forest pests – Page 7 – Center for Invasive Species Prevention

Resistance Breeding – Let’s Do It! (Instead of thinking about it)

TACF back-crossed American-Chinese chestnut; photo by F.T. Campbell

I have advocated for considerably expanding efforts to breed trees resistant to non-native pests (including pathogens) for a decade. Again and again, I and others have pointed out the dire consequences for our forests if we Americans do not rise to the challenge.

In 2014, Scott Schlarbaum – coauthor of Fading Forests III – American Forests: What Choice Will We Make? warned that without restoration becoming an integral part of a strategy addressing non-native plant pests, American ecosystems are doomed to continuing transformation. Once established, a non-native pest is never eliminated, but its impact can be reduced through a combination of measures – as long as support is made available. Scott advised initiating a germplasm conservation strategy when invasion is imminent or once the pest is likely to become a resident pest. (See Chapter 6).

I have posted seven blogs since August 2021 describing the current status of various efforts and urging the U.S. Government and conservation organizations to step up. [To view these blogs, go to www.nivemnic.us, scroll below Archives to “Categories” and click on “resistance breeding.”

More, and Recent, Voices: Implications of Not Acting

More recently, several USDA Forest Service (USFS) experts, including Richard Sniezko, C. Dana Nelson, and Jennifer Koch, have published articles making the same point. These scientists note that many of the decimated species were formerly among the most common trees in our forests. Therefore, the cumulative effect of their disappearance on forest species composition and function is multiplied.

One blog, posted in 2022, is particularly pertinent. It summarizes a special issue of the journal Plants, People, Planet devoted to resistance breeding. The opening essay, by R.J.A. Buggs, concisely reviews six major reasons why so many believe that resistance breeding is a failed strategy.

Port-Orford cedar – one of the trees for which resistance breeding has been successful; photo courtesy of Richard Sniezko, USFS

Others say there have been successes – all through application of classic tree improvement measures, not “genetic engineering.” Pike, Koch and Nelson (2021) list as successes Port-Orford-cedar (Chamaecyparis lawsoniana), the western five-needle pine species, koa (Acacia koa), and resistance to fusiform rust (Cronartium quercuum f. sp. fusiforme) in the commercially-important loblolly (Pinus taeda) and slash (P. elliottii) pines. They also cite encouraging progress by The American Chestnut Foundation (TACF) through backcross breeding of America and Asian chestnuts and a USFS/private foundation effort to expand the genetic base of American elms (Ulmus americana). I regret to say this, but some of these efforts seem to me to be still in experimental stages or — at best — early in widespread – ‘though still experimental — plantings.

Participants in a 2021 Purdue University workshop have again called for greatly expanding breeding. See the special issue of New Forests, Vol. 54 Issue 4. Once again, experts reiterate the urgency of acting, then outline the opportunities and challenges.

In one of the articles (Jacobs et al.) several people – including me! – note that several keystone tree species or genera in North America and Europe have been driven to functional extinction by non-native pests. By this we mean they are no longer sufficiently abundant and/or of adequate size to reproduce sexually or perform their ecological function. Examples include – on both continents – ashes (Fraxinus) and elms; and on North America – American chestnut (Castanea dentata), butternut (Juglans cinerea), and whitebark pine (Pinus albicaulis).If these threats are left unchecked, these at-risk tree species might develop truncated ranges, lose genetic diversity, and face becoming threatened, endangered, or extinct.

In another article, Nelson says the question that should be asked about applying genetic engineering (GE) techniques to tree breeding is whether we should let a species be reduced to a marginal role — or disappear — when GE provides a solution to saving and restoring the species. His case study is a detailed history of TACF’s development of a transgenic American chestnut (called “Darling 58”). He points out that decades of breeding efforts were based on the hope of developing blight resistance within the native gene pool or to obtain resistance from related species through hybridization. However, those efforts have not yet provided trees suitable for restoring the “king of the Appalachian forest” to native landscapes. Nelson wrote his description before TACF discovered flaws in the GE trees they had been working with and decided to pursue different GE “lines” (see below).

Barriers

The overall strategy is clear. Schlarbaum, Sniezko, and Dana Nelson all describe essentially the same steps, built on the same kinds of expertise and facilities.

Of course, each species will require years of input by a range of experts. These challenges are not trivial. However, the experts named above agree that the principal barrier is the absence of sustained, long-term commitment of resources and facilities. With sufficient resources, many of the scientific challenge can be overcome for at least some of the species at risk.

So, what are the scientific challenges? First, scientists must assess whether the tree species contains sufficient genetic variation in resistance. This involves locating candidate resistant trees; developing and applying short-term assay(s) to screen hundreds or thousands of candidate trees; and determining the levels of resistance present. Second, scientists must develop resistant planting stock for use in restoration. This stage includes scaling up the screening protocol; selecting the resistant candidates or progeny to be used; breeding to increase resistance; establishing seed orchards or other methods to deliver large numbers of resistant stock for planting; and additional field trials to further validate and delineate resistance. Sniezko and Koch (2017) and Sniezko and Nelson (2022) discuss the challenges and describe successes.

facilities at Dorena Genetic Resource Center; photos courtesy of Richard Sniezko, USFS

Complicating the restoration phase is the fact that the resistant tree must be able to thrive and compete in an ecosystem that has changed greatly from that in which it formerly resided. Causes of these changes include repercussions from the absence of the tree species – and possibly associated species; the possible presence of other biotic stresses (pests); and climate change. This is discussed by Nelson (2022). See also my blog.

Successfully completing these steps requires a long-term commitment, which includes significant funding and strong supportive infrastructure. Schlarbaum pointed out that the public and politicians don’t understand the complexity of the restoration challenge and the resources required. He documented the shrinking tree improvement infrastructure as of 2014. At that time, funding for all USFS regional breeding programs was just $6 million. State and land grant university breeding programs were fragmented and seriously underfunded. Only 28 states still had some type of tree improvement activity – and some of these programs were only seed orchards, not active breeding and testing programs. Members of university-industrial cooperatives focus on a small number of commercial species – which are not the species threatened by non-native pests. I believe these resources have shrunk even farther in the decade since 2014.

A separate source of funds for resistance breeding is the Forest Health Protection program, which is under the Deputy Chief for State, Private, and Tribal Forestry rather than the Deputy Chief for Research and Development. While nation-wide data on seed or scion collection or screening to identify and evaluate genetic resistance are poorly reported, Coleman et al. indicate that the USFS Dorena Genetic Resource Center screens unspecified “hundreds” of seed lots for resistance to pathogens annually. The Center also participates in seed, cone, and scion collections, especially of white pines vulnerable to white pine blister rust (WPBR). Supplemental Table S3 lists projects funded over the two decades analyzed by Coleman et al. (2011 – 2020). These included efforts to identify and evaluate possible genetic bases for resistance to, e.g., hemlock woolly adelgid, balsam woolly adelgid, laurel wilt, emerald ash borer, butternut canker, rapid ʻōhiʻa death; and gene conservation for eastern hemlock, ashes, chestnut, in addition to the five-needle pines. Currently, FHP allocates $1.2 million annually to support the group of activities called Genetic Conservation, Resistance and Restoration (R. Cooksey, pers. comm.).

American beech grafts to be tested for resistance to beech bark disease; at USFS center in Delaware, Ohio; photo courtesy of Jennifer Koch, USFS

USFS scientists involved in these projects describe challenges arising from efforts to cobble together funding from these many sources to support coherent programs. Overall funding levels still fall short of the need, and failure to obtain funding for one component of a program stymies the entire endeavor.

However, some developments are encouraging. The number of private foundations devoted to tree breeding has increased in the last decade. The American Chestnut Foundation (TACF) and American Chestnut Cooperators Foundation (ACCF) have been joined by the White Pine Ecosystem Foundation, the Great Lakes Basin Forest Health Collaborative, Forest Restoration Alliance, ‘Ohi‘a Disease Resistance Program … These organizations raise awareness, coordinate efforts by multiple parties, and provide opportunities for individuals to contribute funds and volunteer work.

In Hawai`i, disease resistance programs with both koa (Dudley et al.) and ʻōhiʻa ((Metrosideros polymorpha) (Luiz et al.) are active. Work with ash species to find and develop resistance to emerald ash borer is under way but limited due to lack of funds.

Finally, we can persuade Congress to incorporate the provisions of two bills, H.R. 3174 and S. 1238, into the next Farm Bill. The bills would, inter alia, create two grant program. One would fund research addressing specific questions impeding the recovery of native tree species that have suffered severe levels of mortality caused by non-native plant pests. The second would fund implementation of projects to restore these pest-decimated tree species to the forest.

Funded projects would be required to be part of a forest restoration strategy that incorporates a majority of the following components:

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

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

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

(4) Planting of native tree seedlings; and

(5) Post-planting maintenance of native trees.

For a detailed description, see this blog.

Details:

Facilities needed to support successful breeding programs

Sniezko and Nelson identified these needs as follows:

(a) growing space (e.g., greenhouses);

(b) seed handling and cold storage capacity;

(d) field sites for testing;

(e) database capability for collecting, maintaining, and analyzing data;

(f) areas for seed orchard development;

(g) skilled personnel (tree breeders, data managers, technicians, administrative support personnel, and access to expertise in pathology and entomology).

There are very few facilities dedicated primarily to development of populations of trees with resistance to non-native pests; the most notable is the Dorena Genetic Resource Center. Even the existing programs require significant funding increases to accelerate current programs or expand to address additional species. Sniezko and Nelson stress further that a resistance breeding program has different objectives, magnitude and focus than most research projects. It is applied science, that is, an action-oriented effort that is solution-minded—countering the impact of a major disturbance caused by a pest (in our case, a non-native pest).

Schlarbaum provides a shorter but similar list of facilities needed:

production of propagules (seed or clones);
mass propagation in growing facilities, e.g., bare-root seedling nursery or greenhouses;
site preparation of former habitat and planting; and
post-planting maintenance.

Schlarbaum emphasized that each of these activities requires different skill sets, equipment, facilities, and infrastructure.

Genetic Engineering as a Specific Tool

There is considerable interest in the potential role of genetic engineering in pest resistance breeding. None of the successful programs world-wide has yet used genetic engineering (Sniezko and Koch 2017). While incorporating it into holistic breeding programs might result in greater efficiency for certain processes, it raises legal and social acceptability issues. Jacobs et al. discuss the type of education and outreach program needed to generate widespread public support this approach to tree species “rescues.” They call for USDA Forest Service to lead this education effort.

The focus of the 2021 workshop hosted by Purdue University was to explore the pros and cons of using biotechnology in restoring pest-threatened forest tree species. The special issue of New Forests contains several participants’ analyses.

The overall conclusions are that:

“Genetic engineering” – defined as “any technique that uses recombinant, synthesized, or amplified nucleic acids to modify a genome” – is only one type of biotechnology applicable to tree breeding. Other biotechnologies include tissue culture-based propagation, molecular-based genetic markers, gene cloning and sequencing, and genome mapping and sequencing.
These new technologies can increase the efficiency of more traditional breeding techniques, However, biotechnologies cannot substitute for holistic programs that incorporate all helpful methods. Careful consideration goes into selecting which techniques are appropriate for a particular host-pest system.
Each tree species has unique needs regarding seed or scion collection; seedling propagation in nurseries; site preparation and planting techniques; and management of regeneration after its re-introduction into forests. Scientists don’t yet understand these various needs of many threatened species.
In the eastern U.S., the tree-breeding infrastructure is based in the Southeast and focused on a few pine species grown commercially. The facilities do not match the greatest need. That is, many of the at-risk species are hardwoods native to the Northeast.
Current resources are inadequate to support the sustained, long-term commitment of resources and facilities necessary to be successful.

Dana Nelson addressed the role of genetic engineering (GE) in detail. He emphasized repeatedly that GE is not a short-cut to tree improvement. Incorporating a GE component does not avoid the other steps. It can, though, provide new possibilities to address problems. Nelson says the crucial, initial question is – can GE solve the specific forest conservation or management problem more effectively and efficiently than existing methods? There are some important subtleties to consider. First, success does not require achieving immunity (100% resistance); the level of resistance needs to be only sufficient to allow the tree species to survive, reproduce and co-evolve with the pest. Second, “efficiency” is an important consideration. We cannot afford delay because during those years or decades the wild tree loses genetic variability as more trees die. Also, changes in the environment continues to change, and the decimated tree species is not adapting.

If genetic engineering promises to contribute meaningfully, then the breeders must answer several follow-up questions before proceeding to develop a specific plan. Nelson also stresses that the planned activities must be integrated with an ongoing tree breeding program to ensure project success.

Nelson provides a lengthy description of the process of integrating genetic engineering into tree breeding programs.

GE in Chestnut Breeding – Setback

The most prominent breeding effort incorporating genetic engineering in the U.S. has been The American Chestnut Foundation’s (TACF) program to restore American chestnut (Castanea dentata). For decades, TACF has pursued development of trees resistant to the fungus which causes chestnut blight (Cryphonectria parasitica). Over the past decade, hopes have centered on a genetically engineered line into which was inserted a gene from wheat (oxalate oxidase; OxO). The OxO gene detoxifies the oxalic acid produced by the chestnut blight fungus and thus prevents the cankers from killing the tree.

Years of tests have shown the gene to be effective and to cause no environmental harm. In 2023, when trees in outside test plots grew larger, scientists observed disappointing results. Trees’ blight tolerance varied greatly. Worse, resistant trees grew more slowly and exhibited lower overall fitness. [For a full discussion of the issues, visit TACF’s website] Prompted by these disappointments, scientists carried out further molecular analyses. They found that the OxO gene was on a different chromosome than expected.

TACF researchers now suspect that the trees’ variable performance stems primarily from the placement of the OxO gene and the fact that the gene is always “switched on”. That constant expression appears to result in high metabolic costs for the trees. Since all the genetic lines developed to date have this defect, TACF is no longer pursuing research efforts with any of the GE trees developed to date. The Foundation believes it would be irresponsible to continue efforts – by itself and by partners – focused on a genetic line that looks unable to compete successfully when introduced to the forest.

Instead, TACF has begun investigating other transgenic lines that use a “wound inducible” promoter that switches on the OxO gene only in cells where the plant is wounded. Researchers at both the State University of New York College of Environmental Science and Forestry (SUNY-ESF) and the University of Georgia are working with a variety of inducible promoters. TACF is also testing whether inducible OxO expression can be “stacked”onto genes for blight resistance present in the backcross hybrids. Finally, TACF and Virginia Tech are also exploring whether resistance can be enhanced by insertion of genes from Chinese chestnut directly into American chestnut using methods similar to OxO insertion.

It will be years before we know if these approaches provide sufficient levels of resistance. TACF will undertake more extensive testing for efficacy through the tree’s full life cycle – in the lab, greenhouse, and field – before submitting a new GE organism to regulators for review. Meanwhile, it will continue rigorous testing for plant health and environmental risks and will strengthen the cooperative structure to facilitate sharing of intellectual property and provide full transparency.

The Darling GE line was the most important transgenic hybrid chestnut line TACF had invested in. So this is a major setback – and comes when regulatory approval seemed near.

Let’s keep this in perspective, however. As a colleague has said, based on his years of teaching science to middle school students, “There are no failures in science, just reductions in the unknown; Edison failed a thousand times before getting the light bulb right, etc….” The technology is ready when it is ready. In addition, he praised TACF for choosing to explain its decision frankly: “nothing builds credibility like early failures openly admitted.”

Meanwhile, TACF continues to make gains in blight resistance with its traditional American chestnut backcross hybrid breeding program. They have established a genetically diverse, reproducing population of thousands of trees representing hundreds of breeding lines. These trees are planted in TACF’s expansive network of germplasm conservation orchards and regional breeding and backcross orchards. They have substantially increased resistance to both the blight and Phytophthora cinnanomi in these populations. The future inclusion of transgenic and/or gene-edited trees will further increase those gains.

Another Approach

Meantime, the American Chestnut Cooperators Foundation (ACCF), which breeds from persistent pure American chestnut, now has some trees that are nearly 50 years old. The program has bred five generations of pure American chestnuts that show durable blight resistance. Many trees are 60 feet tall or higher; they produce nuts. Vice President Jenny Abla (pers. comm.) reports that they show excellent canker response (swollen and superficial). The picture shows one of their most notable stands, which is in the Jefferson National Forest. Dr. Sniezko is exploring whether this program shows sufficient promise to justify increased support from the USFS.

ACCF chestnut trees; photo courtesy of Jenna Abla

Improving Coordination – will funds follow?

In July 2023, representatives from essentially all the forest tree resistance breeding programs in the U.S. met at Dorena Genetic Resource Center in Oregon to discuss their current successes and how to fast-track all programs. This is the first such meeting since 1982 (Richard Sniezko, pers. comm.). I encourage us all to study the report when it emerges and encourage USFS leadership to support the more unified enterprise.

Status of Efforts to Conserve Other Tree Species

The special issue of New Forests (Vol. 54 Issue 4) included several articles exploring the specifics of breeding elms, ashes, and ʻōhiʻa. These describe difficult challenges … and scientists determined to make progress on overcoming them.

“survival” American elm at Longwood Gardens; photo by F.T. Campbell

Elms (Ulmus spp.) (see article by Martin et al.)

Let’s not forget that elms were keystone species in Europe and North America until attacked by two epidemics of “Dutch” elm disease during the 20^th Century. While hybrid elms are available for urban plantings, many consider them not appropriate for planting in natural forests because these genotypes are not native.

Martin et al. describe a bewildering conglomeration of complexities and possibilities arising from biotic and abiotic factors. Initiation and especially intensity of the disease in a particular tree depend on

the species or strain of the tree, vectoring beetle, and pathogen;
timing of the attack; and
adequacy of water supplies at that time.

Possible targets for manipulation include the pathogen, its beetle vector, and the tree’s response — either in its bark or xylem. Martin et al. suggest that a combination of resistance to the pathogen within the xylem, resistance to beetles’ feeding wounds, and lowering tree clues that attract the beetles could considerably enhance longer-term overall resistance in the field.

However, verifying which approaches produce the best result will be complicated by the trees’ sensitivity to environmental factors such as season and water supply. Apparent resistance might actually be tied to, for example, low water supplies during the spring when the attack occurred.

Restoration strategies, including resistance to pests, must accommodate the diverse ecological conditions in the species’ large range, the rapid evolution of the Ophiostoma pathogens; and other pests and pathogens that attack elms. Nor do scientists know appropriate planting strategies.

Martin et al. believe Dutch elm disease is unlikely to be spread by movement of living elm plants, although other pests could be (and have been).

ash trees to be tested for resistance to emerald ash borer; photo courtesy of Jennifer Koch, USFS

Ashes (Fraxinus spp.)

While a USFS team led by Jennifer Koch link are conducting much of the on-the-ground efforts to breed ash trees resistant to the emerald ash borer (EAB; Agrilus plannipennis), Stanley et al. note that scientists cannot simply cross most North American ash species with the Asian ash, F. mandshurica, because the two groups are sexually incompatible. Scientists have instead focused on trying to enhance the resistance to EAB that is apparently present in a small proportion of ash trees, called “lingering ash.” Scientists funded by USDA Forest Service have already devoted over 14 years to finding such lingering ash to be tested for resistance.

Testing these trees is not simple (see Stanley et al.). But scientists are overcoming some of the obstacles. They have shown that the capability of a few green ash (Fraxinus pennsylvanica) (less than 1%) to defend themselves from EAB attack is genetic. Genes determine the relative abundance of specific metabolites manufactured by the tree; high levels kill more beetle larvae. These trees’ tolerance is not immunity but it might be sufficient to allow the tree to survive and grow. The level of metabolites synthesized by succeeding generations of the tree can probably also be enhanced by breeding.

To restore ash it is necessary to propagate large numbers of clones and to root the resulting embryos. This has been challenging. Merkle et al. describe five years of efforts to develop techniques that allow in vitro propagation to speed up selection and breeding. These techniques will facilitate establishment of numerous groups of propagules with the genetic differences needed to accommodate the large geographic range of several ash trees. For example, the green ash range covers more than half the continental U.S. plus multiple Canadian provinces.

ʻōhiʻa on lava field, Hawaii Volcanoes National Park

‘Ōhi‘a (Metrosideros polymorpha)

‘Ohi‘a is the most widespread tree species on the Hawaiian Islands. It provides vitally important habitat for conservation of countless taxa of endemic birds, insects, and plants. It is also of great cultural importance for Native Hawaiians.

Luiz et al. review the tree species’ importance, the many threats to native Hawaiian forests, and a coalition’s efforts to counter the most recent – and alarming – threat, rapid ʻōhiʻa death (ROD).

Rapid ʻōhiʻa death is caused by two introduced species of in the genus Ceratocystis. C. lukuohia colonizes the tree’s sapwood and kills the tree quickly. This disease is present on two islands, Hawai`i and Kaua‘i. It has the potential to devastate ‘ohi‘a forests across the state. The other pathogen, C. huliohia, invades the phloem, cambium, and outer xylem, resulting in a well-defined area of necrotic tissue and slower mortality. This disease is on Hawai`i and Kaua‘i, plus Maui and O‘ahu. The two pathogens have different origins. C. lukuohia belongs to a genetic line that is based in Latin America, C. huliohia to a genetic line based in Asia and Australia.

Conservationists formed a coalition and developed a strategy to guide the process of identifying and developing disease resistance in M. polymorpha and, if possible, other Metrosideros species on the Islands. Luiz et al. describe the coalition’s many activities. The challenges are familiar ones:

obtaining sufficient facilities to screen large numbers of seedlings;
developing techniques for inoculation, propagation, and speeding up growth of seedlings;
improving techniques for detecting individual infected and healthy trees across difficult terrain;
testing trees native to all parts of the tree’s range, which is not large in area, but covers a great variety of elevations and climates); and
needing to develop trees resistant to both C. lukuohia and C. huliohia.

Luiz et al. reiterate the necessity to manage all threats to healthy ʻōhiʻa stands, for example, by

curtailing human spead of infected wood, using both quarantines and supportive public education;
testing repellants to reduce beetle attack.
reducing injuries to trees by fencing forests and removing feral ungulates. link to website?

SOURCES

Buggs, R.J.A. 2020. Changing perceptions of tree resistance research. Plants, People, Planet. 2020;2:2–4. https://doi.org/10.1002/ppp3.10089

Coleman, T.W., A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States. Journal of Integrated Pest Management. (2023) 14(1): 23; 1–17

Dudley, N.; Jones, T.; Gerber, K.; Ross-Davis, A.L.; Sniezko, R.A.; Cannon, P.; Dobbs, J. 2020. Establishment of a Genetically Diverse, Disease-Resistant Acacia koa A. Gray Seed Orchard in Kokee, Kauai: Early Growth, Form, and Survival. Forests 2020, 11, 1276 https://doi.org/10.3390/f11121276

Jacobs, D.F., R. Kasten Dumroese, A.N. Brennan, F.T. Campbell, A.O. Conrad, J.A. Delborne, et al. 2023. Reintroduction of at-risk forest tree species using biotech depends on regulatory policy, informed

by science and with public support. New Forests (2023) 54:587–604

https://doi.org/10.1007/s11056-023-09980-y

Luiz, B.C., C.P. Giardina, L.M. Keith, D.F. Jacobs, R.A. Sniezko, M.A. Hughes, J.B. Friday, P. Cannon, R. Hauff, K. Francisco, M.M. Chau, N. Dudley, A. Yeh, G. Asner, R.E. Martin, R. Perroy, B.J. Tucker, A. Evangelista, V. Fernandez, C. Martins-Keli.iho.omalu, K. Santos, R. Ohara. 2023. A framework for establishing a rapid ‘Ohi‘a death resistance program. New Forests https://doi.org/10.1007/s11056-021-09896-5

Martín, J.A., J. Domínguez, A. Solla, C.M. Brasier, J.F. Webber, A. Santini, C. Martínez-Arias, L. Bernier, L. Gil1. 2023. Complexities underlying the breeding and deployment of Dutch elm disease resistant elms. New Forests https://doi.org/10.1007/s11056-021-09865-y

Merkle, S.A., J.L. Koch, A.R. Tull, J.E. Dassow, D.W. Carey, B.F. Barnes, M.W.M. Richins, P.M. Montello, K.R. Eidle, L.T. House, D.A. Herms and K.J.K. Gandhi. 2023. Application of somatic embryogenesis for development of emerald ash borer-resistant white ash and green ash varietals. New Forests https://doi.org/10.1007/s11056-022-09903-2

Nelson, C.D. 2023. Tree breeding, a necessary complement to genetic engineering. New Forests

https://doi.org/10.1007/s11056-022-09931-z

Pike, C.C., J. Koch, C.D. Nelson. 2021. Breeding for Resistance to Tree Pests: Successes, Challenges, and a Guide to the Future. Journal of Forestry, Volume 119, Issue 1, January 2021, Pages 96–105, https://doi.org/10.1093/jofore/fvaa049

Sniezko, R.A., J. Koch, J-J. Liu and J. Romero-Severson. 2023. Will Genomic Info Facilitate Forest Tree Breeding for Disease and Pest Resistance? Forests 2023, 14, 2382.

https://doi.org/10.3390/f14122382

Sniezko, R.A. and C.D. Nelson. 2022. Chapter 10, Resistance breeding against tree pathogens. In Asiegbu and Kovalchuk, editors. Forest Microbiology Volume 2: Forest Tree Health; 1^st Edition. Elsevier

Stanley, R.K., Carey, D.W., Mason, M.E., Doran, A., Wolf, J., Otoo, K.O., Poland, T.M., Koch, J.L., Jones, A.D. and Romero-Severson, J. 2023. Emerald ash borer (Agrilus planipennis) infestation bioassays and metabolic profiles of green ash (Fraxinus pennsylvanica) provide evidence for an induced host defensive response to larval infestation. Front. For. Glob. Change 6:1166421. doi: 10.3389/ffgc.2023.1166421

Posted by Faith Campbell

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

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

www.fadingforests.org

Eastern National Parks: Forest Regeneration Failing in 69%

Gettysburg battlefield; now under attack by emerald ash borer (see below)

Kathryn Miller and colleagues (full citation at end of blog) have published a study that examined the status and trends of forest regeneration in 39 National parks from Virginia to Maine. Four-fifths of the forest plots in the study are classified as mature or late successional – so at first glance the forests look healthy. However, the researchers made an alarming finding: in 27 of 39 parks, forest regeneration is failing – either imminently or probably. Acadia National Park is an exception; it is the only park in the study experiencing healthy regeneration. They warn that without intense, sustained – and expensive! – intervention, these forests are likely to be converted to other types of ecosystems. [I blogged recently about findings regarding regeneration in eastern forests: here and here and here and here.

The forests’ understories have too few seedlings and – especially – saplings to maintain themselves. Worse, in many cases the seedlings and saplings are not the same species as the mature trees that form the canopy. The saplings are shorter species that never reach the canopy. That is, species like pawpaw (Asimina triloba), American holly (Ilex opaca), American hornbeam (Carpinus caroliniana), and eastern redbud (Cercis canadensis) are regenerating, rather than the oaks (Quercus spp.), hickories (Carya spp.), maples (Acer spp.), and pines (Pinus spp.) that constitute the canopies of mature forests in these parks.

Miller and colleagues call these “regeneration mismatches.” In about half of the parks, these native canopy tree species make up less than half of current saplings and seedlings. This situation suggests the forests’ species composition will shift substantially, thereby undermining resilience in the face of other challenges, such as invasive plants and pests and climate change.

In many of these National parks, Miller and colleagues found abundant ash regeneration. For example, ash (Fraxinus spp.) constitute more than half of all seedlings in four parks (Johnstown Flood and Friendship Hill in Pennsylvania; Catoctin Mountain in Maryland; Manassas Battlefield in Virginia). Miller and colleagues consigned ash species to the “subcanopy class” because the emerald ash borer (EAB) has caused such high mortality of mature trees. They think regard it unlikely that current and future seedlings will ever reach full size. The devastating impact is most starkly illustrated in Gettysburg National Battlefield Park. Consistent deer management since 1996 has been rewarded: the Park ranks at the top for regeneration among the 39 parks. However, more than half of the seedlings and a quarter of the saplings are ashes. EAB has shifted the Park’s otherwise secure regeneration status into probable failure.

When regeneration fails: too many deer

Throughout the study region, the overwhelming reason regeneration fails is browsing by overabundant deer. The level of deer browse is considered “acceptable” in only four parks. Deer suppress the number of seedlings and saplings. They also skew species composition of native subcanopy species toward those less palatable. Miller and colleagues found that canopy tree density and cover and past human land use had minimal impacts on seedling and sapling numbers or species composition.

Overabundant deer also promote invasion and spread of non-native plants, which are the second most important factor impeding regeneration. Together, invasive plants and non-native earthworms are ecosystem engineers that negatively impact soil and cause cascades of biotic and abiotic impacts throughout forest ecosystems.

Many of the parks experiencing the most severe impacts of chronic deer browse also have the highest invasions by non-native plants. A natural process of regeneration occurs when the death or collapse of mature trees create gaps in the forest canopy. Where deer and invasive shrubs overlap, this process is often hijacked. Instead of nearby native tree species accelerating their growth toward the canopy, thickets of invasive shrubs crowd the space.

For this reason, Miller and colleagues recommend that park management prioritize treating invasive plants in canopy gaps of disturbed stands to avoid forest loss. They recommend deliberate creation of canopy gaps to promote resilience only for parks, or stands within parks, that have low deer and invasive plant abundance or the capacity to intensively manage invasive plants in gaps.

In most parks, non-native tree species are rare, less than 2% of total regeneration. In seven parks, though, non-native trees exceed ten percent of seedlings and/or saplings. In three parks, saplings of non-native trees are increasing. These are primarily tree-of-heaven (Ailanthus altissima) and Norway maple (Acer platanoides). In Saratoga National Historical Park, seedlings of common buckthorn (Rhamnus cathartica) are increasing.

Beech regeneration in Prince William Forest Park

Role of other pests

Miller and colleagues express fear that beech bark disease and beech leaf disease might have effects similar to those of EAB, leading to a greater “regeneration debt” in parks where American beech (Fagus grandifolia) is the dominant regeneration component. They cite specifically Prince William Forest Park in northern Virginia, [25 mi²] Rock Creek Park in the District of Columbia, [2.7mi²] and Saratoga National Historical Park. [5.3 mi²] The authors also suggest that thickets of beech root sprouts formed in response to BBD can suppress regeneration of other native canopy species and so might need to be managed.

Miller and colleagues mention hemlock woolly adelgid (HWA), but provide very little information. They report that Saint-Gaudens National Historical Park in New Hampshire (the home and studio of sculptor Augustus Saint-Gaudens) is at particular risk because of growth of both beech and eastern hemlock (Tsuga canadensis). I know that Delaware Water Gap National Recreation Area [109m²] has experienced major losses of mature hemlocks. [Shenandoah National Park has also, but it was not included in the study.]

Hemlock Ravine, Delaware Water Gap National Recreation Area; photo by Nicholas T via Flickr

Miller and colleagues report that Acadia National Park is seeing recovery of red spruce (Picea rubens) from a major fire in 1947 and possibly also from acid rain. They do not mention the longer-term threat from the brown spruce longhorned beetle. Their focus is on forest dynamics largely unaffected by deer.

In the same way, the authors make no mention of the absence of dogwood trees, presumably because they had been eliminated by dogwood anthracnose decades ago. Nor do they mention vascular streak dieback of redbud; the causal agent still uncertain. [See Annie Self’s presentation to National Plant Board, August 2023.]

dead ash tree in Shenandoah National Park

One omission is large enough that it might affect the study’s findings. At mi² Shenandoah is the largest National Park in the region. It was not included in the study because the Park’s forest monitoring process is not compatible with those in other NPS units. All the other parks – including Acadia (56² mi) – are much smaller, protecting historic sites like Civil War battlefields.

RECOMMENDATIONS

Miller and colleagues recommend that deer management be initiated in parks classified as at imminent or probable regeneration failure, if such programs are not already under way. They warn that effective deer management requires sustained commitment. Studies of deer exclosures show that full forest recovery from chronic deer overabundance can take as long as 40–70 years.

The authors also recommend actions to open the subcanopy to facilitate growth of saplings belonging to desired species. They caution that deer predation must be controlled. Furthermore, either invasive plant cover must be low, or management must ensure that that the park has sufficient resources to sustain an invasive plant control program – especially if invasive plants are combined with abundant deer.

Parks experiencing compositional mismatches and that are dominated by oak–hickory forest types might also benefit from prescribed burning. Again, deer browse pressure must be minimized. In addition, regeneration of oaks and hickories must already be present.

In park forests dominated by species vulnerable to lethal pests, e.g., beech-, ash-, or hemlock-dominated forest stands, Miller and colleagues recommend considering planting alternative native canopy species and protecting those plantings from deer. Park managers should also consider thinning beech thickets formed after beech bark disease kills canopy trees.

Media coverage

The Washington, D.C., public radio station, WAMU, reported on this research on the air (broadcast December 20) and on its website. It is written by Jacob Fenston, with great photographs by Tyrone Turner. The story emphasized the link between deer and invasive plants – since regeneration in eastern deciduous forest happens by saplings taking advantage of gaps formed when mature trees die. The story quotes DC-area people on their efforts to contain vines. The Natural Resource Manager at Catoctin Mountain Park [8 mi²] describes that park’s longstanding deer control program. The story also mentions impacts of EAB and threat of BLD.

News – Funding for these parks to counter the threats!

Lead author Kathryn Miller has informed me that the Bipartisan Infrastructure Law and Inflation Reduction Act has provided the 39 parks involved in this study over $10 million to improve forest resilience largely through reduction of invasive plants and overabundant deer.

Of course, invasive species threats to National parks are not limited to the Northeast – nor are they new. I have raised this problem from the beginning. To see these blogs, on the “nivemnic” website, scroll down below the archives to the “categories”, then click on “national parks”.

SOURCE

Miller, K.M., S.J. Perles, J.P. Schmit, E.R. Matthews, M.R. Marshall. 2023. Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks. Ecological Applications. 2023;33:e2837. https://onlinelibrary.wiley.com/r/eap Open Access

Posted by Faith Campbell

www.fadingforests.org

Succession: “novel drivers” change the trajectory

hardwood regeneration in northern Virginia forest; photo F.T. Campbell

I have posted several blogs recently about tree species’ regeneration. One blog found poor regeneration of many species throughout forests of the eastern United States. Regeneration is particularly poor in the Great Lakes region, western New York and Pennsylvania, along the Mid-Atlantic and New England coasts, and the coastal plain from southern South Carolina to eastern Texas.

A second blog focused on forest succession in New Hampshire. These findings, by Ducey et al., explicitly recognized the impact of non-native tree-killing insects and pathogens. A third article (Payne and Peet, 2023; full citation at the end of this blog) reports similar findings in North Carolina – and explicitly says that the same conditions are found in forests across the eastern United States.

The locations of neither in-depth study – New Hampshire or North Carolina – include those identified by Potter and Riitters (2022) as suffering particularly poor regeneration.

Payne and Peet find that forest succession in the Piedmont region of North Carolina is not proceeding as expected, based on earlier studies conducted in the same region. The differences are apparent at both the canopy and understory levels. Especially notable is the low recruitment of oaks (Quercus species) and hickories (Carya species) – the genera which previous studies indicated would be the climax taxa. One explanation is the disappearance since early in the 20^th Century of fire as a driver of disturbance.

The understory communities are also novel, due largely to invasive species: dramatic loss of flowering dogwood (Cornus florida) killed by the non-native pathogen dogwood anthracnose (Discula destructiva), plus overcrowding of the shrub level by invasive plant species. Other drivers are probably suppression of growth of woody species caused by excessive deer herbivory, and overall accelerated shifts in successional trajectory due to hurricane damage.

flowering dogwood autumn display; F.T. Campbell

Forests in eastern North America in the 21^st Century face several drivers of change that are either novel or greatly heightened. In addition to the disappearance of chronic fire, these are frequency and timing of hurricanes, feeding by herbivore populations, and introduction of non-native tree-killing pests and plants. Payne and Peet say scientists and managers need to consider these additional drivers – and their interactions! – when anticipating successional change.

Like Ducey et al. in New Hampshire, Payne and Peet used 80 years of data from 33 permanent plots established and 55 years of data from another 3 plots. Twenty-eight of the plots are transitioning from loblolly pine (Pinus taeda) to hardwood dominance; eight plots have been mixed-age hardwood stands since before the study plots were established.

In the North Carolina piedmont, the composition of canopy trees in plots evolving from pine compared to hardwood stands continue to be different 90–120 years after succession began. Canopy trees in upland and bottomland hardwood stands also differ. These differences reflect the relative species in the forest at the initiation of succession dynamics. Hurricanes – especially Hurricane Fran in 1996 – apparently accelerated succession in some plots by toppling the oldest pines. Despite the persistent differences, the species compositions of both canopy and subcanopy layers are trending toward increasing similarity.

deer-damaged red maple; photo by Eli Sagor via Flickr

The impact of deer browsing is complicated. Deer populations in the study area quadrupled after measurement began in 1980. Deer herbivory suppressed growth of all plant species when their stems were thin (3 – 10 cm DBH). However, after 1996 rapid growth of plants in openings caused by Hurricane Fran’s passage began to reverse the effects of deer browsing. Also, while deer browsing decreases regeneration, growth, and abundance of oak and hickory seedlings and saplings, it also decreases the abundance of other tree species that have – nevertheless – increased in abundance, e.g., red maple (A. rubrum) and black cherry(Prunus serotina).

Payne and Peet found that soil attributes (wetness, texture, organic matter and chemical components), as well as topographic position were minor factors in determining succession trajectories. Increased light availability due to the new or exacerbated drivers of change (thinning of understory vegetation by disease and deer herbivory and opening of the canopy by hurricanes) overcame the influence of nutrients. At most, a unique soil condition might constraining the impacts of these disturbances. Furthermore, these soil-related conditions and other environmental variables change through time — and as a result so does the vegetation. Specifically, the conditions that once supported establishment of oaks and hickories apparently differ today. Payne and Peet conclude that other drivers might be continuing to impact these species’ maturation.

A partial exception is soil nitrogen, through its influence on mycorrhizal patterns. I review mycorrhizal patterns in the discussion of individual tree species, below.

How are Individual Tree Species Responding?

Oaks and hickories are not expanding as expected – either as canopy-sized trees or as seedlings / saplings in the understory. Payne and Peet agree that century-long suppression of low-intensity ground fires is probably the most significant factor in this compositional shift. This decline has been exacerbated by selective logging and deer herbivory. Hickories have established more widely, possibly because young stems have greater shade tolerance. Only plots located on sandy and acidic soils and plots with the greatest hurricane damage have moderate recruitment of oaks and hickories. Oaks and hickories on the poor soils might be aided by the types of ectomycorrhizal fungi that survive in acidic soils with relatively low nitrogen levels. In addition, these soils’ lower water retention probably impedes competition by more mesic, faster-growing, shade-tolerant species. However, even oaks and hickories that have established as seedlings or saplings only rarely progress to canopy dominance. Payne and Peet conclude that oaks might have lost competitive advantage in many of the undisturbed stands.

More mesophytic hardwoods, especially red maple (Acer rubrum), are becoming more numerous and larger – a trend seen throughout forests of the eastern United States. Damage from Hurricane Fran apparently accelerated this trend. However, red maple growth is significantly inhibited by competition from thicket-forming shrubs, especially in bottomland plots. The invasive non-native species thorny olive or oleaster Elaeagnus pungens increased dramatically following Hurricane Fran in 1996. The situation is likely to worsen: two other invasive species, Amur honeysuckle Lonicera maackii and privet Ligustrum japonicum were first detected in the Duke Forest plots in the 2013 survey.

[In New Hampshire, Ducey et al. detected an unexpected levelling off of red maple increases and decline in sugar maple (Acer saccharum); they were unable to determine a cause.]

beech-dominated understory in northern Virginia; F.T. Campbell

Another mesophytic hardwood – American beech (Fagus grandifolia) – has become very abundant in bottomland hardwood stands, especially in small-stem size classes in the understory. Beech prefers sandy soils and its ectomycorrhizal associations are apparently more tolerant of more acidic soils.

Payne and Peet mention – briefly and vaguely – uncertainty about the future of beech. The reference cited discusses the impact of beech bark disease (BBD) in the northeast. Range maps indicate that BBD is well established in the southern Appalachians along the North Carolina/Tennessee border; it has apparently not spread as far east as the study area. There is no mention of beech leaf disease (BLD), which is the primary threat to seedlings and saplings. BLD is currently known to be in northern Virginia. It is unknown whether the disease has any climatic or other barrier that would prevent its moving farther south.

Another bottomland indicator taxon that is also increasing in abundance is ash (Fraxinus species). Along with sweetgum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera) and black cherry Prunus serotina, ash density and basal area increased dramatically in plots heavily damaged by Hurricane Fran. Payne and Peet expect most ash trees to be killed by emerald ash borer (Agrilus planipennis) by 2022. The beetle was detected in the study area in 2015.

ash killed by EAB on Potomac lowlands; F.T. Campbell

Flowering dogwood (Cornus florida)was one of the most abundant understory species throughout the study area until the late 1980s. The species has declined by more than 80% since then due to the non-native disease dogwood anthracnose (Discula destructiva). No other species has experienced as precipitous a decline. There is now almost no regeneration in most upland sites.

A second species almost eradicated from the study area by a non-native pathogen is American elm (Ulmus americana). Its basal area in 2013 was 5% of peak levels in the 1950s. Most of this loss occurred by the 1960s, shortly after arrived of Dutch elm disease (DED) in North Carolina. A congeneric species, slippery elm U. alata, is reported to beabundant; it is somewhat resistant to DED. There is no mention of the zig-zag sawfly (Aproceros leucopoda) which has been detected in North Carolina, a few counties away from the study area. The foliage-feeding insect’s long-term impact on elm species is not yet understood.

Payne and Peet note that the study area has twice experienced loss of important components due to specialist non-native pathogens: elms and dogwoods. A third similar event looms: ash [The article does not discuss prospects for biological control.] A fourth is less certain: beech. [This numbering assumes that American chestnut and eastern hemlock were not significant components of forests in the study area.] In their view, these events demonstrate the drastic impacts such non-native organisms can have, especially when the host species is highly abundant or otherwise dominant in a specific community. The resulting shifts in community dynamics and modifications to light and water availability due to such losses, can be dramatic and long-lasting, even resulting in novel successional trajectories.

Members of the 23rd Civil Engineer Squadron/23rd Wing chainsaw a tree lying across a street in the NCO housing area- damage to piedmont North Carolina by **Hurricane Fran**. Photo courtesy of U.S. National Archives.

Payne and Peet also emphasize the impact of large, episodic disturbances (in their case, hurricanes). These can have widespread and long-lasting impacts on plant community dynamics. Hurricanes’ frequency, intensity, and timing relative to successional stage are key in determining their impacts on successional trajectories. E.g., strong storms that felled the even-aged pine canopy accelerated succession toward more mixed hardwoods. These changes affect biomass, diversity, competitive dynamics, and invasion by invasive plant species, especially in sites with advantageous soil conditions.

Scientists must also evaluate interactions (both reinforcing and antagonistic) between these drivers. For example, in this study deer herbivory and damage from episodic storms had opposite effects on the density of stems in the understory and therefore the future dynamics of forested stands. Hurricane aftereffects frequently accelerated existing or developing trends resulting from various other drivers (e.g., loss of dogwood to anthracnose disease). [While Ducey et al. also detected lasting impacts from hurricane damage in New Hampshire, these effects did not include changes in tree species composition.] Broader regional and global drivers of change, especially those associated with climate change and nitrogen deposition, interact with these many indicators in novel ways based on their own local loadings.

The Nature Conservancy focuses on fire

The Nature Conservancy magazine for Winter 2023 carries an article describing the organization’s experimental efforts to promote oak succession in the Piedmont forests of North Carolina. Greg Cooper, TNC’s forest ecologist in North Carolina, describes retaining dominance by oaks and hickories – rather than maples and poplars – as vital to protecting the region’s faunal diversity and minimizing impacts from climate change. He says this is because oaks use a quarter of the water of maples and poplars.

Cooper links oaks’ failure to reproduce on fire suppression. TNC kills midstory maples and poplars through hack and squirt methods. This allows more light to penetrate the forest and foster oak seedling recruitment. Then they apply controlled fire. “We currently have 700 acres of [controlled-] burn plots, some of which have been burned twice, some of which have been burned once, [and already] we’re getting more light and an immediate flush of herbaceous diversity. We’re getting a lot more berry species, more wildflowers.” TNC is monitoring plots that have been burned, with and without the pre-burn herbicide treatments, and those that have not been burned. They hope to have results in five to ten years that will indicate whether they are achieving the desired improvement in oak regeneration. If so, they also hope is that in future prescribed burns will be sufficient.

Cooper adds that through the Fire Learning Network and a 23-person fire crew they carry out similar work not just on TNC properties, but also federal and state properties.

SOURCES

Ducey, M.J, O.L., Yamasaki, M. Belair, E.P., Leak, W.B. 2023. Eight decades of compositional change in a managed northern hardwood landscape. Forest Ecosystems 10 (2023) 100121

Payne, C.J. and R.K. Peet. 2023. Revisiting the model system for forest succession: Eighty years of resampling Piedmont forests reveals need for an improved suite of indicators of successional change. Ecological Indicators 154 (2023) 110679

Posted by Faith Campbell

www.fadingforests.org

Imports from Asia rise; perhaps 2,000 or more containers carrying insect pests enter U.S. in one month

Wood packaging – crates, pallets, spools for wire, etc. — has been recognized as a major pathway for introduction of tree-killing pests since the Asian longhorned beetle was detected in New York and Chicago in the late 1990s. As of 2021, 65 new species of non-native wood- or bark-boring Scolytinae had been detected in the United States (Rabaglia; full citation at end of the blog).

As I have often reported [To see my 40+ earlier blogs about wood packaging material, scroll down below archives to “Categories,” click on “wood packaging”.], the international phytosanitary community adopted the International Standard for Phytosanitary Measures (ISPM) #15. The goal of ISPM#15 is to “significantly reduce” [not eliminate] the risk of pests associated with solid wood used for constructing packaging (e.g., crates, pallets), from being introduced to other countries through international trade.

I recently reviewed the first 20+ years of implementation of ISPM#15 including two analyses by Robert Haack and colleagues in a blog in December 2022. I have also provided the broader context of the World Trade Organization (WTO) in my Fading Forests II report.

I last blogged about U.S. import volumes in June. My silence since reflected the significant decline in U.S. imports from Asia. This reduction had reduced the likelihood that a new tree-killing pest would be introduced from that region – or that an already-established pest would be introduced to a U.S. region that had escaped it so far.

However, U.S. imports from Asia have suddenly grown! In October 2023, containerized imports from Asia were 12.4% higher than a year ago – and 6% higher than in September. According to the Journal of Commerce (full citation at end of blog), U.S. retailers anticipate consumers will purchase lots of gifts for the upcoming Christmas season.

The U.S. imported 1.57 million TEU from Asia in October. This volume exceeded even the pre-COVID levels. How great is the associated risk of a pest introduction? To calculate that, I apply the following:

most U.S. imports arrive in 40-foot-long containers, so divide TEU by 2 = 785,000
a decade-old estimate that 75% of containers in maritime shipments contain wood packaging (Meissner et al.) = 588,750 containers with wood packaging (I suspect it is more).
the estimate by Haack et al. 2014 that 0.1% (1/10^th of 1 percent) of consignments (which usually means a single container) harbor tree-killing pests;
the estimate by Haack et al. 2022 that 0.22% of consignments harbor tree-killing pests.

The result of these calculations is an estimate of 648 containers (using the 2009 global estimate), or 1,727 containers (using the 2022 global estimate), or 5,730 containers (using the 2010-2020 estimate for China specifically) entering the country in one month harbored tree-killing pests. Since West Coast ports received 54% of those containers, the estimated number of containers transporting pests that enter California, Washington, or Oregon ranged from 349 to 3,042. The rest are scattered among the dozens of ports on the East and Gulf coasts.

With drought limiting container ship transits through the Panama Canal (Szakonyi 2023), the threat to East and Gulf coast ports might not rise commensurately.

Because of the low levels of imports in previous months, U.S. imports from Asia remain significantly below levels in previous years: 16.6% lower for the January – September period compared to 2022.

The 2022 analysis found that the rate of wood packaging from China that is infested has remained relatively steady since 2003: 1.26% during 2003–2004, and ranged from 0.58 to 1.11% during the next three time periods analyzed. Packaging from China made up 4.6% of all shipments inspected, but 22% of the 180 consignments with infested wood packaging. Thus the proportion of Chinese consignments with infested wood is five times greater than would be expected based on their proportion of imports. Note the great impact of this high infestation rate on the number of containers transporting tree-killing pests to the U.S. in the paragraph above: more than 8,000 containers compared to about 2,000.

I remind you that the U.S. and Canada have required treatment of wood packaging from China since December 1998. Why are the responsible agencies in the United States not taking action to correct this problem? [which has persisted for 2 decades]

The fact is – as I have argued numerous times — a pallet or crate bearing the ISPM#15 mark has not proved to be a reliable indicator as to whether the wood is pest-free. (This might be because the wood had not been treated, or if it was, the treatment failed). All the pests detected in the Haack et al. studies (after 2006) were in wood packaging bearing the ISPM#15 mark. As noted in my past blogs [click on the “wood packaging” category to bring up blogs about wood packaging and enforcement], Customs and Border Protection also report that nearly all the wood packaging in which that they detected insect pests bore the ISPM#15 mark.

According to Angell in November (full citation at end of blog), U.S. imports from India to the east coast fell by 15% in the first 10 months of 2023 compared to last year – to a total of 623,356 TEUs. This might change in the future: a shipper has promised to start weekly arrivals from India beginning in May 2024. the company plans calls at New York-New Jersey, Savannah, Jacksonville, Charleston, and Norfolk. The ships will call, en route, at ports in Saudi Arabia, Egypt, and Spain. What pests might be hitching a ride on these shipments?

SOURCES

Haack RA, Britton KO, Brockerhoff EG, Cavey JF, Garrett LJ, et al. 2014. Effectiveness of the International Phytosanitary Standard ISPM No. 15 on reducing wood borer infestation rates in wood packaging material entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Haack RA, Hardin JA, Caton BP and Petrice TR. 2022. Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Frontiers in Forests and Global Change 5:1069117. doi: 10.3389/ffgc.2022.1069117

Meissner, H., A. Lemay, C. Bertone, K. Schwartzburg, L. Ferguson, L. Newton. 2009. Evaluation of pathways for exotic plant pest movement into and within the greater Caribbean Region.

Mongelluzzo, B. 2023. U.S. imports from Asia hit 2023 high in October despite muted peak season. Journal of Commerce https://www.joc.com/article/us-imports-asia-hit-2023-high-october-despite-muted-peak-season_20231116.html (access limited to subscribers, unfortunately)

Angell, M. 2023. ONE readies India-US East Coast service as part of 2024 network rollout. Journal of Commerce. November 27, 2023

Rabaglia, R. 2021. The increasing number of non-native bark and ambrosia beetles in North America. International Union of Forest Research Organizations. Prague, Czech Republic. September 2021

Szakonyi, M. 2023. Carriers Weigh Options as Panama Canal restrictions become fact of life. Journal of Commerce. November 21, 2023. (Access limited to subscribers, unfortunately)

Posted by Faith Campbell

www.fadingforests.org

USFS Forest Health Protection program: what it funds

affects of mountan pine beetle on lodgepole pine in Rocky Mountain National Park, Colorado photo from Wikimedia; one of pests addressed by USFS FHP

Several USFS scientists have published an assessment of the agency’s program to enhance forest health across the country: the Forest Health (FHP) program. [see Coleman et al., full citation at end of this blog.] The program assists cooperators (including other federal agencies) to prevent, suppress, and eradicate insect and pathogen outbreaks affecting trees, regardless of land ownership.

Each year, I advocate for adequate funding for the FHP program — which comes from annual Congressional appropriations. Funding has remained static at about $100 million per year. I interpret the article as providing support for my call for increased appropriations. First, it reports that the number of projects and extent of area treated have declined from 2011 to 2020. This is because static funding levels are stretched increasingly thin as costs to implement the same activities rise. Second, the program does not address many damaging forest pests already in the country. The result is growth of established threats to forest health. Finally, new insects and pathogens continue to be introduced. Protecting forest health necessitates tackling these new pests – and that requires money and staff.

Coleman et al. analyzed data from the decade 2011- 2020 to determine the most frequently used project types, integrated pest management (IPM) strategies and tactics, dominant forest pests and associated hosts managed, and most comprehensive forest IPM programs in practice. While there is a wide range of possible projects, most of those funded consist of some form of treatment (more below). The databases relied on do not include funding through the National Forest System aimed at improving forest health through such management activities as stand thinning treatments and prescribed fire. Nor are all pest management activities recorded in the centralized databases. I regret especially the fact that “genetic control” (= resistance breeding) are left out.

Port-Orford cedar seedlings in trial for resistance to *Phytophthora lateralis* at Dorena center; photo courtesy of Richard Sniezko, USFS

Summary of Findings

The data are sorted in various categories, depending on whether one wishes to focus on the type of organism being managed or the management approach. All presentations make evident a dramatic imbalance in the projects funded. Again and again, spongy moth (Lymantria dispar dispar), southern pine beetle (SPB, Dendroctonus frontalis), and several bark beetles attacking conifers in the West (in particular mountain pine beetle, [MPB] Dendroctonus ponderosae) dominate, as measured by both funding and area treated.

oak trees in Shenandoah National Park killed by spongy moth; photo by F.T. Campbell

The bulk of the funding went to the above species, plus hemlock woolly adelgid (HWA; Adelges tsugae); emerald ash borer (EAB, Agrilus planipennis), oak wilt (caused by Bretziella fagacearum), and white pine blister rust (WPBR, Cronartium ribicola).
95% of the projects focused on only four taxa: oaks, Quercus spp. [spongy moth suppression and eradication]; loblolly and ponderosa pines [bark beetle prevention and suppression]; and eastern hemlock [HWA suppression].
Projects seeking to suppress an existing pest outbreak covered 87% of the total treatment area. However, 98% of the treated area was linked to only 20 taxa; again, spongy moth dominated.
Projects seeking to prevent introduction or spread of a pest constituted only 30% of all projects and covered only 11% of the total treatment area.
Eradication and restoration projects each equaled less than 5% of total projects and treatment areas.
Native forest pests were targetted by 79% of projects; non-native pests by 21%. However, non-native pests accounted for 84% of the total treatment area (again, the spongy moth).
While 67% of projects took place on USFS lands (focused on MPB and SPB), 89% of the total treatment area was on lands managed by others (state or other federal agencies, or private landowners). Again, the size of the non-USFS area treated was driven primarily by the spongy moth Slow the Spread program.
Insect pests received nearly all of the funding: 70% of funding targetted phloem-feeding insects, especially SPB and MPB; 10% targetted foliage feeders, especially spongy moth; 6% targetted sap feeders. 4% tackled rusts (e.g., WPBR); just 2% addressed wood borers (e.g., Asian longhorned beetle, emerald ash borer).
The ranking by size of area treated differs. In this case, 82% of areas treated face damage by foliage feeders (e.g., spongy moth); 15% of the treated areas are threatened by phloem feeders (e.g., MPB); only 1.4% of the area is damaged by sap feeders (e.g., HWA); 0.6% is threatened by rust; and 0.2% by wood borers.
Re: control strategies, 32% of projects relied on silvicultural strategies; 22% used semiochemical strategies; 21% exploited other chemical controls; and 18% used physical/mechanical control methods.

Coleman et al. regretted that few programs incorporated microbial/biopesticide control strategies; these were applied on only 10% of total treated area. Again, the vast majority of such projects were aerial applications of spongy moth controls, Bacillus thuringiensis var. kurstaki (Btk) and nucleopolyhedrosis viruses (NPV) (Gypchek). Coleman et al. called for more research to support this approach efforts to overcome other obstacles (see below).

Coleman et al. also called for better record-keeping to enable analysis of genetic control/ resistance breeding projects, treatment efficacy, and survey and technical assistance activities.

History

The article provides a brief summary of the history of the Forest Service’ pest management efforts. Before the 1960s, the USFS relied on labor-intensive physical control tactics, classical biocontrol, and widespread chemical applications. Examples include application of pesticides to suppress or eradicate spongy moth; decades of Ribes removal to curtail spread of white pine blister rust; salvage logging and chemical controls to counter phloem feeders / bark beetles in the South and West. These strategies were increasingly replaced by pest-specific management tactics during the 1970s.

Over the decade studied (2011-2020), tree defoliation attributed to various pests (including pathogens) affected an estimated 0.7% of the 333 million ha of U.S. forest land annually. Mortality attributed to pests impacted an estimated 0.8% of that forest annually. See Table 1. Two-thirds of the area affected by tree mortality is attributed to phloem feeders; a distant second agent is wood borers. These data are incomplete because many insects, diseases, and parasitic higher plants are not tracked by aerial surveys.

As I noted above, these data do not include projects that screen tree species to identify and evaluate genetic resistance to a pest; or efforts to collect cones, seed, and scion. I consider these gene conservation and resistance programs to be some of the most important pest-response efforts. I have blogged about the USFS’ Dorena Genetic Resource Center’ efforts to breed five-needle pines, Port-Orford cedar, and ash. link

41% of silvicultural control treatments targetted phloem feeders; 48% addressed cankers and rusts together. Restoration planting was done in response to invasions by ALB, EAB, and WPBR, as well as native bark beetles and mistletoes.

effort to eradicate SOD in southern Oregon; partially funded by USFS FHP. Photo courtesy of Oregon Department of Forestry

Physical/mechanical control projects were most widely applied in the Rocky Mountains in response particularly to diseases: vascular wilts, rusts, and cankers, including WPBR. This type of project was also used to deal with non-native diseases in other parts of the country, e.g., oak wilt, sudden oak death (SOD), Port-Orford cedar root rot, and rapid ʻōhiʻa death. Sanitation treatments (i.e., removal of infected/infested trees) was used for native mistletoes and root rots, and some non-native insects, e.g., EAB and coconut rhinoceros beetle (Oryctes rhinoceros). Pruning is a control strategy for WPBR. Trenching is applied solely to suppress oak wilt.

Chemical controls were limited to small areas. These projects targetted seed/cone/flower fruit feeders, foliage and shoot diseases, sap feeders [e.g., balsam woolly adelgid (BWA), HWA], wood borers (e.g., EAB) and phloem feeders (e.g., Dutch elm disease; DMF oak wilt vectors). Cover sprays have been used against goldspotted oak borer (GSOB); and many native insects. Fungicides are rarely used; some is applied against the oak wilt pathogen in areas inaccessible by heavy equipment.

treating hemlock trees in Conestee Falls, NC; photo courtesy of North Carolina Hemlock Restoration Initiative

Classical biocontrol projects funded by the program targetted almost exclusively HWA. Some 4.3 million predators have been released since the early 1990s; 820,057 in just the past 10 years.

Gene conservation and breeding projects were directed primary at commercially important hosts, e.g., loblolly Pinus taeda and slash pine P. elliottii; and several non-native pests, including chestnut blight, EAB, HWA, and WPBR.

Survey and technical assistance (i.e., indirectly funded activities) conducted by federal, state, and tribal personnel contributed to education/outreach, evaluating effectiveness, identification, monitoring, and record keeping strategies.

As should be evident from the data presented here, suppression treatments dominated by number of projects and treatment area. The poster child project is the national spongy moth Slow the Spread program. The authors say this program is the most advanced forest IPM program in the world. It has successfully slowed spongy moth’s rate of spread by more than 80% for more than 20 years.

A second widely-used subset of suppression programs consists of physical / mechanical control. This is often the principal suppression strategy in high-visitation sites (e.g., administration sites, campgrounds, picnic areas, and recreation areas). Sanitation harvests are one of the few viable management techniques for suppressing or slowing the spread of recently introduced non-native pests. Nevertheless, the largest number of suppression projects and use of sanitation treatments focused on a native pest, mountain pine beetle, at the height of its outbreak in early 2010s.

Silvicultural control, specifically tree thinning, represents the predominant forest pest prevention tactic, especially on lands managed by the USFS. Two programs dominate: the Southern Pine Beetle Prevention Program and the Western Bark Beetle Initiative. Again, Coleman et al. assess these treatments as very successful. Forest thinning treatments also address other management concerns, i.e., reduce threat of catastrophic wildfires and reduce adverse effects of climate change.

Chemical control tactics are applied to suppress most forest insect feeding guilds in high-value sites and seed orchards. Soil or tree injections of systemic pesticides are used to protect ash and hemlock trees. Topical sprays have been applied to protect whitebark pine (Pinus albicaulis) from mountain pine beetle. Whitebark pine was listed as threatened under the Endangered Species Act in December 2022.

dead whitebark pine at Crater Lake NP; photo by F.T. Campbell

Soil or tree injections target two non-native insects, EAB and HWA.

Genetic control via resistance breeding represents the primary strategy to combat several non-native diseases. (More options are typically available for insects than diseases.) Coleman et al. focus on the extensive effort to protect many of the five-needle pines from WPBR. As I have described in earlier blogs, the Dorena Genetic Resource Center in Oregon has engaged on numerous other species, too.

Coleman et al. describe pest-management associated monitoring efforts as consisting largely of coordinated annual aerial detection surveys, detection trapping, stream-baiting of Phytophthora ramorum, and ground surveys to address site-specific issues.

Coleman et al. call for improvement of record-keeping / databases to encompass all pests, management actions, and ownerships. They also advocate for additional decision-making tools, development of microbial/biopesticides, genetic research and breeding, and biocontrol strategies for several pest groups.

They consider the southern pine beetle and spongy moth programs to be models of comprehensive IPM programs that could be adapted to additional forest health threats. They note, however, that development and implementation of these programs require significant time, financial commitments, and collaborations from various supporting agencies. Not all programs enjoy such resources.

SOURCE

Coleman, T.W, A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States

Journal of Integrated Pest Management, (2023) 14(1): 23; 1–17

Posted by Faith Campbell

www.fadingforests.org

International Phytosanitary System: More Evidence of Failure

Rome: home of the International Plant Protection Convention

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

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

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

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

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

SOURCES

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

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

Posted by Faith Campbell

www.fadingforests.org

Predicting Impacts – Can We Do It?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Identifying Key Pathways

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

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

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

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

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

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

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

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

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

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

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

Gaps Despite Above Studies

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

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

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

SOURCES

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

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

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

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

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

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

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

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

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

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

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

Posted by Faith Campbell

www.fadingforests.org

Ash – Science Support Protection Efforts

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

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

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

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

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

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

EAB Will Eventually Kill Them Anyway

Answer:

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

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

Answer:

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

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

Myth: Using Systemic Insecticides to Protect Ash Trees Harms

Non-target Species and the Environment

Answer:

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

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

Myth: It Costs Too Much to Protect Ash Trees

Answer:

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

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

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

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

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

1. Define the problem and identify management objectives

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

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

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

downy woodpecker; photo by Steven Bellovin, Columbia University

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

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

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

SOURCE

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

Posted by Faith Campbell

www.fadingforests.org

What do “Self-Introduced” & “Door-Knocker” Species Tell Us?

*Woldstedtius flavolineatus* – one of at least 13 taxa of non-native ichneumonid wasps established in restoration forests in Hawaiian Forest National wildlife rfefuge; photo by Torgrim Breiehagen for the Norwegian Biodiversity Information Centre; via Wikipedia

As we know, non-native insects and pathogens pose a significant and accelerating threat to biodiversity in forests and other ecosystems. They undermine some conservation programs and reduce ecosystem services and quality of life in urban areas. Nevertheless, damaging introductions continue.

Two recent articles have advocated accelerating biocontrol programs. These articles have reminded us of ongoing failures of international and national biosecurity programs, including that of the US. The articles also make interesting suggestions regarding ways to be more pro-active in preventing introductions.

1. “Self-introductions” of invaders’ enemies

Weber et al. (full citation at end of blog) provide many examples of unintentional “self-introductions” of natural enemies of arthropod pests and invasive plants. In fact, “self-introductions” of natural enemies of arthropod pests might exceed the number of species introduced intentionally. These introductions have been facilitated by the usual factors: the general surge in international trade; lack of surveillance for species that are not associated with live plants or animals; inability to detect or intercept microorganisms; huge invasive host populations that allow rapid establishment of their accidentally introduced natural enemies; and lack of aggressive screening for pests already established.

Among the examples illustrating failures of biosecurity programs:

Across six global regions, nearly two-thirds of parasitoid Hymenoptera species were introduced unintentionally. The proportion varies significantly by region. For example, four-fifths of these insects in New Zealand arrived accidentally.
The unintentional spread of the glassy-winged sharpshooter (Homalodisca vitripennis) and a biocontrol agent Cosmocomoidea ashmeadi has been so rapid among islands in the Pacific Ocean (including Hawai`i) they are considered ‘biomarkers’ of biosecurity failures.
Regarding the United States specifically, an estimated 67% of beneficial insects introduced to Hawai`i and 64% of parasitoid Hymenoptera introduced to the mainland U.S. were accidental “self-introductions.”

Weber et al. consider their figures to be underestimates. The situation is particularly uncertain regarding pathogens that kill arthropods. Many microbial species are not yet described.

spotted lanternfly; photo by Stephen Ausmus, USDA

In some cases, these “self-introduced” arthropods have proved beneficial. Two examples are Entomophaga maimaiga and Lymantria dispar nucleopolyhedrovirus (LdNPV), which help control the spongy moth (Lymantria dispar). In other cases the “self-introduced” creatures are pests themselves. A prominent example is the invasion by the spotted lanternfly (Lycorma delicatula). This was facilitated by the widespread presence of the highly invasive plant Ailanthus altissima. It illustrates what Weber et al. call “receptive bridgehead effects.” That is, once an invasive pest is well-established, the chance that its natural enemies will find a suitable host and also establish in the pest’s invaded range is much higher.

Weber et al. reaffirm that there are many good reasons not to allow such random invasions of diverse non-native species – including their natural enemies. Deliberately introduced biocontrol agents are chosen after determining their efficacy, host-specificity, and climatic suitability. Random introductions, on the other hand, might favor generalist species, which could threaten non-target species. Accidental introductions might also be accompanied by pathogens and hyperparasitoids that could compromise the efficacy of biocontrol agents.

In short, unintentionally introduced natural enemies might have about the same level of success in controlling the target pest’s populations as do intentionally introduced agents. However, unintentional introductions of both pests and pathogens carry additional risks of non-target impacts and contamination with their own natural enemies that would hamper the efficacy of the biocontrol agent. Weber et al. conclude that delays in releasing a deliberately chosen and evaluated biocontrol agent reduce the probability that it will successfully establish instead of an unintentionally introduced organism.

cactus moth larva on *Opuntia*; photo by Doug Beckers via Flickr

It is especially likely that an arthropod – whether or not a biocontrol agent – will spread within a geographic region. Weber et al. say both the U.S. and Canada have received more than a dozen species intentionally introduced into the other country. They also cite spread of the cactus moth, Cactoblastis cactorum, into Florida from several Caribbean countries. The cactus moth has spread and now threatens the center of diversity of flat-padded Opuntia cacti in the American southwest and Mexico.

Another example is California: 44% of invading terrestrial macroinvertebrates that have established in the state came from populations established elsewhere in the US and Canada (Hoddle 2023). This number exceeds the total number of invasive macroinvertebrates in the state that originated anywhere in Eurasia (Weber et al.).

True, it is very difficult to prevent natural spread. But a lot of this spread is facilitated by human activities, e.g., transporting vectors such as living plants, firewood, outdoor furniture or storage “pods.” I have complained often — here and here and here — that interstate movement of invasive plant pests is particularly poorly controlled.

Some scientists and regulators have responded to these situations by improving phytosanitary programs. California officials, in 2019, set up a program to fund projects aimed at developing integrated pest management strategies for species thought to have a high invasion potential before they arrive. I urge other states to do the same. This would probably be most effective in controlling the target species – and in relation to cost — if developed by regional consortia.

Weber et al. suggest that given continuing unintentional introductions of non-native species, phytosanitary agencies need to focus on those invasion pathways that are particularly likely to result in invasions, e.g. live plants, raw lumber (including wood packaging), and bulk commodities e.g. quarried rock.

The authors also suggest research opportunities that arise from biocontrol agents’ “self-introductions”. These include:

Comparing actual host ranges to those predicted by laboratory and other studies;
Quantifying the role of Allee effects, for example by studying the spread of the glassy-winged sharpshooter and its biocontrol agent across the Pacific region;
Using molecular analyses to disentangle multiple routes of entry (e.g., the “invasive bridgehead effect”) and hybridization.

2. Door-knocker species

Hoddle (2023) suggests further that early detection programs should focus on “door-knocker” species — those likely to enter and cause significant negative impacts. In an earlier article (Hoddle, Mace and Steggall 2018) argued that the benefits of a pro-active biocontrol program outweigh the costs. The authors say the information gained would cut the time needed to deploy effective biocontrol. Ultimately, this could reduce the prolonged and even irreversible ecological and economic disruption from invasive pests, associated pesticide applications, and lost ecological services.

Asian citrus psyllid (*Diaphorina citri*); USDA photo by Justin Wendell; Hoddle cites this species as one that a pro-active biocontrol program should have targetted

Hoddle calls funding pro-active biocontrol research programs before they’re needed as analogous to buying insurance. The owners of insurance policies hope not to need them but benefit when catastrophe strikes. Furthermore, the information gained from early research might identify natural enemy species that could “self-introduce” along with the invading host. Finally, proactive research might clarify whether the increasing number of natural enemy species that are “self-introducing” pose a threat to non-target organisms.

Recognizing the difficulty of identifying an “emerging invasive species” before its introduction, Hoddle endorses other components of prevention programs:

Collaborating with non-U.S. scientists to identify and mitigate invasion bridgeheads. Such efforts would both lessen bioinvasion threats and possibly aid in determining native ranges and facilitating location of natural enemies.
Sentinel plantings, such as those established under the International Plant Sentinel Network. These plantings can also support research on natural enemies of key pests.
Integrating online platforms, networks, professional meetings, and incursion monitoring programs into “horizon scans” for potential invasive species. He mentions specifically PestLens; online community science platforms, e.g., iNaturalist; international symposia; and official pest surveillance, e.g., U.S. Forest Service’s bark beetles survey and surveys done by the California Department of Food and Agriculture and border protection stations.

date palm mealybug (*Pseudaspidoproctus hyphaeniacus*); threat to native *Washingtonia* palms of California; one of pests tracked by PestLens

Weber et al. also support the concept of sentinel plant nurseries – especially because accidental plant and herbivore invasions often occur at the same points of entry.

Both Weber et al. and Hoddle urge authorities not to strengthen regulations governing biocontrol introductions. Weber et al. say that would be to make perfect the enemy of the good. The need is to balance solving problems with avoiding creation of new problems.

SOURCES

Hoddle, M.S., K. Mace, J. Steggall. 2018. Proactive biological control: A cost-effective management option for invasive pests. California Agriculture. Volume 72, No. 3

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

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

Posted by Faith Campbell

www.fadingforests.org

The White Mountain forest in New Hampshire: 80 years of change, with more ahead

I have been disappointed that a research symposium focused on the northern hardwood forest workshop gave little attention to non-native pests (see citation at end of this blog). A new study based in the Bartlett Experimental Forest in the White Mountains of New Hampshire is more balanced. Ducey et al. (full citation at the end of this blog) analyzed changes in the forest’s species composition and tree size over the past 80 years.

They found that trees of nearly all species are growing into larger sizes as the forest continues to age since the last widespread clearing at the end of the 19^th Century. The same aging is causing a rapid decline in two shade-intolerant species – paper birch (Betula papyrifera) and aspen (Populus tremuloides and P. grandidentata) – which had grown quickly once the cleared areas were abandoned. The mid-shade -tolerant species yellow birch (Betula alleghaniensis) also is declining. Together, the birch and aspen species have declined from a quarter to a third of basal area in 1931 to 10 – 12% in 2015.

Some developments are unexpected. Red maple (Acer rubrum) increased in abundance until the early 1990s, but that growth then levelled off. Sugar maple (Acer saccharum) has declined in abundance except where the forest is managed to retain it.

There is little evidence of tree species migrating upward on slopes in response to changes in the local climate. Major weather events – a hurricane in 1938 and an ice storm in 1998 — caused significant tree mortality across Bartlett Experimental Forest, but not a dramatic change in forest composition.

Eastern hemlock (Tsuga canadensis) is replacing the disappearing birch and aspen on low elevation sites. Hemlock has increased its proportion of basal area from 8 – 10% to a quarter or more. Despite aggressive management aimed at reducing the tree’s presence, American beech (Fagus grandifolia) is on track to dominate large areas of the Bartlett Experimental Forest. Given the tree-killing pests already present in the region, large increases in eastern hemlock, American beech, and red spruce (Picea rubens) are worrying.

Eastern hemlock creates important wildlife habitat for deer and more than 100 other vertebrate species in New England. However, hemlock woolly adelgid (HWA) has been present in New Hampshire since 2000. It is now within 15-20 km of Bartlett Experimental Forest. There is some hope that the region’s cold temperatures might limit HWA’s spread and impacts, but Ducey et al. expect major change when the adelgid arrives.

Ducey et al. cite a separate study demonstrating that mortality caused by beech bark disease (BBD) can be sufficient to upset carbon storage in old-growth forests. On the Bartlett Forest, nearly 90% of beech trees had become diseased by 1950.

Ducey et al. express concern about the possible impact of beech leaf disease (BLD), as well.

BLD has not yet been detected in the White Mountains or New Hampshire, but is in so New England and coastal Maine. Much remains unknown about the disease, including how it spreads and its long-term impacts.

Ducey et al. do not raise pest concerns about red spruce or balsam fir (Abies balsamea), which co-dominate the Bartlett Forest at higher elevations (above 500 m). This silence is disturbing since red spruce can be killed by the brown spruce longhorned beetle, a European woodborer established in Nova Scotia and threatening to spread south. Balsam firs suffer some mortality from feeding by the balsam woolly adelgid, a Eurasian sap-sucker which has been in New England for more than a century.

White ash (Fraxinus americana) is present as a minor component of the Bartlett Forest. Because it is considered to be a valuable timber species, management has resulted in a modest increase in abundance of ash. Ducey et al. expect dramatic reduction — or even elimination of the species — when the emerald ash borer (EAB) arrives. EAB has been detected within ~ 15 km from Bartlett Experimental Forest.

Ducey et al. conclude that silvicultural management applied at the scope and intensity of that in the Bartlett Experimental Forest has moderated some changes. That is, it is maintaining sugar maple and suppressing the increase of beech. Its effect is secondary, however to overall forest development as the forest ages.

SOURCES

Ducey, M.J, O.L., Yamasaki, M. Belair, E.P., Leak, W.B. 2023. Eight decades of compositional change in a managed northern hardwood landscape. Forest Ecosystems 10 (2023) 100121

Proceedings of the First Biennial Northern Hardwood Conference 2021: Bridging Science and Management for the Future. USDA Forest Service Northern Research Station General Technical Report NRS-P-211, May 2023

Posted by Faith Campbell