Category: forest pathogens

“Ecological memory” determines a forest’s resilience — implications of bioinvasion to New Zealand’s unique flora

kauri dieback

Scientists in New Zealand are saying explicitly that a forest’s unique mixture of species matters when considering the future. This mixture is the result of the forest’s evolutionary history. Losing members of the biological community reduces the forest’s ability to respond to current and future stresses – its resilience.

New Zealand’s forests are part of the broader legacy of the ancient supercontinent of Gondwanaland – the island nation’s plants have close relatives in South America, the Pacific Ocean islands, and Australia. Still, these forests are unique: 80% of New Zealand’s plant species are endemic. The forests are also species-rich. The warm temperate evergreen rain forests of the North Island are home to at least 66 woody plant species that can reach that reach heights above six meters (Simpkins et al. 2024).

These forests have been severely changed by human activity. In just ~ 750 years people have cut down approximately 80% of the original forest cover! (Simpkins et al. 2024) Of the eight million hectares of surviving native forest, a little over five million hectares is managed for the conservation of biodiversity, heritage, and recreation.  Another 2 million hectares are plantations of non-native species.

sites in New Zealand where pine plantations are “wilding”

All these forests are challenged by introduced mammals – from European deer to Australian possums. Climate change is expected to cause further disturbance, both directly (through e.g., drought, extreme weather) and indirectly (e.g., by facilitating weed invasion and shifting fire regimes) (Simpkins et al. 2024).

Pathogen threats are also common threats to the native trees of the Pacific’s biologically unique island systems. For example, Ceratocystis lukuohia and C. huliohia (rapid ‘ōhi‘a death, or ROD). The latter is killing ‘ōhi‘a (Metrosideros polymorpha) on the Hawaiian Islands. More than 40% of native plant species in Western Australia are susceptible to Phytophthora cinnamomi. Here I focus on two pathogens, kauri dieback and myrtle rust, now ravaging New Zealand’s native flora. No landscape-level treatment is available for either pathogen.

When considering this suite of challenges, Simpkins et al. focus on these two pathogens’ probable impact on forest carbon sequestration. They worry in particular about erosion of the forests’ resilience due to loss of “ecological memory” – the life-history traits of the species (e.g., soil seed banks) and the structures left behind after individual disturbances.

one of the largest remaining kauri trees, “Tane Mahuta”, in Waipoua Kauri Forest; photo by F.T. Campbell

Kauri Dieback

The causal agent of Kauri dieback, Phytophthora agathidicida, is a soil-borne pathogen that spreads slowly in the absence of animal or human vectors. The disease affects a single species, Agathis australis (kauri, Araucariaceae). However, kauri is a long-lived, large tree that is a significant carbon sink. It probably modifies local soil conditions, nutrient and water cycles, and associated vegetation. Also, kauri has immense cultural significance.

Simpkins et al. note that kauri dieback threatens stand-level loss of A. australis – that is, local extinctions. In the absence of disturbance Kauri trees can grow to awe-inspiring size. In the 19th Century, before widespread logging, some were measured at 20 meters or more in circumference. Consequently, kauri dieback might cause a decline in aboveground live carbon storage of up to 55%. This loss would occur over a period of hundreds of years, not immediately.

Huge kauri are not likely to be replaced by other long-lived emergent conifers (based on an analysis of one species, Dacrydium cupressinum). Instead, kauri are probably going to be replaced by late-successional angiosperms. The authors discuss the ecological implications for levels of carbon storage and proportions of trees composed of Myrtaceae – exacerbating damage caused by myrtle rust (see below).

The expectation of Simpkins et al. that kauri will suffer at least local extinctions is based on an assumption that no kauri trees are resistant to the pathogen. Fortunately, this might not be true: different Agathis populations show various levels of tolerance to Agathis dieback. Identification and promotion of some levels of resistance could enable A. australis to retain a diminished presence in the landscape.

However, Lantham, et al. make clear that containing kauri dieback remains “challenging,” despite its discovery nearly 20 years ago (in 2006). Scientists and land managers have little information on the distribution of symptomatic trees, much less of the pathogen itself. This means they don’t know where infection foci are or how fast the disease is spreading.

As is often true, the pathogen is probably present in a stand for years, possibly a decade or more, before symptoms are noticed. This means that the current reliance on public reports of diseased trees, or targetting surveillance on easy-to-access sites (e.g., park entrances and along existing track networks), or at highly impacted areas readily identified through aerial methods, fails to detect early stages of infection. Indeed, it seems probable that P. agathidicida had been present in New Zealand’s ecosystems for decades before its formal identification.

The Waipoua forest is one of the largest areas of forest with old kauri stands in the country. A new analysis of aerial surveys done between 1950 and 2019, shows how the forest is changing. The number of dead trees increased more than four-fold and the number of unhealthy-looking trees increased 16-fold over these 70 years. Kauri dieback is now widespread in this forest, especially in areas near human activities like clearing for pasture or planting commercial pine plantations).

Lantham et al. have developed a model which they believe will help identify areas of higher risk so as to prioritize surveillance and inform responses. These could delimit the disease front and help implement quarantines or other measures aimed at limiting the spread of P. agathidicida to uninfected neighboring sites.

I hope New Zealand devotes sufficient resources to expand surveillance and management to levels commensurate with the threat to this ecologically and culturally important tree species.

Leptospermum scoparia; photo by Brian Gatwicke via Flickr

Myrtle Rust

Myrtle rust is a wind-borne disease that affecting numerous species in the Myrtaceae, including some of the dominant early successional species (e.g., Leptospermum spp.). Simpkins et al. expect that myrtle rust might hasten the decline of two such tree species (L. scoparium and Kunzea ericoides). However, these trees’ small size and rapid replacement by other species during succession minimizes the effect of their demise on carbon storage.

Because I am concerned about the irreplaceable loss to biodiversity, I note that Simpkins et al. also feared immediate threats to some trees in the host Myrtaceae family, specifically highly susceptible species such as Leptospermum bullata.

As I reported in a recent blog, a second group of scientists (McCarthy et al.) explored the threat from myrtle rust more broadly. Austropuccinia psidii has spread through Myrtaceae-dominated forests of the Pacific islands for about 20 years.

Trees in the vulnerable plant family, Myrtaceae, are second in importance (based on density and cover) in New Zealand’s forests. Successional shrub communities dominated by the two species named above, Kunzea ericoides and Leptospermum scoparium, are widespread in the northern and central regions of the North Island and in northeastern and interior parts of the South Island. These regions’ vulnerability is exacerbated by the area’s climate, which is highly suitable for A. psidii infection (Simpkins et al. 2024).

McCarthy et al. concluded that if Leptospermum scoparium and Kunzea ericoides prove to be vulnerable to myrtle rust, their loss would cause considerable change in stand-level functional composition across these large areas. They probably would be replaced by non-native shrubs, which are already common on the islands. Any resulting forest will differ from that formed via Leptospermeae succession.

These authors also worry that the risk to native ecosystems would increase if more virulent strains of the myrtle rust pathogen were introduced or evolved. They note that A. psidii is known to have many strains and that these strains attack different host species.

SOURCES

Latham, M.C., A. Lustig, N.M. Williams, A. McDonald, T. Patuawa, J. Chetham, S. Johnson, A. Carrington, W. Wood, and D.P. Anderson. 2025.  Design of risk-based surveillance to demonstrate absence of Phytophthora agathidicida in New Zealand kauri forests. Biol. Invasions (2025) 27, no. 26

McCarthy, J.K., S.J. Richardson, I. Jo, S.K. Wiser, T.A. Easdale, J.D. Shepherd, P.J. Bellingham. 2024. A Functional Assessment of Community Vulnerability to the Loss of Myrtaceae from Myrtle Rust. Diversity and Distributions, https://doi.org/10.1111/ddi.13928

Simpkins, C.E., P.J. Bellingham, K. Reihana, J.M.R. Brock, G.L.W. Perry. 2024. Evaluating the effects of two newly emerging plant pathogens on North Aotearoa-New Zealand forests using an individual-based model.  Ecological Modelling, www.elsevier.com/locate/ecolmodel

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Wood packaging: serious data gaps … but clear opportunities to act

discarded pallets next to developed area in Glacier National Park (!); photo by F.T. Campbell

Since July 2015 I have posted nearly 50 blogs about non-native insects introduced via movement of solid wood packaging material (SWPM). Why? Because SWPM is one of two most important pathways by numbers introduced & by impact of the species introduced. (The other pathway is P4P.) To read those earlier blogs, scroll below “archives” to “categories”, choose “wood packaging”.

Examples of insects introduced via the wood packaging pathway include Asian longhorned beetle, emerald ash borer, redbay ambrosia beetle, Mediterranean oak borer, and possibly, three species of invasive shot hole borers.

dead redbay trees in Everglades National Park; killed by laurel wilt vectored by redbay ambrosia beetle

As I have reported in the earlier blogs and in my “Fading Forests” reports (links at the end of this blog), in 2002, the parties to the International Plant Protection Convention (IPPC) adopted an international “standard” to guide countries’ programs intended to reduce the presence of damaging insects in the wood packaging: International Standard for Phytosanitary Measures (ISPM) #15). The U.S. and Canada adopted the standard through a phase-in process culminating in 2006. [For a discussion of the phase-in periods and process, read either of the studies by Haack et al. cited at the end of this blog.] In other words, the U.S. and Canada have implemented ISPM#15 for almost 20 years. China specifically has been subject to requirements that it treat its SWPM even longer – since December, 1998, i.e., more than 25 years.

Unfortunately, ISPM#15 is not intended to prevent pest introductions.  As stated in Greenwood et al 2023, “Prior to 2009, the goal of compliance with ISPM 15 was to render the risk of wood-borne pests “practically eliminated,” in 2009 the standard was amended to “significantly reduced”.  

Despite almost universal adoption of the standard by countries engaged in international trade, insects have continued to be present in wood packaging. A very high proportion of these infested shipments — 87% – 95% — of the SWPM found by U.S. officials bears the ISPM#15 stamp – that is, is apparently compliant. (See my blogs by clicking on the “Category” “wood packaging” listed below the “Archives”.) The same proportion was found in a narrower study in Europe (Eyre et al. 2018). All the post-2006 examples of infested wood analyzed by Haack et al. (2022) (see below) carry the stamp. I conclude that the ISPM#15 mark has failed in its purpose: to reliably indicate that SWPM accompanying imports has been treated so as to minimize the likelihood that an insect pest will be present.  

Dr. Robert Haack, retired USFS entomologist, has twice tried to estimate the “approach rate” of insects in SWPM entering the United States (both studies are cited at the end of this blog). A study published in 2014 that relied on data from 2009 found that U.S. implementation of ISPM#15 was associated with a reduction in the SWPM infestation rate reported of 36–52%. The authors estimated the infestation rate to be 0.1% (1/10th of 1%, or 1 consignment out of a thousand). (See Haack et al. 2014; citation at the end of this blog.)

In their second study, published in 2022, Haack and colleagues found a 61% decrease in rates of borer detection in wood packaging when comparing numbers of wood borer detections in 2003 – before the U.S. implemented ISPM#15 – to those in 2020. Specifically, detections dropped from 0.34% in 2003 to 0.21% in 2020. This decrease occurred despite the volume of U.S. imports rising 68% between 2003 and 2020. (My blogs document a further increase in import volumes over the years since 2020.) In addition, the number of countries from which the SWPM originated more than doubled from 2003–2004 to 2010–2020. This expansion exposes North America to a wider range of insect species that might be introduced, as well as a wider range of individual countries’ effectiveness in enforcing the standard’s requirements (Haack et al. 2022).

These decreases are encouraging. However, Haack et al. (2022) note some caveats:

  • The reduction in pest presence was greatest during the initial implementation of the program the first phase, 2005-2006 (61%); in subsequent periods pest approach rate inched back up. In the 2010-2020 period, the pest detection rate was only 36% below the pre-ISPM#15 level. Detection rates have been relatively constant since 2005. Does this stasis mean that exporters learned that they could ignore or circumvent the requirements without suffering significant penalties? Or is some of this rise related to increased trade volumes, increasing variety of country of origin for trade, or other global trade patterns unrecognized in the data? (However, see the next bullet point.)
  • Certain types of commercial goods and exporting countries have consistently fallen short. Specifically, the rate of wood packaging from China that is infested remained relatively steady over the 17 years since 2003. The proportion of consignments with infested wood packaging coming from China was more than five times the proportion of all inspected shipments for this period. In other words, China has had a consistent record of poor compliance with phytosanitary regulations since they were imposed in December 1998. Why is USDA not taking action to correct this problem? (As I note below, DHS CBP has ramped up enforcement efforts.) Some other countries, e.g., Italy and Mexico, have reduced the rate at which wood packaging accompanying their consignments is infested. In fact, Mexico’s improved performance largely explains the overall infestation rate estimate of 0.22% during the period 2010-2010. Mexico’s successes affect the overall statistics in a way that makes other countries’ failure to reduce the presence of pests in wood packaging they ship to the United States far less obvious.

Haack et al. (2022) discuss ten possible explanations for their finding that pest approach rates – as determined by their study — have not decreased more. See the article or my blog about the study.

Although USDA APHIS has not taken steps to strengthen its enforcement, U.S. Customs and Border Protection [an agency in the Department of Homeland Security] has done so twice — see here and here.  CBP staff have expressed disappointment that these actions reduced the numbers of shipments in violation of ISPM#15 by only 33% between Fiscal Year 2017 and FY2022. True, more than 60% of these violations consisted of a missing or fraudulent ISPM#15 stamp. However, 194 consignments still harbored live pests prohibited under the standard.

APHIS did agree in 2021 to enable the study by Robert Haack and colleagues, via an interoffice data sharing agreement between USDA APHIS and the Forest Service- this resulted in Haack et al. 2022.

APHIS and CBP also collaborated with an industry initiative to train inspectors that insure other aspects of foreign purchases. The ideas was that CBP or APHIS and their Canadian counterparts would inform importers about which foreign treatment facilities have a record of poor compliance or suspected fraud. The importers could then avoid purchasing SWPM from them. I have heard nothing about this initiative for three years, so I fear it has collapsed.

We lack data on which to base a rigorous analysis

While the two studies by Robert Haack and colleagues are the best available, and they relied on the best data available, the fact is that those available data do not provide a full picture of the risk of pest introduction associated with wood packaging. As pointed out by Leigh Greenwood of The Nature Conservancy in her presentation to 2025 USDA Invasive Species Research Forum, available data have been collected for different purposes than to answer this question. Leigh’s powerpoint is posted here.

Leigh has identified the following data gaps:

  1. In their studies, Haack and colleagues rely on data from the Agriculture Quarantine Inspection Monitoring (AQIM) system. This dataset is based on random sampling of very distinct segments of incoming trade. It is therefore a better measure of insect approach rates than reports of interceptions by either APHIS or CBP.

However, AQIM includes data from only those very distinct segments of trade: perishable goods, SWPM associated with maritime containerized imports, Italian tiles, and “other” goods, AQIM does not contain a segment of trade that includes wood packaging associated with maritime breakbulk or roll-on, roll-off (RORO) cargo. These exclusions have prevented scientists and enforcement officials from determining, inter alia, how great a risk of pest introduction is associated with various types of wood packaging, especially dunnage, as the randomized sample does not include entire pathways for the entrance of dunnage.

Greenwood states that she has not found another country that operates a similar analysis of randomly collected data at ports of entry.

2) USDA does not collect data on consignment size, piece-specific infestation density, nor consignment-wide infestation density. As Haack et al. (2022) point out, reporting detections by consignment doesn’t reveal the number of insects present. If implementation of ISPM#15 resulted in fewer live insects being present in an “infested” consignment, this would reduce the establishment risk because there is lower propagule pressure. However, we cannot know whether this is true.

3) Neither USDA nor CBP reports the inspection effort. Nor do they conduct a “leakage survey” to see how often target pests are missed. This means, inter alia, that we cannot estimate inspectors’ efficiency in detecting infested wood packaging. If their proficiency has improved as a result of improvements in training, inspection techniques, or technology, the apparent impact of ISPM#15 would be under-reported in recent years.

4) USDA does not require port inspectors to report the type of SWPM in which the pest was detected. Leigh participated in an effort that included industry representatives, DHS CBP and USDA APHIS to define the types of wood packaging in legal terminology so that they could be incorporated in the drop-down menu on inspectors’ reporting system. This was first successfully included in the legal glossary within USDA APHIS system of record, ACIR Glossary. Last fall the team was working to integrate the requirement for using these definitions into the inspection data collection system used by DHS CBP, which would then make this data available in Agricultural Risk Management, ARM (see Abstract here for adequate primer on ARM). However, it is unclear now whether the new administration will do so. One potential barrier is that asking the port of entry inspection staff to record these data will add to the time and training required for reporting inspection results.

In summary, Leigh reports that current data systems do not support

  • estimating probabilities of pest infestation of via volume or type of SWPM (e.g. pallet vs dunnage)
  • measuring the risk of arrival associated with a specific hazard (in this case, a hazard being a live pest or pathogen associated with SWPM)
  • extrapolating or supporting findings for some types of wood packaging to other types of wood packaging

Scientists from Canada, Mexico, and the United States have formed a working group under the auspices of the North American Plant Protection Organization (NAPPO). The group is trying to determine whether various types of wood packaging are more likely to harbor pests. This study is currently hampered by the many data gaps, including those Leigh outlined above. The best data available, cited by Haack et al. (2022), found that in maritime containerized shipping, crates were more likely to harbor pests than pallets- however, other forms of SWPM (dunnage, bracing, etc.) had such low sample size that no analysis of those is possible. One of the main objectives of the NAPPO study is to evaluate if dunnage poses the same or higher risk, so this is a major impediment.

Two issues need to be resolved.

One is scientific: why are insects continuing to be detected in wood packaging marked as having been treated? What is the relative importance of insects surviving the treatment versus treatment facilities applying the treatments incorrectly or inadequately?

The second issue is legal and political: what proportion of the detections is due to treatment facilities committing outright fraud – claiming to treat the wood, stamping it with an IPPC stamp, while not actually performing any treatments at all?

Knowing which measures will most effectively solve these quandaries / reduce pest presence in wood packaging depends on knowing what the relative importance of these factors are in causing the problem.  The lack of basic data on which to base any analysis certainly hampers efforts to improve protection.

Leigh calls for researchers to recognize these data needs and work to fill them.

•Understand, account for, and communicate data realities

•Work collectively to increase useable data quality

•Use additional research to validate, or to demonstrate disparities

Why Wait for the Science?

In the meantime, however, I assert that more vigorous enforcement efforts by responsible agencies should help reduce the occurrence of fraud. I have suggested the following actions:

  • U.S. and Canada refuse to accept wood packaging from foreign suppliers that have a record of repeated violations – whatever the apparent cause of the non-compliance. Institute severe penalties to deter foreign suppliers from taking devious steps to escape being associated with their violation record.
  • APHIS and CBP and their Canadian counterparts follow through on the industry-initiated program described above and here aimed at helping importers avoid using wood packaging from unreliable suppliers in the exporting country.
  • Encourage a rapid switch to materials that won’t transport wood-borers. Plastic is one such material. While no one wants to encourage production of more plastic, the Earth is drowning under discarded plastic. Some firms are recycling plastic waste into pallets.

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

What will replace hemlocks? Intractions with other plants & introduced pathogens complicate the situation

eastern hemlocks in Cook Forest State Forest Pennsylvania; photo by F.T. Campbell

As Eastern hemlock (Tsuga canadensis) suffers high levels of mortality across nearly all its range due to hemlock woolly adelgid (HWA; Adelges tsugae),  scientists scramble to determine what the successor forests will look like. The transformation will be stark: from deeply shaded evergreen coniferous forest with a sparse understory to something very different. As this process takes place, most scientists expect cascading effects on not only terrestrial and aquatic wildlife but also onecosystem functions, including soils and nutrient and hydrologic cycles (Dharmadi et al. 2019 Plotkin et al. 2024).

New England

In southern New England, hemlock groves are being replaced by stands of deciduous hardwood forests dominated by black birch (Betula lenta). While birch are expected to continue to dominate, other species comprise at least one third of seedlings in the Harvard Forest experimental sites, primarily eastern white pine (Pinus strobus) and red maple (Acer rubrum). Plotkin et al. (2024) note that conversion of hemlock forests to pine forests would be a less dramatic ecosystem shift since both are evergreen conifers.

symptoms of beech leaf disease; photo by the Ohio State University

In both southern New England and farther north, in Vermont and New Hampshire, maples and American beech have increased in prominence. In the latter case, this is despite the prevalence of beech bark disease and managers’ efforts to suppress beech. I have noted that beech leaf disease now threatens to disrupt this process.

Landowners in the region often seek to get some financial return from their forests before a pest kills the trees. About a quarter of the almost 9,000 ha of hemlock stands in the southern Connecticut River Valley have been harvested as HWA spread into the area. To test the effect of pre-mortality logging of hemlock stands, Plotkin et al. tried to mimic HWA-caused mortality by girdling all the hemlocks in some plots in Harvard Forest. In other plots they harvested most hemlocks and some of the other tree species. The girdled plots had a dramatic increase in standing and downed deadwood and a longer period of elevated understory light levels than the logged plots. They note that standing snags and on-ground dead wood provide critical ecosystem functions. Many wildlife and microbial species depend on dead wood for nutrition and a variety of micro habitats. Plotkin et al. found that the slowly decomposing dead wood also stored a large amount of carbon: girdled plots stored 18% more above-ground carbon than logged sites, even after accounting for carbon stored in harvested wood products.

a beech snag with nesting cavities; photo by F.T. Campbell

The magnitude of these differences might be even larger than demonstrated in this experiment. In New England, hemlocks infested with HWA die over a decade, not the two years seen after girdling. The delayed mortality provides a longer window of opportunity for succeeding vegetation to adapt and preserve higher levels of biodiversity. Plotkin et al. (2024) suggest that logging pest-threatened hemlock forests might remove structural resources that would support forest response to ongoing climate stress and future disturbances.

Considering the disturbed plots’ invasibility by non-native plants, Plotkin et al. (2024) found that more non-native shrubs invaded the girdled plots than the logged plots. They suggest that birds that disperse the shrubs’ fleshy fruits were attracted by perch sites provided by the standing dead trees.

Southern Appalachians

In the Southern Appalachians, post-HWA forests will apparently be quite different. At the USDA Forest Service’ Coweeta Hydrologic Laboratory in the Nantahala Mountain Range of western North Carolina, eastern hemlock died much faster than in New England. Hemlocks comprised more than 40% of the basal area before arrival of HWA (detected in 2003). Within two years all hemlock trees were infested. Half were dead by 2010, 97% by 2014 (Dharmadi et al. 2919).

In some part of the southern Appalachian forests the shrub layer is dominated by Rhododendron maximum (rosebay rhododendron). This dense shrub layer is preventing recruitment of deciduous tree species that had been expected to replace the dead hemlocks. Tree seedlings died rather than grew into saplings. Scientists working in the Coweeta experimental forest attribute the seedlings’ demise to limited access to key resources, e.g., water, nutrients (especially inorganic nitrogen), and light (Dharmadi, Elliott and Miniat 2019).

In the Coweeta Basin, hemlock loss is the most recent of a series of severe disturbances that have apparently led to a cascade of responses in the overstory, midstory, and soil that have promoted expansion of rhododendron. (The earlier disturbances were widespread logging in the 19th Century and the loss of American chestnut to chestnut blight in the first part of the 20th Century. Therefore, the response of future forests to changes in temperature and rainfall might now depend on these novel tree-shrub interactions .

R. maximum hampers succession by forming a dense subcanopy layer that greatly limits light reaching the forest floor and reduces soil moisture and temperature. These changes impede seed germination and seedling survival. In addition, rhododendron leaves that fall to the ground create a thick organic soil layer that decomposes very slowly. This affects soil chemistry, specifically availability of the key nutrient nitrogen.

The rhododendron shrubs in the region are younger than the deciduous trees now making up the canopy above them (Dharmadi, Elliott and Miniat 2019). The dense rhododendron stands resulted from the widespread mortality of American chestnut (Castanea dentata) in the early 20th century and of hemlock in the first years of the 21st Century. What’s more, even the mature deciduous trees appear to be suppressed by dense rhododendron stands. Canopy trees above rhododendrons are on average 6m shorter than those growing on sites without rhododendron thickets (Dharmadi, Elliott and Miniat 2019). In fact, by 2014, 10% of standing trees other than hemlocks had died. The tree suffering the highest level of mortality was flowering dogwood (Cornus florida). The authors do not mention a probable factor, the introduced disease dogwood anthracnose. Other species experiencing high levels of mortality are not, to my knowledge, under attack by non-native pests, so their demise seems more clearly linked to resource competition with rhododendron. These were striped maple (Acer pennsylvanicum), pitch pine (Pinus rigida), witch hazel (Hamamelis virginiana), and that staple of New England aftermath forests, black birch (Betula lenta).

Dharmadi, Elliott and Miniat (2019) suggested that managers should step in to increase recruitment in both understory and overstory layers. They proposed active management: removing rhododendrons and the soil organic layer. USFS scientists are applying these ideas experimentally at the Coweeta research station. I am unclear as to whether there is one study or more. In any case, rhododendronplants have been removed with the goal of restoring vegetation structure and composition – presumably both understory plant diversity and recruitment of tree species capable of growing into the canopy. In at least some cases, the rhododendron removal is followed by prescribed fire. One study is looking also at whether this action increased water yield.

Apparently this lack of tree regeneration is extensive – although published data are not easily accessible. Staff of the North Carolina Hemlock Restoration Initiative report they encounter similar issues (O.W. Hall, Hemlock Restoration Initiative, pers. comm.)

Several experiments have demonstrated that even in the southern Appalachians, where there are abundant moisture and rainfall, the trees and shrubs compete for water and other nutrients. However, Dharmadi et al. (2022) found that removal of the rhododendron shrub layer is unlikely to significantly alter streamflow, atr least during the growing season. In winter, when deciduous trees lack leaves, reduction in interception of precipitation might result in increased streamflow (Dharmadi et al. 2022). I ask whether increasing stream flow in winter is a goal? I thought the concern was stream flow levels in summer.

Nor is removal of the rhododendron shrub layer likely to alter stream chemistry during the growing season.

Removal of living Rhododendron and leaf litter apparently can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.

Removing Privet

However, other management actions might bring about desired changes more effectively or broadly. Specifically Dharmadi and colleagues mentioned removal of privet (Ligustrum) – a very widespread invasive shrub in forests of the Southeast. (Fifteen years ago it was estimated that just one privet species, Chinese privet, occupied more than a million hectares in 12 southeastern states [Hanula 2009].)

Chinese privet

I ask also whether prescribed fire to remove the rhododendron-dominated soil organic layer is useful. Dharmadi and colleagues found that such fires reduced leaf litter temporarily, but annual leaf-fall replaced the litter layer the next year, so this management effort is unlikely to affect plot evapotranspiration rates.

Supporting Pollinators

Another study (Ulyshenet al. 2022) examined whether removing rosebay rhododendron would benefit bees and other pollinators. They found that removal of Rhododendron alone (without fire) did not dramatically improve pollinator habitat in the southern Appalachians. In fact, about a quarter of the bee species studied visited R. maximum flowers and might decline if the shrub’s population is reduced. Ulyshen and colleagues suggest that some factors that correlate with fire severity probably promotes growth of insect-pollinated plants. They suggest specifically the greater presence of downed woody debris, which provides nesting sites and other resources used by insects. They recommended creation of open areas to support wildflowers as a more effective way to benefit bees in this region. Again, rhododendron removal pales in effectiveness compared to eradication of privet.

SOURCES

Dharmadi, S.N., K.J. Elliott, C.F. Miniat. 2019. Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians. Forest Ecology and Management 440 (2019) 122–130.

Dharmadi, S.N., K.J. Elliott, C.F. Miniat. 2022.  Larger hardwood trees benefit from removing Rhododendron maximum following Tsuga canadensis mortality. Forest Ecology and Management

Hanula, J.L., S. Horn, and J.W. Taylor. 2009. Chinese Privet (Ligustrum sinense) Removal and its Effect on Native Plant Communities of Riparian Forests. Invasive Plant Science and Management 2009 2:292–300.

Plotkin, A.B., A.M. Ellison, D.A. Orwig, M.G. MacLean. 2024. Logging response alters trajectories of reorganization after loss of a foundation tree species. Ecological Applications. 2024;e2957.

Ulyshen, M., K. Elliott, J. Scott, S. Horn, P. Clinton, N. Liu, C.F. Miniat, P. Caldwell, C. Oishi,  J.  Knoepp, P. Bolstad. 2022. Effects of Rhododendron removal and prescribed fire on bees and plants in the southern Appalachians. Ecology and Evolution. 2022;12:e8677.

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

A systemic treatment for beech leaf disease!

Reminder: beech leaf disease (BLD) came to attention in 2012 near Cleveland. It has since spread rapidly to the East and more slowly to the North, South, and West. It has been detected in 15 states and the Province of Ontario. The disease is caused by the foliar nematode Litylenchus crenatae mccannii (Lcm). Damage to the leaves can significantly reduce the tree’s ability to photosynthesize, resulting in a progressive depletion of carbohydrate reserves following successive years of infection. Scientists continue efforts to determine how it is spread and the extent of tree mortality.

Last summer I blogged about an Integrated Pest Management (IPM) strategy developed by Bartlett Tree Research Laboratories – the research arm of Bartlett Tree Experts – to treat individual beech trees afflicted with beech leaf disease (BLD). This treatment relied on a foliar spray. The challenge is that the entire canopy must be sprayed; this is difficult for large trees. Also, some trees cannot be sprayed because of their proximity to water bodies or other issues. For these large trees, Bartlett sought to develop a systemic treatment that can be applied as a drench or root flare injection.

photo by Matt Borden, Bartlett, via Flickr

I rejoice to tell you that Bartlett has now confirmed effectiveness of a systemic root flare injection treatment. Again, the project is led by Dr. Andrew Loyd and Dr. Matthew Borden. The full citation for their publication is at the end of this blog.ph

This second treatment utilizes Thiabendazole (TBZ), which has a long history of use in arboriculture to manage Dutch elm disease and sycamore anthracnose. In addition to being a fungicide TBZ is also a potent nematicide.  

Bartlett tested the efficacy of TBZ by monitoring 62 symptomatic American beech (Fagus grandifolia) trees across three sites comprising natural mixed hardwood forests with beech as a dominant species. Half of the trees were injected with TBZ, and the rest were monitored as non-treated controls. In the late winter after applying the injection, the researchers sampled twigs from all the trees and counted the number of nematodes in late-season dormant buds, where most of the damage occurs. They also quantified canopy density and BLD symptom expression before treatment and around 11 months after the tree injections the following year, to gauge year-over-year change. At one site, in Ohio, the trees were assessed again in the second season, or 22 months, after the initial injection. They also assessed damage to the root flare caused by the injection process.

Bartlett’s researchers found that at both 11 and 22 months after treatment, injected trees had significantly better visual ratings of canopy condition and lower numbers of Lcm in dormant buds. The untreated controls continued to have high disease severity and large numbers of nematodes in their buds.

Detailed Results

At the time of the initial inventory and treatment, about 65% of the canopies of beech trees at the two Ohio sites displayed foliar BLD symptoms. The proportion was lower at the New Jersey site, where BLD has been present only a season or two before treating – 42%.

During the first growing season post-treatment, the percent of the symptomatic beech canopies at the two Ohio sites fell by 70 – 85%. At the site where trees were evaluated again the second season (22 months) post-treatment, the percent of the canopy exhibiting leaf symptoms continued to decline. The scientists hypothesize that this continuing decrease could be due to TBZ residues being translocated to new leaves and buds, or to a reduction in local inoculum sources within the individual trees and surrounding forest due to treating a significant portion of the community.

At the New Jersey site, where injection was performed later in the season, the percent of the canopy exhibiting leaf symptoms increased by 66%. However, by another measure – percent of canopy with fine twig dieback – these trees improved by 71% while on untreated trees twig dieback increased by 95% and were already experiencing severe canopy loss.

New Jersey site contrasting treated & untreated trees; photo by Matt Borden, Bartlett

On average, there were significantly fewer Lcm in dormant bud tissues in treated trees compared to the untreated control trees. At the two sites in Ohio, the reductions were by 86% and 99%. At the New Jersey site, the decline was not as great, but still encouraging: 70%.

These results suggest that one treatment can substantially reduce symptoms. Scientists now need to determine at what point BLD symptoms return to damaging levels at both “low” & “high” concentrations of thiabendazole in order to determine retreatment intervals and expectations.

While the disease severity (measured by the percent canopy displaying BLD leaf symptoms) of all trees increased at Hillsborough, the canopies of trees injected with the “low rate” of TBZ was significantly better than those of the untreated trees. This was because of a significant reduction in fine twig dieback in the former as opposed to a significant increase in fine twig dieback in untreated controls. Fine twig dieback symptom expression is presumed to be associated with bud abortion caused by Lcm.

The New Jersey treatments occurred at the end of August. The scientists think that this period might follow rather than precede dispersal of many nematodes from the leaves to the buds, as evidenced by the reduced but still substantial numbers of nematodes found in the buds.

While there was some damage visible at injection sites, the Bartlett team considers the frequency of these symptoms to be low. Cracking of the bark was seen on 19% of injected trees; evidence of fluxing was present on 12%. Injection sites were closing rapidly at the site reviewed after 22 months post-treatment. Additionally, based on observations made during this study, they believe that cracking can be further reduced.

Loyd et al. conclude that TBZ injection is an effective treatment option for large beech (> 25-cm dbh) where full coverage sprays with fluopyram are difficult, or for trees growing near water, or where pesticide drift may be of concern.

a forest in Northern Virginia dominated by beech; photo by F.T. Campbell

While this treatment can be used in natural landscapes, treatments of whole forests will probably not be feasible due to the cost. Scientists continue investigating whether some combination of silvicultural practices such as reduction in stand density and with pesticide application of select mature beech might prove effective. In fact, scientists are establishing new plots this year to test a silviculture management approach in forests of Pennsylvania and Rhode Island where BLD is prevalent.

SOURCE

Loyd, A.L., M.A. Borden, C.A. Littlejohn, C.M. Rigsby, B. Brantley, M. Ware, C. McCurry, & K. Fite. 2025. Thiabendazole as a Therapeutic Root Flare Injection for Beech Leaf Disease Management Arboriculture & Urban Forestry 2025 https://doi.org/10.48044/jauf.2025.007

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

More pests in Europe & Mideast – hazard to North American trees

giant sequoia; photo by Matthew Dillon via Flickr

The pest alert system “PestLens” has again alerted us to plant pests in Europe or Asia that feed on species closely related to tree species native to North American forests.  Two of the insects named in the alert apparently pose a hazard to icons of the forests of America’s Pacific coast forests, giant sequoia and redwood.

I hope APHIS is using this information to alert port and on-the-ground staff and perhaps initiating more in-depth risk assessments.

The posting on February 27, 2025 reported that cotton jassid, Jacobiasca lybica (Hemiptera: Cicadellidae), affects not just cotton and citrus but also Cupressus sempervirens (Mediterranean cypress) [Cupressaceae]. More than a dozen North American trees species are in this family, including

  • Sequoiadendron giganteum or giant sequoia. Giant sequoia is listed as an endangered species by the IUCN with fewer than 80,000 remaining in its native California.
  • Chamaecyparis thyoides and C. lawsoniana (Port-Orford cedar). Port-Orford cedar has been decimated in its native range by an introduced pathogen, Phytopthora lateralis. A major breeding effort has developed trees that are resistant to the pathogen; they are now available for people to plant.
  • Thuja occidentalis, also known as northern white-cedar, eastern white-cedar, or arborvitae,
  • Taxodium ascendens, also known as pond cypress
  • several Juniperus
  • Hesperocyparis macrocarpa also known as Cupressus macrocarpa, or the Monterey cypress. NatureServe ranks the cypress as GI – critically imperiled.

Cotton jassid been reported from several countries in Europe, Africa, and the Middle East.

China has reported the existence of a previously unknown bark beetle species, Phloeosinus metasequoiae (Coleoptera: Curculionidae). It was found infesting Metasequoia glyptostroboides (dawn redwood) trees in China. Affected trees exhibited reddened leaves and holes and tunnels in branches.

China has also discovered a several new hosts utilized by the fungus Pestalotiopsis lushanensis (Sordariomycetes: Amphisphaeriales). Formerly known to infect tea (Camellia sinensis) and several other plant species, P. lushanensis has now been found shoot causing blight and leaf drop on a conifer, deodar cedar (Cedrus deodara) and leaf spots on an angiosperm with congeners in North America — the rare Chinese species, Magnolia decidua. There are eight species of Magnolia native to North America.

Magnolia grandiflora; photo by DavetheMage via Wikimedia

APHIS’ ability to respond to alerts remains uncertain.

The agency’s probationary employees have been fired – just as at other agencies. APHIS staff were prohibited from participating in last week’s annual USDA Invasive Species Research Forum – the 33rd such meeting. The bird flu emergency is demanding all the attention and funds.

So – how can the rest of us fill in?

At the USDA Research Forum I again presented a poster urging greater attention to tree-killing pathogens. Scientists have made considerable progress in identifying factors that indicate whether a non-native insect might pose a significant threat (see blogs on conifer and deciduous species; more to come!). However, USDA had not funded a similar effort to improve understanding of pathogens. The most promising strategy so far are sentinel plantings. However, these systems have weaknesses; I will blog in the near future about another analysis.

I propose that APHIS start by working with independent scientists to determine the actual, current level of pathogens associated with various types of incoming goods. Contact me directly if you wish to read the text of my poster.

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Pest Alerts – is USDA (able to) pay attention?

redbay (Persea borbonia killed by laurel wilt

The pest alert system “PestLens” provides information about new reports of plant pests around the world. Notices are published weekly. These provide North American stakeholders advance notice of pests to be on the lookout for. While I have followed these postings for several years, I have been alarmed by recent notices report documenting the presence of insects or pathogens that feed on species in the same genera as tree species native to North American forests. The alerts cover pests of all types of crops, not just trees.

I note that several of these not-yet-introduced pests attack the genus Persea, which contains several native tree and shrub species that are already severely affected by laurel wilt disease.

The report for 19 December, 2024, provided information about two pathogens.

flowering dogwood Cornus florida; photo by F.T. Campbell
  1. The bacterium Pectobacterium aroidearum (Gammaproteobacteria: Enterobacteriales) was detected in China. The bacterium infests several crops and Persea americana (avocado). Although the detection in China is new, the bacterium is apparently already widespread, since it has been earlier been reported from parts of Africa, the Middle East, Asia, Brazil, and Jamaica.
  2. The dagger nematodes Xiphinema simile and X. zagrosense (Longidoridae) were reported in Syria.  X. simile is associated with economically important plants, including Cornus spp. (dogwood; North American species already decimated by the introduced pathogen dogwood anthracnose), Malus spp.(apple), Prunus spp. (stone fruit), Quercus spp. (oaks – already under attack by many non-native organisms), and Vitis vinifera (grape). X. zagrosense is also associated with Poaceae. X. simile has earlier been reported from parts of Europe, Kenya, Iran, and Russia. X. zagrosense has also been reported from Iran.

The report for 9 January, 2025, conveyed information about 1 pathogen and 1 insect.

  1. It noted the presence in Thailand of the fungus Pseudoplagiostoma perseae (Sordariomycetes: Diaporthales) on Persea americana.
  2. The South American palm borer, Paysandisia archon (Lepidoptera: Castniidae), is infesting several palms at multiple locations in Switzerland. It attacks several economically important palm species and the native genus Washingtonia spp. (fan palm).
native California fan palm, Washingtonia filifera; photo by F.T. Campbell

The report for 13 February, 2025, gave information about 1 pathogen and 1 insect.

  1. An anthracnose fungus Colletotrichum aenigma (Sordariomycetes: Glomerellales) infecting Carya illinoinensis (pecan) and Ilex cornuta (Chinese holly) in China. Colletotrichum aenigma infects other economically important plants. These include the following genera with native species in North America: Vaccinium (blueberry), Malus (apple), Persea americana (avocado), and Vitis vinifera (grape). Colletotrichum aenigma is also widespread; it has been reported from parts of Europe, the Middle East, Asia, New Zealand, and South America.
  2. South African citrus thrips, Scirtothrips aurantii (Thysanoptera: Thripidae) in a greenhouse in the Netherlands. The thrips infests several woody plants, including Ilex crenata (Japanese holly), Rosa spp., Malus (apple), Persea americana (avocado), Prunus spp. (stone fruit), and Vitis vinifera (grape). S. aurantii  it is under eradication in Portugal and Spain. It has also been reported from parts of Africa, Yemen, and Australia.
Scirtothrips aurantii; photo by Pablo Alvarado Aldea, Spain

A few weeks ago I wanted to conclude this blog by stating my hope that APHIS is using this information to alert port and on-the-ground staff and perhaps initiating more in-depth risk assessments. Now – as we learn about mindless firings of USDA staff, I fear I must limit my hopes to a future for APHIS’ programs and skilled staff in more general terms.

Do we face shut-down of pest prevention/response efforts across the board?

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 here or here.

New Shothole Borer in California — Alert! & Opportunity to Advise Whether the State or County Should Lead the Response

several Euwallaceae species; E. interjectus is 2nd from the top. Photo from Gomez et al. 2018; ZooKeys 768 19-68

In December 2024, California officials announced detection of a third species of invasive shothole borer beetle in the state. This invasion was found in Santa Cruz County in October 2024. The beetle has been identified as Euwallacea interjectus; the associated fungus is Fusarium floridanum. Like other non-native shothole borers in the same genus already known to be in California, Euwallacea interjectus is native to Southeast Asia.

So far, the infestation extends across at least 75 acres (CDFA proposal). It is affecting primarily box elders (Acer negundo). Other tree species have also been attacked: California sycamore (Platanus racemose), coast live oak (Quercus agrifolia), arroyo willow (Salix lasiolepis), red willow (Salix laevigata), and black cottonwood (Populus balsamifera ssp. trichocarpa). [See the CDFA risk assessment referred to below]. While it is too early to know precisely, E. interjectus is expected to pose a risk of tree dieback in urban, wildland and agricultural landscapes similar to that already caused by its relatives — the Polyphagous shot hole borer (Euwallacea fornicatus s.s. [PSHB]) and Kuroshio shot hole borer (Euwallacea kuroshio [(KSHB)].

The Santa Cruz County Department of Agriculture and University of California Cooperative Extension Service are coordinating with the California Department of Food and Agriculture (CDFA) to monitor and respond to the infestation. Research is being conducted by the University of California to evaluate the full range of potential tree species that may be affected by the beetle.

CDFA is seeking input on whether to designate Euwallacea interjectus as a category “B” pest. Under this category, response to the pest would be carried out by the counties at their own discretion, not by the state. You can advise CDFA’s on this decision until 17 February. Go here.

In its proposal, CDFA notes that several tree hosts of the beetle grow throughout California. The analysis gave a risk ranking of “High (3)” in four categories: climate/host interaction, host range, dispersal and reproduction, and ecosystem-level impacts. The economic risk rank is “Medium (2)” because it might attack only stressed trees – although CDFA concedes that drought stress is common in California. The overall determination is that the consequences of Euwallacea interjectus’ introduction to California is “High (14)”. Still, CDFA proposes to leave response to this introduction up to affected counties.

Santa Cruz County is outside the areas identified by a model developed by Lynch et al. (full citation below) as being at high risk of establishment of the Euwallacea-Fusarium complex, based on analysis of sites where Euwallacea fornicatus and E. kuroshio are already established. Nearby areas are ranked at high risk; these include drier areas in the San Francisco Bay region.

There are at least three four beetles in the Euwallacea fornicatus species complex. Several look almost identical to one another; the only reliable way to tell them apart is by looking at their DNA. However, E. interjectus is substantially larger than E. fornicatus and E. kuroshio, the two already-established shothole borers causing damage in southern California.

Various members of the Euwallacea fornicatus species complex have invaded countries around the world and other parts of the United States. While many of these introductions occurred decades ago – e.g., Hawai`i, Florida, possibly Israel, there appears to have been a spurt of introductions around or after 2000. The PSHB was first detected in California in 2003; the KSHB in 2013. As of 2022, disease caused by these two complexes had spread throughout Orange, San Diego, Los Angeles, Riverside, San Bernardino and Ventura counties. Outbreaks have been detected as far north as Santa Barbara /Santa Clarita. The KSHB had “jumped” to more distant locations in San Luis Obispo and Santa Clara counties. So far, the two later detections apparently do not represent established populations. In November 2023, the PSHB beetle–pathogen complex was confirmed killing hundreds of trees in riparian forests in San Jose, in the San Francisco Bay region. Two host trees – California sycamore and valley oaks – are important in the urban forest canopy of the region

NOTE: the invasive shot hole borers and their associated fungi attacking trees in California are completely unrelated to the laurel wilt complex killing trees in the Lauraceae family in eastern States.  This complex involves an ambrosia beetle Xyleborus glabratus and associated fungus Harringtonia (formerly Raffaelea) lauricola.

SOURCES

California Department of Food and Agriculture, California Pest Rating Proposal Euwallaceae interjectus (Blanford): Boxelder ambrosia beetle https://blogs.cdfa.ca.gov/Section3162/wp-content/uploads/2025/01/Euwallacea-interjectus.pdf  

Comments due by February 17, 2025.

Lynch, S.C., E. Reyes-Gonzalez, E.L. Bossard, K.S. Alarcon, N.L.R. Love, A.D. Hollander, B.E. Nobua-Behrmann & G.S. Gilbert. 2024. A phylogenetic epidemiology approach to predicting the establishment of multi-host plant pests  Communications Biology

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Urban forests – resource under many threats

ash tree in Michigan killed by emerald ash borer; photo courtesy of (then) Mayor John Hieftje

The Forest Service is promoting its efforts to protect urban forests [see the Northeast Region’s “Roots in Research” in mid-December 2024]. The rationale is that urban forests provide substantial environmental and economic benefits that deserve more attention. These include air purification, temperature regulation and energy savings, water absorption, and improved public health. At the same time, urban forests face multiple and overlapping threats – including the one of greatest concern to us, introduction of tree-killing non-native insects and pathogens.

The article on which the Roots in Research “Science Brief” is based was actually published in 2022 in the Journal of Forestry. In it, David Nowak, Eric Greenfield, and Alexis Ellis evaluated historical and current threats to urban forests across the contiguous states and projected them 50 years into the future. Threats included urban expansion, climate change, insect infestation, and extreme weather events. Their goal was to help urban forest managers and policymakers prioritize resources and planning efforts.

I believe stakeholders should view these projects as underestimates because the sources Nowak et al. relied on for both future climatic conditions and non-native pest impacts are incomplete or outdated. I am not criticizing the choice of sources – they are the standard ones. But events have raised questions about their accuracy.

Nowak, Greenfield, and Ellis expected that urban tree cover will decline significantly by 2060. The principle cause is urban expansion — development of previously wooded areas. Development has traditionally been the leading cause of urban forest loss.

Newer threats have become obvious in recent decades – i.e., pest and disease attacks and extreme weather events.

coast live oak infected by GSOB; Heisey County Park, San Diego County photo by F.T. Campbell

The most troubling example of the sources’ weaknesses is the Alien Forest Pest Explorer (AFPE), on which the authors rely for their list of non-native insects and pathogens present in the United States. However, the compilers of this database decided not to include pests that are native to some parts of North America but are behaving as bioinvaders in other regions. The premier example is the goldspotted oak borer (GSOB), Agrilus auroguttatus. This insect kills three species of oaks native to southern California – coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis). Twelve years ago scientists estimated that GSOB had killed at least 100,000 trees in San Diego County; it has since been detected in widespread infestations in four other counties in southern California.

Not including GSOB (or Mediterranean oak borer; see below) skews the findings because of the importance of the oaks in California’s urban forests. Their genus is the second most-abundant native genus in the state’s urban forest, making up 6.5% of the trees. Because many of these trees are large, they contribute significantly to the ecosystem benefits provided by urban forests. Out of the 152,594 coast live oaks in 287 cities statewide, at least 30,000 of them meet GSOB’s preferred size limit (DBH greater than 18 – 20 inches [~45 cm]) (Love et al. 2022). The highest presence of oaks in urban forests in the South is in Santa Barbara – which has not yet been invaded by GSOB. However, built-up sections of Los Angeles – which are heavily invaded already — have between 250,000 and 300,000 coast live oak trees.

The Alien Forest Pest Explorer also does not include pests of palms. Palms are the first and second most the abundant species in urban areas of both the Southern California Coast and Southwest Desert regions (Love et al. 2022). Of course, palms contribute little to the ecosystem benefit associated with urban forests, but they are iconic symbols of the region. California’s palms are under attack by the South American palm weevil. https://cisr.ucr.edu/invasive-species/south-american-palm-weevil

More difficult to understand is the AFPC’s failure to include the Mediterranean oak borer, (MOB) (Xyleborus monographus). MOB has been introduced from Europe, so it fits the AFPE’s criteria for inclusion. MOB is killing valley (Quercus lobata) and blue oaks (Q. douglasii) in Lake, Napa, Sacramento, and Sonoma counties in California and Oregon oak (Q. garryana) in Troutdale, Salem, and other towns in Oregon.

Quercus lobata, killed by Mediterranean oak beetle

As to the data sources relied on for projections of future climatic factors, several measurements of the changing climate already exceed projections in the models. They expect intensified threats from changes in air temperature, precipitation, aridity, wildfire risk, flooding, and sea level rise. By 2060, temperatures in urban areas are expected to increase by 1.2 – 3.5° C. Nowak and colleagues expected this warming to exacerbate threats from heat stress, flooding, increased salinity, drought, and wildfire. Less certain but possible are more intense storms and pest outbreaks. As I noted above, perhaps even these projections understate the threats.

For example, in discussing flooding the authors relied on measurements of the historic 100-year flood plain. I understand that experts now say this standard is inadequate, given existing records and projected further increases in precipitation (especially high-intensity storms). Urban areas in 98% of the 2,424 counties Nowak et al. analyzed contain flood-prone areas.

Nowak et al. do mention two additional elements exacerbating the flood risk: the spread of impervious surfaces and location of many cities next to bays or wide rivers. In these latter cases, risks might include salt intrusion linked to higher water levels, even in the absence of flooding. The National Oceanographic and Atmospheric Administration’s “intermediate high” scenario projects sea level will rise 61 cm by 2060. 

Nowak, Greenfield, and Ellis said the greatest overall threat is in the eastern states, especially New England other than Vermont and Maine; the mid-Atlantic; South Carolina; and Ohio. They say this arises from the combination of high levels of urbanization and accumulation of several threats. The specific threats include projected precipitation changes, storms (hurricanes in the southeast; ice storms in the Appalachians); sea level rise; and the abundance of non-native pests. I think that reliance on data from the past results in understating the hurricane risk in the Northeast (especially the Hudson and Connecticut river basins) and in North Carolina.

Nowak, Greenfield, and Ellis reminded us that a healthy urban forest canopy can help mitigate some of the threats associated with climate change. This applies particularly to local air temperatures. Reducing urban heat islands not only addresses a direct threat; it can also moderate such other threats as pest infestations, wildfire, aridity, and storm damage, especially runoff. They advocate science-based tree management programs including preserving existing trees and planting species that can thrive in the expected new local and regional environment, e.g., withstand droughts, flooding, saltwater exposure, or extreme temperatures.

I think their recommendation on pest threats is lame: they suggested “monitoring and managing local pest threats.” Non-native pests demand additional actions at all levels of authority — local, state, and federal.  (See the “Fading Forests” reports linked to at the end of this blog, and earlier blogs under the category “invasive species policy”.) I have already noted troubling exclusion of some pests already present in urban areas of the continental United States. I understand that it is impossible to predict which additional pests might be introduced in the next 50 years. But I would have appreciated a sentence stating the near certainty that more pests will be introduced and cause damage to urban forests in the next 50 years.  

Given the recent fires in the Los Angeles region, I believe we need new analyses of the risk of wildfire in cities and the positive and negative interactions with the urban forest.

SOURCES

Threats to Urban Forests in the United States Roots in Research Issue 45 | December 2024 https://research.fs.usda.gov/nrs/  products/rooted-research/threats-urban-forests-united-states?utm_source=MarketingCloud&utm_medium=email  accessed 24-12/31

Nowak, D.J., E.J. Greenfield, and A. Ellis. 2022. Assessing Urban Forest Threats across the conterminous United States. Journal of Forestry, 2022, Vol. 120, No. 6

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

APHIS funding for pests that kill trees (& cacti)

emerald ash borer; some of PPA grants are funding evaluation of biocontrol efficacy

USDA APHIS has released information about its most recent annual allocation of funds under the Plant Pest and Disease Management & Disaster Prevention Program under §7721 of the Plant Protection Act. (Also see Fading Forests II and III; links provided at the end of this blog.) These funds support both critical needs and opportunities to strengthen the nation’s infrastructure for pest detection, surveillance, identification, and threat mitigation. Since 2009, this USDA program has provided nearly $940 million to more than 5,890 projects.

For FY25 APHIS allocated $62.725 million to fund 339 projects, about 58% of the proposals submitted. About $10 million has reserved for responding to pest and plant health emergencies throughout the year.

According to APHIS’ press release, the highest amount of funds (almost $16 million) is allocated to the category “Enhanced Plant Pest/Disease Survey.” Projects on “Enhanced Mitigation Capabilities” received $13.6 million. “Targetting Domestic Inspection Efforts to Vulnerable Points” received nearly $6 million. “Improving Pest Identification and Detection Technology” was funded at $5 million. Outreach & education received $4 million.  I am not sure why these do not total $63 million.

Funding for States and Specific Pests

Wood-boring insects received about $2.3 million. These included more than $869,800 to assess the efficacy of biocontrol for controlling emerald ash borer (EAB) Agrilus planipennis, $687,410 was provided for various detection projects, and $450,000 for outreach efforts related to various pests. Ohio State received $93,000 to optimize traps for the detection of non-native scolytines (bark beetles).

Biocontrol efficacy will also be assessed for hemlock woolly adelgid, invasive shot hole borers, cactus moth, and several invasive plants (including Brazilian pepper). (Contact me to obtain a copy of CISP’s comments on this biocontrol program.)

Opuntia basilaris in Anza Boreggo; one of flat-padded Opuntia vulnerable to the cactus moth; photo by F.T. Campbell

Funding for other pests exceeded $1 million for spotted lanternfly (nearly $1.4 million), Asian defoliators ($1.2 million) and box tree moth (just over $1 million).

$630,000 was provided for detection surveys and studies of the sudden oak death pathogen Phytophthora ramorum, especially how it infects nursery stock. Nursery surveys are funded in Alabama, Louisiana, North Carolina, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Virginia, and West Virginia. Most of these states are in regions considered most at risk to SOD infection of wildland plants.    

sudden oak mortality of tanoak trees in southern Oregon; photo by Oregon Department of Forestry

Oregon received much-deserved $41,000 to evaluate the threat of the NA2 and EU2 lineages of P. ramorum to nurseries and forests Oregon also received $104,000 to respond to the detection of Phytophthora austrocedri in nurseries in the state. The Oregon outbreak has been traced to Ohio, but I see no record of funds to assist that state in determining how it was introduced.

Asian defoliator (e.g., Lymantrid moths) surveys have been funded for several years. This year’s projects are in Alaska, Arkansas, California, Kentucky, Maryland, Massachusetts, Mississippi, Montana, Nevada, North Carolina, Oregon, Tennessee, Texas, Washington, and West Virginia. While I agree that the introduction risk is not limited to coastal states with maritime ports, I don’t what criteria were applied in choosing the non-coastal states which are funded to search for these insects

Spotted lanternfly surveys (including technological improvements) or related outreach are funded in Alabama, Connecticut, Delaware, Kentucky, New Hampshire, New Jersey, North Carolina, Oregon, Pennsylvania, and Tennessee. California’s project is focused on postharvest treatments.

The Don’t Move Firewood project continues to be funded by APHIS. Several states also direct attention specifically to the firewood pathway: Kentucky, Maine, and Michigan.

I applaud the precautionary funding of the Agriculture Research Service to generate of high-quality genomic resources for managing the causal agent of Japanese oak wilt Dryadomyces quercivorous

Florida Department of Agriculture, North Carolina State University, and West Virginia University each received more than $100,000 to improve detection and management of invasive hornets.

Tennessee State University got $100,000 to continue efforts to detect and understand Vascular Streak Dieback in redbud Cercis canadensis.

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  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

How beech leaf disease spreads in the forest

BLD symptoms; photo by Matt Borden, Bartlett Tree Experts

As beech leaf disease (BLD) is detected in an ever-expanding number of counties from Michigan to Maine south to Virginia, scientists are trying to clarify how the causal nematode — Litylenchus crenatae ssp. mccannii (Lcm) – spreads. One focus is on local spread from tree to tree. Mankanwal Goraya and colleagues set up an experiment in Stone Valley Forest, a recreation and research site managed by Penn State in Huntington County, Pennsylvania. BLD is present – although I have not been able to determine for how many years. [The full citation to Goraya et al. is provided at the end of this blog.]

Goraya et al. (2024) set up four stands, each bearing three funnels, at varying distances from naturally BLD-infected American beech (Fagus grandifolia) trees. Two stands were at 3.51 m from symptomatic trees of starkly different sizes: one of the trees had a dbh of 50 cm, the other of only 5.6 cm. A third close-up stand was set up at 2.20 m from another large tree, having a dbh of 46 cm. The fourth stand was set up at a significantly longer distance, 11.74 m from a symptomatic beech tree; this tree was also small, with a dbh of 5 cm. This arrangement allowed the scientists to detect influences of both distance from the source of infection and relative canopy size of the source tree. They consider dbh to be an adequate substitute for canopy size. There was apparently no other effort to determine or vary the height of “source” trees, although I think that might influence speed of the wind flowing through the canopy.

Goraya et al. also tested whether it is possible to detect the presence of Lcm in association with other invertebrates that live in beech forests. To do this, they counted numbers of nematodes in frass from six species of caterpillars that had been feeding on leaves of infected trees, and in two spider webs spun in the branches of symptomatic trees. They also determined whether these nematodes were alive (active) or inactive – presumably dead.

The study makes clear that Lcm’s life cycle and impact are not as surprising as initially thought. Several species in the family Anguinidae – to which Lcm belongs – are considered significant pests. These nematodes can parasitize aerial parts of the plants (leaves, stems, inflorescences and seeds), causing swellings and galls. Furthermore, they are migratory; they can move across the surface of host tissues using water films. Once they have penetrated the host tissues, they can induce host cell hyperplasia and hypertrophy, resulting in leaf or bulb deformities, shorter internodes, and neoplastic tissues. Furthermore, heavy rainfall and wind are known to play significant roles in the dissemination of plant-infecting nematodes. In their desiccated state on infected seeds, some species of this family can survive passage through animals’ gastrointestinal digestive tract (e.g., domestic livestock, insects, & birds).

A crucial factor is that Lcm can reach densities of thousands of nematodes per leaf by late summer or early fall, increasing the likelihood of their exposure to facilitating environmental conditions at the time they migrate from leaves to buds. And once established within the bud tissues, the nematodes feed on bud scales and newly forming leaves to develop & increase their pop #s. They also use the bud as protection from adverse environmental conditions.

Goraya and colleagues collected samples every other day from September 9 to November 23, 2023 – the period when Lcm migrate from highly infected leaves to newly forming buds. [I note that it in the mid-Atlantic – where Lcm is spreading – we had an extensive drought in autumn 2024 – more than 30 days without any rain from early October into November. I hope scientists are monitoring BLD spread sufficient closely to see whether this drought affected dispersal.]

Nematodes dispersal linked to weather

Goraya and colleagues collected 324 samples from the funnels. Eighty-two percent (n =266) of the samples had nematodes; up to 92% were identified as Lcm. Non-Lcm nematodes were distributed across different genera, mostly classified as free-living nematodes. While several hundred nematodes were found in the funnels on most days, numbers peaked noticeably on some days in September and October.   A startling 2,452 nematodes were recovered from a single funnel in October. Depending on the sample, up to 67% of Lcm recovered from the funnels were active.

Analysis of the environmental (weather) variables found that increases in wind speed, humidity, and precipitation (rainfall) coincided with higher numbers of Lcm being recovered from the funnels.  However, the effect of wind speed becomes less positive as precipitation increases or vice versa. Goraya et al. suggest a pronounced negative interaction between wind and rain. At low precipitation levels, increased wind speed might facilitate Lcm dispersal. As rainfall increases, higher wind speeds might carry the Lcm nematodes farther away. Support is seen in the fact that fewer nematodes were found in the funnels closer to the BLD-infected trees during these periods. Really heavy rain might push a significant preponderance of nematodes to the ground. The scientists point to a very complex interplay between weather patterns and Lcm population dynamics and dispersal.

BLD symptoms on beech tree in Fairfax County, Virginia – a dozen miles from known infestation; photo by F.T. Campbell

The model did not show any significant influence of maximum temperature on nematode numbers in autumn. Goraya et al. do not speculate on whether temperatures might play a role during summer, as distinct from cooler autumn periods.

Goraya et al.’s findings differ from those of previous studies. Earlier documentation of wind dispersal of nematodes concerned primarily free-living species. It was unexpected to find consistently much higher numbers of Lcm – especially because Lcm is a plant-parasitic nematode. Another surprise is the high proportion of nematodes that are active.

Goraya et al. conclude that because Lcm is actively migrating in large numbers during autumn months, it is primed to take advantage of favorable weather. This nematode will likely survive and thrive in the environmental conditions of beech forests in northeastern North America.

Considering the effect of distance, some findings fit expectations: significantly more Lcm were recovered from funnels placed near symptomatic “source” trees than from those farther away. However, this was not a simple relationship. For example, in two cases the scenarios seemed nearly alike: both “source” trees were large (dbh 46 or 50 cm) and symptoms were “medium-high” (more than half of leaves presenting dark-green interveinal bands). Distance of funnels from the “source” tree differed minimally: 2.2 m versus 3.51 m. Still, the number of nematodes retrieved from the two sets of funnels differed significantly: one set of funnels recovered the highest number of Lcm nematodes obtained during the entire experiment – 2,452; the second contained only up to 600 nematodes. The authors do not offer an explanation.

I am not surprised by the apparently strong correlation between numbers and proximity to the disease source (a symptomatic tree). Nor am I surprised that Lcm nematodes were also found in funnels 11 meters away. I do wonder, however, why they are certain that no source was closer. Detecting early stage infections is notoriously difficult.

beech with large canopy; photo by F.T. Campbell

Goraya et al. also evaluated the effect of size of the source tree. They used dbh a substitute for larger canopies. Trees with larger canopies can host more nematodes, so are likely to contribute more to dispersal events. Two sets of funnels were equidistant from separate “source” trees – 3.51 m. One tree was small – 5.6 cm dbh, 11% as large as the other tree (50 cm). They collected many fewer Lcm nematodes from the smaller tree – the maximum was only 132 compared to 600 (a decrease of 78%).

Still, small trees can apparently support spread of the nematode to a reasonable distance. The fourth set of funnels was set up more than three times farther away (11.74 m) from an infected tree of a similar size (dbh = 5 cm) but recovered almost the same number of Lcm nematodes (0 – 119).

I find it alarming that both small trees in this part of the experiment had low BLD symptoms – only a few leaves were banded. Yet they apparently are the source of Lcm spread. The alternative, as I noted above, is that other “source” trees were in the vicinity but were not detected, possibly because they did not yet display symptoms?

Goraya et al. conclude that “source” tree size directly impacts the number of recovered nematodes. In addition, wind plays a pivotal role in their local distribution. This suggests a complex dispersal pattern in which proximity to the source leads to higher numbers of nematodes but longer-distance spread is possible.

Tussock moth; photo by Jon Yuschock via Bugwood

 Nematodes’ association with other organisms

Goraya et al. (2024) collected one each of six caterpillar species from BLD-symptomatic trees. The frass of one – the tussock moth caterpillar (Halysidota tessellaris) — contained 12 nematode specimens — 10 of them Lcm. Two of the Lcm were alive and active. Their presence indicates that Lcm can survive passage through the caterpillar’s gastrointestinal tract. The authors conclude that caterpillars feeding on symptomatic leaves might contribute to local dispersal of Lcm.

Hundreds of Lcm were recovered from the two spider webs collected from the branches of a BLD-infected beech tree. From one web, 255 nematodes were captured; 58 were active. In the second web there were only 34 Lcm, but one-third — 10 – were active.

Goraya et al. (2024) hypothesized that any biotic form having the ability to move from a BLD-infected tree would be able to transport Lcm to other non-infected trees. Beyond caterpillars, they speculate that birds consuming these caterpillars might also disperse Lcm. Doug Tallamy has documented that many birds feed on caterpillars, link although he is focused on those that consume caterpillars in the spring, not the autumn. They note that others are studying that the bird species that feed on beech buds (e.g., finches) might transport nematodes. They note the need for additional research to clarify whether the nematode can survive birds’ digestive system.

Re: detection of live Lcm in spider webs, Goraya et al. suggest two possible interpretations: 1) this finding demonstrates that nematodes might fall from leaves, potentially spreading the infection to other trees beneath the canopy. (Supporting this idea is the fact that sub-canopy trees are often heavily infected with BLD and are frequently the first to exhibit BLD symptoms.) 2) Nematodes in spider webs are very likely to be transported by other “incidental organisms” (e.g., insects, birds, mammals) that feed on invertebrates trapped in webs — thereby potentially increasing the number and impact of nonspecific nematode vectors.

In conclusion, Goraya et al. found that many factors, e.g., distance & size of infected beech trees, wind speed, & humidity, contribute significantly to Lcm dispersal. The multitude of organisms interacting beneath the canopy also play a role.

They suggest that several major questions still need to be explored. These include how Lcm navigate environmental factors in their spread; and whether Lcm can survive – perhaps in a anhydrobioses state –transport over long distances, whether by abiotic or biotic vectors.

I remind my readers of the importance of beech in the hardwood forests in northeastern North America. Many wild animals, including squirrels, wild turkeys, white-tailed deer, and bears depend on beechnuts for fats and proteins. Moreover, some insects birds rely on beech tree canopies for shelter & nesting.

Other Hosts

Beech leaf disease attacks not just American beech (Fagus grandifolia). In North America, it has also attacked planted European beech(F. sylvatica), Chinese beech (F. engleriana), and Oriental beech (F. orientalis). Thus if it spreads it could have severe impacts across forests of much of the Northern Hemisphere.

range of European beech; from Royal Botanic Gardens, Kew

I appreciate that this project was funded by the USDA Forest Service International Program. I will pursue information concerning efforts by USFS Research and Development and the Forest Health Protection program.

SOURCE

Goraya, M., C. Kantor, P. Vieira, D. Martin, M. Kantor. 2024 Deciphering the vectors: Unveiling the local dispersal of Litylenchus crenatae ssp mccanni in the American beech (Fagus grandifolia) forest ecosystem  PLOS ONE |https://doi.org/10.1371/journal.pone.0311830 November 8, 2024 1 / 16

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 https://treeimprovement.tennessee.edu/

or

www.fadingforests.org