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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

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

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

Hot off the presses: How beech leaf disease kills trees

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

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

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

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

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

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

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

Vieira et al. found that:

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

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

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

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

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

SOURCES

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

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

Posted by Faith Campbell

www.fadingforests.org

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

www.fadingforests.org

Invasive Plant Species: Good News, Bad News — including on the News

garlic mustard – a widespread invasive plant in forests of the Northeast and Midwest; photo by Chris Evans, University of Illinois; via Bugwood

This blog summarizes important new research on invasive plant species. You’ll find a USFS update on the spread of invasive plants in regional forests. The second paper used a new funding source to assess the impacts of removing invasive shrub honeysuckles on forest canopies – and to begin restoration. The third paper uses Google searches to study news coverage of plant invaders (spoiler alert: it’s poor). I welcome the attention!

A. Update on Invasive Plants in the Northeast: 44 Species, in 24 States, on Forested Land

The USDA Forest Service Northern Research Station (NRS) continues to publish lots of studies of invasive plants in the forests of the Northeast and Midwest. Kurtz (see USDA citation at end of blog) summarizes data on the extent and intensity of plant invasions collected by the USFS Forest Inventory and Analysis (FIA) program. Since 2012, scientists have compiled data on 44 invasive species on forested land. Crews recorded these plants’ presence on 6,361 plots in 2014 and 4,244 in 2019. [The species surveyed are listed in Table 2 of the publication.]

1. Presence —

Overall, the number of forested plots on which one or more of the 44 monitored invasive species occurs rose from 52% in 2014 to 55% in 2019. Eighteen of the 24 states in the region experienced an increase in the percentage of plots with invasive plants. The number of invasive plants per plot also increased.

The plots with the greatest number of these invasive plants are Ohio (98%), Indiana (97%), North Dakota (93%), and Illinois and Iowa (92% each). The North Dakota data bear a high uncertainty because only 15 plots were surveyed in 2019. The states with the lowest number of invaded plots are New Hampshire (16%) and Vermont (22%). South Dakota also ranks near the bottom with 38% of plots invaded.

A different picture emerges when considering the number of invasive plants per plot. States ranking highest on this criterion were Pennsylvania (13 species on at least one plot) and Illinois (11 species on at least one plot). (See map in USDA publication from References) Plots with five to eight species appear in a band from the Ohio/West Virginia border, across western Maryland and western and eastern Pennsylvania, and into western New York and western Massachusetts. Note the apparent abse nce of invasive plants in the Adirondacks!

2. Species

Of the 44 species monitored, 41 species were observed on plots during these surveys. Two of the three species not found — punktree/Melaleuca and Chinese tallow tree (Triadica sebifera) – grow in the deep South so I am not surprised by their absence. The third – Bohemian knotweed (Polygonum xbohemicum) is invasive in colder climates, e.g., Washington State. Multiflora rose (Rosa multiflora) is the most common species – again, not surprising since it was long planted deliberately to provide food for wildlife. Three species were observed in one but not both inventories: Chinaberry (Melia azedarach) was only found in 2014; saltcedar (Tamarix spp.) and European swallow-wort (Cynanchum rossicum) were found only in 2019.

Amur honeysuckle; photo by pverdunk via Flickr

Most of the 44 plant species increased their presence as measured by the proportion of plots on which they were observed (see Table 2). Among these, Amur honeysuckle (Lonicera maackii) increased by 5.38%; garlic mustard (Alliaria petiolata) by 3.21%; Japanese stiltgrass (Microstegium vimineum) by 2.95%; and bush honeysuckles (Lonicera species.) by 2.77%. Twelve species decreased – all by less than one percent.

3. One of the Worst: Amur Honeysuckle

Amur honeysuckle is one of most actively spreading invasive plant in the entire region – especially in urban forests. The proportion of plots invaded by this species rose from half a percent to six percent between 2014 and 2019. Amur honeysuckle dominates due to competitive growth, allelopathy (producing a biochemical that impedes the germination, growth, and survival of other plants), and its ability to resprout after cutting. The presence of honeysuckle negatively affects native plant communities. It suppresses plant recruitment, homogenizes forest communities, and alters ecosystem processes. It also shapes the canopy structure by affecting the growth and composition of overstory trees, as well as the amount of leaf material (Fotis et al.).

The riparian areas which Amur honeysuckle often invades provide numerous ecosystem services. These include filtering nutrients, preventing soil erosion, filtering sediment from runoff, offering shade and wildlife habitat, and reducing the likelihood of floods to croplands and downstream communities (Fotis et al.).

B. Managing Honeysuckle, Restoring Riparian Forests

Forest managers in central Ohio, near Columbus, have begun a significant restoration experiment to improve the resilience and function of disturbed riparian forests. Fotis and colleagues took advantage of this to track and characterize the immediate, short-term, and long-term impacts of removing Amur honeysuckle on forest canopies. They used new technology: portable canopy imaging, detection, and ranging (LiDAR).

Fotis et al. found that

Honeysuckle presence had a stronger influence on tree species diversity than on the size or number of trees.
Removing honeysuckle from areas where its abundance is high and native tree density is low promoted native tree growth (e.g., the height of tallest trees) and increases in the tree canopy’s structural complexity for up to 10 years.
Honeysuckle removal, followed by treating honeysuckle stumps with herbicides to prevent resprouting, is key for establishing a healthy restored riparian forest.
Planting a diverse suite of native species to fill different ecological niches helps create more resilient forest systems.
Forest recovery began within two years of honeysuckle removal.

At some sites, the researchers planted a variety of woody plants, including understory shrubs and mid- and full-canopy trees, after removing the honeysuckle. The article did not discuss the results.

giant hogweed; photo by NY State DEC fia Flickr

C. Poor Media Coverage of Invasive Plants Undermines Management Efforts

Woodworth et al. (full citation at end of blog) studied how media coverage of invasive plants interacts with low public interest, which the authors claim hampers management efforts and efficacy.

Woodworth et al. note that strong public awareness of urgent environmental issues is linked to the development of new public policy. However, public awareness of invasive species is low despite their causing high monetary costs and significant damage to the environment and human health.

The authors recognize the public generally has a low level of “plant awareness”. They argue that some plants are “charismatic” in one way or another, e.g., some form widespread and conspicuous monocultures; some are attractive and sold in the ornamental trade; and some are harmful to people by producing allergens or bearing thorns/spines/prickles. Such plants often attract more attention.

They asked four questions: Is public interest in these invasive plant species driven by (1) their abundance? (2) their traits? (3) the quantity and sentiment of news articles written about them? Finally, (4) How do these factors combine to drive interest – or lack thereof! – in invasive plant species?

1. Searching for Answers

Woodworth et al. analyzed data on Google searches (in Google Trends) for 209 plant species, 2010 – 2020. The searches revealed whether members of the public sought information about invasive plants, whether news media covered invasive plant issues, whether such coverage had a positive or negative slant, and whether it affected public awareness and attitudes toward invasive plants.

They found that public search interest was highest for the species that are most abundant at both national and state levels. Plant abundance was the second strongest predictor of Google search interest. Also, the more widespread the invader, the more articles were published and the more negative the terms used to describe it. For some species, high search interest was limited to the locations where the plant occurs. An example is garlic mustard (Alliaria petiolata). For other species, there was high search interest across the US although the plant occurs only in some localities. This was true, for example, for giant hog-weed (Heracleum mantegazzianum).

In a related finding, plant species posing a risk to human health ranked high in Google searches. These include giant hogweed (which causes serious skin rashes), allergen-producing grasses, plants that harbor ticks, and those that contribute to fires in the West like cheatgrass (Bromus tectorum). Regarding ticks, they cite multiflora rose but not barberry (Berberis spp). These results suggest to the authors that public interest in invasive plants is motivated primarily by the likelihood of encountering these species, with direct consequences for health and well-being. These concerns can be amplified by the media. The authors suggest that articles spelling out health risks of invasive plants might increase public support for wider management efforts.

cheatgrass invasion of BLM lands in SW Idaho; photo by Thane Tuoson via Flickr

The greater the number of articles published about a species, the more frequently readers utilized Google to search for information. Woodworth et al. suggest that more attention by the media translates to more public interest. They caution, however, that there are two other possible explanations. First, more searches might drive science journalists [I add: or newspaper editors!] to write more articles on species already known to be of interest to the public. Or, search and media interest might be unrelated to each other, but driven by the same external factors, such as a recent fire. In other words, journalists and the general public might be interested in the same aspects of invasive plants.

2. Which Invaders Got Attention

The media and public focus on only some invasive plants. Media articles discussed only 175 (84%) of the 209 species included in the study. More than 50% of news articles were written about only 10 species (5% of all the species); 80% on just the top 25 species. Public interest was even narrower. No one searched for information on 60 of the species (29%) over the 10-year period.

The authors – and I – are distressed that invasive plants that are sold as ornamentals were both written about and searched for less than invasive plants that are not so marketed. Worse, articles about the ornamental species had a more positive tone than articles about non-marketed invasives.

English ivy (*Hedera helix*) – an invasive plant widely used in horticulture; Washington State Weed Control Board

Finally, they found that species that form monocultures did not garner more media attention, more negative coverage, or search interest. Their example is Japanese stiltgrass (Microstegium vimineum), which they describe as being a problematic and prolific invader of eastern forest understories. There are exceptions to this finding: common reed Phragmites and kudzu. I note that these species are very noticeable, and thus “charismatic.” The former is a large plant and masses along roadsides and in open habitats. Kudzu has been notorious for decades as the “vine that ate the South”.

Woodworth et al. concluded that the media’s narrow focus on “notorious” invasive plant species, when combined with the lower and more positive coverage of ornamental introductions, could send mixed messages and weaken public awareness of their threats. The authors believe, however, that there is ample opportunity to improve messaging and increase public awareness. This requires more media coverage and a greater focus on invasive plants’ negative impacts. One potentially “sticky” message is about “loss of control.”

SOURCES

Fotis, A., Flower, C.E.; Atkins, J.W. Pinchot, C.C., Rodewald, A.D., Matthews, S. 2022. The short-term and long-term effects of honeysuckle removal on canopy structure and implications for urban forest management. Forest Ecology and Management. 517(6): 120251. 10 p. https://doi.org/10.1016/j.foreco.2022.120251 .

USDA Forest Service Northern Research Station Rooted in Research ISSUE 18 | SEPTEMBER 2023

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

Woodworth, E. A. Tian, K. Blair, J. Pullen, J.S. Lefcheck and J.D. Parker. 2023. Media myopia distorts public interest in US invasive plants. Biol Invasions (2023) 25:3193–3205 https://doi.org/10.1007/s10530-023-03101-8

Posted by Faith Campbell

www.fadingforests.org

Bug Apocalypse Update

*Aedes aegypti* photo by James Gathany via Flickr

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

SOURCES

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

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

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

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

Posted by Faith Campbell

www.fadingforests.org

USFS Lays Out Incomplete Picture of the Future

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Treatment of Invasive Species

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

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

Specifically: Invading Plants

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

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

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

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

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

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

Specifically: Insects and Pathogens

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Tree Species’ Regeneration

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

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

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

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

SOURCES

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

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

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

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

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

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

Posted by Faith Campbell

www.fadingforests.org

Invading deer, earthworms, shrubs & more! Managing Forests with Many Risks

a West Virginia forest; photo by Jarek Tuszyński

The Eastern deciduous forest is large and important ecologically. The forest is important for biological diversity: it shelters many endangered species, especially plants, molluscs and fish, mammals, and reptiles. In addition, the majority of forest carbon stocks in the U.S. are those of the eastern states.

But the Eastern deciduous forest is also under many anthropogenic stresses – including high numbers of non-native insects and pathogens, Liebhold map high numbers of invasive plants, blog invasive earthworms, blog browsing by overabundant deer, and timber extraction. In the southern portions of the forest, human populations are expanding, resulting in landscape fragmentation (USDA FS 2023b RPA, full reference at end of this blog).

map showing number of non-native pests in each county, as of ~2010

Agency and academic scientists in the USDA Forest Service Eastern Region (Maine to Minnesota; Delaware to West Virginia, then north of the Ohio River to Missouri) are trying to understand how long-term, continuous stressors, like deer browsing and invasive plants and earthworms, – interact with short-term gap-forming events. They call the long-term stressors “press” disturbances to distinguish them from the short-term “pulse” disturbances (Reed, Bronson, et al.; full citation at end of this blog). Understanding the processes by which forests recover from disturbance is increasingly important. Climate change is expected to raise the frequency and intensity of catastrophic natural disturbances (Spicer and Reed, Royo et al.).

The scientists emphasize that the impacts of these stressors – and effective solutions — vary depending on context.

Invasive Earthworms

USDA APHIS is responsible for regulating introduction of new species. For earthworms, APHIS’ principal concern is clearly the possibility that imported worms or soil might transport pathogens. However, the agency’s website does mention worms’ ability to disrupt the soil and possibly cause undesirable impacts on plant growth and diversity. At the 2023 National Plant Board meeting in early August 2023, Gregg Goodman, Senior Agriculturalist in APHIS PPQ NPB website for agenda? discussed issues that he considers when evaluating whether to grant permits for importing earthworms. APHIS allows imports to be used for fish bait. Dr. Goodman explained that APHIS surveyed fishermen to determine where they dump unused bait. He found no damage to plants along streams, etc. where they are dumped. A state plant health official from a northern state and I objected that the ecosystem damage caused by earthworms is well documented and we doubted that dumping of bait is not a pathway for introducing worms into natural areas.

Reed, Bronson et al. found lower earthworm biomass and density in both deer exclosures and canopy gaps. They hypothesize that the new plant growth associated with canopy gaps attracts deer, resulting in increased browse pressure. That browse pressure then affects the plant community, succession and forest structure. The changed plant community affects soil properties that then affect soil-dwelling fauna like earthworms. They believe the higher worm densities in closed-canopy sites might be the result of nutrient-rich tree leaf litter which provides both shelter and food. Another factor might be lack of recent soil disturbances in closed canopy sites.

While they say need more research is needed on the complex, combined effects of earthworms and deer, Reed, Bronson et al. still suggest that reducing deer populations or – where that is not possible – creating gaps might help manage earthworm invasions.

Deer Interactions

The long-term, chronic effect of excessive deer herbivory are well documented. See the many presentations at the recent Northern Hardwood research forum (USDA FS 2023b Proceedings). Most studies show that deer browsing overwhelms other disturbances, such as fire and canopy gaps that typically promote seedling diversity. However, recent results refine our understanding.

Samuel P. Reed and colleagues (Reed, Royo et al.) found that on the Allegheny Plateau of western Pennsylvania high deer densities at the time of stand initiation resulted in long-term reduced tree species diversity, density, and basal area. These responses were still detectable nearly four decades later. Stands are dominated by the unpalatable black cherry (Prunus serotina). The reduced stand density and the cherries’ narrower crowns lead to less above-ground biomass and reductions in above-ground carbon stocks. These scientists recommend that managers reduce deer populations to prevent changes in forest structure with probably long-term and important ramifications for many ecosystem functions.

*Prunus serotina*; photo by Awinch1001 via Flickr

Hoven et al. concurred with the importance of reducing deer densities, but suggested focussing on wet sites where, in their study, deer browsing had its greatest effects. On drier sites deer browsing had no effect on the diversity of woody plant seedlings.

Spicer et al. seek particularly to maintain a heterogeneous landscape to allow coexistence of both early- and late-successional species. In the Eastern Deciduous Forest biome, herbs, shrubs, and vines comprise 93% of the species richness of vascular plants

These authors found that the impact of deer browsing diverged depending on vegetation management actions. In wind-throw gaps where the plant community was retained, deer caused a 14% decline in shrub cover. In contrast, when scientists removed the extant vegetation at the beginning of recovery, deer exclusion caused a 67% increase in shrub cover. The authors speculate that vegetation removal stimulated abundant blackberry (Rubus species) regrowth. Where they had access (in gaps lacking exclosures), deer heavily browsed young Rubus stalks that sprouted after the competing vegetation was cut down. However, when the pre-established vegetation was not removed, older Rubus thickets might have protected other herbs and shrubs from browsing. Spicer et al. did not observe any major shifts in browse-tolerant species in deer-exclusion plots.

Invasive Shrubs

Hoven et al. found that in drier forest plots, the presence of non-native shrubs reduced native seedling abundance, richness, and diversity. Instead there were more seedlings of introduced species, including Lonicera maackii, L. morrowii, Ligustrum sp., and Rosa multiflora. They are concerned that replacement by invasive honeysuckles might be particularly strong in gaps resulting from death of ash trees caused by emerald ash borer. Woodlands could become dominated introduced shrubs, reducing diversity. Consequently, they recommend removing non-native shrubs in drier forests to promote seedling numbers and diversity.

In contrast, in wetter forests basal area of non-native shrubs did not affect introduced seedling abundance. However, the shrubs’ size did promote greater proportions of Lonicera maackii and Ligustrum seedlings. They suggest this might be the outcome of either abundant seed sources or allelopathic properties of some invasive shrubs e.g., L. maackii. In such sites, seedling diversity is already limited to plants that tolerate waterlogging. A hopeful note is that one native shrub, Lindera benzoin, seems able to prevent establishment of L. maackii.

*Lonicera maackii*; photo by pverdonk via Flickr

Hoven et al. do worry that death of ash trees might lead to declining transpiration rates, raising water tables, and further reducing seedling species richness and diversity.

Impact of Salvage Logging and Vegetation Removal

Spicer et al. studied how anthropogenic stressors affect succession. These scientists took advantage of tornado-caused gaps to compare interactions with deer browsing, salvage logging, and mechanical removal of the understory.

Contrary to expectations, none of these anthropogenic disturbances delayed community recovery or reduced diversity in comparison to the natural disturbance (tornado blowdown). Instead, adding either salvage logging or mechanical removal of understory vegetation substantially enhanced herbaceous species richness and shrub cover.

However, each major plant growth form responded differently. First, none of the manipulations affected species diversity or abundance of tree seedlings and saplings. Second, salvage logging in the wind-throw gaps increased species richness of herbs by 30%. Shrub abundance was doubled and cover almost tripled, but species richness did not change. Third, removing competing understory vegetation caused an increase of 23% in mean herbaceous cover. I have already discussed the impact of excluding deer.

Spicer et al. greet these increases in species richness with enthusiasm; they recommend managing to create a patchwork of combined natural and anthropogenic disturbances to promote plant diversity. However, I have some questions about which species are being promoted.

This study identified a total of 264 vascular plant species: 40 trees, 190 herbs, 15 shrubs, 17 vines, and 2 of unknown growth form. Only about half of these, 123 species, grew in portions of the mature forest not affected by either the tornado or one of the anthropogenic manipulations.

Gaps contained more plant species – as is to be expected. Natural blowdown areas where no manipulation was carried out had 49 more species than the undisturbed forest community (172 species). Blowdown sites subjected to salvage logging added another 53 species for a total of 225 species, or 102 more than the undisturbed reference forest.

A total of 17 species occurred only once in the authors’ data [= unique species]. Eight of these species grew only in the undisturbed forest. Two grew only in the tornado-impacted plots. Spicer et al. do not elaborate on whether these species are officially rare in that part of Pennsylvania – although it seems they might be. I wish Spicer et al. had addressed whether these possibly rare species might be affected by the forest management they recommend, i.e., intentionally creating a patchwork of various disturbances. An additional seven unique species were found in plots that had been subjected to an anthropogenic disturbance — either salvage logging or removal of remnant vegetation.

nodding trillium (*Trillium cernuum*); imperiled by restricted range or low populations; photo by Jason Ryndock, Pennsylvania Natural Heritage Program

In tornado-disturbed sites, one native species associated with areas where vegetation was left intact is one of the gorgeous wildflowers of eastern deciduous forests: a Trillium (species not indicated). The one native plant associated with plots from which vegetation was removed was a grass (unspecified).

Spicer et al. report that the proportion of the flora composed of non-native species was very similar between the salvage-logged area (7%) and the undisturbed reference forest (5%). Half of the non-native plant species (3.5% or 9 species) are listed as invasive in Pennsylvania (the article does not list them).

Spicer et al. say these non-native species are relatively uncommon and that they pose a minimal threat. They do concede that the invasive thorny shrub barberry (Berberis thunbergii) was more common in disturbed than intact areas. [I saw plenty of barberry along forest edges in Cook State Forest, which is only 100 miles away from the study site.] I think Spicer and others are too blasé since invasive plant populations can build up quickly when seed sources are present.

Spicer et al. raise two caveats. First, their results regarding the beneficial effects of salvage logging and vegetation manipulation probably will not apply to situations in which vast areas are logged.

More pertinent to us, they warn that their results would also not apply to forest areas in which propagules have been drastically depleted. This can result from previous human land-use or repeated catastrophic disturbances, such as canopy fires. Nor would their results apply to forests that are more threatened by invasive species. They note that a widespread and dense understory of multiple non-native species can create invasional meltdowns, resulting in a lasting depauperate state. This is especially the case when invaders at higher trophic levels, such as earthworms, are part of the mix.

Other Lessons

Reed, Bronson et al. conclude that forest canopies’ responses to disturbance are too variable to be measured by a single method. Evaluating proposals for management will require multiple measures. The overwhelming recommendation of presenters at the recent northern hardwoods research symposium (USDA FS 2023a Proceedings) was to adapt more flexible management strategies to promote forest sustainability and species diversity.

Hovena et al.’s principal finding is that interactions among site wetness, non-native shrubs and the total basal area of trees in the stand had the largest impacts on the species composition of seedlings. In Ohio, site wetness and chronic stressors like deer and introduced shrubs are acting together to shift seedling communities towards fewer native species. Of these three long-term “press” stresses, the interaction between introduced shrubs and soil wetness overshadowed even the impact of deer herbivory on seedling species richness and abundance. Surprisingly, site-specific characteristics – e.g., wetness, canopy tree competition, deer herbivory and introduced shrubs – were more influential than ash mortality in shaping woody seedling communities.

SOURCES

Hoven, B.M., K.S. Knight, V.E. Peters, D.L. Gorchov. 2022. Woody seedling community responses to deer herbivory, introduced shrubs, and ash mortality depend on canopy competition and site wetness. Forest Ecology and Management 523 (2022) 120488

Reed, S.P., D.R. Bronson, J.A. Forrester, L.M. Prudent, A.M. Yang, A.M. Yantes, P.B. Reich, and L.E. Frelich. 2023. Linked disturbance in the temperate forest: Earthworms, deer, and canopy gaps. Ecology. 2023;104:e4040. https://onlinelibrary.wiley.com/r/ecy

Reed, S.P, A.A. Royo, A.T. Fotis, K.S. Knight, C.E. Flower, and P.S. Curtis. 2022. The long-term impacts of deer herbivory in determining temperate forest stand and canopy structural complexity. Journal of Applied Ecology. 2022; 59:812-821

Spicer, M.E., A.A. Royo, J.W. Wenzel, and W.P. Carson. 2023. Understory plant growth forms respond independently to combined natural and anthropogenic disturbances. Forest Ecology and Management 543 (2023) 12077

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

United States Department of Agriculture. Forest Service. 2023b. Future of America’s Forests and Rangelands. Forest Service 2020 Resources Planning Act Assessment. GTR-WO-102. July 2023 https://www.fs.usda.gov/research/treesearch/66413

Posted by Faith Campbell

www.fadingforests.org

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

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

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

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

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

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

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

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

Naming the invaders

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

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

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

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

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

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

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

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

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

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

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

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

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

SOURCES

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

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Especially the Appendix on the Southeast and Caribbean. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

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

Posted by Faith Campbell

www.fadingforests.org

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

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

Fire and Invasive Grasses

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

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

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

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

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

On the other hand, there are hopeful signs.

endangered ‘akikiki photo by Carter Atkinson, USGS

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

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

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

Plant Invasions in Hawaiian Forests

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

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

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

*Psidium cattleyanum*; photo by Forrest and Kim Starr

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

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

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

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

Protecting Native Trees

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

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

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

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

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

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

Native Trees at Risk to Nonnative Insects

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

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

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

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

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

In Conclusion

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

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

SOURCE

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

Posted by Faith Campbell