Category: forest pathogens

Global Overview of Bioinvasion in Forests

black locust – one of the most widespread invasive tree species on Earth; photo via Flickr

In recent years there has been an encouraging effort to examine bioinvasions writ large see earlier blogs re: costs of invasive species – here and here. One of these products is the Routledge Handbook of Biosecurity and Invasive Species (full citation at end of this blog). I have seen only the chapter on bioinvasion in forest ecosystems written by Sitzia et al. While they describe this situation around the globe, their examples are mostly from Europe.

Similar to other overviews, this article re-states the widely-accepted attribution of rising numbers of species introductions to globalization, especially trade. In so doing, Sitzia et al. assert that the solution is not to curtail trade and movement of people, but to improve scientific knowledge with the goal of strengthening biosecurity and control programs. As readers of this blog know, I have long advocated more aggressive application of stronger restrictions on the most high-risk pathways. Still, I applaud efforts to apply science to risk assessment.

Sitzia et al. attempt to provide a global perspective. They remind readers that all major forest ecosystems of Earth are undergoing significant change as a result of conversion to different land-uses; invasion by a wide range of non-native introduced species—including plants, insects, and mammals; and climate change. These change agents act individually and synergistically. Sitzia et al. give greater emphasis than other writers to managing the tree component of forests. They explain this focus by asserting that forest management could be either the major disturbance favoring spread of non-native species or, conversely, the only way to prevent further invasions. They explore these relationships with the goal of improving conservation of forest habitats.

Japanese stiltgrass invasion; photo by mightyjoepye via Flickr

Sitzia et al. focus first on plant invasions. They contend that – contrary to some expectations – plants can invade even dense forests despite competition for resources. They cite a recent assessment by Rejmánek & Richardson that identified 434 tree species that are invasive around Earth. Many of these species are from Asia, South America, Europe, and Australia. These non-native trees can drive not only changes in composition but also in conservation trajectories in natural forests. However, the example they cite, Japanese stilt grass (Microstegium vimineum) in the United States, is not a tree! Sitzia et al. note that in other cases it is difficult to separate the impacts of management decisions, native competitive species, and non-native species.

Sitzia et al. note that plant invasions might have a wide array of ecological impacts on forests. They attempt to distinguish between

  • “drivers” of environmental change – including those with such powerful effects that they call them “transformers”;  
  • “passengers” whose invasions are facilitated by other changes in ecosystem properties; and
  • “backseat drivers” that benefit from changes to ecosystem processes or properties and cause additional changes to native plant communities.

An example of the last is black locust (Robinia pseudoacacia). This North American tree has naturalized on all continents. It is a good example of the management complexities raised by conflicting views of an invasive species’ value, since it is used for timber, firewood, and honey production.

Sitzia et al. then consider invasions by plant pathogens. They say that these invasions are one of the main causes of decline or extirpations in tree populations. I applaud their explicit recognition that even when a host is not driven to extinction, the strong and sudden reduction in tree numbers produces significant changes in the impacted ecosystems.

American chestnut – not extinct but ecological role gone; photo by F.T. Campbell

Sitzia et al. contend that social and economic factors determine the likelihood of a species’ transportation and introduction. Specifically, global trade in plants for planting is widely recognized as being responsible for the majority of introductions. Introductions via this pathway are difficult to regulate because of the economic importance (and political clout) of the ornamental plants industry, large volumes of plants traded, rapid changes in varieties available, and multiple origins of trade. As noted above, the authors seek to resolve these challenges by improving the scientific knowledge guiding biosecurity and control programs. In the case of plant pathogens, they suggest adopting innovative molecular techniques to improve interception efficiency, esp. in the case of latent fungi in asymptomatic plants.

The likelihood that a pathogen transported to a new region will establish is determined by biogeographic and ecological factors. Like other recent studies, Sitzia et al. attempt to identify important factors. They name a large and confusing combination of pathogen- and host-specific traits and ecosystem conditions. These include the fungus’ virulence, host specificity, and modes of action, reproduction, and dispersal, as well as the host’s abundance, demography, and phytosociology. A key attribute is the non-native fungus’ ability to exploit micro-organism-insect interactions in the introduced range. (A separate study by Raffa et al. listed Dutch elm disease as an example of this phenomenon.)  I find it interesting that they also say that pathogens that attack both ornamental and forest trees spread faster. They do not discuss why this might be so. I suggest a possible explanation: the ornamental hosts are probably shipped over wide areas by the plant trade.

surviving elms in an urban environment; photo by F.T. Campbell

Sitzia et al. devote considerable attention to bioinvasions that involve symbiotic relationships between bark and ambrosia beetles and their associated fungi. These beetles are highly invasive and present high ecological risk in forest ecosystems. Since ambrosia beetle larvae feed on symbiotic fungi carried on and farmed by the adults inside the host trees, they are often polyphagous. Bark beetles feed on the tree host’s tissues directly, so they tend to develop in a more restricted number of hosts. Both can be transported in almost all kinds of wood products, where they are protected from environmental extremes and detection by inspectors. Sitzia et al. specify the usual suspects: wood packaging and plants for planting, as ideal pathways. These invasions threaten indigenous species by shifting the distribution and abundance of certain plants, altering habitats, and changing food supplies. The resulting damage to native forests induces severe alterations of the landscape and causes economic losses in tree plantations and managed forests. The latter losses are primarily in the high costs of eradication efforts – and their frequent failure.

Eucalyptus plantation in Kwa-Zulu-Natal, South Africa; photo by Kwa-Zulu-Natal Department of Transportation

Perhaps their greatest contribution is their warning about probable damage caused by invasive forest pests in tropical forests. (See an earlier blog about invasive pests in Africa.) Sitzia et al. believe that bark and ambrosia beetles introduced to tropical forests threaten to cause damage of the same magnitude as climate change and clear cutting, but there is little information about such introductions. Tropical forests are exposed to invading beetles in several ways:  

1) A long history of plant movement has occurred between tropical regions. Sitzia et al. contend that the same traits sought for commercial production contribute to risk of invasion.

2) Logging and conversion of tropical forests into plantation forestry and agriculture entails movement of potentially invasive plants to new areas. Canopies, understory plant communities, and soils are all disturbed. Seeds, insects, and pathogens can be introduced via contaminated equipment.

3) Less developed nations are often at a disadvantage in managing potential invasion. Resources may be fewer, competing priorities more compelling, or potential threats less obvious.

Sitzia et al. call for development of invasive species management strategies that are relevant to and realistic for less developed countries. These strategies must account for interactions between non-native species and other aspects of global environmental change. Professional foresters have a role here. One clear need is to set out practices for dealing with conflicts between actors driven by contrasting forestry and conservation interests. These approaches should incorporate the goals of shielding protected areas, habitat types and species from bioinvasion risk. Sitzia et al. also discuss how to address the fact that many widely used forestry trees are invasive. (See my earlier blog about pines planted in New Zealand.)

planted forest in Sardinia, Italy; photo by Torvlag via Flickr

In Europe, bark beetle invasions have damaged an estimated ~124 M m2 between 1958 and 2001. Sitzia et al. report that the introduction rate of non-native scolytins has increased sharply. As in the US, many are from Asia. They expect this trend to increase in the future, following rising global trade and climate change. Southern – Mediterranean – Europe is especially vulnerable. The region has great habitat diversity; a large number of potential host trees; and the climate is dry and warm with mild winters. The region has a legacy of widespread planting of non-native trees which are now important components of the region’s economy, history and culture. These include a significant number of tree species that are controversial because they are – or appear to be – invasive. Thus, new problems related to invasive plants are likely to emerge.

Noting that different species and invasion stages require different action, Sitzia et al. point to forest planning as an important tool. Again the discussion centers on Europe. Individual states set forest policies. Two complications are the facts that nearly half of European forests are privately owned; and stakeholders differ in their understanding of the concept of “sustainability”. Does it mean ‘sustainable yield’ of timber? Or providing multiple goods and services? Or sustaining evolution of forest ecosystems with restrictions on the use of non-native species? Resolving these issues requires engagement of all the stakeholders.

Sitzia et al. say there has recently been progress. The Council of Europe issued a voluntary Code of Conduct on Invasive Alien Trees in 2017 that provides guidelines on key pathways. A workshop in 2019 elaborated global guidelines for the sustainable use of non-native tree species, based on the Bern Convention Code of Conduct on Invasive Alien Trees. The workshop issued eight recommendations:

  • Use native trees, or non-invasive non-native trees;
  • Comply with international, national, and regional regulations concerning non-native trees;
  • Be aware of the risk of bioinvasion and consider global change trends;
  • Design and adopt tailored practices for plantation site selection and silvicultural management;
  • Promote and implement early detection and rapid response programs;
  • Design and adopt practices for invasive non-native tree control, habitat restoration, and for dealing with highly modified ecosystems;
  • Engage with stakeholders on the risks posed by invasive NIS trees, the impacts caused, and the options for management; and
  • Develop and support global networks, collaborative research, and information sharing on native and non-native trees.

SOURCE

Sitzia, T., T. Campagnaro, G. Brundu, M. Faccoli, A. Santini and B.L. Webber. 2021 Forest Ecosystems. in Barker, K. and R.A. Francis. Routledge Handbook of Biosecurity and Invasive Species. ISBN 9780367763213

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Analysis of Methods to Predict a New Pest’s Invasiveness: Which Work Best Under What Conditions?

spotted lanternfly – could we have predicted its arrival? Its Impacts? Photo by Holly Ragusa, Pennsylvania Department of Agriculture

As readers of my blogs know, I wish to prevent introduction and spread of tree-killing insects and pathogens and advocate tighter and more pro-active regulation as the most promising approach. I cannot claim to have had great success.

Of course, international trade agreements have powerful defenders and the benefit of inertia. And in any case, prevention will be enhanced by improving the accuracy of predictions as to which specific pests are likely to cause significant damage, which are likely to have little impact in a naïve ecosystem. This knowledge would allow countries to can then focus their prevention, containment, and eradication efforts on this smaller number of organisms.

I applaud a group of eminent forest entomologists and pathologists’ recent analysis of widely-used predictive methods’ efficacy [see Raffa et al.; full citation at the end of this blog]. I am particularly glad that they have included pathogens, not just insects. See earlier blogs here, here, here, and here.

I review their findings in some detail in order to demonstrate their importance. National and international phytosanitary agencies need to incorporate this information and adopt new strategies to carry out their duty to protect Earth’s forests from devastation by introduced pests.

Raffa et al. note the usual challenges to plant health officials:

  • the high volumes of international trade that can transport tree-killing pests;
  • the high diversity of possible pest taxa, exacerbated by the lack of knowledge about many of them, especially pathogens;
  • the restrictions on precautionary approaches imposed by the World Trade Organization’s Sanitary and Phytosanitary Agreement (the international phytosanitary system) – here, here, and here.   
  • the high cost and frequent failure of  control efforts.
ash trees killed by emerald ash borer; Mattawoman Creek, Maryland; photo by Leslie A. Brice

The Four Approaches to Predicting Damaging Invaders

At present, four approaches are widely used to predict behavior of a species introduced to a naïve environment:

(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.

Raffa et al. first identify each method’s underlying assumptions, then discuss the strengths and weaknesses of each approach for addressing three categories of biological factors that they believe explain why some organisms that are relatively benign, sparse, or unknown in their native region become highly damaging in naïve regions:

(1) the lack of effective natural enemies in the new region compared with the community of predators, parasites, pathogens, and competitors in the historical region (i.e., the loss of top-down control);

(2) the lack of evolutionary adaptation by naïve trees in the new region compared with long-term native interactions that select for effective defenses or tolerance (i.e., the loss of bottom-up control); and

(3) novel insect–microbe associations formed in invaded regions in which one or both members of the complex are non-native, resulting in increased vectoring of or infection courts for disease-causing pathogens (i.e., novel symbioses). I summarize these findings in some detail later in this blog.

Most important, Raffa et al. say none of these four predictive approaches can, by itself, provide a sufficiently high level of combined precision and generality to be useful in predictions. Therefore, Raffa et al. outline a framework for applying the strengths of the several approaches (see Figure 4). The framework can also be updated to address the challenges posed by global climate change.

Raffa et al. repeatedly note that lack of information about pests undercuts evaluation efforts. This is especially true for pathogens and the processes determine which microbes that are innocuous symbionts in co-evolved hosts become damaging pathogens when introduced to naïve hosts in new ecosystems.

Findings in Brief

Raffa et al. found that:

  1. Previous pest history in invaded environments provides greater predictive power than population dynamics in the organism’s native regions.
  2. Models comparing pest–host interactions across taxa are more predictive when they incorporate phylogenies of both pest and host. Traits better predict a pest’s likelihood of transport and establishment than its impact.
  3. Sentinel plantings are most applicable for pests that are not primarily limited to older trees. Ex patria sentinel plantations are more likely to detect pest species liberated by loss of bottom-up controls than top-down controls, i.e., most fungi and woodborers but not insect defoliators.
  4. Laboratory tests are most promising for pest species whose performance on seedlings and detached parts (e.g., leaves) accurately reflects their performance on live mature trees. They are thus better at predicting impacts of insect folivores and sap feeders than woodborers or vascular wilt pathogens.

Raffa et al. also ask some fundamental questions:

  • How realistic is it to expect reliable predictions, given the uniqueness of each biotic system?
  • When should negative data – lack of data showing a species is invasive – justify decisions not to act? Especially when there are so many data gaps?  
  • Who should make decisions about whether to act? How should the varying values of different social sectors be incorporated into decisions?

Raffa et al. identify critical areas for improved understanding:

1) Statistical tools and estimates of sample size needed for reliable forecasts by the various approaches.

2) Reliability, breadth, and efficiency of bioassays.

3) Processes by which some microorganisms transition from saprophytic to pathogenic lifestyles.

4) Procedures for scaling up results from bioassays and plantings to ecosystem- and landscape-level dynamics.

5) Targetting and synergizing predictive approaches and methods for more rapid and complete information transfer across jurisdictional boundaries.

I am struck by two generalizations:

  1. While most introduced forest insects are first detected in urban areas, introduced pathogens are more commonly detected in forests. I suggest that more intensive surveys of urban trees and “sentinel gardens” might result in detection of pathogens before they reach the forest.
  2. Enemy release is rarely documented as the primary basis for pathogens that cause little or no impact in their native region but become damaging in an introduced region. Enemy release appears generally more important with folivores and sap feeders than with woodborers.

Detailed Evaluation of Predictive Methodologies

white pine blister rust-killed whitebark pine at Crater Lake National Park; photo by F.T. Campbell
  1. Empirical assessment of pest status in previously occupied habitats

This is the most commonly applied method now, partly because it seems to follow logically from the World Trade Organization’s requirement that national governments provide scientific evidence of risk to justify adopting phytosanitary measures. The underlying assumption is that species that have caused damage in either their native or previously invaded ranges are those most likely to cause damage if introduced elsewhere. The corollary is that species that have not previously caused damage are unlikely to cause significant harm in a new ecosystem.

As noted above, Raffa et al. found that a species’ damaging activity in a previously invaded area can help indicate likely pest status in other regions. However, its status — pest or not — in its native range is not predictive. See Table 1 for numerous examples of both pests and non-pests. For example, Lymantria dispar has proved damaging in both native and introduced ranges. Ips typographus has not invaded new territories despite being damaging in its nature range and frequently being transported in wood. White pine blister rust is not an important mortality source on native species in its native range but is extremely damaging in North America.

Raffa et al. also note the importance of whether effective detection and management strategies exist in determining a pest’s impact ranking. Insects are more easily detected than pathogens; some respond to long distance attractants such as pheromones or plant volatiles. These methods can include insect vectors of damaging pathogens.

Re: the difficulty of assessing insect–microbe associations, they name several examples of symbionts which have caused widespread damage to naïve hosts: laurel wilt in North America; Sirex noctilio and Amylosterum areolatum around the Southern Hemisphere; Monochamus spp. and Bursaphelenchus xylophilus in Asian and European pines. Dutch elm disease illustrates a widespread epidemic caused by replacement of a nonaggressive native microorganism in an existing association with a non-native pathogen. Beech bark disease resulted from independent co-occurrence of an otherwise harmless fungus and harmless insect.

In sum, “watch” lists are disappointingly poor at identifying species that are largely benign in their native region but become pests when transported to naive ecosystems. Many of our most damaging pests are in this group. Raffa et al. note that this is not surprising because naïve systems lack the very powerful top-down, bottom-up, and lateral forces that suppress pests’ populations in co-adapted system. Countries often try to overcome this uncertainty by shifting to pathway mitigation and other “horizontal measures” – as I have often advocated. Raffa et al. emphasize that such approaches are costly to implement and constrain free trade.

  • Predictive models based on traits of pests and hosts

Predictive models provide the most all-encompassing and logistically adaptable of the forecasting approaches. Typically, models consider various components of risk, e.g. probability of transport, probability of establishment, anticipated level of damage.

The overriding assumption is that patterns emerging from either previous invasions or basic biological relationships can provide reliable predictions of impacts that might result from future invasions. However, Raffa et al. note that the models’ reliability and specificity are hampered by small sample sizes and data gaps.

They found that specific life history traits have proved to be more predictive of insect — and to a lesser extent fungal – establishment than of impact. Earlier studies [Mech et al. (2019) and Schulz et al. (2021)] found no association between life history traits and impacts for either conifer-feeding or angiosperm-feeding insects.   

Some traits of pathogens have been linked to invasion success, e.g., dispersal distance, type of reproduction, spore characteristics, and some temperature characteristics for growth and parasitic specialization. Raffa et al. say that root-infecting oomycete pathogens have a broader host range and invasive range than those that attack aboveground parts. Oomycetes that grow faster and produce thick-walled resting structures have broader host ranges. Phenotypic plasticity is also important. Raffa et al. say that those organisms that require alternate hosts can be limited in their ability to establish. However, they don’t mention that – once introduced — they can have huge impacts, as the example of white pine blister rust illustrates.

Raffa et al. say that phylogenetic distance of native and introduced hosts is more predictive for foliar ascomycetes than for basidiomycete and oomycete pathogens with broad host ranges. They suggest predictive ability can be improved by incorporating other factors, e.g., feeding guild. They note that the findings of Mech et al. and Schulz et al. (see links above) show the importance of both host associations with pests and phylogenetic relationships between native and naïve hosts for predicting impacts.

Geography is important: while there is a greater chance of Northern Hemisphere pests invading in the same hemisphere, this is not universal, as shown by Sirex (of course, the woodwasp is attacking hosts native to the Northern Hemisphere – pines).

Genomic analyses have been used more often with pathogens. There are two general approaches:

trees killed by chestnut blight; USDA Forest Service photo

1) Comparing the genomes of different species to identify the determinants associated with certain traits or lifestyles. For example, a post hoc analysis of the genus Cryphonectria could distinguish nonpathogenic species from the chestnut blight fungus C. parasitica.

2) Using genomic variation within a single species to identify markers associated with traits. Genome sequencing of a worldwide collection of the pathogens that cause Dutch elm disease revealed that some genome regions that originated from hybridization between fungal species contained genes involved in host–pathogen interactions and reproduction, such as enhanced pathogenicity and growth rate.

Raffa et al. point out that the growth of databases will facilitate genomic approaches to identify important invasiveness and impact traits, such as sporulation, sexual reproduction, and host specificity.

At present, Raffa et al. believe that models based on traits, phylogeny, and genomics offer potential for a rapid first pass to predicting levels of pest damage. However, assessors must first have a list of candidate pest species and detailed information about each. Plus there is still too much uncertainty to rely exclusively on the models.

  • Sentinel trees

Raffa et al. say that sentinel trees can potentially provide the most direct tests of tree susceptibility and the putative impact of introduced pests. Three types of plantations offer different types of information:

  1. In patria sentinels [= sentinel nurseries] = native trees strategically located in an exporting country and exposed to native pests. The intention is to detect problematic hitchhikers before they are transported to a new region. These plantings are useful for commodity risk assessment.  However, all the taxa associated with the sentinel trees must be identified to ascertain whether they can become a threat to plants in the new ecosystem.
  • Ex patria plantings [= sentinel plantations] = trees from an importing country are planted in an exporting country with the aim of assessing new pest–host associations. These plantings are most useful for identifying threats that arise primarily from lack of coevolved host tree resistance (i.e., loss of bottom-up control). They cannot predict the effects of lack of co-adapted natural enemies in the importing region (i.e., loss of top-down control). Plantings are thus more helpful in predicting impacts by pathogens and woodborers than folivores and sap feeders. However, ex patria plantings cannot predict pest problems that arise from novel microbial associations, or increased susceptibility to native pests.
  • Trees in botanic gardens, arboreta, large-scale plantations, and urban parks and yards can provide information on both existing native-to-native associations and new pest–host associations. Analyzing these plantings can be useful for studying host-shift events and novel pest–host associations. Again, all the taxa associated with the sentinel trees must be identified to ascertain whether they can become a threat to plants in the new ecosystem. Monitoring these planting have detected previously unknown plant–host associations (such as polyphagous shot hole borer and tree species in California and South Africa), and entirely unknown taxa. Pest surveillance in urban areas can also facilitate early detection, thereby strengthening the possibility of eradication.
PSHB attack on Erythrina caffri; photo by Paap

Sentinel tree programs are limited by 1) small sample sizes; 2) immature trees; and 3) the fact that trees planted outside their native range might not be accurate surrogates for the same species in native conditions. Some of these issues can be reduced by establishing reciprocal international agreements among trading partners; the International Plant Sentinel Network helps to coordinate these collaborations.

Botanic gardens and arboreta have the advantage of containing adult trees; this is important because pest impacts can vary between sapling to mature trees. However, they probably contain only a few individuals per plant species, usually composed of narrow genetic base.

Large-scale plantations of exotic tree species, e.g., exotic commercial plantations, comprise large numbers of trees planted over large areas with varied environmental conditions, and they stand for longer times. Still, they commonly have a narrow genetic base that might not be representative of wild native plants. Also, only a few species are represented in commercial plantations.

Raffa et al. report that experience in commercial Eucalyptus plantations in Brazil alerted Australia to the threat from myrtle rust (Austropuccinia psidii). However, in an earlier blog I showed that Australia did not act quickly based on this knowledge.

  • Laboratory assays using plant parts or seedlings

Laboratory tests artificially challenge seedlings, plant parts (e.g., leaves, branches, logs), or other forms of germplasm of potential hosts to determine their vulnerability. These tests are potentially powerful because they are amenable to experimental control, standardized challenge, and replication. They also avoid many of the logistical constraints of sentinel plantings. Finally, they can be performed relatively rapidly.

The key underlying assumption is that results can be extrapolated to predict injury to live, mature trees under natural conditions. The validity of this assumption depends on the degree to which exogenous biotic and abiotic stressors affect the outcomes. Raffa et al. report that environmental stressors tend to more strongly influence tree interactions with woodborers than folivores.

These assumption are more likely to be met by pathogens that infect shoots or young tissues, such as the myrtle rust pathogen Austropuccinia psidii, ash dieback pathogen Hymenoscyphus fraxineus, and the sudden oak death pathogen Phytophthora ramorum. 

The host range of and relative susceptibilities to insects is usually tested on twigs bearing foliage for defoliators and sap suckers; bark disks, logs, or branches for bark beetles, ambrosia beetles, and wood borers. These methods do not work as well for bark beetle species that attack mature trees in which active induced responses and transport of resins through established ducts are critically important.

The major advantages of laboratory tests is that they readily incorporate both positive (known hosts) and negative (known nonhosts) controls, can provide a range of environmental conditions, can be performed relatively rapidly, are statistically replicable at relatively low costs, and can test multiple host species and genotypes simultaneously. The ability to statistically replicate a multiplicity of environmental combinations and species is particularly valuable for evaluating relationships under anticipated future climatic conditions.

However, there are several important limitations. In testing pathogens, environmental conditions required for infection are often unknown. Choice of non-conducive conditions might result in false negatives; choice of too-conducive conditions might result in exaggerating the likelihood of infection. Results of tests of insect pests can vary depending on whether the insects are allowed to choose among potential host plants. Other complications arise when the pest being evaluated requires alternate hosts. In addition, seedlings are not always good surrogates for mature trees – especially as regards pathogens and bark, wood-boring and root collar insects. Folivores are less affected by conditions. Plus, the costs can be significant since they involve maintaining a relatively large number of viable and virulent pathogen cultures, insects, and candidate trees in quarantine.

Finally, although lab assays are well suited for identifying new host associations, results might not be amenable to scaling up to predict a pest’s population-level performance in a new ecosystem. Scaling up is especially problematic for those insect species whose dynamics are strongly affected by trophic interactions.

SOURCE

Raffa, K.F., E.G. Brockerhoff, J-C Gregoire, R.C. Hamelin, A.M. Liebhold, A. Santini, R.C. Venette, and M.J. Wingfield. 2023. Approaches to Forecasting Damage by Invasive Forest P&P: A Cross-Assessment.  BioScience Vol. 73 No. 2: 85–111    https://doi.org/10.1093/biosci/biac108  

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Help Fight for $$ to Protect Forests

Help Fight for Money to Protect Forests

This blog asks YOU!!! to support funding for some of the key USDA programs. This blog focuses on USDA’s Animal and Plant Health Inspection Service (APHIS). APHIS is responsible for preventing introduction of pests that harm agriculture, including forests; and for immediate efforts to eradicate or contain those pests that do enter. While most port inspections are carried out by the Department of Homeland Security Bureau of Customs and Border Protection, APHIS sets the policy guidance. APHIS also inspects imports of living plants.

Please help by contacting your members of the House and Senate Appropriations Committees. I provide a list of members – by state – at the end of this blog. APHIS is funded by the House and Senate Appropriations Subcommittees on Agriculture and Related Agencies. These Subcommittees have scheduled hearings on the topic and I’ve drafted written testimony for them. I expect CISP will be joined by additional members of the Sustainable Urban Forest Coalition in signing the testimony. You can add the crucial voice of constituent’s support.

I will blog soon about funding for USDA’s Forest Service (USFS) – I don’t yet have necessary information to suggest specific funding levels.

Your letter or email need be no more than a couple paragraphs. To make the case for greater funding, feel free to pick-and-choose from the information that follows. Your greatest impact comes from speaking specifically about what you know and where you live.

These are the specific dollar amounts we’d like you to ask for. The rationale for each is below.

Appropriations for APHIS programs (in $ millions)

ProgramFY 2022 (millions)FY 2023FY 2024 Pres.’ request Our ask
Tree & Wood Pest$61$63$64$65 M
Specialty Crops$210$216$222$222 M
Pest Detection$28$29$30$30 M
Methods Development$21$23$23$25 M

The Costs of Introduced Pests

Introduced pests threaten many forest products and services benefitting all Americans, including wood products, wildlife habitat, carbon sequestration, clean water and air, storm water management, lower energy costs, improved health, aesthetic enjoyment, and related jobs. Already, the 15 most damaging non-native pests threaten at least 41% of forest biomass in the “lower 48” states. In total, these 15 species have caused an additional annual conversion of live biomass to dead wood at a rate similar in magnitude to that attributed to fire (5.53 TgC per year for pests versus 5.4 to 14.2 TgC per year for fire) [Fei et al.; full citation at end of blog; see also earlier].

tanoaks killed by SOD; Oregon Department of Forestry photo

These pests also impose significant costs that are borne principally by municipal governments and homeowners. As more pests have been accidentally introduced over time, these costs have risen. A study published last year [Hudgins et al.] projected that by 2050 1.4 million street trees in urban areas and communities will be killed by introduced insect pests. Municipalities on the forefront include Milwaukee and Madison Wisconsin; the Chicago area; Cleveland; and Baltimore, Towson, and Salisbury, Maryland. Removing and replacing these trees is projected to cost cities $30 million per year. Additional urban trees – in parks, on homeowners’ properties, and in urban woodlands – are also expected to die and require removal and replacement.

Pathways of Introduction

Tree-killing pests are linked to the international supply chain. Many pests—especially the highly damaging wood-borers like emerald ash borer, Asian longhorned beetle, polyphagous and Kuroshio shot hole borers, and redbay ambrosia beetle—arrive in inadequately treated crates, pallets, and other forms of packaging made of wood. Other pests—especially plant diseases like sudden oak death and sap sucking insects like hemlock woolly adelgid—come on imported plants. Some pests take shelter, or lay their eggs, in or on virtually any exposed hard surface, such as steel, decorative stone, or shipping containers.

infested wood from a crate; Oregon Department of Agriculture photo

Wood Packaging

Imports from Asia have historically transported the most damaging pests, e.g., Asian longhorned beetle, emerald ash borer, redbay ambrosia beetle, and the invasive shot hole borers. For decades goods from Asia have dominated imports. As of February 2022, U.S. imports from Asia were running at a rate of 20 million shipping containers per year. A recent analysis [Haack et al.; see also here] indicates that at least 33,000 of these shipping containers, perhaps twice that number, are carrying a tree-killing pest. These facts have led scientists to project [Leung et al.] that by 2050, the number of non-native wood-boring insects established in the US could triple. Hudgins et al. say the greatest damage would occur if an Asian wood-boring insect that attacks maples or oaks were introduced. Such a pest could kill 6.1 million trees and cost American cities $4.9 billion over 30 years. The risk would be highest if this pest were introduced to the South – and U.S. southern ports are receiving more direct shipments from Asia after the expansion of the Panama Canal in 2016. https://www.nivemnic.us/?m=202207

After introduction of the ALB, APHIS acted to curtail further introductions in wood packaging from China. First – in 1998 – APHIS required China to treat its wood packaging. Second, it worked with foreign governments to develop the International Standard for Phytosanitary Measures (ISPM) #15. The U.S. and Canada began phasing in ISPM#15 in 2005 with full implementation in 2006. Under ISPM#15, all countries shipping goods to North America must treat their wood packaging according to specified protocols with the goal of “significantly reducing” the risk that pests will be present.

However, as I have often blogged [see blogs under “wood packaging” category on this site] ISPM#15 has fallen short. Haack et al. found that as recently as 2020, 0.22% [1/5th of 1 percent] of the shipping containers entering the U.S. were infested by a tree-killing insect. This equates to tens of thousands of containers harboring tree-killing insects.

Worse, the data indicate that our trade partners’ compliance with the rules has deteriorated; the “approach rate” of pest-infested wood packaging fell in 2005-2006, but has since gone back up.

The most troubling offender is China. Although required since 1998 to treat its wood packaging, China consistently has one of the highest pest approach rates: it was 0.73% [or ¾ of 1%] during the 2010- 2020 period. This is three times the global average for the period. Since China supplied 40.7% of U.S. imports in 2022 [Szakonyi], or 5,655,000 containers. Thus China alone might be sending to the U.S. 30,000 containers infested with tree-killing insects. These pests threaten our urban, rural, and wildland forests and reduce forest productivity, carbon sequestration, the rural job base, water supplies and quality, and many other ecosystem services. 

ISPM#15 falls short at the global level. The fact that a pallet or crate bears the mark indicating that it complies with ISPM#15 has not proved to be reliable.

You might ask your Member of Congress or Senators to ask APHIS what steps it will take to correct the problem of Chinese non-compliance. (Remind him or her that that the Asian longhorned beetle, emerald ash borer, and many other insects of so-far lesser impact were introduced in wood packaging from China.

Asian longhorned beetle

Remind them also that the Department of Homeland Security’s Bureau of Customs and Border Protection has twice enhanced its enforcement of wood packaging rules. In 2017 it began penalizing importers of non-compliant wood packaging under Title 19 United States Code (USC) §1595a(b) or under 19 USC §1592. In 2021, it incorporated the wood packaging requirements into its voluntary C-TPAC program.)  

You might also urge them to ask APHIS what steps it is taking at the global level to improve the efficacy of ISPM#15 – or to replace it if necessary to ensure that pests are not being introduced.

spread of beech leaf disease

Imported Plants (“Plants for Planting”)

Some pest types—especially plant diseases like sudden oak death and sap-sucking insects like hemlock woolly adelgid—come on imported plants. The U.S. imported about 5 billion plants in 2021 [MacLachlan]. Recent introductions probably via this pathway include several pathogens — Phytophthoras, rapid ʻōhiʻa death in Hawai`i, beech leaf disease (established from Ohio to Maine), and boxwood blight. Insects have also been introduced on imported plants recently; one example is the elm zigzag sawfly (present in North Carolina, Virginia, and New York and Ontario). https://www.nivemnic.us/?p=4115

An analysis of data from 2009 [Liebhold et al.] found that approximately 12% of plant shipments were infested by a pest. This pest approach rate is more than 50 times higher than the 0.22% approach rate for wood packaging. APHIS has adopted several changes to its phytosanitary system for imported plants in the decade since 2009. A few studies have been published, but they have focussed on insects and excluded pathogens. We have noted that pathogens continue to be introduced via the plant trade. Therefore, please ask your Member or Senators to ask APHIS to facilitate an independent analysis of the efficacy of the agency’s current phytosanitary programs to prevent introductions of pests on important plants, with an emphasis on introductions of plant pathogens.

APHIS is responsible for preventing spread of the SOD pathogen, Phytophthora ramorum, through trade in nursery plants. In recent years California has had few detections in nurseries and little expansion in forests – but the situation suggests that this good news is probably more the result of the drought than of program efficacy. In cooler, wetter conditions in Oregon and Washington, detections in nurseries and alarming detections in the forest or plantings continue.

In 2022, the APHIS SOD Program supported detection and regulatory activities in 25 states. P. ramorum was detected at 18 establishment, 12 of which were first-time detections. The California nursery regulatory program – which is funded by APHIS – saw reduced funding in 2022. We think these cuts are unwise since this year’s very wet winter will probably lead to a new disease outbreaks. Programs in Oregon and Washington continue to detect infestations in additional retailers brought in by plants bought from other nurseries. Washington responded to four separate “trace forward” incidents, one involving more than 160 residential sites. Clearly, the federal-state program is not succeeding in eradicating P. ramorum from nurseries. Please suggest that your Congressperson and Senators ask APHIS what steps it is taking to improve the efficacy of the SOD program.

SOD-infected rhodoendron on plants in Indiana; photo by Indiana Department of Natural Resources

In the East, P. ramorum was found in three of 65 streams sampled in 10 states in 2022 (reaching across the Southeast from Mississippi through North Carolina, plus Texas, Maryland, Pennsylvania, and Illinois). One stream is troubling: a first-time detection in South Carolina, with no obvious nursery source. Since stream sampling began, P. ramorum has been detected from eight streams in four states, Alabama, Mississippi, North Carolina, and now South Carolina. The pathogen has been present in some of these streams for more than 10 years.

Oregon faces particularly high risks. Three of the four known strains of P. ramorum are established in Oregon forests. One of them, the EU1 lineage, is more aggressive than the NA1 clonal lineage already present in forests. In addition, the EU1 strain might facilitate sexual reproduction of the pathogen, thus exacerbating Oregon’s struggle to contain the disease.

As we know, introduced pests do not stay in the cities where they first arrived — they spread! Often that spread is facilitated by our movement of firewood, plants, or outdoor household goods such as patio furniture.

The beech trees so important to wildlife conservation in the Northeast are under attack by two pathogens and at risk to an insect. Most alarming is the spread – in a dozen years! — of beech leaf disease DMF from Ohio to Maine. A leaf-feeding weevil is spreading south in eastern Canada. Please suggest that your Member or Senators to ask APHIS what steps it is taking to prevent the weevil’s introduction to the U.S.

‘Ōhi‘a trees make up 80% of the biomass of forests in both wet and dry areas of the Hawaiian archipelago. It is under attack by two diseases caused by introduced pathogens first detected in 2010. ‘Ōhi‘a forests support more threatened and endangered species than any other forest system in the U.S. They also play a uniquely important role in providing other ecosystem services, including water supplies.

Asking for the Money Pest Problems Deserve


To respond effectively to these pests and to the others that will be introduced in coming years, the key APHIS programs identified above must have adequate funds. The funding levels I request – and hope you will support – are lower than I would wish, but everyone expects the Congress to refuse significant increases in funding (see table at beginning of this blog).

The Tree and Wood Pests account supports eradication and control efforts targeting principally the ALB and spongy (= gypsy) moth. Eradicating the ALB normally receives about two-thirds of the funds. The programs in Massachusetts, New York, Ohio, and South Carolina must continue until eradication succeeds.

Oregon detected the EAB in 2022. Although the state and Portland have been preparing for a decade for this eventuality, there will still be significant impacts. Four percent of Portland’s street trees are ash – more than 9,000 trees. Young ash constitute three percent of young trees in parks. Loss of Oregon’s ash will also have severe ecosystem impacts. In Willamette Valley wetlands, ash constitutes up to 100% of the forest trees. Washington and California are also concerned. Indeed, the Hudgins study identified Seattle and Takoma as likely to lose thousands of ash trees. The numerous ash in riparian forests, windbreaks, and towns of North Dakota are also at risk since the EAB is established in South Dakota, Minnesota, and Manitoba.

APHIS manages damaging pests introduced on imported plants or other items through its Specialty Crops program. The principal example is its efforts to prevent spread of the SOD pathogen through the interstate trade in nursery plants. We noted above that this program is not as successful as it should be. We support the Administration’s request for $222 million; however, you might suggest that your Member or Senator urge APHIS to allot adequate funding under this budget line to management of SOD, rapid ʻōhiʻa death pathogens in Hawai`i, and beech leaf disease and elm zig-zag sawfly in the East.

The Pest Detection program is key to the prompt detection of newly introduced pests that is critical to successful pest eradication or containment. The “Methods Development” program enables APHIS to improve development of essential detection and eradication tools.

The Administration’s request include a $1 million emergency fund. This is far below the level needed to respond when a new pest is discovered. Funding constraints have hampered APHIS’ response to past pest incursions.

Please note that many of the members of the Agriculture Appropriations Subcommittee are from states where non-native pests are probably not top of mind. It is important that everyone that knows about these threats communicate with your Member/Senators!!

Members of House or Senate Subcommittees that Fund APHIS

(Names of Senators are italicized)

STATEMEMBERAPHIS APPROPHOUSESENATE
AKLisa Murkowski  X
ALJerry Carl Katie BrittXX  X
CalifBarbara Lee David Valadao Josh Harder Diane FeinsteinX X   XX X X        X
FLDebbie Wasserman Scultz Scott FranklinX XX X 
GASanford BishopXX 
IDMike Simpson X 
ILLauren UnderwoodXX 
KSJerry MoranX X
KYMitch McConnellX X
LAJulia Letlow Ashley HinsonX XX X 
MDAndy Harris Chris Van HollenXX    X
MEChellie Pingree Susan CollinsX XX  X
MIJohn Moolenaar Gary PetersX Xx  X
MNBetty McCollumXX 
MSCindy Hyde-SmithX X
MTJon Tester Ryan ZinkeX    XX
NBDeb Fischer  X
NDJohn HoevenX X
NMMartin HeinrichX X
NVMark Amodei X 
OHMarcy KapturXX 
ORJeff MerkleyXXX
PAGuy ReschenthalerXX 
RIJack Reed  X
TXMichael Cloud Jake EllzeyXX X 
UTChris Stewart X 
VABen ClineXX 
WADan Newhouse Derek KilmerXX X 
WVShelly Moore Capito Joe Manchin  X X X
WIMark Pocan Tammy BaldwinX XX  X

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. PNAS August 27, 2019. Vol. 116 No. 35  17371–17376

Haack R.A., J.A. Hardin, B.P. Caton and T.R. Petrice .2022. Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Front. For. Glob. Change 5:1069117. doi: 10.3389/ffgc.2022.1069117

Hudgins, E.J., F.H. Koch, M.J. Ambrose, and B. Leung. 2022.  Hotspots of pest-induced US urban tree death, 2020–2050. Journal of Applied Ecology

Leung, B., M.R. Springborn, J.A. Turner, and E.G. Brockerhoff. 2014. Pathway-level risk analysis: the net present value of an invasive species policy in the US. Front Ecol Environ 2014; doi:10.1890/130311

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. Frontiers in Ecology.

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).

Szakonyi, M. 2023. Sourcing shift from China pulls US import share to more than a decade low.

One State’s Program Illustrates Importance of Federal Funding

Dead ash along Mattawoman Creek in 2019; Mattawoman Creek is a Maryland tributary of the Potomac River, hence of the Chesapeake Bay. Photo courtesy of Leslie A. Brice

In this blog I describe one state’s forest health efforts – Virginia. The pertinent lesson is the importance of external funding, especially that provided by USFS Forest Health Protection program, in supporting states’ efforts. Is your state’s forest health program as dependent upon federal funding as Virginia’s is? If so, there is a role for everyone: lobby your Congressional representative and senators to increase funding for this program!

I have based most of this blog on the Virginia Department of Forestry’s annual report for 2022.

Forests grow on more than 16 million acres in Virginia, or 62% of the Commonwealth’s land area. Eighty percent of these forests are hardwood or hardwood-pine. They break down as follows: 61% oak-hickory; 11% oak-pine; 5% bottomland hardwood; and 2% maple-beech-birch. A fifth of the forest is pine, composed of pine plantation (14%) and natural pine (7%). The long term trend is growth, especially among hardwoods.

The report devotes much of its attention to the agency’s programs to advise private landowners (individuals own 80% of the Commonwealth’s forestland); fire management (including prescribed burns); and state and federal conservation programs (e.g., easements). A major program shares reforestation costs on harvested pine lands. In 2022, this program assisted reforestation practices on 74,702 acres. Virginia has an impressive tree-raising program. VDOF grows more than 40 species, including longleaf and shortleaf pine, several spruce species, and dozens of hardwoods. The aim is to provide stock suited for the Commonwealth’s soils and climate. Many of the hardwood species are grown from acorns and seeds collected and donated by volunteers.

VDOF also helps to protect and improve the Commonwealth’s water quality through tree planting and sound forest management. VDOF has an unusual responsibility: enforcing the Virginia Silvicultural Water Quality Law.

The report also summarizes several urban and community forestry programs focused on education, community engagement, tree selection, and grants for tree planting to ensure canopy retention & management.

Forest Health – Importance of Federal Funding

Spongy Moth

Slightly over 1 million acres was mapped by aerial surveys in FY22. I believe the funding for these surveys came largely from the USFS. The surveys detected heavy to moderate defoliation by the spongy moth on 24,493 acres (almost twice the area detected in FY21). The spongy moth infestation is primarily in counties on the western side of the state, in the mountainous region, which has the highest densities of oaks and other hardwoods.

Spotted Lanternfly

The spotted lanternfly (SLF) was detected in Virginia early – in 2018 in Winchester at the northern end of the Shenandoah Valley. Winchester is connected to central Pennsylvania by Interstate 81, so rapid movement of SLF to Virginia from outbreaks slightly to the east of I-81 in Pennsylvania doesn’t surprise me. SLF has been spreading south along the mountains and over the Blue Ridge to Loudoun and Fairfax counties (in 2022). Fairfax County has announced a four-year, $200,000 effort to try to slow SLF spread by eradicating high densities of its preferred host, Ailanthus, from two county parks in the far south and north ends of the county. Ailanthus removal requires not just cutting the trees, but applying herbicide to prevent sprouting from the roots. This work is funded by the county, the local park authority and a $20,000 grant from the regional energy company, Dominion Energy Charitable Foundation.

Emerald Ash Borer

Virginia has six species of ash: white and green (both common), and smaller populations of black, blue, pumpkin and Carolina. EAB is now confirmed in 84 counties – most of the Commonwealth except the far southeast. The Department of Forestry treats 130 – 150 trees per year – half or more on state lands. At least in FY21, the funding came from federal sources. The report also notes outreach efforts at two minor league baseball games. Virginia recently adopted a priority of protecting the Chesapeake Bay watershed by promoting tree planting in riparian forest buffers. The EAB threatens this goal; see the photo (at top) of ash mortality along a Maryland tributary of the Bay. In 2021, EAB was detected in Gloucester County – a peninsula east of the York River that has Bay shoreline on the eastern side, tributary on the west (see photo).

Gloucester Point – Virginia Institute of Marine Sciences “living shoreline”; EAB was detected in Gloucester County in 2021, threatening riparian areas. Photo courtesy of the Chesapeake Bay Program

Threats to Beech

Beech bark disease is present in the western mountainous parts of the Commonwealth. One new county – Augusta – was detected in 2022. Three other counties are infested with the scale, but the fungal pathogen has not yet been detected. The alarming new threat, beech leaf disease, was detected in Prince William County in 2021. In 2022, it was confirmed in neighboring Fairfax County. The source of funding is not specified.

beech in a typical northern Virginia second-growth forest; photo by F.T. Campbell

Laurel Wilt Disease

Sassafras; photo by David Moynihan

I am pleased that the Commonwealth is paying attention to laurel wilt disease, which has been spreading north on sassafras. The closest outbreaks are in Tennessee, to the southwest of Virginia. The Commonwealth hosted a detection training program attend by 26 participants from six agencies from three states. The report does not specify the source of the funding.

Southern Pine Beetle

Virginia has also utilized funding from the USFS FHP program to manage the southern pine beetle. Since the program’s inception in 2004, Virginia has thinned pines on more than 70,000 acres, including 4,240 acres in FY22.

Invasive Plants

USFS FHP invasive species grants funded control treatments of invasive plants on somewhat less than 1,300 acres of state lands. Different figures on different pages of the report cause confusion. However, it is clear that nearly all the funds came from the USFS FHP program. Ailanthus was the main target; other species mentioned are privet, mimosa, autumn olive and Miscanthus.

State Funding of Conservation Initiatives; Will They Continue?

While the state’s government was controlled by Democrats, the governor and state legislature launched new programs with broader conservation goals. It is unclear whether they will continue now that Republicans have won the governorship and control of the House of Delegates.

Among the programs enjoying increased funding from the state budget during the current two-year cycle are

  • Efforts to restore depleted populations of two groups of tree taxa, shortleaf and longleaf pines. The emphasis has shifted to longleaf pine: the number of projects and acreages rose from 220 acres in FY21 to 1,212 acres in FY22. Restoration of shortleaf pine forests was limited to slightly over 600 acres in both years.
  • Programs to improve management of hardwood stands. These projects included crop tree release, control of “invasive species” (I think probably targetting invasive plants), prescribed burning and commercial thinning. There were also several demonstration projects on state-owned lands, a small land-owner planning assistance program, and training of state foresters and private consulting foresters in hardwood management. Apparently these aspects had been largely ignored in the past.
  • Creation of a dedicated Watershed program focused on increasing riparian forest buffers. This section of the report does not mention the threat posed by loss of ash to the emerald ash borer (EAB) [see EAB section above]
  • Urban forestry projects, many linked to protecting surface and ground water (including Chesapeake Bay watershed).

Posted by Faith Campbell

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

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

or

www.fadingforests.org

see also the article about beech leaf disease in the mid-Atlantic region written by Gabe Popkin; posted here

Saving Old Trees is Key to CO2 Storage

The Old Man – a giant ash tree in Wytham Woods; photo from https://theoldmanofwytham.com/2018/11/29/ash-dieback-in-wytham-woods/

I campaign for protecting trees – especially trees growing to their natural capacity in the habitats in which they have evolved. I focus on the threat to these trees from non-native insects and various pathogens (fungi, nematodes …). I have often expressed my distress because others appear to place a low priority on this goal. I have also asked whether protecting trees might be given a higher priority by more decision-makers if they recognize trees’ vitally important role in countering climate change.

For this reason, I have blogged several times about studies examining the role trees play in sequestering carbon — see here & here & here.

A new study demonstrates that protecting large, old trees – almost by definition in their natural environment – is vitally important. Planting new, small, trees is helpful but cannot substitute for the venerable trees.

Calders and colleagues (full citation at the end of the blog; open access!) have used new technology to update assessments of the amount of carbon sequestered in trees. They conducted their study in a temperate hardwood forest – Wytham Woods, a typical broadleaf temperate forest in Oxfordshire, southern Great Britain. [Wytham Woods is also the site of two of the “Inspector Morse” mysteries –  “Secret of Bay 5B” and “A Way Through the Woods”.]

They found that these trees sequester 1.77 times more carbon in their above-ground biomass (AGB) than previously believed based on currently-used models.

One consequence of their findings is that countries using the standard assessment method (which was developed by Robert Bunce in 1968) are reporting inaccurate carbon sequestration estimates to the United Nations per the Paris climate accords. (Calders et al. believe that calculations for conifer species are probably more accurate than those for deciduous forests.)

A second consequence is that death of large trees – from whatever cause – will result in greater loss of carbon storage than previously thought.

Old v. New Measurements

The underlying Bunce dataset and algorithm applied in most European biomass estimates were based on a small sample: 200 trees belonging to five taxa growing in one forest area. The models were derived by cutting down trees and weighing them to determine tree biomass. Smaller trees were used because they are easier to process. The scientists then extrapolated the biomass of bigger trees based on the assumption that correlation between tree size and mass is independent of tree size. This assumption has rarely been tested because of the difficulty and expense of carrying out this type of destructive sampling.

The higher estimates of carbon storage in Calders et al. arise in part from the bias towards small trees in calibration of the earlier models. Calders et al. found that trees do not follow a size-invariant scaling relationship, particularly at larger size; it is important to include crown area. Thus, Calders and colleagues calculated a higher sequestration rate for trees in Wytham Woods that fell within the size range used in developing the Bunce allometric model.

In addition, changes in forest management have increased the abundance of larger trees compared to the 1960s when Bunce carried out his study. Indeed, many of the trees in Wytham Woods are nearly twice as large as the trees used in the original calculation of biomass. The median dbh in Bunce (1968) is 8.4 cm; the mean dbh for the TLS dataset (based on a 2015 inventory) is 15.9 cm. The large trees represent a high proportion of the above-ground biomass: 50% of AGB in Wytham Woods was associated with fewer than 7% of the trees (those with dbh greater than 53.1 cm). All these trees were larger than the trees used to calibrate the widely used allometric model.

Calders et al. say that the distribution of tree size (trunk diameter) in Wytham Woods is representative of broadleaved species throughout Great Britain. Basal area had doubled in 40 years from 1974. Thus, the growth trajectory reflected at Wytham Woods – and presumably across Britain – resulted in a net carbon sink of ~1.77tha-1year-1ha in Calder et al’s 3D analysis. This is almost double the ~1tha-1year-1ha derived using the traditional allometric models. .

Methodology

graphic from Calders et al. large maple (green) and oak (blue) trees illustrated by LiDAR images – profiles and location in the forest (indicated by arrows); copyright Ecological Solutions and Evidence

Calder et al. used terrestrial laser scanning (TLS; terrestrial LiDAR) methods & 3-dimensional analysis to derive tree volume and convert this to above-ground biomass (AGB) and carbon sequestration. They scanned 815 live standing trees in Wytham Woods during winter so leaves did not complicate computations. They found:

  •  total volume of these 815 trees was 742.6±3.9m3ha-1.
  • TLS-derived AGB =  409.9tha-1. This is significantly greater than the 231.9tha-1 resulting from applying the Bunce allometric models.
  • In sum, 1.77 times more carbon is stored per ha according to this model than carbon values derived through the allometric AGB models developed by Bunce. 

A Fly in the Ointment

ash dieback in Great Britain; cc-by-sa/2.0 – © Adrian Diack – geograph.org.uk/p/6497286

Calder et al. describe the threat to European carbon sequestration projections caused by ash dieback. Ash dieback has been spreading across Europe since the 1990s – although the causal agent was not determined until 2006 (Paap et al.).  It is killing European ash across the continent. Some of these trees are large – that is, store impressive amounts of carbon. In Wytham Woods specifically, ash dieback threatens some of the largest trees.

Ash dieback disease was first observed in the United Kingdom in 2012; it reached Wytham Woods in 2017. Ash contributed ~13.2% of the biomass carbon sequestration in the study area. However, the species’ presence in all of Wytham Woods might approach ~34%. Ash comprised 75% of seedlings in 2012. Ash is one of three species that contribute >26% of broadleaved tree AGB & carbon for Great Britain as a whole. The British Woodland Trust expects the UK to lose 80% of its ash trees.  As a result, Wytham Woods, Britain, and, by extension, a significant amount of European temperate deciduous forests, are likely to become a substantial carbon source in the next decades.

A dead elm tree on Skelston Moor; photo by Walter Baxter; CC BY-SA 2.0

I note that Europe has already lost any sequestration benefits it would have enjoyed from large elm trees due to “Dutch” elm disease. Various Phytophtoras are killing trees in Britain and Ireland.

CC BY-SA 2.0

I recently described threats to plane trees, pines, and other trees across Europe.

I interpret these findings as demonstrating that protecting large trees growing in natural ecosystems is highly important as we try to cope with climate change. This will require determined, sustained, and strategic actions in the face of disturbances predicted to increase as result of changes in climate and the human activities that contribute to climate change – e.g., overexploitation of natural resources, conversion of natural systems to human use, shipping goods around the globe, …

Calders and colleagues say we cannot afford to lose substantial reservoirs of carbon currently sequestered in temperate forests. Such forests currently account for ~14% of global forest carbon stocks in their biomass and soil. Their importance is growing because of widespread deforestation in the tropics.

What is To Be Done? (to cite Lenin)

Calders and colleagues call for several actions to address potential biases in biomass carbon estimates and drastically improve estimates of forest biomass:

(i) Research to improve knowledge about carbon sequestration levels in trees. This will require

a) greater sampling using such nondestructive methods as TLS to estimate AGB of a wider variety of forest types,

b) improved understanding of wood density, and

c) properly testing the fundamental assumption of size dependency in allometric models.

(ii) Develop empirical models of AGB that do not assume size invariance. This might require. This implies more destructive harvesting to obtain data from a variety of forest compositions, locations, etc,

(iii) Establish a biomass reference network of permanent sample plots specifically designed for estimating AGB. The improved data can then be fed into satellite-derived biomass estimates, which are likely to become the de facto standard for assessing the state and change of forest AGB at large scales. The GEO-TREES database can help. It aims to build on existing long-term ecological plot networks, by including TLS, airborne laser scanning & other ancillary data (including harvest measurements) to specifically allow for upscaling of AGB & development of new empirical models.

(iv) Ensure much better traceability in the use of allometric models. If applying a model to a site at several removes from the original data, e.g., published allometric models, clearly identify where and when the underpinning data were collected, the number and size range of trees from which models were derived, and clarify any assumptions regarding environmental conditions, wood density etc. Database initiatives such as GlobAllomeTree can help.

North American Situation

remains of Michigan’s champion green ash

A study in 2019 (Fei et al. 2019; full citation at end of the blog) has already estimated that 41% of total live (woody) biomass in forests of the “lower 48” states is at risk from the most damaging of introduced pests. The greatest biomass loss was caused by emerald ash borer, Dutch elm disease, beech bark disease, and hemlock woolly adelgid. Before arrival of these non-native pests, mature ash, elms, beech and hemlock were  large – providing significant storage of carbon (and other ecosystem services). A complication is that elms and beech, at least, began dying decades before the underlying (Forest Inventory and Analysis; FIA) data began to be collected. Consequently, the reported mortality rates underestimate the actual loss in biomass associated with these pests.

Did Fei et al. rely on biomass estimates based on measurements and algorithms now questioned by Calders et al.? One of the co-authors, Dr. Randall Morin, has told me that USFS scientists are shifting to new models that will result in a slight bump in overall biomass for the U.S. largely because of increased recognition of the biomass in crowns and limbs.  However, the new models are based partly on a felled-tree study, so I wonder if they will have similar issues.

Certainly in some situations that threat posed by non-native pests is not yet being adequately incorporated. Badgley et al. (2022) analyzed the California cap-and-trade program to determine whether forest projects enrolled under its provisions can provide sufficiently permanent carbon sequestration. They determined that sequestration losses tied to mortality of one tree species (tanoak; Notholithocarpus densiflorus) due to one disease – sudden oak death – would fully deplete the “buffer pool” set aside to compensate for losses due to disease and insect infestations. This leaves the program unable to provide the promised benefits in carbon sequestration. SOD continues to spread and tanoaks (and other tree species) to die. California along is home to other tree-killing pathogens and insects, e.g., white pine blister rust, Port-Orford cedar root disease,  Fusarium dieback, goldspotted oak borer … 

California live oak killed by GSOB; photo by F.T. Campbell

Furthermore, the program allows enrollment of forests across the United States, so the multiple pests threatening ash, hemlocks, oaks, and other tree taxa across North America must also be accommodated. I have not even mentioned the likelihood that additional tree-killing pests will be introduced in the future.

How can scientists enhance the credibility of well-intentioned efforts to incorporate forest conservation into strategies aimed at mitigating climate change?

[A separate study by Oxford University has estimated that 2 billion tonnes of CO2 are removed from the atmosphere every year – 99% of it by trees. They point out that this is not sufficient to help Earth avoid temperatures rising above Paris-set levels. See an article by Lottie Limb, Reuters, published 19 January 2023 (sorry – I don’t have a direct link).]

SOURCES

Badgley, G., Chay, F., Chegwidden, O.S., Hamman, J.J., Freeman J. and Cullenward, D. 2022. Calif’s forest carbon offsets buffer pool is severely undercapitalized. Front. For. Glob. Change 5:930426. doi: 10.3389/ffgc.2022.930426

Bunce, R. G. H. (1968). Biomass and production of trees in a mixed deciduous woodland: I. Girth and height as parameters for the estimation of tree dry weight. Journal of Ecology, 56, 759–775.

Calders, K., H. Verbeeck, A. Burt, N. Origo, J. Nightingale, Y. Malhi, P. Wilkes, P. Raumonen, R.G.H. Bunce, M. Disney.  Laser scanning reveals potential underestimation of biomass carbon in temperate forest. Ecol Solut Evid. 2022;3:e12197. wileyonlinelibrary.com/journal/eso3  open access!

Paap, T., M.J. Wingfield, T.I. Burgess, J.R.U. Wilson, D.M. Richardson, A. Santini.  2022. Invasion Frameworks: a Forest Pathogen Perspective. FOREST PATHOLOGY Current Forestry Reports https://doi.org/10.1007/s40725-021-00157-4

(UK) Woodland Trust https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/tree-pests-and-diseases/key-tree-pests-and-diseases/ash-dieback/

Posted by Faith Campbell

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

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

or

www.fadingforests.org

FY 23 Funding of Tree Pest Projects

Phytophthora ramorum-infected rhododendron plant; photo by Jennifer Parke, Oregon State University

APHIS has released the list of projects funded under §7721 of the Plant Protection Act in Fiscal Year 2023.  Projects funded under the Plant Pest and Disease Management and Disaster Prevention Program (PPDMDPP) are intend to strengthen the nation’s infrastructure for pest detection and surveillance, identification, threat mitigation, and safeguard the nursery production system.

APHIS has allocated $62.975 M to fund 322 projects in 48 states, Guam, & Puerto Rico. ~ $13.5 M has been reserved for responding to pest and plant health emergencies throughout the year. USDA is funding ~70% of the more than 460 PPDMDPP proposals submitted.

Funding by Goal Area

  • 1A – Enhance Plant Pest/Disease Analysis                               $2,057,174
  • 1S – Enhance Plant Pest/Disease Survey                                 $14,375,000
  • 2 – Target Domestic Inspection Activities at Vulnerable Points              $6,356,964
  • 3 – Pest Identification and Detection Technology Enhancement            $5,295,125
  • 4 – Safeguard Nursery Production                                                                 $2,079,119
  • 5 – Outreach and Education                                                                            $4,131,333
  • 6 – Enhance Mitigation Capabilities                                                             $13,875,775

By my calculation (subject to error!), the total for projects on forest pests is ~$6.5 M – or a little over 10% of the total. The top recipient was survey and management of sudden oak death: ~$700,000 for research at NORS-DUC and NCSU plus detection efforts in nurseries of 14 states. Other well-funded efforts were surveys for bark beetles and forest pests (projects in 14 states); surveys for Asian defoliators (projects in 14 states); and outreach programs targetting the spotted lanternfly (10 states, plus surveys in California).

Three states (Iowa, Kentucky and Maryland) received funding for surveys targetting thousand cankers disease of walnut; two states (Kentucky and Maine) obtained funding for outreach about the risk associated with firewood. Funding for the Nature Conservancy’s “Don’t Move Firewood” campaign appears under the home state of its leader, Montana.

Massachusetts obtained funding for outreach re: Asian longhorned beetle. Ohio State received funding for developing a risk map for beech leaf disease.

Ten states received funding for no forest pest projects; I don’t know whether they sought funding for this purpose. These states are Arizona, Colorado, Florida, Hawai`i, Idaho, Minnesota, Nebraska, New Mexico, North Dakota, and Puerto Rico. The “National” funding category also contained no forest pest projects.

Looking at the overall funding level might give a somewhat skewed impression because several of the projects with total funding of ~ $500,000 are actually carried out by USDA agencies. These awards are listed under the state in which the USDA facility happens to be located. Nearly half this money ($213,000) goes to a project by an Agriculture Research Service unit in Delaware to study the efficacy of the biocontrol targetting emerald ash borer.  Another $105,588 is allocated to detection of the SOD pathogen (Phytophthora ramorum) in irrigation water, undertaken – I think – at the ARS quarantine facility in Frederick, Maryland. A smaller project at a USFS research facility in Connecticut is studying egg diapause in the spotted lanternfly. The Delaware ARS unit is also pursuing biological control of the red-necked longhorn beetle (RNB) Aromia bungi, which attacks primarily stone fruits. Native to China and other countries in Asia, RNB has been intercepted in wood packaging by the U.S. and Europe; it has become established in Italy and Japan [Kim Alan Hoelmer, ARS, pers. comm.] The APHIS lab in Massachusetts is developing a light trap for detection of the Asian spongy moths Lymantria dispar.

I am intrigued that two states (Mississippi and Nevada) are conducting “palm commodity” surveys. Palms are important components of the environment in some states – although I am not certain these are the two most important!

As you might remember, I am also interested in some invaders other than forest pests. Washington has obtained $998,000 to support two projects integral to its efforts to find and eradicate the Asian (or Northern) Giant hornet. Oregon has obtained funding to carry out a survey for these hornets.  

Cactus moth larvae feeding on prickly pear cactus; photo by Doug Beckers, via Flickr

I rejoice to see that the Florida Department of Agriculture continues efforts to deploy biocontrol agents targetting the cactus moth. The Agriculture Research Service is evaluating the establishment of biocontrol agents released to counter two highly invasive plants. Re: Brazilian peppertree, I don’t question the damage it has caused in southern Florida but I have grave concerns should the psyllid and thrips reach Hawai`i. I am most distressed to see that Hawaiian Division of Forestry and Wildlife and Department of Agriculture are actively pursuing deliberate introduction of the thrips. ARS is also searching for potential biocontrol agents targetting the invasive cogongrass (Imperata cylindrica). Penn State is working on registering a soil fungus native to North America, Verticillium nonalfalfae, as a biocontrol targetting the highly invasive tree of heaven (Ailanthus).  

Phragmites invading Merkle Wildlife Sanctuary, Upper Marlboro, Maryland; photo by Alicia Pimental, (c) Chesapeake Bay Foundation

APHIS is pursuing biocontrol for “Roseau” cane scale. This situation presents a conflict of geographic regions because the plant to be controlled is Phragmites australis. Phragmites is highly invasive in the Mid-Atlantic, Northeast, and Great Lakes states . On the Mississippi delta it is considered important in maintaining wetlands crucial to protecting the Louisiana coast from rising seas.

Finally, USDA is pursuing management tools to contain the Box Tree Moth – a threat to the most widely planted ornamental shrub.  

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Can we work together to curtail introductions of new diseases?

Phytopthora ramorum-infected potted plants; photo by Washington State University

At this year’s USDA Invasive Species Forum I will be seeking to promote a discussion of what American and other stakeholders can do to suppress spread of forest pathogens. I have raised this issue many times before.  To see my blogs about the P4P pathway, scroll down below the archives to the “categories”.  See especially here  and here. 

I note that:

  • Non-native invasive pathogens and pests are decimating forests worldwide, threatening biodiversity & limiting efforts to rely on forests to alleviate impacts of climate change.
  • Many of the most damaging non-native organisms are pathogens that are especially difficult to detect at borders or to contain or eradicate once introduced.
  • A principal pathway by which pathogens are introduced is the international trade in living plants, or “plants for planting” (P4P).
  • Forest pathologists have long advocated a more pro-active approach – but national and international plant health officials have not taken up the challenge. [think Clive Brasier, Bitty Roy, Thomas Jung, Michael Winfield …]
Austropuccinia psidii on Melalecua in Australia; John Tann via Flickr

At the global level I suggest that we need:

  1. National agricultural agencies, stakeholders, FAO & International Plant Protection Convention (IPPC) to consider amending IPPC requirement that scientists identify a disease’s causal agents before regulating it. I think experience shows that this policy virtually guarantees that pathogens will continue to enter, establish, & damage natural and agricultural environments.
  2. National governments & FAO / IPPC to fund greatly expanded research to identify microbes resident in regions that are important sources of origin for traded plants, vulnerability of hosts in importing countries, and new technologies for detecting pathogens (e.g., molecular tools, volatile organic compounds [VOCs]).
  3. Researchers & agencies to expand international “sentinel plants” networks; incorporate data from forestry plantations, urban plantings, etc. of non-native trees.
  4. Application of ISPM#36 to promote use of HACCP programs for plants in trade. (See also my discussion in Fading Forests III – link at end of this blog.)
‘ohi‘a trees killed by rapid ‘ohi‘a death; photo by Richard Sniezko, USFS

We Americans need to

  1. Evaluate efficacy of current regulations – incorporating NAPPRA & Q-37 revision.  Rely on AQIM data. Include arthropods, fungal pathogens, oomycetes, bacteria, viruses, nematodes. Include threats to U.S. tropical islands (Hawai`i,  Puerto Rico, Guam, etc.) which are centers of plant endemism.
  2. Apply existing programs (e.g., NAPPRA, Clean Stock Network, post-entry quarantine) to strictly regulate trade in plant taxa most likely to transport pests that threaten our native plants; e.g., plants belonging to genera shared between North American trees & plants on other continents.
  3. Recognize that plant nurseries are incubators for microbial growth, hybridization, and evolution; require nurseries to adopt sanitary operation procedures regardless of whether they sell in inter-state or intra-state commerce

I will explain my sense of urgency by noting the many recent introductions of pathogens – most probably via P4P or cut vegetation:

  • 13 outbreaks of Phytophthora-caused disease in forests and natural ecosystems of Europe, Australia and the Americas. Three of four known strains of P. ramorum are established in U.S. forests.
  • Myrtle rust (Austropuccinia psidii) has been introduced to 27 countries, including the U.S., Australia, and South Africa.
  • Two new species of Ceratocystis (C. lukohia & C. huliohia)—causal agents of rapid ‘ohi‘a death (ROD) – spreading on the Hawaiian Islands. The former species appears to have originated in the Caribbean; the latter in Asia.
  • Since 2012, beech leaf disease has spread from northeastern Ohio to Maine.   
  • Boxwood blight (caused by 2 ascomycete fungi, Calonectria pseudonaviculata & C. henricotiae) introduced to at least 24 countries in 3 geographic areas: Europe / western Asia; New Zealand, North America.
  • ash dieback fungus (Hymenoscyphus fraxineus) has spread across Europe after introduction from Asia.

What do you think? Can we find more effective methods to curtail introductions?

beech leaf disease

Posted by Faith Campbell

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

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

or

www.fadingforests.org

America & Russia – Sharing the Pests

Platanus orientalis in Turkey; photo by Zeynek Zebeci

A current issue of the journal Forests (2022 Vol. 13) is a special issue focused on forest pests. This topic was chosen because of increased pest incursions. Choi and Park (full citations at the end of the blog) link this to climate change and increased international trade, as well as difficulties of predicting which pests will cause damage where.

The journal issue contains 15 papers. Several patterns appear throughout. First is the important role of international trade in living plants – “plants for planting” – in introductions. This is hardly news! A second pattern is that at least two North American species were introduced to Europe during the 1940s, probably in wood packaging used to transport military supplies during World War II.

This compilation provides the opportunity to review which organisms of North American origin have become damaging invaders in Eurasia — and sometimes other continents. For example, the journal carries four articles discussing pine wilt disease (PWD). It is caused by the North American nematode Bursaphelenchus xylophilus, and is vectored by wood-boring insects in the genus Monochamus. Beetles introduced from North America and those native to the invaded area are both involved. This disease is considered a severe threat to forest health globally. No apparent association with WWII exists for PWD.

Two fungal pathogens from North America cause serious damage in urban and natural forests of Europe and central Asia. Neither is discussed in the special issue:

  • Ceratocystis platani has devastated urban trees in the Platanus genus, especially the “London plane” hybrid, and the native European tree, Platanus orientalis. This fungus was accidentally introduced to southern Europe during WWII – as were the two insects described by Musolin et al. It was first reported in northern Italy and Mediterranean France in the early 1970s, but disease symptoms had been observed years earlier. C. platani is established across the northern rim of the Mediterranean and to the east in Armenia and Iran. The worst damage has been in Greece, especially in natural forest stands in riparian areas. Spread of the pathogen there is facilitated by root grafts and by tree wounds caused by floating wooden debris during floods (Tsopelas et al. 2017.)
Platanus orientalis along Voidomatis River in Greece; photo by Onno Zweers, via Wikimedia
  • Heterobasidion irregulare infects conifers. It has spread and killed large numbers of Italian stone pine (Pinus pinea). The disease was inadvertently introduced to central Italy in the 1940s. H. irregulare has greater sporulation potential and decays wood more quickly than the native congener H. annosum. H. irregulare appears to be replacing the European species; scientists fear it will exacerbate tree infection and mortality rates (Garbelotto, Leone, and Martiniuc. date?)

A third North American pathogen, sooty bark disease (Cryptostroma corticale) has been introduced to Europe. This disease, found on sugar maple in eastern North America, was detected in Great Britain in 1945; it is now throughout Europe (Tanney 2022). EPPO reports that it is widespread in western Europe and in some Balkan countries. The website provides no information on its impact in Europe.

Pests in Russia

A paper authored by Musolin, et al. discusses 14 species of invasive or emerging tree pests found in Russian forest and urban ecosystems. Of these, two are native to North America. Another eight pose a threat to North America if they are introduced here.

As Musolin et al. point out, Russia covers a huge territory across Europe and Asia – stretching 10,500 km, or 6,500 miles. These encompass a great variety of ecological zones. Russia is also actively involved in international trade. It is not surprising, then, numerous non-native organisms have been introduced.

As of 2011, 192 species of phytophagous non-native insects from 48 families and eight orders were documented in the European part of Russia. This number does not include the vast areas in Asian Russia. Additional introductions have probably occurred in the most recent decade. Some of these introduced species have cause significant economic losses. Still, Russia appears to rarely mount a serious control effort.

Of course, the opposite is also true: pests native to some part of Russia can be transported to new regions of Russia or beyond its borders. We North Americans have focused on various species of tussock moths (Lymantria spp., etc.). There are many others. Musolin et al. describe eight in detail. All the information in this blog are from that article unless otherwise indicated.

Two North American Species’ Damage in Eurasia

Both these introductions were detected around the year 2000. Was there some event – other than simply expanding trade – that might explain these introductions?

Leptoglossus occidentalis; photo by nutmeg66 via Flickr
  • Western Coniferous Seed Bug, Leptoglossus occidentalis

This insect from western North America has invaded Eurasia, North Africa, and Central America. The first detection in Europe was in 1999 in Italy. It spread quickly and is present now from Morocco to Japan, as well as in South Africa and South America. The seed bug is spreading northward in European Russia, including into the forest-steppe zone. Its ability to spread to the East is uncertain.

L. occidentalis attacks a wide range of Pinaceae and Cupressaceae. In the Mediterranean region it has had serious impacts on the pine nut supply (Ana Farinha, IUFRO, Prague, September 2021). In southern parts of Russia it has caused “significant damage”. L. occidentalis also vectors a pathogenic fungus Sphaeropsis sapinea (=Diplodia pinea), which causes diplodia tip blight. The cumulative damage of insect and pathogen to pines can be significant.

The introduction pathway to Russia is unknown. It might have flown from established populations in Europe, or it might have been transported on plants for planting or Christmas decorations.

  • Oak Lace Bug, Corythucha arcuata  

This insect is widespread in the United States and southern Canada. It was first detected in Europe – again, Italy – in 2000. Twenty years later it has spread to almost 20 countries.

Russia was invaded relatively recently; the first outbreak was detected in 2015 in the subtropical zone along the Black Sea coast and Caucasus. Musolin et al. expect the lace bug to spread to natural forests of Central Asia and other countries of the Caucasus. Its spread will be assisted by air currents and movement of plants for planting. The insect is causing considerable aesthetic damage, but other impacts have not been estimated.

Hosts include many species of oak (Quercus spp.), European and American chestnuts (Castanea spp.) plus trees from other botanical families: willows and maples (Salicaceae), redbay (Fagaceae), and alder (Betulaceae).  

Pests in Russia that Could Damage North America if Introduced Here

Malus sierversii; photo by Lukacz Szczurowski via Wikimedia

Threat to Apples — Apple Buprestid, Agrilus mali

This Asian beetle has caused extensive mortality of wild apple (Malus sieversii) forests in Xinjiang, China. Wild apple trees are important components of deciduous forests in the Central Asian mountains. The species is also an ancestor of the domestic apple tree. Consequently, the borer is considered a potential threat to cultivated apple trees – presumably everywhere. A. mali might also attack other fruit trees in the Rose family, i.e., Prunus (plums, cherries, peaches, apricots, almonds) and Pyrus (pears).

Unlike most of the other species described here, A. mali is a quarantine pest in Russia and across Europe and the Mediterranean regions – the region where phytosanitary policies are coordinated by the European and Mediterranean Plant Protection Organization (EPPO). Russia bans imports of apple seedlings from infested areas.

China is reported to be experimenting with a possible biocontrol agent, Sclerodermus pupariae (a parasitoid of emerald ash borer).

Threat to Pines and Firs, Already Under Invasive Species Threats

  • Small Spruce Bark Beetle, Ips amitinus

This European beetle has been considered a secondary pest of dying conifers. Over the last 100 years, it has moved farther North. The first Russian record was 100 years ago, in the region where Russia, Belarus, and Ukraine meet. (Did military action during World War I play a role? This is not discussed by the authors.) By 2022, the beetle occupies 31 million ha. It is probably spread through transport of logs by rail.

In Western Siberia, the spruce beetle has attacked a new host, Siberian pine (Pinus sibirica).

The danger to North America arises from this beetle’s preference for five-needle pines (genus Pinus section Quinquefoliae). North America’s five-needle pines are already under severe pressure from the introduced pathogen white pine blister rust (Cornartium ribicola) and the native mountain pine beetle (Dendroctonus ponderosae). 

  • Four-Eyed Fir Bark Beetle, Polygraphus proximus

This East Asian beetle feeds on firs (Abies spp.). Less commonly, it feeds on other genera in the Pinaceae: spruce (Picea ), pines (Pinus), larch (Larix), hemlock (Tsuga).

This beetle has been spreading west; the first substantiated record in European Russia was 2006 in Moscow. The beetle was probably present in western Siberia in the 1960s, although it was not detected until 2008. Again, the probable pathway of spread is movement of lumber by railroad.

P. proximus vectors an obligate symbiotic fungus, which can rapidly weaken the host. Musolin et al. comment on the beetle’s impacts – which they rarely do in this article. (Does this signify more damaging impacts, or availability of past studies?) They note significant changes in the forests’ ecosystem structure and microclimate, vegetation cover, and local insect fauna.

The danger to North America arises from this beetle’s preference for firs from the sections Balsamea and Grandis. Many North American firs are in these sections, including Fraser fir (Abies fraseri), balsam fir (A. balsamea), subalpine fir (A. lasiocarpa), grand fir (A. grandis), white fir (A. concolor), and others. Several of these firs already are challenged by the introduced balsam woolly adelgid. Firs in central and western Europe are less vulnerable since they are in the section Abies, which the beetle prefers less.

Threats to Poplars

  • Spotted Poplar Borer, Agrilus fleischeri

This boring beetle is native to northern Asia. It has caused significant mortality in native and exotic Populus plantations in China. Although there have been no reports of this beetle moving beyond its native range, many other Agrilus species have. Canada has twice intercepted adult spotted poplar borers on wood packaging. Musolin et al. fear that the adoption of non-native hosts might trigger an outbreak that would facilitate spread.

  • Poplar Leafminer, Phyllonorycter populifoliella
balsam poplar; photo by Matt Lavin via Flickr

This micromoth is widely distributed across the Palearctic. It was recently detected on introduced poplars growing in India.  

The danger to North America arises from the beetle’s preference for black and balsam poplars. Several species in these taxonomic groups are common in North America, including Populus balsamifera, P. trichocarpa, P. deltoides, and Populus × Canadensis.

Threat to Oaks — Leaf Blotch Miner Moth, Acrocercops brongniardella

This micromoth is widely distributed in Europe and expanding to the north. The pest mines the leaves of several oak species (Quercus spp.), especially English oak, Q. robur; and sometimes European chestnut (Castanea sativa). Leaf blotch miner is considered one of the most important folivore insect pests of oaks in Russia. Damage has been greater in Omsk Oblast (Siberia), where both English oak and the micromoth are introduced species, than in St. Petersburg, which is on the northern limit of their natural range. Musolin et al. fear that the warming climate will lead to the pest causing greater damage in the northern portions of its range.

Threat to Basswood — Lime Leaf Miner, Phyllonorycter issikii

This Asian moth has been moving west since the mid-1980s. It now occupies most of European Russia with some outbreaks in Siberia. In Europe, it is a conspicuous pest of Tilia species.

In these invaded regions, the leaf miner has shifted to novel hosts, including American basswood (T. americana). Basswood is a common plant in the eastern deciduous forest of North America.

Threat to Horse Chestnuts & Urban Trees — Horse-Chestnut Leaf Miner, Cameraria ohridella

This tiny moth was unknown to science before the first recorded outbreak in the late 1980s. Over the next three decades it spread to most of Europe, where horse chestnut (Aesculus hippocastanum)has been widely planted for three centuries. It has caused significant damage.

The first Russian detection was in Kaliningrad, on the shores of the Baltic Sea, in 2003. The leaf miner now occupies 69% of administrative units of European Russia. It is considered one of the Top 100 most dangerous invasive species in Russia.

In North America, the moth might attack native horse chestnuts, Ae. octandra (=flava) and Ae. glabra. Urban plantings are at particular risk because the leaf miner might attack both European horse chestnuts and two non-native maples that have been planted widely, sycamore maple (Acer pseudoplatanus) and Norway maple (A. platanoides). Data cited by Musolin et al. are contradictory regarding larval development on the maples. Once introduced, the leaf miner is difficult to contain because it spreads through natural flight of adults, wind-blown leaves, hitchhiking on vehicles, and movement of infected plants. 

Shared Pests

Russia has been invaded by two species that have been introduced in many countries (beyond pine wilt nematode). These two entered the country on plants for planting being imported to landscape venues for the XXII Winter Olympic Games – held in Sochi in 2014.

First to arrive was the Box Tree Moth, Cydalima perspectalis. This East Asian species was first detected outside its native range in Germany in 2006. By 2011 it was widespread in European and Mediterranean countries. In 2021, the boxwood moth was found in North America (first Canada, then the United States).  [I discuss the boxwood moth briefly here.]

boxtree moth; photographer unknown

In Russia, box tree moth larvae were first recorded in 2012 on the planting stock of its principal host, Buxus sempervirens. The moth quickly spread around the Black Sea region and to the North Caucasus. It spread farther, too: it reached the Kaliningrad Oblast (southeast coast of the Baltic Sea) in 2020. The main pathway of C. perspectalis invasion was the introduction of infested box-wood planting material.

Further spread of C. perspectalis is likely from Russia into the natural forests across the Caucasus (Transcaucasia) and to countries located further south. This is most distressing because the region has extensive natural forests of Buxus sempervirens. In 2015–2017, C. perspectalis almost completely destroyed the natural boxwood populationsin these regions of Russia and further eastwards in Abkhazia. Boxwood stands in Georgia and northern Iran are already suffering intensive defoliation as the result of infection by two non-native pathogens, Calonectria pseudonaviculata [synonym Cylindrocladium buxicola] and Calonectria henricotiae. Damage to these forests could lead to reductions in soil stability and subsequent declines in water quality and flood protection, changes in forest structure and composition, and declines in Buxus-associated biodiversity (at least 63 species of lichens, fungi, chromista and invertebrates might be obligate). (In December 2022, Iryna Matsiakh presented a compelling overview of threats to these forests in a webinar sponsored by the Horticulture Research Initiative; apparently no recording is available.)

The second global invader to appear was the Brown Marmorated Stink Bug, Halyomorpha halys.

This insect from southeast and east Asia invaded the United States in 1996. The first detection in Europe was in Liechtenstein in 2004. In both cases, it spread quickly across these continents.

Russia’s first detection of stinkbug was in 2014 in parks in Sochi and elsewhere along the Black Sea coast. The spread in Russia appears to have been limited to the Black Sea – Caucasus area.

The brown marmorated stinkbug is highly polyphagous, feeding on more than 300 species of plants.  In southern Russia, 107 species have been documented as hosts. At times, stinkbug feeding has caused severe losses in yields of fruit and vegetable crops.

Patterns

Musolin et al. stress the importance of the pest shifting to new hosts–usually from the same or a closely related genus. They cite several examples of these shifts occurring in the pest’s native range, including Agrilus planipennis (from local Asian ash species to introduced North American ash species); Phyllonorycter populifoliella and Agrilus fleischeri (from local poplars to widely cultivated introduced North American poplars and hybrids); Agrilus mali (from cultivated to wild apples).

As I noted above, the introduction and spread pathways are the usual ones: plants for planting (three species) and shipments of logs. There is one indication of wood packaging – Spotted Poplar Borer, Agrilus fleischeri at the Canadian border.

SOURCES

Choi, W.I.; Park, Y.-S. Management of Forest Pests and Diseases. Forests 2022, 13, 1765. https://doi.org/10.3390/f13111765

Garbelotto, M., G. Lione, and A.V. Martiniuc. date?  The alien invasive forest pathogen Heterobasidion irregulare is replacing the native Heterobasidion annosum. Biological Invasions https://doi.org/10.1007/s10530-022-02775-w

Musolin, D.L.; Kirichenko, N.I.; Karpun, N.N.; Aksenenko, E.V.; Golub, V.B.; Kerchev, I.A.; Mandelshtam, M.Y.; Vasaitis, R.; Volkovitsh, M.G.; Zhuravleva, E.N.; et al. Invasive insect pests of forests and urban trees in Russia: Origin, pathways, damage, and management. Forests 2022, 13, 521.

Tanney, J. Forest Health Challenges Exacerbated by a Changing Climate: Swiss Needle Cast and Sooty Bark Disease in B.C. 65th ANNUAL FOREST PEST MANAGEMENT FORUM (Canada). December 7, 2022.

Tsopelas, P., A. Santini, M.J. Wingfield, and Z.W. de Beer. Canker Stain: A Lethal Disease Destroying Iconic Plane Trees. Plant Disease 2017. 101-645-658 American Phytopathological Society

US invasive species — updated USGS database now on-line

ōhiʻa rust on Hawai`i; photo by J.B. Friday

The U.S. Geological Survey (USGS) has published an updated register of introduced species in the United States. The master list contains 14,700 records, of which 12,571 are unique scientific names. The database is divided into three sub-lists: Alaska, with 545 records; Hawai`i, with 5,628 records; and conterminous (lower 48) United States, with 8,527 records.

The project tracks all introduced (non-native) species that become established, because they might eventually become invasive. The list includes all taxa that are non-native everywhere in the locality (Alaska, Hawai`i, or 48 conterminous states) and established (reproducing) anywhere in that locality.

Each record has information on taxonomy, a vernacular name, establishment means (e.g.,  unintentionally, or assisted colonization), degree of establishment (established, invasive, or widespread invasive), hybrid status, pathway of introduction (if known), habitat (if known), whether a biocontrol species, dates of introduction (if known; currently 47% of the records), associated taxa (where applicable), native and introduced distributions (when known), and citations for the authoritative source(s) from which this information is drawn. 

The 2022 version is more complete re: plant pathogens than earlier iterations; I thank the hard-working compilers for their efforts!

Hawai`i

wiliwili tree (Erythrina sandwicensis); photo by Forest and Kim Starr

Among the non-native species listed as being in Hawai`i are 3,603 Arthropods, including the following about which I have blogged:

The list also includes 25 fungi, among them the two species of Ceratocystis that cause rapid ʻōhiʻa death; DMF & blog 270 and the ʻōhiʻa or myrtle rust, Austropuccinia psidii.

Also listed are 95 mollusk species and 20 earthworm species. I wonder who is studying the worms’ impacts? I doubt any is native to the Islands.

The Hawaiian list contains 1,557 non-native plant species. Families with largest representation are Poaceae (grass) – 223 species; Fabaceae (beans) – 156 species; and Asteraceae – 116 species. About a third of the plant species – 529 species – are designated as “widespread invaders”. This number is fifteen times higher than the numbers in lists maintained by either the Hawaiian Ecosystems At Risk project (106 species) [HEAR unfortunately had to shut down a decade ago due to lack of funds]; or Hawaiian Invasive Species Council (80 species). Furthermore, some of the species listed by HEAR and HISC are not yet widespread; the lists are intended to facilitate rapid responses to new detections.  We always knew Hawai`i was being overrun by invasive species!

Among the 529 most “widespread invaders” are the following from the most introduced families:

  • Poaceae – Agrostis stolonifera, 6 Cenchrus spp, 2 Cortaderia spp, 3 Eragrostis,8 Paspalum, 4 Setaria spp, 2 Urochloa (Poacae)
  • Fabaceae – 3 Acacia, 2 Prosopis

Other families have fewer introduced species overall, but notable numbers of the most widespread invaders:

  • Euphorbiaceae – 8 spp. of Euphorbia
  • Cyperaceae – 6 spp. of Cyperus
  • Myrtaceae – Melaleuca quinquenervia, 2 Psidium, Rhodomyrtus tomentosa rose myrtle, 3 Syzygium [rose myrtle has been hard-hit by the introduced myrtle rust fungus]
  • Zingiberaceae – 3spp. Hedychium (ginger)
  • Anacardiaceae — Schinus molle (Peruvian peppertree); USGS considers congeneric S. terebinthifolia to be somewhat less widespread.

Plus many plant taxa familiar to those of us on the continent: English ivy, privet, castor bean, butterfly bush, Ipomoea vines  … and in more limited regions, Japanese climbing fern Lygodium japonicum.

Rhus sandwicensis; photo by Forest and Kim Starr

I learned something alarming from the species profiles posted on the HISC website: the Hawaiʻi Division of Forestry and Wildlife and Hawaiʻi Department of Agriculture are considering introduction of a species of thrips, Pseudophilothrips ichini, as a biocontrol agent targetting S. terebinthifolia. I learned in early 2019, when preparing comments on Florida’s proposed release of this thrips, that Pseudophilothrips ichini can reproduce in low numbers on several non-target plant species, including two native Hawaiian plants that play important roles in revegetating disturbed areas. These are Hawaiian sumac Rhus sandwicensis and Dodonea viscosa. The latter in particular is being propagated and outplanted in large numbers to restore upland and dryland native ecosystems. While the environmental assessment prepared by the USDA Animal and Plant Service says the thrips causes minimal damage to D. viscosa, I am concerned because of the plant species’ ecological importance.  Of course, the two Schinus species are very damaging invasive species in Hawai`i … but I think introducing this thrips is too risky. [To obtain a copy of CISP’s comments, put a request in comments section. Be sure to include your email address in your comment; the section algorithm does not include email addresses (how inconvenient!).]

Continental (lower 48) states

Among the 8,500 species listed in the USGS Register for the 48 continental states are 4,369 animals, among them 3,800 arthropods; 3,999 plants; and just 89 fungi. Among the arthropods, there are 1,045 beetles and 308 lepidopterans. The beetles listed include 12 Agrilus (the genus which includes emerald ash borer and goldspotted oak borer.) It does not include the elm zig-zag sawfly USGS staff have not found any publications documenting its U.S. occurrences. Among the microbes are six Phytophthora (P. cinnamomi, P. lateralis, P. pseudocryptogea, P. quercina, P. ramorum, P. tentaculata). Profiles of several of these species are posted at www.dontmovefirewood.org; click on “invasive species”, then scroll using either Latin or common name.

elm zig-zag sawfly; photo by Gyorgy Czoka via Bugwood

Citation:

Simpson, Annie, Pam Fuller, Kevin Faccenda, Neal Evenhuis, Janis Matsunaga, and Matt Bowser, 2022, United States Register of Introduced and Invasive Species (US-RIIS) (ver. 2.0, November 2022): U.S. Geological Survey data release, https://doi.org/10.5066/P9KFFTOD

United States Register of Introduced and Invasive Species; US-RIIS ver. 2.0, 2022

 If you would like to contribute to future versions of the US-RIIS, please email the project leaders at us-riis@usgs.gov. 

Posted by Faith Campbell

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

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

or

www.fadingforests.org

A Forest without Big Trees — Which Animals Will be Decimated?

In an earlier blog about tree extinctions, I commented that less drastic impacts by pests can also be important. I mentioned specifically that clumps of beech root sprouts cannot duplicate the quantities of nuts and cavities provided by mature beech trees.

This thought prompted me to search for information about use of tree cavities by wildlife. The articles I have found are decades old and largely focus on implications for management of forests for timber. Timber production conflicts with a goal of ensuring the presence of large (“overmature”), trees, especially those with dead branches, and completely dead trees (“snags”). These articles were written too long ago to address the possible impacts of non-native insects and pathogens – although there is some discussion of widespread mortality of pines caused by the mountain pine beetle.

These sources make clear that species that make cavities are keystone species. Many other wildlife species depend on them — birds, bats and terrestrial animals – mammals and herps. Furthermore, these cavity-associated species require forests with significant numbers of large, old, declining trees. When non-native insects or pathogens kill those trees, there might be a short-term bonanza of dying trees – suitable for nesting and foraging; and wood-feeding insects to provide food. But afterwards – for decades or longer – there will probably be small-diameter trees, and different species. Can the cavity-dependent species find habitat or food under these circumstances?

[By coincidence, the PBS program “Nature” broadcast an episode on woodpeckers on the 2nd of November! The title is “The Hole Story”. ]

Cavities provide a variety of habitats for many species – including some not usually thought of as “forest” species. Among the 85 North American bird species identified by Scott et al. as associated with cavities are seven species of ducks, two vultures, three falcons, 12 owls, two swifts, six flycatchers, two swallows, purple martin, seven chickadees, three titmice, four nuthatches, brown creeper, five wrens, three bluebirds, and two warblers. They point out that the majority of these birds are insectivores. Woodpeckers are especially important predators of tree-killing bark beetles.

Goodburn and Lorimer found that more than 40 species of birds and mammals in hardwood forests of Wisconsin and Michigan use cavities in snags and dead portions of live trees for nest sites, dens, escape cover, and winter shelter. Bunnell reported that 67 vertebrate species commonly use cavities in the Pacific Northwest. Chepps et al., Daily et al., and Wiggins focus on specific species in the Rocky Mountains. (Full citations for all sources are at the end of the blog.)

While Scott et al. (published in 1977) do not address the impact of non-native pests, their profiles of individual bird species sometimes name specific types of trees favored. Several of these tree taxa have been decimated by such non-native pests, or face such attack in the near future. Thus, concern appears warranted for:

pileated woodpecker; photo by Jo Zimni via Flickr
  • birds nesting in American elm, including two that are quite large so they require large trees to accommodate their nests: common goldeneye (a duck) and pileated woodpecker (larger than a crow).
  • the pileated woodpecker also nests in ash and beech and here
  • the yellow-bellied sapsucker nests in butternut.

How many species depended on American chestnut, which – before the blight — grew to diameters up to 5 feet, heights of 70 to 100 feet, and had hollow centers (USDA 2022)?

In the West, some nesting tree species are under imminent threat from invasive shot hole borers, goldspotted oak borer, or sudden oak death. Detection of the emerald ash borer in Oregon portends a longer-term threat. Birds likely to feel these impacts include the acorn woodpecker, ash-throated flycatcher, and purple martin. The golden-fronted woodpecker is associated with oaks in parts of Texas where oak wilt is severely affecting live oaks.

ash-throated flycatcher; photo by Mick Thompson via Flickr

At the beginning of the 21st Century – before widespread mortality caused by the emerald ash borer — densities of snags in the managed forests in the Lake States were apparently already insufficient to sustain population densities of cavity nesting birds. Pileated woodpeckers and chimney swifts both prefer snags greater than 50 cm dbh, which are significantly less abundant in harvested stands. For six of eight bird species studied, the number of breeding pairs was significantly higher in old-growth northern hardwood stands than in those under management (Goodburn and Lorimer).

Strong Primary Excavators are Keystone Species

Cavity nesters are commonly divided into:

1) primary excavators that excavate their own cavities. These are further divided into strong excavators – those species that forage by drilling, boring, or hammering into wood or soil; and weak excavators – those species that probe or glean bark, branches, and leaves to acquire prey.

2) secondary cavity users, that use holes made by primary cavity excavators (Bunnell).

Strong primary excavators tend to be large, e.g., most woodpeckers, sapsuckers, and the northern flicker. Weak excavators are mostly smaller species, such as chickadees and nuthatches; plus those woodpeckers that forage primarily by probing and gleaning, extracting seeds, or capturing insects in flight [e.g., acorn woodpecker (Melanerpes formicivorus), downy woodpecker (Picoides pubescens)] (Bunnell).

Bunnell considers strong excavators to be keystone species because so many other cavity users depend on them. Their loss would seriously disrupt forest ecosystems. For example, in the Pacific Northwest, only nine of 22 avian primary excavators are strong excavators. Another 45 species are secondary cavity users. These include waterfowl, tree swallows, and some mammals such as flying squirrels. Some cavity nesters support an even wider group of species: in the Pacific Northwest, at least 23 bird species, six mammal species, and numerous arthropods (nine orders and 22 families) feed on sap and insects collected at holes drilled by sapsuckers (Bunnell). [I discuss sapsuckers’ ecosystem role in greater detail later.]

Tree Characteristics

There is general agreement that animals dependent on tree cavities “prefer” (actually, require) trees that are large – tall, of large circumference, and sturdy – while having decayed interiors.

Size:

As Bunnell notes, larger snags provide more room and tend to stand longer without breaking, so they provide greater opportunities for cavity use. They also tend to be taller, so they offer higher nest sites that provide better protection from ground-dwelling predators. While larger-diameter trees remain standing longer regardless of the cause of mortality, snags created by fire usually fall sooner than do other snags. Beetle-killed trees are more attractive to cavity nesters that tend to excavate nest sites in trees on which they have foraged.

In the upper Midwest, cavity trees were a scare resource, even in unmanaged forests. Mean diameters for live cavity trees were twice as large as the mean diameter of the live trees in stands under a management regime. Such larger-diameter snags were more numerous in old-growth than in managed stands, especially in mixed hemlock-hardwood stands (Goodburn and Lorimer).

The Importance of Decay

Excavating a cavity demands considerable energy, so birds seek sites where a fungal infection has softened the interior wood. The exterior wood must remain strong to prevent collapse of the nest. These rots take time to develop, so they appear more often in older, even dying, trees. Bunnell, Scott et al., Chepps et al., and Goodburn and Lorimer all emphasize the role of decay in providing suitable cavity sites. Chepps et al. compared the aspen trees used by four species of cavity-nesting birds in central Arizona. Not only were nest trees softer than neighboring trees; they were softer at the spot where the nests were excavated than at other heights. [Spring (1965) provides a fun discussion of different species’ adaptations to the energy demands of hard pecking and climbing vertical trunks.]

Live v. Dead Trees

However, the need for decay does not necessarily mean birds prefer dead trees. Goodburn and Lorimer found that in Wisconsin and Michigan, a large percentage of all cavities found were in live trees.  

Bunnell found that strong excavators select trees with less visible signs of decay. Where possible, secondary users will also use live trees. However, intense competition often forces them to use dead trees.

Hardwoods v. Conifers

Bunnell states that deciduous trees more often contain internal rot surrounded by a sound outer shell than do conifers (at least this is true in the Pacific Northwest). He found that cavity nesters chose hardwoods for 80–95% of their nest sites even where hardwoods comprised only 5–15% of the available tree stems. He concluded that availability of living hardwoods had a significant influence on strong excavators in the West, although probably was less important in hardwood stands in the East.

Taxa Dependent on Other Types of Cavity

Some species depend on cavities created by forces other than bird excavations, such as decay or fire. These include most of the mammals, especially the larger ones e.g., American martens, fishers, porcupines, and black bears. These natural cavities are often uncommon. Vaux’s swifts nest and roost in hollow snags large enough that they can fly in a spiral formation to enter and leave (Bunnell).

little brown bat Myotis sp. photo by S.M. Bishop via Wikimedia Commons

Bats are a special case. Bats are unique among mammals of their size in having long lives, low reproductive rates, and relatively long periods of infant dependency. They also play a key ecological role as the major predators of nocturnal flying insects (van den Driesche 1999). Also many species are in perilous conservation status: half of the 16 bat species in British Columbia were listed as threatened or endangered as of 1998 (van den Driesche). This was before the deadly disease whitenose syndrome had been detected in North America.

Bats require larger trees. In the Pacific Northwest at least, that choice often means conifers (Bunnell). Roosts are difficult to find, so samples are small. A study on the west coast of Vancouver Island (van den Driessche), located only nine roosts despite searching during three summers. Five roosts were in large-diameter (old) western red cedar, with dead tops and extensive cracks.

Brown creepers and some amphibians and reptiles nest or seek cover under slabs of loose bark, which are typically found on dead or dying trees. The same large, mature and old-growth conifer trees also provide preferred foraging habitat, since there is a higher density of arthropod prey on their deeply furrowed bark. While Wiggins (2005) studied bird populations in the Rocky Mountains, he cited studies in the eastern United States, specifically in the Blue Ridge and Allegheny mountains, that have found similar results. Goodburn and Lorimer found that in National forests in Wisconsin and Michigan, only 15% of trees consisted of the necessary snags with loose bark plates. Suitable trees were most frequent old-growth hemlock-hardwood stands, and on larger-diameter snags. A high proportion of the snags with loose bark were yellow birch (Betula alleghaniensis).

Importance of foraging sites

As Bunnell points out, a bird must feed itself before it can nest. Foraging trees and snags are usually smaller than nesting trees. Furthermore, birds need many more foraging sites than nesting sites. The situation perhaps most pertinent to our usual focus on invasive pests concerns bird species’ response to mountain pine beetle outbreaks. Red-breasted nuthatches and mountain chickadees increasing dramatically in apparent response to the beetle epidemic. When most of the conifers had been killed, and numbers of beetles diminished, numbers of these bird species also declined–despite the increased availability of conifer snags for nesting. Indeed, the birds continued to nest primarily in aspen during the epidemic.

Bunnell reiterates that snags of all sizes are needed; they provide perching, foraging, and hawking sites for bird species beyond cavity nesters as well as sustenance for bryophytes, insects, and terrestrial breeding salamanders. He says more than 200 studies reported harvesting of standing dead trees in beetle-killed forests had negative effects on bird, mammal, and fish species.  

Other Dependencies – Food Sources

yellow-bellied flycather; photo by Dennis Church via Flickr

A few studies looked at the role of cavity-creating birds in providing food sources. The focus was on sapsuckers. They drill sapwells into trees’ phloem; sap flowing into these wells attracts many other species. In Michigan, Rissler determined that yellow-bellied sapsuckers’ sapwells attracted insects in seven orders and 20 families, especially Coleoptera, Diptera (other than Tephritidae), bald-faced hornets, and Lepidoptera. Daily et al. (1993) cites other studies showing that ruby throat and rufous hummingbirds have extended their breeding ranges by relying on these sapwells for nutrition in early spring before flowers open. [The “Nature” program covers this behavior.]

In a subalpine ecosystem in Colorado, Daily et al. found that red-naped sapsuckers support other species in two ways. First, they excavate nest cavities in fungus-infected aspens that are utilized by at least seven secondary cavity nesting bird species. When they feed, they drill sapwells that nourish more than 40 species – including hummingbirds, warblers, and chipmunks. Daily et al. called this a keystone species complex comprised of sapsuckers, willows, aspens, and a heartwood fungus. Disappearance of any element of the complex could cause an unanticipated unraveling of the community.

Goodburn and Lorimer looked at the availability of downed wood but did not discuss the implications of the presence of only small-diameter coarse woody debris.

Efforts to Accommodate Biodiversity Needs

Scott et al. reported in 1977 that the USDA Forest Service had required staff at regional and National Forest levels to develop snag retention policies. Twenty years later, Goodburn and Lorimer noted that Forest Service management guidelines for some Wisconsin and Michigan National forests since the early 1980s have called for the retention of all active cavity trees and  5-10 snags (larger than 30 cm dbh)/ha. However, as I noted above, they fear that these recommended snag retention levels might still be too limited to support cavity nesters. They found that two species that prefer snags greater than 50 cm dbh, pileated woodpeckers and chimney swifts, were significantly more abundant in old-growth than in selection stands. Furthermore, the number of breeding pairs of six species was at least 30% higher in old-growth northern hardwood than in selection stands and more than 85% higher in selection cuts than even-aged.

Goodburn and Lorimer cited others’ findings that removal of some live timber and snags in an Arizona ponderosa pine forest reduced cavity-nesting bird populations by 50%. Species affected were primarily violet-green swallows, pygmy nuthatches, and northern three-toed woodpeckers.

Female mountain bluebird by Jacob W. Frank. Original public domain image from Flickr

As I noted, none of these experts has addressed the impacts of wide-spread pest-caused tree mortality. If I may speculate, it seems likely that when the first wave of mortality sweeps through a forest, the result might be an expansion of both nesting opportunities (in dead or dying trees) and food availability for those that feed on wood borers. These would probably be more plentiful even in trees killed by pathogens or nematodes. Sapsuckers and those that depend on them might experience an immediate decline in sap sources. Over the longer term it seems likely that all cavity-dependent species will confront a much lower supply of large mature trees. I note that many deciduous/hardwood tree species are being affected by introduced pests.

Are there current studies in Michigan, where so many ash have died?

SOURCES

Bunnell, F.L. 2013. Sustaining Cavity-Using Species: Patterns of Cavity Use and Implications to Forest Management. Hindawi Publishing Corporation. ISRN Forestry. Volume 2013, Article ID 457698

Chepps, J., S. Lohr, and T.E. Martin. 1999. Does Tree Hardness Influence Nest-Tree Selection by Primary Cavity Nesters? The Auk 116(3):658-665, 1999

Daily, G.C., P.R. Ehrlich, and N.M. Haddad. 1993. Double keystone bird in a keystone species complex. Proc. Natl. Acad. Sci. USA Vol. 90, pp. 592-594, January 1993 Ecology

Goodburn, J.M. and C.G. Lorimer. 1998. Cavity trees and coarse woody debris in old-growth and managed northern hardwood forests in Wisconsin and Michigan. Can. For. Res. 28: 427.438 (1998)

Rissler, L.J., D.N. Karowe, F. Cuthbert, B. Scholtens. 1995. Wilson Bull., 107(4), 1995, pp. 746-752

Spring, L.W.  1965. Climbing and Pecking Adaptations in Some North American Woodpeckers.

Scott, V.E., K.E. Evans, D.R. Patton, C.P. Stone. 1977. Cavity-Nesting Birds of North American Forests. Agriculture Handbook 511 USDA Forest Service. https://www.gutenberg.org/files/49172/49172-h/49172-h.htm

United States Department of Agriculture, Animal and Plant Health Inspection Service. Draft Enviromental Impact Statement. 2022. State University of New York College of Enviromental Science and Forestry Petition (19-309-01p) for Determination of Nonregulated Status for Blight-Tolerant Darling 58 c’nut (Castanea dentata)

van den Driessche, R., M. Mather, T. Chatwin. 1999. Habitat use by bats in temperate old-growth forests, Clayoquot Sound, British Columbia 

Wiggins, D.A. (2005, January 27). Brown Creeper (Certhia americana): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/browncreeper.pdf [date of access].

Posted by Faith Campbell

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

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

or

www.fadingforests.org