and timely action in response to volunteers’ findings.

SOURCE

Sadof, C.S., D.G. McCullough, and M.D. Ginzel. 2023. Urban ash management and emerald ash borer (Coleoptera: Buprestidae): facts, myths, and an operational synthesis. Journal of Integrated Pest Management, 2023, Vol. 14, No. 1 https://doi.org/10.1093/jipm/pmad012

Posted by Faith Campbell

www.fadingforests.org

Hot off the presses: How beech leaf disease kills trees

beech leaf disease symptoms; photo courtesy of Jennifer Koch, USFS

As we know, beech leaf disease (BLD) has spread rapidly in the decade since its discovery in northeast Ohio. It has been detected as far east as the Maine coast, as far south as northern Virginia, as far north as southern Ontario, and as far west as eastern Michigan and northern Indiana. It has been found in 12 states.

BLD is associated with a nematode, Litylenchus crenatae subsp. mccannii (Lcm), although whether this is the sole causal agent is not yet clear.

BLD’s North American host, American beech (Fagus grandifolia),is an important native deciduous hardwood species. It plays important roles in nutrient cycling, erosion control, and carbon storage and sequestration in forests. Wildlife species depend on the trees’ canopies and especially cavities blog for nesting sites, shelter, and nutritious nuts. American beech – with sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) – dominate the northern hardwood ecosystem of northeastern United States and southeastern Canada. These forests occupy a huge area; in just New England and New York they occupy 20 million acres (Leak, Yamasaki and Holleran. 2014; full citation at end of blog).

Beech leaf disease also affects European beech, (F. sylvatica), Chinese beech (F. engleriana), and Oriental beech (F. orientalis) planted in North America. The disease has not yet been detected in Asia or Europe. Japanese beech (F. crenata) sporadically display symptomatic leaves, but the disease has not been reported there.

Scientists working to understand the disease, how it spreads, and its ecological impact confer every other month. The next time is in early December.

Paulo Vieira, of the USDA Agriculture Research Service, leads one group seeking to better understand how the disease infects its host. They published a new study (see full citation at end of blog) examining how the nematode provokes changes in the cells of the trees’ leaves. As they point out, leaves are plants’ primary organs for photosynthesis – hence providing energy for growth. The leaf is composed of a several cell types organized into different tissues with specific function related to photosynthesis, gas exchange, and/or the transportation of water and nutrients. Thus, changes in leaf morphology affect the normal functioning of the leaf and therefore the tree’s growth and survival.

Vieira et al. found that:

The BLD nematode enters the leaf bud as it forms in late summer. In early autumn, all nematode developmental stages were found in the buds, including eggs at various stages of embryonic development, juveniles, and adults. Adult males were found in fewer than 20% of the buds, suggesting that the nematode can reproduce asexually.
Feeding by the BLD nematode induces abnormal and extensive cell proliferation, resulting in a significant increase of the number of cell layers inside the leaf. These changes improve the nutrition that the leaves provide to the nematode. However, the BLD-induced distortions of the bud persist as the leaf grows. Symptomatic leaf “banding” results. These areas have a proliferation of abnormally large and irregularly shaped cells with more chloroplasts. Intercellular spaces are also larger; this is where the nematodes are found. in. (The publication has dramatic photographs.)
Sites damaged by nematodes are a major resource for metabolites needed for plant performance. So their damage imposes a considerable drain.
Colonization of roots by ectomycorrhizal fungal is also reduced in severely diseased trees.
Immature female nematodes are the principal winter survivors. However, many die, making it difficult to culture nematodes in the spring. The nematodes reproduce during the growing season. Buildup of nematode numbers makes culturing easier, so facilitating confirmation of the disease’s presence.
Nematodes can migrate along the stem to other leaves, thus spreading the infection.

Vieira et al. tell us fascinating facts about the nematode. The BLD nematode, Litylenchus crenatae subsp. mccannii (Lcm) is now considered one of the top ten most important plant-parasitic nematodes in the United States. To date, species of this genus have been found only in Japan and New Zealand. The species L. crenatae was first described from Japan. A second species — L. coprosma – was detected in 2012 in New Zealand in association with small chlorotic patches on leaves of two native plants in the Coprosma genus.

Litylenchus belongs to the family Anguinidae. Several species in the family are designated quarantine pests because they cause economically significant damage to food and ornamental corps, including grains (wheat, barley, rice) and potatoes. Anguinidae nematodes often parasitize aerial parts of the hosts (e.g., leaves, stems, inflorescences, seeds); less frequently they infest roots. They can migrate along the host tissue surfaces in water films. Their host ranges vary from broad to narrow. Other Anguinidae nematodes apparently share the ability to manipulate the host’s cellular machinery, which often results in the induction of cell hyperplasia [the enlargement of an organ or tissue caused by an increase in the reproduction rate of its cells], and hypertrophy [increase and growth of cells] of the tissues on which they feed.

Vieira et al. assert that the rapid spread of Litylenchus crenatae subsp. mccannii – combined with the apparent lack of resistance in native beech trees – suggests that this nematode was recently introduced to North America. Furthermore, the ability of this subspecies to change the host’s cell cycle machinery supports the link between the presence of the nematode and the disease.

The mechanisms by which nematodes change host-plant cells are unknown. I hope that scientists will pursue these questions. Perhaps the nematode family’s threat to grains and other food crops will prompt funding for such work. Unfortunately, I don’t think the threat to an ecologically-important native tree species will have the same power.

SOURCES

Leak, W.B, M. Yamasaki and R. Holleran. 2014 Silvicultural Guide for Northern Hardwoods in the Northeast. United States Department of Ariculture Forest Service Northern Research Station. General Technical Report NRS-132. April 2014.

Vieira P, M.R. Kantor MR, A. Jansen, Z.A. Handoo, J.D. Eisenback. (2023) Cellular insights of beech leaf disease reveal abnormal ectopic cell division of symptomatic interveinal leaf areas. PLoS ONE October 5, 2023. 18(10) https://doi.org/10.1371/pone.0292588

Posted by Faith Campbell

www.fadingforests.org

Bug Apocalypse Update

*Aedes aegypti* photo by James Gathany via Flickr

In October 2018 I posted a blog on the decline of global insect numbers and biodiversity.

This month a Washington Post columnist, Michael J. Corin, published a piece decrying people’s use of “bug zappers” [full citation at end of this blog] in an effort to prevent biting insects from ruining their evenings.

After quoting some of the manufacturers’ advertising claims, Corin point to the scientific consensus that these zappers don’t kill mosquitoes. In some studies, the zappers killed tens of thousands of insects, but far fewer than 1% were mosquitoes. A study by Iowa State University (citation at end of blog) that estimated even a fraction of the bug zappers sold in the United States kill more than 70 billion insects annually.

Corin even says, “bug zappers make it more likely you’ll be bitten by mosquitoes while sitting in your backyard.” Any mosquitoes drawn to the vicinity of a bug zapper will redirect their attention to the proximal warmblooded mammals — usually humans.

This information is important to us because bug zappers are exceptional killers of (other) insects. Corin cites a study by the University of Delaware (Frick and Tallamy, full citation below) in which the zappers caught nearly 14,000 insects over a summer. Roughly half the catch — 6,670 insects — were harmless aquatic species from nearby rivers and streams, fish food in the aquatic food chain. Many of the others were parasitic wasps and beetles that naturally prey on mosquitoes.

caddisfly (one of groups often killed by bug zappers, according to Doug Tallamy) photo by Anita Gould via Flickr

So, these traps are exacerbating the “insect apocalypse” and undermining biocontrol programs!

These studies were published in the middle-1990s – nearly 30 years ago. Dr. Douglas Tallamy of the University of Delaware told me that there are now traps that use baits (octanol and CO₂). Carbon dioxide and octenol (a derivative of mammalian body odor) are known to attract biting insects, including mosquitoes. However, these traps cost up to $500 and do not sell well. The standard zapper still only catches non-targets.

A study by Kim et al. found that neither the Stinger Electric Zapper nor the Mosquito Deleto works effectively.

Apparently most bug zappers are not working as advertised. Why, then, are they still marketed using false claims? The Federal Trade Commission is supposed to investigate misleading advertising claims and take legal action against manufacturers who don’t correct them. Corin says the agency suggested to him that the public should submit any complaints through the agency’s website.

Should we not provide information to the FTC and urge them to take action? Do you have information – or access to research capabilities – to support such an effort?

Corin provides the usual advice to minimize mosquitoes around your house:

1) eliminate standing water – in which mosquitoes breed. Or install a “Bucket of Doom” (a design developed by the Centers for Disease Control). Fill a 5-gallon bucket with water and add leaf litter or straw.Mosquitoes love to lay eggs here. Add granules of Bacillus thuringiensis to kill the mosquito larvae. Some commercial versions are Ovi-Catch AGO trap sold by Catchmaster or the “GAT” trap sold by Biogents.

2) wear long sleeves and pants. Add repellent – especially those containing DEET.

3) Turn on a fan to create a steady breeze.

SOURCES

Coren, M.J. 2023. Trying to kill mosquitoes? Don’t buy a bug zapper. The Washington Post. September 14, 2023. https://www.washingtonpost.com/climate-environment/2023/09/12/bug-zappers-mosquito-repellent/

Frick, T.B. and D.W. Tallamy. 1996. Density and Diversity of Nontarget Insects Killed by Suburban Electric Insect Traps. Ent. News Vol 107, No. 2, March & April 1996

Kim, J. et al. 2002. A study comparing efficiency of insect capture between Stinger electric zapper and Mosquito-deleto at varying locations and heights in northern Michigan. https://deepblue.lib.umich.edu/handle/2027.42/54969

Lewis, D. 1996. Bug Zappers are Harmful, Not Helpful. Iowa State University Extension. Bug Zappers are Harmful, Not Helpful | Horticulture and Home Pest News (iastate.edu)

Posted by Faith Campbell

www.fadingforests.org

USFS Lays Out Incomplete Picture of the Future

tanoak trees in southern Oregon killed by sudden oak death; photo by Oregon Department of Forestry; this pathogen is not mentioned by USFS RPA report

In August the USDA Forest Service published the agency’s 2020 assessment of the future of America’s forests under the auspices of the Resources Planning Act. [See United States Department of Agriculture Forest Service Future of America’s Forests and Rangelands, full citation at the end of the blog.] To my amazement, this report is the first in the series (which are published every ten years) to address disturbance agents, specifically invasive species. In 2023! Worse, I think its coverage of the threat does not reflect the true state of affairs – as documented by Forest Service scientists among others.

This is most unfortunate because policy-makers presumably rely on this report when considering which threats to focus on.

Here I discuss some of the USFS RPA report and what other authors say about the same topics.

The RPA Report’s Principle Foci: Extent of the Forest and Carbon Sequestration

The USFS RPA report informs us that America’s forested area will probably decrease 1- 2% over the next 50 years (from 635.3 million acres to between 619 and 627 million acres), due largely to conversion to other uses. This decline in extent, plus trees’ aging and increases in disturbance will result in a slow-down in carbon sequestration by forests. In fact, if demand for wood products is high, or land conversion to other uses proceeds apace, U.S. forest ecosystems are projected to become a net source of atmospheric CO₂by 2070.

Eastern forests sequester the majority of U.S. forest carbon stocks. These forests are expected to continue aging – thereby increasing their carbon storage. Yet we know that these forests have suffered the greatest impact from non-native pests.

I don’t understand why the USFS RPA report does not explicitly address the implications of non-native pests. In 2019, Songlin Fei and three USFS research scientists did address this topic. Fei et al. estimated that tree mortality due to the 15 most damaging introduced pest species have resulted in releases of an additional 5.53 terragrams of carbon per year. Fei and colleagues conceded this is probably an underestimate. They say that annual levels of biomass loss are virtually certain to increase because current pests are still spreading to new host ranges (as demonstrated by detection of the emerald ash borer in Oregon). Also, infestations in already-invaded ranges will intensify, and additional pests will be introduced (for example, beech leaf disease).

I see this importance of eastern forests in sequestering carbon as one more reason to expand efforts to protect them from new pest introductions, and the spread of those already in the country, etc.

A second issue is the role of non-native tree species in supporting the structure and ecological functions of forests. Ariel Lugo and colleagues report that 18.8 million acres (7.6 million ha, or 2.8% of the forest area in the continental U.S.) is occupied by non-native tree species. (I know of no overall estimate for all invasive plants.) They found that non-native tree species constitute 12–23% (!) of the basal area of those forest stands in which they occur.

Norway maple (*Acer platanoides*); one of the most widespread invasive species in the East. Photo by Hermann Falkner via Flickr

Lugo and colleagues confine their analysis of ecosystem impacts to carbon sequestration. They found that the contribution of non-native trees to carbon storage is not significant at the national level. In the forests of the continental states (lower 48 states), these trees provide 10% of the total carbon storage in the forest plots where they occur. (While Lugo and colleagues state that the proportion of live tree biomass made up of non-native tree species varies greatly among ecological subregions, they do not provide examples of areas on the continent where their biomass – and contribution to carbon storage — is greater than this average.) In contrast, on Hawai`i, non-native tree species provide an estimated 29% of live tree carbon storage. On Puerto Rico, they provide an even higher proportion: 36%.

Brazilian pepper (Schinus terebinthifolius) – widespread invasive in Hawai`i and Florida; early stage invasive in Puerto Rico. Photo by Javier Alexandro via Flickr

In the future, non-native trees will play an even bigger role. Since tree invasions on the continent are expanding at ~500,000 acres (202,343 ha) per year, it is not surprising that non-native species’ saplings provide 19% of the total carbon storage for that size of trees in the lower 48 states (Lugo et al.).

Forming a More Complete Picture: Biodiversity, Disturbance, and Combining Data.

The USFS RPA report has a chapter on biodiversity. However, the chapter does not discuss historic or future diversity of tree species within biomes, nor the genetic diversity within tree species.

Treatment of Invasive Species

The USFS 2020 RPA report is the first to include a chapter on disturbance, including invasive species. I applaud its inclusion while wondering why they have included it only now? Why is the coverage so minimal? I think these lapses undercut the report’s purpose. The RPA is supposed to inform decision-makers and stakeholders about the status, trends, and projected future of renewable natural resources and related economic sectors for which USFS has management responsibilities. These include: forests, forest products, rangelands, water, biological diversity, and outdoor recreation. The report also has not met its claim to “capitalize on” areas where the USFS has research capacity. One excuse might be that several important publications have appeared after the cut-off date for the assessment (2020). Still, the report’s authors cite some of the evaluations that were in preparation as of 2020, e.g., Poland et al.

I suggest also that it would be helpful to integrate data from other agencies, especially the invasive species database compiled by the U.S. Geological Survey, into the RPA. For example, the USGS lists just over 4,000 non-native plant species in the continental U.S. (defined as the lower 48 plus Alaska). On Hawai`i, the USGS lists 530 non-native plant species as widespread. Caveat: many of the species included in these lists probably coexist with the native plants and make up minor components of the plant community.

Specifically: Invading Plants

The USFS RPA report gives much more attention to invasive plants than non-native insects and pathogens. The report relies on the findings of Oswalt et al., who based their data on forested plots sampled by the Forest Inventory and Analysis (FIA) program. (The RPA also reports on invasive plants detected on rangelands, primarily grasslands.) Oswalt et al. found that 39% of FIA plots nationwide contained at least one plant species that the FIA protocol considers to be invasive and monitors. The highest intensity of plant invasions is in Hawai`i – 70% of the plots are invaded. The second-greatest intensity is in the eastern forests: 46%. However, the map showing which plots were inventoried for invasive plants makes clear how incomplete these data are – a situation I had not realized previously.

I appreciate that the USFS RPA report mentions that propagule pressure is an important factor in plant invasions. This aspect has often been left out in past analyses. I also appreciate the statement that international trade in plants for ornamental horticulture will probably lead to additional introductions in the future. Third, I concur with the report’s conclusions that once forest land is invaded, it is unlikely to become un-invaded. Invasive plant management in forests often results in one non-native species being replaced by another. In sum, the report envisions a future in which plant invasion rates are likely to increase on forest land.

If you wish to learn more about invasive plant presence and impacts, see the discussion of invasive plants in Poland et al., my blogs based on the work by Doug Tallamy, and several other of my blogs compiled under the category “invasive plants” on this website.

I believe all sources expect that the area invaded by non-native plant species, and the intensity of existing invasions, will increase in the future.

The USFS RPA links these invasions to expansion of the “wildland-urban interface” (“WUI”). These areas increased rapidly before 2010. At that time, they occupied 14% of forest land. The report published in 2023 did not assess their future expansion over the period 2020 to 2070. However, it did project increased fragmentation in many regions, especially in the RPA Western and Southeastern regions. Since “fragmentation” is very similar to wildland-urban interfaces, the report seems implicitly to project more widespread plant invasions in the future.

plant invasions facilitated by fragmentation; northern Virginia; photo by F.T. Campbell

Specifically: Insects and Pathogens

The USFS RPA report on insects and pathogens is brief and contains puzzling errors and gaps. It says that the tree canopy area affected by both native and non-native mortality-causing agents has been consistently large over the three most recent five-year FIA assessment periods. It notes that individual insects or diseases have extirpated entire tree species or genera and fundamentally altered forests across broad regions. Examples cited are chestnut blight and emerald ash borer.

The USFS RPA report warns that pest-related mortality might be underreported in the South, masked by more intense management cycles and higher rates of tree growth and decay. On the other hand, the report asserts that pest-related mortality is probably overrepresented in the Northern Region in the 2002 – 2006 period because surveyors drew polygons to encompass large areas affected by EAB and balsam woolly adelgid (Adelges piceae) infestations. The latter puzzles me; I think it is probably an error, and should have referred to hemlock woolly adegid, A. tsugae. Documented mortality has generally been much more widespread from insects than diseases, e.g., bark beetles, including several native ones, across all regions and over time, especially in the West – where the most significant morality agents are several native beetles. The USFS RPA report mentions that the Northern Region has been particularly affected by non-native pests, including EAB, HWA, BWA, beech bark disease, and oak wilt. It mentions that Hawai`i has also suffered substantial impacts from rapid ʻōhiʻa death.

Defoliating insects have affected relatively consistent area over time. This area usually equaled or exceeded the area affected by the mortality agents. Principal non-native defoliators in the Northern Region have been the spongy moth (Lymantria dispar); larch casebearer (Coleophora laricella); and winter moth (Operophtera brumata). In the South they list the spongy moth.

More disturbing to me is the USFS RPA report’s conclusion that the future impact of forest insects is highly uncertain. The authorsblame the complexity of interactions among changing climate, those changes’ effects on insect and tree species’ distributions, and overall forest health. Also, they name uncertainty about which new non-native species will be introduced to the United States. I appreciate the report’s avoidance of blanket statements regarding the effects of climate change. However, other studies – e.g., Poland et al. – have incorporated these complexities while still offering conclusions about a number of currently established non-native pests. Finally, I am particularly dismayed that the USFS RPA does not provide analysis of any forest pathogens beyond the single mention of a few.

I am confused as to why the USFS RPA report makes no mention of Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation). This is a multi-partner effort to prioritize U.S. tree species for conservation actions based on invasive pests’ threats and the trees’ ability to adapt to them. Several USFS units participated, including the Southern Research Station, the Eastern Forest Environmental Threat Assessment Center, and the Forest Health Protection program. The findings were published in 2019. See here. Lead scientist Kevin Potter was one of the authors of the RPA’s chapter on disturbance.

redbay (*Persea borbonia*) trees in Georgia killed by laurel wilt; photo by Scott Cameron. Redbay is ranked by Project CAPTURE as 5th most severely at risk due to a non-native pest

“Project CAPTURE” provided useful summaries of non-native pests’ impacts, including the facts that

54% of the tree species on the continent are infested by one or more non-native insect or pathogen;
nearly 70% of the host/agent combinations involve angiosperm (broadleaf) species, 30% gymnosperms (e.g., conifers). When considering only non-native pests, pests attacking angiosperms had greater average severity.
Disease impacts are more severe, on average, than insect pests. Wood-borers are more damaging than other types of insect pests.
Non-native agents have, on average, considerably more severe impacts than native pests.

Project CAPTURE also ranked priority tree species based on the threat from non-native pests (Potter et al., 2019). Tree families at the highest risk to non-native pests are: a) Fagaceae (oaks, tanoaks, chestnuts, beech), b) Sapindaceae (soapberry family; includes maples, Aesculus (buckeye, horsechestnut); c) in some cases, Pinaceae (pines); d) Salicaceae (willows, poplars, aspens); e) Ulmaceae (elms) and f) Oleaceae (includes Fraxinus). I believe this information should have been included in the Resources Planning Act report in order to insure that decision-makers consider these threats in guiding USFS programs.

I also wish the USFS RPA had at least prominently referred readers to Poland et al. Among that study’s key points are:

Invasive (non-native) insects and diseases can reduce productivity of desired species, interactions at other trophic levels, and watershed hydrology. They also impose enormously high management costs.
Some non-native pests potentially threaten the survival of entire tree genera, not just individual species, e.g., emerald ash borer and Dutch elm disease. I add white pine blister rust and laurel wilt.
Emerald ash borer and hemlock woolly adelgid are listed as among the most significant threats to forests in the Eastern US.
White pine blister rust and hemlock woolly adelgid are described as so profoundly affecting ecosystem structure and function as to cause an irreversible change of ecological state.
Restoration of severely impacted forests requires first, controlling the non-native pest, then identifying and enriching – through selection and breeding – levels of genetic resistance in native populations of the impacted host tree. Programs of varying length and success target five-needle pines killed by Cronartium ribicola; Port-Orford cedar killed by the oomycete Phytophthora lateralis; chestnut blight; Dutch elm disease; butternut canker (causal agent Ophiognomonia clavigignenti juglandacearum), emerald ash borer; and hemlock woolly adelgid.
Climate change will almost certainly lead to changes in the distribution of invasive species, as their populations respond to increased variability and longer-term changes in temperature, moisture, and biotic interactions. Predicting how particular species will respond is difficult but essential to developing effective prevention, control, and restoration strategies.

Poland et al. summarizes major bioinvaders in several regions. Each region except Hawai`i (!!) includes tree-killing insects or pathogens.

It is easier to understand the RPA report’s not mentioning priority-setting efforts by two other entities, the Morton Arboretum and International Union for the Conservation of Nature (IUCN). These studies were published in 2021 and their lead entities were not the Forest Service – although the USFS helped to fund the U.S. portion of the studies.

The Morton Arboretum led in the analysis of U.S. tree species. It published studies evaluating the status of tree species belonging to nine genera, considering all threats. The Morton study ranked as of conservation concern one third of native pine species; 31% of native oak species; significant proportion of species in the Lauraceae. The report on American beech — the only North American species in the genus Fagus – made no mention of beech leaf disease – despite it being a major concern in Ohio – only two states away from the location of the Morton Arboretum near Chicago.

valley oak (*Quercus lobata*) in Alameda Co, California; photo by Belinda Lo via Flickr

Most of the species listed by the Morton Arboretum are of conservation concern because of their small populations and restricted ranges. The report’s coverage of native pests is inconsistent, spotty, and sometimes focuses on odd examples.

Tree Species’ Regeneration

Too late for consideration by the authors of the USFS RPA report come new studies by Potter and Riitters that evaluate species at risk due to poor regeneration. This effort evaluated 280 forest tree species native to the continental United States – two-thirds of the species evaluated in the Kevin Potter’s earlier analysis of pest impacts.

The results of Potter and Riitters 2023 only partially matched those of the IUCN/Morton studies. The Morton study did not mention three genera with the highest proportions of poorly reproducing species according to Potter and Riitters: Platanus, Nyssa, and Juniperus. Potter, Morton, and the IUCN largely agree on the proportion of Pinus species at risk. Potter et al. 2023 found about 11% of oak species to be reproducing poorly, while Morton designated a third of 91 oak species to be of conservation concern.

I believe Potter and Riitters and the Morton study agree that the Southeast and California are geographic hot spots of tree species at risk.

Potter and Riiters found that several species with wide distributions might be at risk because they are reproducing at inadequate rates. Three of these exhibit poor reproduction across their full range: Populus deltoids (eastern cottonwood), Platanus occidentalis (American sycamore), and ponderosa pine(Pinus ponderosa). Four more species are reported to exhibit poor reproduction rates in all seed zones in which they grow (the difference from the former group is not explained). These are two Juniperus, Pinus pungens, and Quercus lobata. As I point out in my earlier blog, valley oak is also under attack by the Mediterranean oak borer.

SOURCES

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. Proceedings of the National Academy of Sciences. Vol. 116, No. 35. August 27, 2019.

Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, and C.M. Kurtz. 2022. The Contribution of Nonnative Tree Species to the Structure and Composition of Forests in the Conterminous United States in Comparison with Tropical Islands in the Pacific and Caribbean. USDA USFS General Technical Report IITF-54

Poland, T.M., T. Patel-Weynand, D.M. Finch, C.F. Miniat, D.C. Hayes, V.M. Lopez, eds. 2021. Invasive Species in Forests and Rangelands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

Potter, K.M., M.E. Escanferla, R.M. Jetton, G. Man, and 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.

Potter, K.M. and Riitters, K. 2023. A National Multi-Scale Assessment of Regeneration Deficit as an Indicator of Potential Risk of Forest Genetic Variation Loss. Forests 2022, 13, 19. https://doi.org/10.3390/f13010019

United States Department of Agriculture Forest Service. 2023. Future of America’s Forests and Rangelands: The Forest Service 2020 Resource Planning Act Assessment. GTR-WO-102 July 2023

Posted by Faith Campbell

www.fadingforests.org

Invasive Tree Species in the U.S. Caribbean: New Attention!

African Tulip Tree (*Spathodea campanulata*) on Puerto Rico; photo by Joe Schlabotnik via Flickr

While it is widely accepted that tropical island ecosystems are especially vulnerable to invasions, there has been little attention to terrestrial bioinvaders in the Caribbean; there has been more attention to marine bioinvaders such as lionfish. I am glad that is starting to change. Here I review a new study by Potter et al. (full citation at end of this blog), supplemented by information from other recent studies, especially Poland et al.

Potter et al. used USFS Forest Inventory and Analysis (FIA) survey data to examine regeneration rates by non-native tree species introduced to the continental United States, Hawai`i, and Puerto Rico. I rejoice that they have included these tropical islands, often left out of studies. They are part of the United States and are centers of plant endemism!

Potter et al. sought to learn which individual non-indigenous tree species are regenerating sufficiently to raise concern that they will cause significant ecological and economic damage in the future. That is, those they consider highly invasive. They defined such species as those for which at least 75% of stems of that species detected by FIA surveys are in their small tree categories – saplings or seedlings. They concluded that these species are successfully reproducing after reaching the canopy so they might be more likely to alter forest ecosystem functions and services. They labelled species exhibiting 60 – 75% of stems in the “small” categories as moderately invasive.

The authors recognize that many factors might affect tree species’ regeneration success, especially at the stand level. They assert that successful reproduction reflects a suite of factors such as propagule pressure, time since invasion, and ability of a species to adapt to different environments.

As I reported in an earlier blog, link 17% of the total flora of the islands of the Caribbean archipelago – including but not limited to Puerto Rico – are not native (Potter et al.). In Puerto Rico, two-thirds of forests comprise novel tree assemblages. The FIA records the presence of 57 non-native tree species on Puerto Rico. Potter et al. identified 17 non-native tree species as highly invasive, 16 as potentially highly invasive, and two as moderately invasive. That is, 33 of 57 nonnative tree species, or 58% of those species tallied by FIA surveyors, are actual or potential high-impact bioinvaders. While on the continent only seven non-native tree species occurred on at least 2% of FIA plots across the ecoregions in which they were inventoried, on Puerto Rico 21 species occurred on at least 2% of the FIA plots (38%). They could not assess the invasiveness of the eight species that occurred only as small stems on a couple of survey plots. These species might be in the early stages of widespread invasion, or they might never be able to reproduce & spread.

The high invasion density probably reflects Puerto Rico’s small size (5,325 mi² / 1,379,000 ha); 500 years of exposure to colonial settlement and global trade; and wide-scale abandonment of agricultural land since the middle of the 20^th Century

Naming the invaders

The most widespread and common of the highly invasive non-native tree species are river tamarind (Leucaena leucocephala), on 12.6% of 294 forested plots; algarroba (Prosopis pallida) on 10.9%; and African tuliptree (Spathodea campanulata)on 6.1%. Potter et al. attribute the prevalence of some species largely to land-use history, i.e., reforestation of formerly agricultural lands. In addition, some of the moderately to highly invasive species currently provide timber and non-timber forest products, including S. campanulata, L. leucocephala, Syzgium jambos (rose apple) and Mangifera indica (mango).

Potter et al. contrast the threat posed by Spathodea campanulata with that posed by Syzgium jambo. The latteris shade tolerant and can form dense, monotypic stands under closed canopies. Because it can reproduce under its own canopy, it might be able to remain indefinitely in forests unless it is managed. In contrast S. campanulata commonly colonizes abandoned pastures. Since it is shade intolerant, it might decline in the future as other species overtop it. Meanwhile, they suggest, S. campanulata might provide habitat appropriate for the colonization of native tree species.

Second-growth forest in Caribbean National Forest “El Yunque”

Poland et al. say the threat from Syzgium jambos might be reduced by the accidentally introduced rust fungus Puccinia psidii (= Austropuccinia psidii), which has been killing rose apple in Puerto Rico. In Hawai`i, the same fungus has devastated rose apple in wetter areas.

Potter et al. note that stands dominated by L. leucocephala and Prosopis pallida in the island’s dry forests are sometimes arrested by chronic disturbance – presumably fire. However, they do not report whether other species – native or introduced – tend to replace these two after disturbance. The authors also say that areas with highly eroded soils might persist in a degraded state without trees. The prospect of longlasting bare soil or trashy scrub is certainly is alarming.

Potter et al. warn that the FIA’s sampling protocol is not designed to detect species that are early in the invasion process. However, they do advise targetting eradication or control efforts on the eight species that occurred only as small stems on a couple of survey plots. While their invasiveness cannot yet be determined, these species might be more easily managed because presumably few trees have yet reached reproductive age. They single out Schinus terebinthifolius (Brazilian pepper), since it is already recognized as moderately invasive in Hawai`i. I add that this species is seriously invasive in nearby peninsular Florida and here! APHIS recently approved release of a biocontrol insect in Florida targetting Brazilian pepper. It might easily reach nearby Puerto Rico or other islands in the Caribbean. I am not aware of native plant species in the Caribbean region that might be damaged by the biocontrol agent. However, two native Hawaiian shrubs might be harmed if/when this thrips reaches the Hawaiian Islands. Contact me for specifics, or read the accompanying blog about Potter et al. findings in Hawai`i.

Poland et al. looked at the full taxonomic range of possible bioinvaders in forest and grassland ecosystems. The Caribbean islands receive very brief coverage in the chapter on the Southeast (see Regional Summary Appendices). This chapter contains a statement that I consider unfortunate: “Introduction of species has enriched the flora and fauna of Puerto Rico and the Virgin Islands.” The chapter’s authors assert that many of the naturalized species are restoring forest conditions on formerly agricultural lands. They say that these islands’ experience demonstrates that introduced and native species can cohabitate and complement one another. I ask – but in what kind of forest? These forests, are novel communities that bear little relationship to pre-colonial biodiversity of the islands. Was not this chapter the right place to note that loss? Forests are more than CO₂sinks.

I also regret that the chapter does not mention that the Continental United States can be the source of potentially invasive species (see several examples below).

Mealybug-infested cactus at Cabo Rojo National Wildlife Refuge, Puerto Rico. Photo by Yorelyz Rodríguez-Reyes

The chapter does concede that some introduced species are causing ecological damage now. See Table A8.1. Some of these troublesome introduced species are insects:

the South American Harrisia cactus mealybug (Hypogeococcus pungens) is killing columnar cacti in the islands’ dry forests. The chapter discusses impacts on several cactus species and control efforts, especially the search for biocontrol agents.
the agave snout weevil (Scyphophorus acupunctatus), native to the U.S. Southwest and Mexico , is threatening the endemic and endangered century plant (Agave eggersiana) in St. Croix & Puerto Rico.
Tabebuia thrips (Holopothrips tabebuia) is of unknown origin. It is widespread around mainland Puerto Rico. Its impacts so far are primarily esthetic, but it does apparently feed on both native and introduced tree species in the Tabebuia and Crescentia genera.

The Caribbean discussion also devotes welcome attention to belowground invaders, i.e., earthworms. At least one species has been found in relatively undisturbed cloud forests, so it is apparently widespread. Little is known about its impact; more generally, introduced earthworms can increase soil carbon dioxide (CO₂) emissions as through speeded-up litter decomposition and soil respiration.

A factsheet issued by the British forestry research arm DEFRA reports that the pine tortoise scale Toumeyella parvicornis has caused the death of 95% of the native Caicos pine (Pinus caribaea var. bahamensis) forests in the Turks and Caicos Islands (a UK Overseas Territory). The scale is native to North America. It has recently been introduced to Italy as well as to Puerto Rico, and the Turks and Caicos Islands.

SOURCES

Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, C.M. Kurtz. 2022. The Contribution of Non-native Tree Species to the Structure & Composition of Forests in the Conterminous United States in Comparison with Tropical Islands in the Pacific & Caribbean. USFS International Institute of Tropical Forestry General Technical Report IITF-54.

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. Especially the Appendix on the Southeast and Caribbean. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

Potter K.M., Riitters, K.H. & Guo. Q. 2022. Non-nativetree regeneration indicates regional & national risks from current invasions. Frontiers in Forests & Global Change Front. For. Glob. Change 5:966407. doi: 10.3389/ffgc.2022.966407

Posted by Faith Campbell

www.fadingforests.org

Sobering News: Invasive Grasses, Trees, and Killer Pests in Hawai`i

At CISP, our hearts go out to all those affected by the terrible August fires on Maui. May the departed rest in peace. May the living find comfort and all that is needed for recovery.

Fire and Invasive Grasses

A fire in non-native grasses on Maui in 2009; photo by Forrest and Kim Starr

Major U.S. and international media continue to detail the fires’ devastation, especially in Lahaina. As time has passed, more news has highlighted the role that the widespread presence of introduced, fire-prone grasses played in the rapid growth and spread of Maui’s fires.

For example, The Washington Post devoted seven paragraphs in one story to the issue of grasses. The story quotes several experts: Alison Nugent, an associate atmospheric scientist at the University of Hawaii’s Water Resources Research Center; Jeff Masters, a meteorologist for Yale Climate Connections; and Clay Trauernicht, a fire researcher at the University of Hawaii.

These and others have been widely quoted in the many recent articles. I am glad that they – and the media – are making clear that climate change is not the sole factor causing damaging wildfires. It is clear that Maui’s recent weather patterns – including the high-velocity winds and drought – have been within the range of normal climate patterns. Fluctuations in the Pacific’s weather have also been normal, especially under the influence of the current El Niño.

The dangers caused by Hawai’i’s fire-prone grasses are also clear – and have been for years. Experts have identified policy weaknesses at the county and state level. Also, they have specified changes to land management that could better prevent or mitigate wildfires. There has been far too little action.

On the other hand, there are hopeful signs.

endangered ‘akikiki photo by Carter Atkinson, USGS

The Hawai’i Wildfire Management Organization, a nonprofit, is educating and engaging communities state-wide. Elizabeth Pickett, a Co-Executive Director, presented an overview of wildfire at the Hawai’i Invasive Species Awareness Month in February 2023. The Big Island Invasive Species Committee has successfully eradicated two species of pampas grass on Hawai’i Island – after 13 years’ work. A native species has been planted where pampas formerly grew.

Another Post article reported on efforts by staff and fire departments to protect the Maui Bird Conservation Center, which houses critically endangered Hawaiian birds found nowhere else on Earth, including some currently extinct in the wild. As I have blogged previously, the palila, kiwikiu, ‘akikiki, ‘alalā [Hawaiian crow; extinct in the wild] and other birds are dying from avian malaria, carried by nonnative mosquitoes. The Center on Maui and another on the Big Island are run by the San Diego Zoo Wildlife Alliance. Conservationists have completed field trials of a proposed mosquito suppression process for Maui and are seeking public comments for a similar program on Kaua’i. These programs represent groundbreaking and long-awaited progress on countering a principal threat to the survival of Hawai`i’s unique avifauna. Loss of the Center and its birds would have devastated post-suppression efforts to rebuild and restore bird populations in the wild.

The Post carried a second story about the effort to protect Hawai`i’s endangered birds – a full page of print, even longer – with many photos, on the web. The article mentions the “Birds, Not Mosquitoes” program and varying views about it. I rejoice that the dire situation for the Islands’ biodiversity is getting attention in the Nation’s capital. Again, see my earlier blog.

Plant Invasions in Hawaiian Forests

A team of scientists from the USDA Forest Service and Natural Resources Conservation Service, plus the Hawaii Division of Forestry and Wildlife, has carried out a new assessment of the extent of invasive plant species in forests on the Hawaiian Islands (Potter et al. 2023; full citation at end of blog).

The results of their analysis are – in their words – “sobering”. They portend “a more dire future for Hawai`i`s native forests.”

First, regarding the recent fires, Potter et al. found significantly higher cover by invasive grasses on Forest and Inventory Analysis (FIA) plots on Hawai‘i and Maui than on O‘ahu, Kaua‘i, and Lana‘i. Grass invasions were particularly high on the eastern coast of Maui – near Lahaina. Even so, the authors say their study’s methods resulted in a gross underestimate of areas invaded by fire-prone grasses. That is, most of Hawai’i’s xerophytic dry forests were converted to grasslands before the FIA program began. Therefore these grasslands are not included in FIA surveys.

*Psidium cattleyanum*; photo by Forrest and Kim Starr

The extent of current invasions in wetter forests is already significant – but trends point to an even more worrying future.

Naturalized non-native plant taxa constitute half of the Hawaiian flora.
56% of Hawaii’s 553,000 ha of forest land contained non-native tree species; about 39% of these forest lands are dominated by non-native tree species. Invasive plant species of particular concern were found in the understory of 27% of surveyed forest plots.
Across all islands, six of the ten most abundant species are non-native: Psidium cattleyanum, Schinus terebinthifolius, Leucaena leucocepahala, Ardisia elliptica, Psidium guajava, and Acacia confusa.
While less than one-third (29%) of large trees across the Islands are non-native, this proportion increases to about two-thirds of saplings (63%) and seedlings (66%). Potter et al. focus on the likelihood that plant succession will result in transformation of these forests’ canopies from native tree species to non-native species.
75% of forests in lower-elevation areas of all islands are already dominated by non-native tree species. “Only” 31% of higher-elevation forests are so dominated. These montane forests have been viewed as refugia for native species, but all are invaded to some extent – and likely to become more degraded.
Potter et al. say the high elevation forests might be more resistant to domination by non-natives. Such a result would be counter to well-documented experience, though. Even the authors report that the montane rainforests and mesophytic forests of O‘ahu and Kaua‘i are heavily invaded by non-native tree species. Such species constitute 86% or more of large trees, saplings, and seedlings in mesophytic forests; 45% of large trees and 66% of seedlings in their montane rainforests.
The most abundant tree species in Hawai`i is the invasive species Psidium cattleyanum (strawberry guava). It was recorded on 88, or37%, of 238 FIA plots. There are nearly twice as many P. cattleyanum saplings as Hawai`i’s most widespread native species, ‘ohi’a lehua (Metrosideros polymorpha).
Widescale replacement of native trees by non-native species is likely. Several factors favor these changes: 1) tree disease – rapid ‘ohi’a death has had drastic impacts on ‘ohi’a populations on several islands; 2) invasions by forbs and grasses; 3) soil damage and other disturbances caused by invasive ungulates; and 4) climate change. If succession conforms to these trends, non-native tree species could eventually constitute 75% or more of the forest tree stems and basal area on all islands and across forest types and elevations.

Loss of Hawai’i’s native tree species would be disastrous for biodiversity at the global level. More than 95% of native Hawaiian tree species are endemic, occurring nowhere else in the world.

The authors analyzed plant presence data from 238 FIA plots. Plots spanned the state’s various climates, soils, elevations, gradients, ownership, and management. However, access issues precluded inclusion of forests from several islands: Moloka‘i, Kaho’olawe, and Ni‘ihau. I know that Moloka‘i, at least, has a protected forest reserve (a Nature Conservancy property) at the island’s highest elevations.

Protecting Native Trees

Federal, state, and private landowners have carried out numerous actions to protect native forests. These efforts might be having some success. For example, forests on public lands, in conservation reserves, or in areas fenced to exclude ungulates were less impacted by non-native plants than unfenced plots, on average. However, the authors could not determine how much of this difference was the result of management or because protections were established in forests with the lowest presence of IAS species. Fencing did not prevent invasions by forbs and grasses – possibly because they are so widespread that seed sources are everywhere.

Hawaii’s two National parks (Hawai`i Volcanoes and Haleakala) have made major efforts to control invasive plants. Hawai`i Volcanoes, on the Big Island, began its efforts in the 1980s; Haleakala (on Maui) more recently. This might be one explanation for the fact that a smaller proportion of the forests on these two islands have been invaded. These efforts have not fully protected the parks, however. Low elevation native rainforests now have a high presence of non-native shrubs. Such forests on Hawai`i Island also have significant invasions by non-native woody vines, forbs and grasses.

More discouraging, intensive efforts have not returned lowland wet forest stands to a native-dominated state. Native tree species are not regenerating—even where there is plentiful seed from native canopy trees and managers have repeatedly removed competing non-native understory plants.

Potter et al. conclude that other approaches will be needed. They suggest deliberate planting of native and non-invasive non-native species or creation of small artificial gaps that might facilitate recovery of native tree species. In montane forests on Hawai`i and Maui, where native tree seedlings account for more than 70% of all tree seedlings, they propose enhancing early detection/rapid response efforts targetting invasive forbs. This would include both National parks.Certainly Haleakala National Park has this priority in mind. It launched a serious effort to try to eradicate Miconia calvescens when this tree first was detected.

Lloyd Loope, much-mourned scientist with US Geological Survey, attacking *Miconia* on Maui

Potter et al. note the challenge of managing remnant xerophytic dry forests, where natural regeneration of native plants has been strongly limited by invasive grasses; loss of native pollinators and seed dispersers; and the increasing frequency and intensity of droughts. They note that expanded management efforts must be implemented for decades, or longer, to be successful.

Native Trees at Risk to Nonnative Insects

Beyond the scope of the Potter et al. study is the fact that at least two dry forest endemic trees have faced their own threats from non-native insects.

The Erythrina gall wasp, Quadrastichus erythrinae, appeared in Hawai`i in 2005; it originates in east Africa. It attacks the endemic tree, wiliwili, Erythrina sandwicensis. I believe a biocontrol agent, Eurytoma erythrinae, first released in 2008, has effectively protected the wiliwili tree, lessening this threat.

The Myoporum thrips, Klambothrips myopori, from Tasmania, was detected on the Big Island in 2009. It threatens a second native tree. Naio, (Myoporum sandwicense), grows in dry forests, lowlands, upland shrublands, and mesic and wet forest habitats from sea level to 3000 m. The loss of this species would be both a signifcant loss of native biodiversity and a structural loss to native forest habitats. The thrips continues to spread; a decade after the first detection, it was found on the leeward (dry) side of Hawai`i Island with rising levels of infestation and tree dieback.

*Rhus sandwicensis* on Maui; photo by Forrest and Kim Starr

Two native shrubs, Hawaiian sumac Rhus sandwicensis and Dodonea viscosa, might be at risk from a biocontrol agent in the future. APHIS has approved a biocontrol for the highly invasive Brazilian pepper, Schinus terebinthifolia. Brazilian pepper is the second-most abundant non-native tree species in the State. It was found on 28 of 238 (12%) FIA plots. However, the APHIS-approved biocontrol agent is a thrips—Pseudophilothrips ichini. It is known to attack both of these two native Hawaiian shrubs. The APHIS approval allowed release of the thrips only on the mainland US. However, many insects have been introduced unintentionally from the mainland to Hawai`i. Furthermore, Hawaiian authorities were reported to be considering deliberate introduction of P. ichini to control peppertree on the Islands.

In Conclusion

In conclusion, Potter et al. found that most Hawaiian forests are now hybrid communities of native and non-native species; indeed, a large fraction are novel forests dominated by non-native trees. Business-as-usual management will probably mean that the hybrid forests – and probably those in which the canopy is currently dominated by native species—will follow successional trajectories to novel, non-native- dominated woodlands. This likelihood results in a more dire future for native plants in Hawaiian forests than has been previously described.

Potter at al. hope that their findings can guide research and conservation on other islands, especially those in the Pacific. However, Pacific islands already have the most naturalized species globally for their size—despite what was originally considered their protective geographic isolation.

SOURCE

Potter, K.M., C. Giardina, R.F. Hughes, S. Cordell, O. Kuegler, A. Koch, E. Yuen. 2023. How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement. Landsc Ecol https://doi.org/10.1007/s10980-023-01662-6

Posted by Faith Campbell

www.fadingforests.org

Tree Regeneration Rates: A Tool for Prioritizing Tree Conservation Efforts

Ponderosa pine, Coconico National Forest; photograph by Brady Smith, USFS

Have you noticed, as I have, a spurt of interest in conservation of trees? I can rejoice that more people now focus on this!!!!

I have blogged previously about international and national efforts to determine not only native species deserving conservation priority – by the Morton Arboretum and IUCN but also species most threatened by non-native pests. I have also reported on growing attention to breeding tree resistance to non-native pests.

Some scientists are now focusing on species’ regeneration as a way to understand the probable future of both native and introduced species. I hope that scientists will integrate these new data with existing information on the impacts of invasive non-tree plants and tree-killing introduced pests. We need such a comprehensive picture. That will be a challenge!

Also, I hope attempts to set conservation priorities will influence decisions by governmental and non-governmental funders – and those who influence them! So far, I see little evidence that these key players are paying attention. Some Forest Service scientists and academics are pushing for expanded resistance-breeding efforts. Others are writing sophisticated analyses of non-native pests’ ecosystem impacts. But is the USDA leadership supporting stronger pest-prevention measures? Or funding for research on restoration of species? Are conservation NGOs addressing introduced forest pests?

Here, I summarize new work by Kevin Potter and his colleagues, published in two papers (full references at the end of this blog). After reading my summary, I’d like to know: What do you think? Do you agree with the focus on individual species’ regeneration to set conservation and control priorities? Do you agree with the priority species and geographic regions they suggest?? How should we resolve inconsistencies compared to the priorities suggested by the IUCN and Morton Arboretum? If you do agree, how would you suggest we move forward? If not, what approach do you think would be more useful?

A New Approach to Evaluating Species at Risk

Potter and Riitters (2022) point out that a species’ successful regeneration is key to its population’s future genetic diversity. That, in turn, determines the organisms’ ability to adapt to environmental stress and change. The latter includes, but is not limited to, climate change. Because trees are immobile and long-lived, their populations probably require substantially more genetic variation than those of other kinds of plants.

Potter and colleagues (both articles) used FIA survey data to examine regeneration rates by both tree species native to the continental United States (= CONUS) and non-native tree species introduced to CONUS, Hawai`i, or Puerto Rico. I rejoice that they have included these tropical islands, which are part of the United States and are centers of plant endemism. (Two other blogs provide details on their findings in Hawai`i and Puerto Rico.

Native Trees at Risk: Focus on Poor Regeneration

For CONUS, Potter and Riitters (2022) asked whether 280 native forest tree species are regenerating at sustainable levels, both across their full ranges and in regional portions of their ranges, defined by provisional seed zones (an area within which plant materials are assumed to be adapted). Tree species for which FIA surveys placed 75% of the stems in the sapling or seedling classes are determined to be regenerating at sustainable levels. Tree species exhibiting lower proportions of their stems in these “small tree” classes are said to be failing to regenerate adequately.

Potter and Riitters (2022) found that 46 of the 280 native tree species (16.4%) might be at risk of losing important levels of genetic variation (see the list of species in Table 2 of the article). These included high proportions of species evaluated in the following genera: two of three Platanus species; two of four Nyssa species; about 40% of Juniperus and Pinus; and five of 46 Quercus species (10.9%).

[Many areas of the eastern forest, especially in the Mid-Atlantic region, are reported by Stout, Hille, and Royo (2023) to be have insufficient advance regeneration to replace canopy trees.]

Some species appear to be headed toward outright extinction, not only loss of genetic diversity. These include four relatively rare species in California: Pinus muricata, Platanus racemosa, Pseudotsuga macrocarpa, and Sequioadendron giganteum. No seedlings or saplings are recorded on the plots on which they occurred. I note that Platanus racemosa in southern California is being attacked and killed by the Fusarium dieback vectored by the polygamous and Kuroshio shot hole borers.

*Platanus racemosa* riddled by invasive shot hole borer; photo by Beatriz Nobua-Behrmann, University of California Cooperative Extension

I find it alarming that a few of the possibly at-risk species have extremely wide distributions. These are Populus deltoides (eastern cottonwood), Platanus occidentalis (American sycamore), and ponderosa pine (Pinus ponderosa). Another group of species are classified as at potential risk in all their seed zones: Juniperus californica, Juniperus osteosperma, Pinus pungens, and Quercus lobata (valley oak). I note that valley oak is also under attack by the recently introduced Mediterranean oak borer. Its vulnerability is exacerbated by its relatively small range.

Potter and Riitters (2022) found distinct geographic hot spots: 15 at-risk species occur primarily in the Southeast and 14 species are in California; both represent nearly a third of the at-risk species.

In general, high rates of regeneration failure are seen in the West. Nine at-risk species (19.6% of the 46) grow in the Southwest, eight in Texas (17.4%), and four in the Rocky Mountains (8.7%). However, the Northeast and Midwest are not immune. Seven species from the former and six from the latter are also regenerating poorly. Considering pines alone, seven of 14 at-risk speciesare in the West and five in the Southeast.

Seed Zones: a Proxy for Local Genotypes

As I noted at the beginning, Potter and Riitters (2022) used USDA Forest Service provisional seed zones as a proxy for areas in which a species is presumably locally adapted. In addition to the 46 species considered failing to regenerate adequately throughout their entire ranges, Potter and Riitters (2022) determined that another 39 species are at potential risk of losing locally adapted genotypes. That is, their regeneration levels fell below the threshold in at least half of the seed zones in which they occurred. These potentially at-risk species are in the same taxonomic groups: 13 pines (33.3% of the 39 species in the category), six junipers (15.3%), and three oaks (7.7 %). These, too are concentrated in the Southeast and California: 40% are in the former — including both bald-cypress species — and 30.8% are in California. Another seven species (17.9% of the 39) are in Texas. The Midwest is home to seven species, the Northeast and Southwest each has five species (12.8%), and the Rocky Mountain region has three species (7.7%).

Bald-cypress; photo by Kej605 via WikiMedia

The seed zones with the largest numbers of species regenerating poorly are in the East, specifically the central Great Lakes region, western New York and Pennsylvania, along the Mid-Atlantic and New England coasts, and the coastal plain from southern South Carolina to eastern Texas. Potter and Riitters (2022) say these areas have such high numbers of at-risk species because they are home to so many tree species. I note [although Potter and Riitters (2022) do not] that these regions have also experienced severe levels of tree mortality due to the emerald ash borer (mature and young trees), beech leaf disease (primarily young trees), and laurel wilt disease (sub-canopy trees).

A different geographic pattern appears when considering the proportion — rather than the number — of species facing deficits in regeneration. In several Western regions, 60 – 100% of the tree species fell below the study’s threshold of 75% of recorded stems being in the sapling or seedling sizes. These seed zones are found particularly in parts of California, the Southwest, the Great Basin, and the Pacific Northwest. In none of the seed zones in the East are more than 50% of tree species in the category of potentially losing genetic variation. The implication is that while more species might be lost from parts of the East, the loss of fewer species in some Western seed zones could result in larger impacts on the composition, structure, and function of forest ecosystems there.

Potter and Riitters (2022) say that their approach has limitations because it relies on an assumption that a lack of smaller (i.e., younger) trees is an indication that a species has inadequate regeneration across all or part of its distribution and thus is vulnerable to losing genetic variation. They are not able to quantify directly the genetic variation within most forest tree species. In addition, the choice of 75% or fewer of all trees being seedlings or saplings threshold as the threshold is arbitrary. They believe these decisions are defensible.

Potter and Riitters (2022) hope that indicators of forest sustainability such as this can bridge the gap between scientists, forest managers, policy makers, and other stakeholders.

Further, the authors hope that this approach will help prioritize species most in need of: 1) monitoring for genetic diversity, 2) in situ conservation, and 3) ex situ propagule collections. In a future blog I will compare the species highlighted by Potter and Riitters (2022) to the earlier priority list developed by the IUCN and Morton Arboretum. Finally, the focus on regeneration levels could help scientists design representative sampling protocols for range-wide ex situ propagule collections for genetic diversity studies using molecular markers.

Applying This Analysis to Invasions by Non-native Trees

In a second study, Potter, Riitters, and Guo (full citation at end of this blog) flipped the focus: they used the same approach to quantify the degree of invasion by non-native trees in the U.S. I’ve blogged about this study, in general, here. Also see my separate blogs for its welcome application to Hawai`i and Puerto Rico.

Again, Potter, Riitters, and Guo hope their approach will assist in the crucial, difficult task of distinguishing between high-impact and less threatening non-native species. They warn, however, that the FIA survey procotol does not suit the needs of an early detection system.

Differentiating Invasive Tree Species’ Impacts

Potter, Riitters, and Guo note that thousands of non-native tree species have been planted around world to provide an extensive list of ecosystem services. Globally, 400 tree species have been recognized as naturalized (= consistently reproducing) or invasive (= spreading) in areas outside their native ranges. Contrary to some expectations, even relatively undisturbed forests are affected by invasive plants. In the continental United States, many fewer invasive plant species are trees than other forms/habits – shrubs, forbs, gramminoids. On the tropical islands, a much higher proportion of invasive plants are trees.

Lugo et al. (2022; full citation at end of this blog) find non-native tree species occupy a tiny fraction of the forest area of the continental United States [= CONUS], i.e., only 2.8% of the area, and only 0.4% of all tree species recorded in the FIA plots. However, these non-native tree species are widespread. They are found in 61% of forested ecosections in CONUS. Also, they are becoming more common in invaded sites. [Ecosections are divisions within 37 ecological provinces in the hierarchical framework developed by Cleland et al. (2007). There are 190 ecosections in U.S. forest biomes.]

Potter, Riitters, and Guo categorized those non-native tree species with at least 75% of stems detected by FIA surveys to be in sapling or seedling size as highly invasive. In other words, these species are successfully reproducing after reaching the canopy. So they might be more likely to alter forest functions and ecosystem services than those reproducing less robustly. They classified as species with 60 – 75% of recorded stems in these “small tree” categories as “moderately invasive.”

Potter, Riitters, and Guo suggest that control might more productively target the moderately invasive species in geographic regions where they have spread less so far – so presumably fewer seed-bearing mature specimens are present. They list as examples Picea abies, Pinus sylvestris, and Paulownia tomentosa.

In CONUS, FIA protocols specify reporting of 30 non-indigenous tree species.

Acer platanoides

Ailanthus altissima

Albizia julibrissin

Alnus glutinosa

Castanea mollissima

Casuarina lepidophloia

Cinnamomum camphora

Citrus sp.

Elaeagnus angustifolia

Eucalyptus globulus

Eucalyptus grandis

Ginko biloba

Melaleuca quinquenervia

Melia azedarach

Morus alba

Paulownia tomentosa

Picea abies

Pinus nigra

Pinus sylvestris

Populus alba

Prunus avium

Prunus persica

Salix alba

Salix sepulcralis

Sorbus aucuparia

Tamarix spp

Triadica sebifera

Ulmus pumila

Vernicia fordii

About half of these –16 species – qualified under the Potter, Riitters, and Guo criteria as highly invasive: Acer platanoides, Ailanthus altissima, Albizia julibrissin, Cinnamomum camphora, Elaegnus angustifolia, Melia azedarach, Melaleuca quinquenervia, Morus alba, Picea abies, Pinus nigra, Prunus avium, Salix alba, Salix sepulcralis, Triadica sebifera, Ulmus pumila, Vernicia fordii. An additional four taxa are ranked as potentially highly invasive: Tamarix; Eucalyptus grandis and E. globulus, Populus alba.

ring-billed gulls eating berries of Chinese tallowtree (*Triadica sebifera*); photo by TexasEagle via Flickr

I ask : Do YOU agree that these taxa are the most important to be tracking as potentially invasive in forests of the continental United States?

Potter, Riitters, and Guo distinguish between the most “common” and the most “widespread” invasive tree species – although they do not define the differences. Some of the most “common” or “widespread” species are not a surprise: Ailanthus altissima, Triadica sebifera (syn. Sapium sebiferum), and Acer platanoides. Ailanthus is categorized as highly invasive in 39 of 44 ecoregions in which it occurs. It is also notoriously difficult to manage. Triadica sebifera is classified as highly invasive in every one of the 20 ecoregions in which it occurs. It produces prolific seed crops that are widely dispersed by birds and water. It can invade both disturbed and undisturbed habitats. Some of the common or widespread species do surprise me: Ulmus pumila, Morus alba and Picea abies.

Most of the non-native tree species occur on only 2% of plots in the ecoregions in which they occur. However, some highly invasive trees exceed this level:

Triadica sebifera is detected on 8.6% of plots on average across 20 ecoregions;

Ulmus pumila is detected on 3.7% of plots across 39 ecoregions;

Elaeagnus angustifolia is detected on 3.3% of plots in 13 ecoregions;

Melaleuca quinquenervia is detected on 2.7% of plots in 4 ecoregions.

A. altissima is detected on only 2% of plots in the 44 ecoregions. This is surprising to me. I see it everywhere in the Mid-Atlantic – and elsewhere!

[In USFS Region 9 (24 states in the Northeast and Midwest), FIA surveys in 2019 detected Ailanthus on only 3% of plots, Norway maple and Siberian elm each on only 1% of plots (Kurz 2023).]

Eastern U.S. forests are invaded at rates several times those in Western forests, both as a proportion of plots that are invaded and the diversity of plant growth forms. The probability of invasion is highest in Eastern forests that are relatively productive and located in fragmented landscapes that contain developed or agricultural land. Non-native invasive trees are most prevalent along the Gulf Coast and in Mid-Atlantic and Midwestern States. Highly invasive non-native trees are most diverse in the ecoregions of the Mid-Atlantic and Southeast. I note that these regions also rank high in numbers of native tree species determined by Potter et al.’s other study to be reproducing an unsustainable levels.

The study found that non-native trees are almost entirely absent from the Rocky Mountain States and Alaska. However, I have seen Ailanthus in riparian areas of Utah, Arizona, and New Mexico. While few non-native tree species are recorded from ecoregions along the Pacific Coast, those areas are heavily invaded by other types of plants. Lugo et al. say those shrubs and forbs are not interfering with forest regeneration. Do YOU agree?

BLM & USFS botanists removing Spanish broom from Rogue River Canyon; photo by Stacy Johnson, BLM

On tropical islands included in the study – Hawai`i and Puerto Rico – the situation is very different. Together, these islands’ tree canopy covers less than 0.5% that of the area in the lower 48. Hawai`i is recognized as a global hotspot of non-native species richness. Naturalized non-native plant taxa constitute about half of the Hawaiian flora. The US Forest Service tracks twice as many non-native tree species in Hawai`i (62) than over the entire continental U.S. plus Alaska.

Of these 62 species, Potter, Riitters, and Guo identified 26 tree species as either highly or moderately invasive, either already or potentially highly invasive, three as moderately invasive, seven as potentially moderately invasive. In general, the richness of non-native tree species is higher in lower-elevation ecoregions, especially the lowland/leeward dry and mesic forests on O’ahu and lowland wet and mesic forests of the Big Island. [The article makes a brief reference to the probable role of rapid ʻōhiʻa death opening the canopy of the mesic and wet forests, thereby facilitating plant invasions.] Most Hawaiian ecoregions, especially those on O’ahu and Hawai’i Island, had higher non-native tree species richness than even the most highly invaded ecoregions in the lower 48 states. Parts of O’ahu & Maui had the most non-native tree species classified as highly invasive.

The Caribbean archipelago – including but not limited to Puerto Rico – has a lower proportion of non-native plant species than Hawai’i — 17% of plant species are not native. However, their presence is even higher: two-thirds of Puerto Rico’s forests comprise novel tree assemblages. This is probably because Puerto Rico has half the land area of the Hawaiian archipelago and has been part of global trade networks for 500 years instead of 200. Potter and colleagues identified 17 non-native tree species as highly invasive, 16 as potentially highly invasive, and two as moderately invasive.

On the continent only seven of 30 non-native tree species occurr on at least 2% of FIA plots across the ecoregions in which they are inventoried. Hawai’i is stunningly different: 56 of 62 species occurr on at least 2% of plots across ecoregions on average; 24 species are present on at least 10% of plots on average. One species, Psidium cattleyanum, is present on nearly half of surveyed plots across 13 ecoregions! In Puerto Rico, 21 species occurred on at least 2% of the FIA plots.

Acacia confusa – highly invasive in dry forests of Hawai`i; photo by Forrest and Kim Starr

Potter, Riitters, and Guo could not assess the invasiveness of several species that occurred only as small stems in a couple of plots. There are 11 such species on Hawai`i, eight on Puerto Rico. These species might be in the early stages of widespread invasion, or they might never be able to reproduce and spread. Despite the uncertainty, the authors suggest that eradication or control efforts targetting these species might be more cost-effective since presumably few trees have reached reproductive age yet. In Puerto Rico, they single out Schinus terebinthifolius, since it is already recognized as moderately invasive in Hawai`i [I add – seriously invasive in nearby Florida!]. However, they also emphasize the threat from one of the widespread species, Syzgium jambos, because it is a shade-tolerant species that can form dense, monotypic stands under closed canopies

I have posted separate blogs providing more details on the invasive tree species in Hawai`i and Puerto Rico.

Limits of the FIA Dataset

As in the study of native species regeneration, Potter, Riitters, and Guo specify limits arising from use of the FIA dataset. Two seem particularly pertinent to evaluation of the situation on the tropical islands.

First, they cataloged only those non-native tree species chosen by the FIA program administrators to track in the three major regions. Again, I ask YOU whether you agree with the species being recorded. Should others species be included? Should some of these species be dropped?

Second, the survey protocol does not differentiate between sites with significantly different status and history. For example, non-native trees growing on abandoned agricultural sites are counted the same way as those growing in presumably old-growth forests. They conclude that including such sites might explain the records of Eucalyptus and pine species in surveys on the islands.

Finally, as noted in the other study, the program incorporates plots that contain at least 10% canopy cover by live trees or had such cover in the past. The inventory has not included urban parks – although in recent years an urban inventory protocol has been developed.

I remind you that Potter, Riitters, and Guo warned that the FIA inventory is not designed to detect newly introduced species that are early in the invasion process.

SOURCES

Kurtz, C.M. 2023. An assessment of invasive plant species in northern U.S. forests. Res. Note NRS-311. http://doi.org/10.2737/NRS-RN-311

Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, and C.M. Kurtz. 2022. The Contribution of NIS Tree Species to the Structure and Composition of Forests in the Conterminous United States in Comparison with Tropical Islands in the Pacific & Caribbean. USDA USFS General Technical Report IITF-54.

Potter, K.M and Riitters, K. 2022. A National Multi-Scale Assessment of Regeneration Deficit as an Indicator of Potential Risk of Forest Genetic Variation Loss. Forests 2022, 13, 19. https://doi.org/10.3390/f13010019.

Potter K.M., Riitters, K.H. and Guo, Q. 2022. Non-native tree regeneration indicates regional and national risks from current invasions. Frontiers in Forests and Global Change doi: 10.3389/ffgc.2022.966407

Stout, S.L., A.T. Hille, and A.A. Royo. 2023. Science-Management Collaboration is Essential to Address Current & Future Forestry Challenges. IN United States Department of Agriculture. Forest Service. 2023. Proceedings of the First Biennial Northern Hardwood Conference 2021: Bridging Science and Management for the Future. Northern Research Station General Technical Report NRS-P-211 May 2023

Posted by Faith Campbell

www.fadingforests.org

Support House & Senate bills to Enhance Response to Forest Pests

white ash: a species that might be restored under the programs envisioned in the proposed bills

Bills have been introduced into both the House and Senate to enhance USDA APHIS and Forest Service programs intended to curtail introduction and spread of non-native forest pests and disease and – especially – programs aimed at restoring pest-decimated trees to the forest.

The House bill is H.R. 3174; it was introduced by Reps. Becca Balint (VT).

The Senate bill is S. 1238; it was introduced by Senators Peter Welch (VT), Mike Braun (IN), and Maggie Hassen (NH). [Both senators Welch and Braun are on the Agriculture Committee – which will write the bill.]

CISP hopes that the contents of these two bills will be incorporated in the Farm Bill that Congress is expected to adopt this year or next. The proposals have the support of the Forests in the Farm Bill coalition. [Unfortunately, neither the “Consolidated Recommendations” nor “Summarized Recommendations appears to be posted on the internet at present.]

In the last Congress, a nearly identical bill introduced by then-Representative Peter Welch was endorsed by the organizations listed below. We hope they will endorse the new bills now! If you are a member of one of these organizations, please ask them to do so.

Organizations that endorsed the previous bill: Vermont Woodlands Association, American Forest Foundation, Center for Invasive Species Prevention, Reduce Risk from Invasive Species Coalition, National Woodland Owners Association (NWOA), National Association of State Foresters (NASF), The Society of American Foresters (SAF), the North American Invasive Species Management Association (NAISMA), the Ecological Society of America, Entomological Society of America, a broad group of university professors and scientists, The Nature Conservancy (TNC) Vermont, Audubon Vermont, the Massachusetts Forest Alliance, the New Hampshire Timberland Owners Association, the Maine Woodland Owners Association, and the Pennsylvania Forestry Association.

I seek your help in generating support for incorporating these proposals into the 2023 Farm Bill. Please urge your representative and senators to co-sponsor the bills or otherwise support that action.

beech in a breeding experiment at The Holden Arboretum; photo by Jennifer Koch

Key points of the two bills:

They strengthen APHIS’ access to emergency funds. APHIS has had the authority to access emergency funds from the Commodity Credit Corporation since 2000. However, the Office of Management and Budget has often blocked its requests. See § 2, of the bills, EMERGENCY AUTHORITY WITH RESPECT TO INVASIVE SPECIES.
It creates two separate but related grant programs.
- The first grant program – in § 3. FOREST RECLAMATION GRANTS – funds research addressing specific questions impeding the recovery of tree species that are native to the US and have suffered severe levels of mortality caused by non-native plant pests or noxious weeds.
- The second grant program – in § 4. FOREST RESTORATION IMPLEMENTATION GRANTS – funds implementation of projects to restore these pest-decimated tree species to the forest. These projects must be part of a forest restoration strategy that incorporates a majority of the following components:

(1) Collection and conservation of native tree genetic material.

(2) Production of propagules of the target tree species in numbers sufficient for landscape-scale restoration.

(3) Preparation of planting sites in the target tree species’ former habitats.

(4) Planting of native tree seedlings.

(5) Post-planting maintenance of native trees.

§ 5 states that the absence of a national policy on addressing nonnative forest pests has resulted in their receiving a low priority within all Federal agencies. It then mandates a study to analyze agencies’ available resources, raise the issue’s priority, and improve coordination among agencies. This study is to be carried out by an independent institution, for example the National Academy of Sciences. The authors are to consult with specialists in entomology, genetics, forest pathology, tree breeding, forest and urban ecology, and invasive species management.
Funding for all three action components – the emergency response and both grant programs – would come from the Commodity Credit Corporation, so it would not be subject to the vagaries of annual appropriations bills.

Forest Restoration Alliance volunteers potting hemlock seedlings; photo provided by Fred Hains

Entities which could apply for the research grants (§ 3 of the bills) include Federal agencies; State cooperative institutions; academic institutions offering degrees in the study of food, forestry, and agricultural sciences; and non-profit organizations exempt from taxes under §501(c)(3) of the tax code. Types of research funded could include:

‘‘(A) biocontrol of nonnative pests & diseases or noxious weeds severely damaging native tree species [the bill does not specify, but Project CAPTURE identifies many qualifying species; see also my earlier blog];

‘‘(B) exploration of genetic manipulation of the plant pests or noxious weeds;

‘‘(C) enhancement of pest-resistance mechanisms of hosts; and

‘‘(D) development of other strategies for restoring individual tree species.

The maximum amount of such grants is $400,000 per year.

Entities which could apply for the implementation grants (§ 4 of the bills) include a cooperating forestry school; a land-grant college or university; a State agricultural experimental station; a 501(c)(3) organization. Funding would begin at $3 million for FY 2023 and rise to $10 million for FY 2026.

The Secretary of Agriculture would be guided in implementing these programs by two committees. One – the committee of experts – would constitute representatives of the USFS, APHIS, ARS & State forestry agencies. The second – the advisory committee – would be composed of representatives of land-grant colleges and universities and affiliated State agriculture experiment stations, forest products industry, recreationists, and professional forester, conservation, and conservation scientist organizations.

Port-Orford cedar seedlings at USFS Dorena Center – a model for success! Photo provided by Richard Sniezko

Please contact your Member of Congress (Representative) and senators to urge them to support inclusion of these provisions in the Farm Bill. [Remember: they work for us!] Telling them of your support for these bills is especially important if your Representative or Senator is on the Agriculture Committee. I list those legislators here:

State	HOUSE AGRIC COMM	SENATE AGRIC COMM
AL	Barry Moore	Tommy Tuberville
AR	Rick Crawford	John Boozman
CA	Doug Lamalfa John Duarte Jim Costa Salud Carbajal
CO	Yadira Caraveo	Michael Bennet
CT	Jahana Hayes
FL	Kat Cammack Darren Soto
GA	Austin Scott David Scott Sanford Bishop	Raphael Warnock
HI	Jill Tokuda
IA	Randy Feenstra Zach Nunn	Joni Ernst Charles Grassley
IL	Mike Bost Mary Miller Nikki Budzinski Eric Sorensen Jonathan Jackson	Richard Durbin
IN	Jim Baird	Mike Braun
KS	Tracey Mann Sharice Davids	Roger Marshall
KY		Mitch McConnell
MA	Jim McGovern
ME	Chellie Pingree
MI	Elissa Slotkin	Debbie Stabenow
MN	Angie Craig	Amy Klobuchar Tina Smith
MO	Mark Alford
MS	Trent Kelly	Cindy Hyde-Smith
NC	David Rouzer Alma Adams
ND		John Hoeven
NE	Don Bacon	Deb Fischer
NJ		Cory Booker
NM	Gabe Vasquez	Ben Ray Lujan
NY	Marc Molinaro Nick Langworthy	Kirsten Gillibrand
OH	Max Miller Shontel Brown	Sherrod Brown
OK	Frank Lucas
OR	Lori Chavez-Deremer Andrea Salinas
PA	Glenn Thompson	John Fetterman

SD	Dusty Johnson	John Thune
TN	Scott Desjarlais Brad Finstad
TX	Ronny Jackson Monica de la Cruz Jasmine Crockett
VA	Abigail Spanberger
VT	Peter Welch
WA	Marie Gluesenkamp Perez
WI	Derrick van Orden

Posted by Faith Campbell