Month: April 2022

Harvest + Tree-Killing Pests = Threat to Forest Composion

EAB-killed ash in Ontario; photo by Michael Hunger

Lately I have become aware of articles discussing how silviculturists and timber managers in the East are responding to the threat from introduced pests.

As Holt et al. (2022; full citation at end of blog) point out, private landowners control 56% of U.S. forestland – and 80% in the East. Their collective decisions about managing those forests are one of two factors that largely determine the composition and structure of the forested landscape and the ecosystem services those woodlands provide. The second determining factor is invasive pests. If an invasive pest prompts many landowners across the East to harvest their timber, the collective impact will be enormous. In this way, invasive species carry a double threat: direct mortality of one or more tree species or genera; and stimulation of removal of the host species from the forest by land managers trying to maximize or protect their current and future monetary investment.

Projections suggest that the number of non-native woodborers established in North America will increase three- or four-fold by 2050. If these prove true (see Leung et al. 2016), the impact on eastern North America forests and associated ecosystem services would be profound.

Holt et al. explore how private landowners have responded to an actual invasive species, the emerald ash borer (EAB). They analyze the influence of EAB’s presence on:

(1) annual probability that a landowner would decide to harvest timber on his/her own lands;

(2) intensity of any such harvest (percentage of trees cut); and

(3) diameter of harvested trees.

They examined harvesting of both the host (ash) and non-host species that co-occur.

Using data from U.S. Forest Service permanent inventory plots, they compared harvest levels in counties in which EAB was detected before 2007 to harvest levels in counties that were infected after 2012. To simplify, they omitted counties in which EAB was detected during the period 2007–2012. They excluded plots that did not contain any ash trees; and plots owned by federal or state agencies. They also excluded trees with diameters less than 12.7 cm (5 inches) dbh.

Ash harvests were apparently less widespread than non-ash harvests. Ash trees were harvested on 6% of the USFS Forest Inventory and Analysis (FIA) plots compared to 9% of plots for harvests of non-ash trees. However, a higher proportion of ash basal area was removed in these harvests — 63% of ash basal area versus 32% of non-ash basal area (remember, ash trees were present in all plots).  

The presence of EAB resulted in

  • an increased amount of biomass harvested – by approximately 25% of basal area;
  • harvests contained greater quantities of ash, relative to non-ash species.
  • harvested trees in EAB-infested areas had smaller diameters, on average; this was true of both ash and non-ash species.

Two demographic variables were analyzed. Higher median household income resulted in a lower probability of non-ash harvest. Human population density had no significant effect.

Holt et al. say their findings indicate that a wave of ash removals will follow EAB spread with a potential to alter forest development trajectories and change structural legacies, with consequences for ecosystem services and biodiversity. They consider tree species that co-occur with ash, and that are preferred timber species, are the most likely to be removed in excessive numbers as a result of EAB-induced harvest.

Holt et al. note that ash removals were perhaps underestimated by the study because landowners might have cut their ash before EAB actually was detected in their county.

Managing the Northern Forest – Emphasis on reducing the beech component

Meantime, two other groups are suggesting how forest managers should respond to current challenges, including invasive pests. Both suggest steps to reverse – or at least slow – trends under which American beech (Fagus grandifolia) is becoming more dominant. (Given beech’s ecological importance, this stance bothers me!  I don’t quarrel that many timber-oriented people don’t want more beech.) Neither of these studies considers the possible impact of beech leaf disease and beech leaf miner.  I recently posted a blog link reporting Reed et al.’s (2022) analysis of interactions between BBD and BLD.

Rogers et al. (2022), the first group, note that successful silviculture is the art and science of managing forests intended to achieve human defined goals. Usually this means assuring the “desired” species composition and structure. However, to succeed, silviculture must also consider site conditions, including competing vegetation and changing climates.

They focus on the northern hardwood forest – also called the beech-birch-maple forest. It is broadly defined by the dominance of sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), and American beech. The northern hardwood forest occupies about 20 M ha across northern United States and southern Canada. From a traditional management perspective, maple and birch are the desired species; American beech is widely considered undesirable.

Unfortunately, from the timber point of view, Rogers et al. expect the abundance of sugar maple and yellow birch to decrease and American beech to increase. Important factors in this trend are soil types; deer numbers and preference for tree species other than beech; and high number of root sprouts stimulated by beech bark disease (BBD). Rogers et al. call for modification of traditional silvicultural approaches in the region. They call specifically for “adaptation planting” (also called “assisted migration”). They note that increased canopy openings – e.g., “irregular shelterwood system” — are important for establishing shade intolerant and mid-tolerant species, among them white ash (Fraxinus americana). They do mention the threat from emerald ash borer.

In an earlier blog I noted that the second group, Clark and D’Amato(2021), called for silvicultural management of New England forests (part of the same northern hardwood forest). Their goal was to maximize carbon sequestration. They advised management to promote retention of eastern white pine (Pinus strobus) and slow takeover by American beech and eastern hemlock (Tsuga canadensis). They say these species will fare poorly in warmer climates. Of course, all these species face non-native pests. See above for beech; hemlock is being decimated by hemlock woolly adelgid. Eastern white pine has apparently survived its own non-native pest, white pine blister rust.

I hope these pest-related hindrances to traditional timber-focused forestry will help convince the U.S. Department of Agriculture and Congressional agriculture and natural resource committees that non-native pests are a significant threat. Clearly past documentation of impacts to biological diversity and native ecosystems have not prompted them to adopt adequate protective measures or to respond effectively to established invaders. See earlier blogs, my recent article, and the Fading Forests reports (link at end of blog) for suggestions on what actions should be taken.

SOURCES

Clark, P.W. and A.W. D’Amato. 2021. Long-term development of transition hardwood and Pinus strobus – Quercus mixedwood forests with implications for future adaptation and mitigation potential. Forest Ecology and Management 501 (2021) 119654

Holt, J.R., J.R. Smetzer, M.E. Borsuk, D. Laflower, D.A. Orwig, J.R. Thompson. 2022. EAB intensifies harvest regimes on private land. Ecological Applications. 2022;32:e2508.

Leung, B., M.R. Springborn, J.A. Turner, E.G. Brockerhoff. 2014. Pathway-level risk analysis: the net present value of an invasive species policy in the US. The Ecological Society of America. Frontiers of Ecology.org

Rogers, N.S., AW. D’Amato, C.C. Kern, S. B`edardd. 2022.  Northern hardwood silviculture at a crossroads: Sustaining a valuable resource under future change

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

Spring wildflowers – why is one valley invaded while neighboring one is not?

I post here photos from two creek valleys in northern Virginia.

The Accotink creek valley is completely overrun by invasive plants … the herbaceous layer is made up of lesser celandine (Ficaria verna Huds; Ranunculus ficaria L.) and – in some places — Leucojum.

Neighboring Pohick creek valley still supports native hebaceous plants – skunk cabbage, spring beauties, trout lillies.

They both flow through wealthier suburbs in Fairfax County.

?????

P.S. In a ditch connecting to Pohick creek I have found this aquatic plant:

Plant is rooted, but leaves float on the water surface. In March the leaves were wide with scalloped edges; by April they are longer – lanceolate? I have seen it nowhere else. Anyone know what it is? Local authorities say it is not water chestnut (Trapa).

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

Plants Depend on Animals – and They are Disappearing

black berry eating hawthorn berries; photo by Paul D. Vitucci

Articles by Evan Fricke and colleagues remind us to look more broadly at bioinvasion to consider the impact on ecosystem function and evolution. They focus on animal interactions with plants in the shared environment, especially animals’ role as seed dispersers.

The authors also remind us that natural barriers explain why there are different species in different areas and thus how evolution and speciation follow different paths in different places. Think of Galapagos finches evolving in isolation from a few ancestors that somehow made it over the ocean from mainland South America.

These points are made in two recent articles.

In the first, Fricke and Svenning 2020 (full citation at end of this blog) note that about half of all plant species depend on animals to disperse their seeds. Animal seed dispersal is influenced by several drivers of global change, including local or generalized extinction (= defaunation); bioinvasion; and habitat fragmentation. The decline of large vertebrates has a particularly important role in these interactions.

Their study focused on fleshy-fruited plants that are dispersed by animals. (The study does not include nuts, e.g., acorns, which are presumably subject to some of the same pressures.) They expect evolution of the affected plants and animals to proceed differently as a result of the new partnerships, but they did not study any such interactions.

Their study covered animal seed-dispersal interactions with plants at 410 locations. The data encompassed 24,455 unique animal-plant pairs involving 1,631 animal and 3,208 plant species. Three quarters of the animals were birds; most of the rest were mammals, primarily bats and primates. Only 1% were in other animal groups – lizards, tortoises, or fish.

fruit bats on Luzon, Philippines; photo by Francesco Vernonesi; Flickr.com

They found that introduced plants and animals are twice as likely as native species to interact with introduced partners. The resulting interactions are likely to amplify biotic homogenization in future ecosystems. Already, introduced species have largely replaced missing native frugivore species in some places. In fact, mutualisms in which either or both the plant and animal is an introduced species are now about seven times higher than decades ago.

These mutual-benefit interactions of introduced species are even more prevalent in areas where human modification of the environment is greater. The proportion of introduced species and of novel interactions caused by introduced plant or animal species was higher for oceanic island systems than for continental bioregions. This finding adds a new dimension to the already recognized heightened susceptibility of remote islands to invasion and their loss of native species. Continental bioregions’ networks typically had few introduced animals and a greater prevalence of intro plants than animals.

Fricke and colleagues think plant-frugivore networks are likely to increasingly favor a relatively few introduced generalists over many native species, reducing the uniqueness of future biotas. The result might be to reduce resilience of terrestrial ecosystems by, first, allowing perturbations to propagate more quickly; and, second, by exposing disparate ecosystems to similar drivers. They called for giving higher priority to managing increasing ecological homogenization.

In the second article, Fricke, Ordonez, Rogers, and Svenning (2022) note that climate change requires many plant species to shift their populations hundreds of meters to tens of kilometers per year to track their climatic niche. Earth is also experiencing the formation of novel communities as species introductions and shifting ranges result in co-occurrence of species that do not share co-evolutionary history. They conclude that the novel mutualistic interaction networks will influence whether certain plant species persist and spread.

These authors examined four scenarios to assess how current long-distance dispersal has been affected by past defaunation and invasion and how it is threatened by species endangerment. These scenarios are as follows:

1st scenario (current scenario) = natural and introduced ranges of extant species today.

2nd scenario (natural scenario) = mammal and bird ranges as they would be if unaffected by extinctions, range contractions, or introductions.

3rd scenario (extinction scenario) = those bird and mammal species listed as vulnerable or endangered by the IUCN go extinct.

4th scenario (extirpation of introduced species scenario) = introduced species are extirpated.

Fricke and colleagues estimate that extinction of at least local populations of seed-dispersing mammals and birds has already reduced the capacity of plants to track climate change by 60% globally. The effect is strongest in temperate regions and regions with little topographic complexity. Two examples are eastern North America and Europe. These regions face a double threat: rapid climate change and loss of large mammals that provided long-distance dispersal.

The extinction scenario is most evident in Southeast Asia and Madagascar. The remaining animal seed dispersers are already threatened or endangered. Fricke and colleagues project that future loss of vulnerable and endangered species from their current ranges would result in a further reduction of 15% in the capacity of plants to track climate change.

The contrary situation is found on islands which have few native mammals. Introduced species are now important long-distance seed dispersers. In some cases, the introduced animals are dispersing invasive plant seeds, e.g., on Hawai`i feral hogs are spreading the invasive plant strawberry guava (Psidium cattleianum).

strawberry guava on Maui; photo by Forest and Kim Starr

People’s actions have resulted in ecoregions disproportionately losing the species that provide long-distance seed dispersal function, i.e., large mammals. In other words, human activities have caused not only rapid climate change—requiring broad-scale range shifts by plants—but also defaunation of the birds and mammals needed by plants to do so. Habitat fragmentation and other land-use changes will likely amplify existing constraints on plant range shifts.

Fricke and colleagues say their findings emphasize the importance of not only promoting habitat connectivity to maximize the functional potential of current seed dispersers but also restoring biotic connectivity through the recovery of large-bodied animals to increase the resilience of vegetation communities under climate change.

SOURCES

Fricke, E. C., & Svenning, J. C. (2020). Accelerating homogenization of the global plant–frugivore meta-network. Nature, 585(7823), 74-78. https://www.nature.com/articles/s41586-020-2640-y

Fricke, E. C., Ordonez, A., Rogers, H. S., & Svenning, J. C. (2022). The effects of defaunation on plants’ capacity to track climate change. Science, 375(6577), 210-214. https://www.science.org/doi/full/10.1126/science.abk3510

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

APHIS – 50 years + plant pest detection month

beech leaf disease – Not one of the plant pests that APHIS is regulating! Photo by Jennifer Koch, USFS

APHIS has reminded us that 2022 is the agency’s 50th year. In its press release, APHIS claims several accomplishments over this period:

  • Eradicating plant pests like European grapevine moth and plum pox from the country, while reducing the impact of others plant diseases, including boll weevil and Mediterranean and Mexican fruit flies;
  • Eradicating serious animal diseases, including highly pathogenic avian influenza, virulent Newcastle disease, and pseudorabies, from the country’s herds and flocks, while reducing the prevalence of other animal diseases like bovine tuberculosis and brucellosis;
  • Improving care for laboratory animals, exhibited animals and other animals;
  • Ensuring genetically engineered plants do not pose a risk to plant health, while keeping up with the ever-changing technology in this field;
  • Reducing the impact of wildlife damage on agriculture and natural resources; and
  • Ensuring safe trade of agriculture commodities across the globe

APHIS also launched a new page on its website to share a series of visual timelines of its history and important milestones.

APHIS also states that USDA) has declared April 2022 to be Invasive Plant Pest and Disease Awareness Month (IPPDAM). The link Invasive Plant Pest and Disease Awareness Month connects you to APHIS’ webpage. Secretary Vilsack asks people to be alert. He noted particularly the risk that pests will hitch a ride on untreated firewood, outdoor gear and vehicles, and soil, seeds, homegrown produce, and plants.

The notice urges people to:

  • Familiarize yourself with the invasive pests that are in your area, and their symptoms. [Faith says – also look for pests not “here” yet – early detection!]
  • Look for signs of new invasive plant pests and diseases and report them to your local Extension office, State department of agriculture or your USDA State Plant Health Director’s office.
  • When returning from travel overseas, declare all agricultural items to U.S. Customs and Border Protection so they can ensure your items won’t harm U.S. agriculture or the environment.
  • Don’t move untreated firewood. Buy local or use certified heat-treated firewood, or responsibly gather it on site where permitted.
  • Source your plants and seeds responsibly. When ordering online, don’t assume items available from foreign retailers are legal to import into the United States. Learn how to safely and legally order plants and seeds online.
  • Don’t mail homegrown plants, fruits and vegetables. You may live in an area under quarantine for a harmful invasive plant pest. You could inadvertently mail a pest.
  • When in doubt, contact your local USDA State Plant Health Director’s office to find out what you need to do before buying seeds or plants online from an international vendor or before mailing your homegrown agricultural goods.

West Coast Responding to EAB

nearly pure stand of Oregon ash in Ankeny National Wildlife Refuge, Oregon; photo by Wyatt Williams, Oregon Department of Forestry

While Michiganders document the impacts of the emerald ash borer (EAB) there, conservationists on the West Coast are jump-starting efforts to save their regional species, Oregon ash (Fraxinus latifolia). Earlier field tests in the Midwest showed that EAB will attack Oregon ash (press release) – something West Coast state would like to counter as early and effectively as possible.

Oregon ash is a wide-ranging species, occurring from California to Washington and possibly into British Columbia. The species has not been studied extensively (it is not a timber species!), but it is clearly an imponearlrtant component of riparian forests. In wetter parts of the Willamette Valley, ash is the predominant tree species. See the photo of the riparian forest in the Ankeny National Wildlife Refuge; this forest is nearly 100% Oregon ash (ODA/ODF EAB Response Plan).

As is true in the Midwest, ash provides important food and habitat resources along creeks and rivers where seasonally high water-tables can exclude nearly all other tree species. Standing and fallen dead ash biomass can alter soil chemistry and affect rates of decomposition, nutrient, and water cycling, i.e., nutrient resource availability for the remaining trees. Gaps in tree canopy can increase soil erosion, stormwater runoff and elevated stream temperatures. In dense stands of Oregon ash, understory vegetation is often sparse, consisting primarily of sedges. The authors of the Response Plan anticipate invasion by non-native plants into canopy gaps caused by the loss of ash trees as a result of an EAB invasion. In Michigan, though, it is the sedges that dominate these gaps.

The Oregon Department of Forestry, the state Department of Agriculture, and other entities have actively participated in “don’t move firewood” campaigns for at least a decade. The Departments of Forestry and Agriculture also led a team that prepared the EAB Response Plan in 2018 (full citation at the end of this blog). It lays out in considerable detail the roles of both government agencies and non-governmental stakeholders. Oregon’s quarantine is broad, covering all insects not on an approved list (Williams, pers. comm.)

California has inspected incoming firewood for years. In April 2021 – after APHIS terminated the federal quarantine on EAB — California Department of Food and Agriculture established a state quarantine on the beetle and articles that could transport it into the state. In doing so, CDFA noted that commercially grown olive trees might also be at risk to EAB.

Washington State operates a statewide trapping program for invasive insects. There has also been significant attention to non-native insect threats to urban forests. These have included a study in 2016 led by the Washington Invasive Species Council (WISC). It involved a partnership of WISC with the Washington Department of Natural Resources Urban and Community Forestry Program as well as and statewide stakeholder meetings [Bush, pers. comm.].

Of these various state-wide initiatives, the institutions in Oregon appear to be most pro-active. The Tualatin Soil and Water Conservation District provided $10,000 to fund some of the genetics work and testing for EAB resistance. Other funding came from the USDA Forest Service Forest Health Protection unit of State and Private Forestry (not from USFS’ Research Program). As described by USFS geneticist Richard Sneizko in an article in the publication TreeLine (full citation at end of blog), participants hope to find at least some level of genetic resistance to EAB. Any such resistance might be deployed in several ways: 1) promoting reproduction by resistant trees to enhance their numbers before EAB gets to Oregon; 2) using seeds from resistant trees for restoration of natural areas; or 3) cross-breeding resistant trees to build genetically diverse stocks of resistant trees for future restoration.

Participants think it is vitally important to work from seeds collected over much of the range of Oregon ash – first, to search for probably very rare resistant trees; and second, to preserve the full diversity of the tree species’ genome so that restored ash will be adapted to the wide variety of conditions in which ash grow.

Participants in this effort include the forest genetics/tree improvement community – specifically, the USDA Forest Service Dorena Genetic Resource Center (located in Cottage Grove, Oregon) and Washington State University at Puyallup Research & Extension Center. Also engaged is the public gardens community, specifically the Huntington Botanical Gardens in San Marino, Los Angeles County.  The garden is collecting seed of Oregon and other western ashes from California and Washington State.

The first step in assessing resistance is collecting seed from ash trees across the range of Oregon ash. This began in 2019. Carried out by, inter alia, some USFS and Interior’s Bureau of Land Management units, Oregon State University, citizen scientists [Sniezko] and the Oregon Department of Forestry [press release & Sniezko pers. comm.] Also, some seeds were collected in Washington State in 2020. Additional collections in Oregon are scheduled for 2022.

The collected seeds have been evaluated for vitality and stored by the USFS Dorena Center and at the USFS National Seed Lab (Macon, GA).

Oregon ash planting at Dorena; photo by Emily Boes

The USFS Dorena Center and Washington State University have begun germinating and growing some of the seedlings for various tests of possible resistance. There is concern that the 2021 drought might have killed some of the seedlings in Oregon; those in Washington are not affected. The initial seedlings are mostly from Oregon but there is space to add additional families from a wider geographical area. Experimenters plan to collect data annually on bud break, yearly growth, and any diseases or pests that develop on the trees. (Chastagner pers. comm.)

The next step is systematic testing whether some of the ash show genetic resistance to EAB. Richard Sneizko has sent seedlings of 17 ash families to USFS colleague Dr. Jennifer Koch. She operates a breeding facility in northern Ohio where they can be tested for resistance. Testing is expected to begin this year. [Tree Line]

The Dorena Center is also helping a researcher at Penn State University, Dr. Jill Hamilton, to set up a landscape genomics project. She will evaluate the genetic variability in the species by using leaf samples from about 20 trees from many populations across the Oregon ash’s range (California to British Columbia).  This potentially includes a collection from the Dorena population of ash in late Spring 2022. [Sniezko]

These various ash plantings can also be “sentinel” plantings to assist in early detection of newly arriving EAB. [Tree Line]

SOURCES

Bush J. Executive Coordinator | Washington Invasive Species Council

ODF and ODA Emerald Ash Borer Readiness and Response Plan. 2018. 

Click to access EAB+Plan+2018.pdf

ODF press release Feb 24, 2022

Treeline Newsletter May 13, 2021. Richard Sniezko. Is There a Future for Oregon Ash? Forest Genetics to the Rescue? Genetic & Emerald Ash Borer Resistance Projects https://www.nnrg.org/wp-content/uploads/2022/02/Treeline_newsletter-June-2021.pdf

The newsletter is issued by Bonneville Environmental Foundation for a consortium of conservation agencies

Sniezko pers comm Feb 2022  22-2/24

A video explaining the campaign to save Oregon ash is at https://youtu.be/uZmfLrxEA7g or https://youtu.be/S8y-XK285S8

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

In Michigan: Devastating News for Black Ash; Merely Bad News for Green Ash

results of EAB infestation; photo by Nate Siegert, USFS

A series of studies by Patrick J. Engelken, M. Eric Benbow, Deborah G. McCullough, Nate Siegert, Randall Kolka, Melissa Youngquist and others examine the status of ash (Fraxinus spp.) in the aftermath of the emerald ash borer (EAB) invasion. Initial studies documented the crash of biomass supporting EAB numbers when the large ash trees died (Siegert, Engelken, McCullough. 2021; full citation at end of blog.) More recent studies have focused on bogs and forests in the riparian areas where ash were especially numerous and arguably ecologically most important. I posted a blog about black ash bogs earlier.

I will focus here on the studies in riparian areas of southern and northwest Michigan. Information about impacts in forests of southern Michigan are from Engelken, Benbow and McCullough (2020); information about impacts in northwest Michigan are from Engelken and McCullough (2020). Full citations for both are at the end of the blog.

All study areas had high ash densities before EAB’s arrival. One study (Engelken and McCullough 2020) found ash densities high in the immediate riparian areas (in one case, a strip reaching 100 meters from the streambank) but scattered in surrounding forests.

In all these study areas, populations of mature (reproductive age) ash crashed within 10-15 years after EAB invasion:

  • In northeast Michigan, EAB carrying capacity was reduced by 94% – 99%; total ash basal area was reduced by 87 – 97.7% (Siegert, Engelken, McCullough. 2021);
  • In southern Michigan, more than 85% of the basal area of green (F. pennsylvanica) and black ash (F. nigra) had been killed by 2020. An estimated 96% of the overstory ash phloem area had died, thus radically reducing EAB carrying capacity (Engelken, Benbow and McCullough 2020);
  • In northwest Michigan, more than 95% of the overstory ash have been killed. (Engelken and McCullough 2020).

The worst impact has been on black ash– which plays such an important ecological role in riparian areas and wetlands and has enormous importance in Native American cultures. In all these study areas, there is no stump sprouting by black ash (Siegert, Engelken, McCullough. 2021; Engelken, Benbow and McCullough 2020; Engelken and McCullough 2020). In three watersheds of northwest Michigan where black ash constituted up to a quarter of the overstory species before the EAB invasion, scientists found no black ash recruits, only eight saplings, and a single seedling.

Green ash (F. pennsylvanica) has survived in much higher numbers – so far. However, this species’ ability to grow into reproductive size is still uncertain. In northwest Michigan, green ash saplings are abundant in canopy gaps created by EAB-caused mortality of mature ash. These saplings had established before the EAB invasion so some call them the “orphaned cohort”.  However, there are few seedlings of any woody plant species in these gaps because sedges form such dense mats.

Green ash reproduction faces many challenges before persistence of the species can be considered assured.  First, populations of EAB – now reduced by the lack of mature ash to support them – might resurge when young ash grow to larger sizes. It is not yet clear the extent to which introduced biocontrol agents and native predators, e.g., woodpeckers, will protect these trees as they grow to reproductive size. Here, again, green ash has an advantage over black ash. While green ash produce seed at a relatively young age, black ash don’t produce seed until they reach 30–40 years. Even then, they produce seeds only sporadically, with intervals of five or more years.

A second challenge is the lack of seed sources – at least until and unless young trees are able to reach reproductive size.

A third challenge is competition for resources from other plants. The canopy gaps eliminate competition for light for the taller plants, i.e., the existing ash saplings. However, the sapling cohort is not supported by a seedling cohort. There are very few seedlings of all woody plant species (including invasive species!). Seed germination is suppressed by the dense mats of wetland-adapted sedges and possibly the higher water tables (which resulted from reduced evapotranspiration following mortality of the mature trees).

Competition for resources is also a factor in the forests outside the immediate riparian zone. There, ash seedlings sprout, but shade created by lateral ingrowth suppresses their growth. In southern Michigan, Engelken, Benbow and McCullough (2020) note that the forests are apparently transitioning from red oak dominated forests to red maple and black cherry dominated forests. This transition is apparently intensified by forest mesophication resulting from reduced fire frequency, decreasing light availability in forest understories and increasing soil moisture content.

Fourth, while stump sprouting of green ash was noted in southern Michigan, in the northwestern forests all the sprouts died. I have already noted the absence of stump sprouting by black ash at all sites.

Beaver & Green Ash in Northern Virginia

photos of beaver feeding on ash saplings in northern Virginia; photos by F.T. Campbell

In spring 2022 I noticed along one stream in northern Virginia that beavers had cut down green ash saplings; McCullough and Siegert report that this does not appear to be a problem in their study areas.

By December 2022, the beaver-cut trees tried to recover: see the sprouts from a stump [below]. (I think deer or rabbits ate the tips of the sprouts.)

The beavers also continued feeding on the ash — the tree photographed in the spring when it was half-chewed through has now been felled and its branches removed [see below].

Ecosystem Impacts, Especially on Streams

Across much of the upper Midwest, massive ash mortality is causing widespread changes in forest systems.

Riparian forests, i.e., areas adjacent to waterways where periodic inundation occurs, are functionally linked to the aquatic systems. Loss of such a significant proportion of the overstory changes the transfer of energy to adjacent waterways that takes the form of inputs of nutrients from leaf litter and coarse woody debris. Intact forests also stabilize stream banks and maintain channel depth by preventing erosion. Forests moderate temperature of the water. Finally, forests with “coarse woody debris” increase habitat structure. These impacts might be especially important along first order streams, (defined as perennial streams that have no permanently flowing tributaries). These streams are too small to buffer the impacts of major tree loss. The scientists say they are uncertain whether these changes continue to affect larger streams downstream.

Unshaded streams have higher water temperatures that can affect populations of fish, in particular salmonids, by delaying migration, reducing egg viability and increasing egg mortality. Higher temperatures can also alter primary productivity of aquatic algae, potentially increasing eutrophication (Engelken and McCullough 2020).

The scientists expect increasing abundance of coarse woody debris in the forests and streams of northwest Michigan as the 75% of dead ash that are still standing fall. Such debris provides nutrients and habitat for an array of plants and animals, thereby influencing the abundance, activity and species compositions of several ground dwelling insects and seedling establishment. In streams, coarse woody debris provides complex habitat and refuges. It also retains organic matter. Recreationists do find that debris impedes boating.

Loss of ash specifically 

As described by Engelken, Benbow and McCullough (2020), and in my earlier blog, ash leaf litter – particularly black ash leaf litter – is highly nutritious. Ash leaf litter has efficient turnover rates and contributes important soil nutrients such as nitrogen, organic carbon and exchangeable cations. Invertebrate communities in headwater streams feed largely on coarse organic material such as leaf litter (Engelken and McCullough 2020). Consequently, loss of the annual influx of ash leaf litter will likely have adverse effects on nutrient availability in riparian forests and adjacent streams.

SOURCES

Engelken, P.J., M.E. Benbow, D.G. McCullough. 2020. Legacy effects of emerald ash borer on riparian forest vegetation and structure.  Forest Ecology and Management 457 (2020) 117684

Engelken, P.J. and D.G. McCullough. 2020. Riparian Forest Conditions Along Three Northern Michigan Rivers Following Emerald Ash Borer Invasion. Canadian Journal of Forest Research.

Siegert, N.W., P.J. Engelken, D.G. McCullough. 2021 Changes in demography and carrying capacity of green ash and black ash ten years after emerald ash borer invasion of two ash-dominant forests. Forest Ecology and Management Vol 494, August 2021

Posted by Faith Campbell

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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