Download Restoration of Landscapes Degraded by Invasive Insects and

Restoration of Landscapes Degraded by Invasive Insects and Pathogens
Jennifer Koch, Research Biologist, NRS-16, Delaware OH [email protected]
Exotic insects and pathogens are among the most serious threats to forest landscapes and forest
tree species in the United States. When invading pests or pathogens successfully establish, high mortality
rates often result and can lead to extinction or extirpation of the host species. Dozens of forest tree species
in the United States are currently under threat of widespread mortality due to such threats. Extensive
mortality results in shifts in forest species composition as dying trees are replaced. Large canopy gaps
create opportunities for establishment of invasive plant species. These changes impact forest nutrient
cycling, productivity, decomposition, hydrology, food web dynamics, wildlife and species diversity.
Impacts can be even greater when species that co-occur are each affected by an invasive threat, such as
eastern hemlock (hemlock wooly adelgid, HWA) and American beech (beech bark disease, BBD), or
when multiple pests or pathogens target the same host species such as ink disease and chestnut blight
infection of American chestnut. Resilience of forests in the face of additional biotic and abiotic stresses,
such as climate change and introductions of new invasive threats, can be tremendously reduced.
A number of different approaches can be taken to restore landscapes degraded by invasive species
including eradication or suppression of the insect or pathogen (covered by Poland & Juzwik), silvicultural
treatments, seedling plantings, and various combinations of these. In some cases, it may be most
appropriate to do nothing, after careful consideration of funding limitations, the degree of impact, and the
potential for recovery without costly interventions. The American chestnut, for example, has already
been eliminated as a functional part of the ecosystem and is now an issue of reintroduction of the species.
Taking no action for a single lost species may not cause additional harm, but continued losses of
additional species can lead to decreased forest productivity and resilience. Alternatively, the potential
impacts of reintroduction of chestnut on the current ecosystem are unknown. Despite decades of pressure
by Dutch elm disease (DED), inspection of FIA data indicates basal area in all size classes of American
elm are increasing in some regions, possibly the result of natural selection leading to a landscape level
increase in resistance (Morin & Liebhold, unpublished). Intervention may only be necessary in regions
where both DED and elm yellows (a lethal phytoplasma) are present and basal area continues to slowly
decline.
Silvicultural treatment involving the removal of susceptible trees, can be effective especially
when infected trees survive for long periods of time, as is the case with beech bark disease (BBD). Forest
Service researchers have shown that single tree selection (removal of diseased beech) can lead to
significant increases in standing timber quality and a higher frequency of resistance in the subsequent
seedling generation1,2. The success of this approach is dependent on the mode and complexity of
inheritance of resistance as well as the density and frequency of resistant trees in the stand. In most cases,
natural regeneration alone is not effective and restoration requires seedling plantings. When resistant
planting stock is not yet available, the goal of restoration may be to preserve ecosystem services through
the use of species that are not hosts for the invasive pest or pathogen. For example, rapid emerald ash
borer (EAB) induced mortality in flood plains or swamp forests where ash is the dominant species can
result in hydrology changes and an influx of invasive plants. Factors impacting success of restorative
seedling plantings in ash dominated floodplains with varying levels of EAB-induced mortality, including
seedling species, are currently being assessed by Forest Service researchers (Knight, NRS). Careful
selection of non-host species replacements can minimize ecological impacts and potentially increase
resilience3. However, as the number of invasive species continues to increase, the choices of appropriate
non-host species may become very limited.
Restoration plantings using genetically resistant planting stock are most desired for conservation
of both the species and ecosystem services. It may be the only choice when silvicultural options or
appropriate non-host species are lacking. In the Pacific northwest, the Dorena Genetic Resource Center
supplies genetic services to 19 national forests and cooperators. Their programs include the development
and/or deployment of sugar pine, western white pine, and whitebark pine with resistance to white pine
blister rust and Port-Orford-cedar with resistance to Phytophthora lateralis. In the Southern Region the
Resistance Screening Center facilitates efforts of university and industry based tree improvement
programs to develop and deploy planting stock with resistance to fusiform rust, pitch canker and other
diseases. Similarly coordinated programs and infrastructure are lacking in the central and eastern U.S but
despite this, FS researchers are leading efforts to develop resistance breeding programs for American
beech (BBD), American elm (DED), ash and butternut, largely through piecing together short-term
grants. The Forest Service has also partnered with the American Chestnut Foundation (TACF), a nonprofit organization focused on a hybrid breeding approach to develop resistance to chestnut blight.
Both the SRS and NRS are involved in research on reintroduction of the American chestnut,
primarily focusing on identifying factors important for seedling survival4. These studies rely on and can
be limited by the materials available from the TACF program. Recently published results by TACF on the
resistance of the BC3F3 generation (the advanced hybrid generation expected to produce the highest level
of resistance) reported that only 16 % were highly resistant5. Other Forest Service studies indicate that
inadequate cold tolerance in the TACF hybrids may limit restoration in the northeastern U.S. and current
research is evaluating silviculture and genetic selection methods to increase cold tolerance6. Regardless, it
is highly unlikely that efforts to reintroduce the American chestnut will succeed unless regionally adapted
seedlings with durable resistance to chestnut blight and ink disease are available, yet there is only one FS
research project involved in actively breeding chestnut for resistance (HTRIC, NRS).
As new invasive species are identified and more species face possible extirpation, funding has
historically been primarily focused on faster fixes and basic research, even though host-resistance is often
the best and most sustainable approach to restoration of degraded landscapes. As a result, genetics and
breeding of host-resistance is often lagging behind in both funding and progress. The recent invasion of
EAB brought about initial investment of resources in unsuccessful quarantines and eradication efforts.
Parasitoids are being reared and released in an effort to control and reduce EAB populations, yet high
mortality levels of planted green ash reported in China, despite naturally high levels of parasitism,
indicate that some level of host-resistance will likely also be necessary to insure survival of ash in the
U.S.7 Considerable resources have supported work identifying differences between EAB-resistant and
susceptible species in an attempt to uncover potential mechanisms of resistance8. Though such findings
are interesting, they are highly unlikely to promote the development of resistant material because they
lack integration with a tree improvement program and meaningful, informative associations with the
EAB-resistance phenotype. Several years into the EAB invasion, NRS researchers received short-term
funding to begin exploring the use of hybrid breeding with EAB-resistant Asian ash species9. More
recently, researchers have confirmed there is genetic variation in resistance to EAB within green ash that
can be used as the basis for a breeding program, and collaborated in the development of genomic tools to
assist breeding10,11. A funding approach that is balanced between short and long-term solutions could
prevent delays.
Reinvigoration and prioritization of Forest Service forest genetics research and tree improvement
programs will be necessary to address the most critical gap in restoration research: the need for regionally
appropriate, genetically diverse, resistant planting stock. Research is necessary to develop quality
standards for resistance/tolerance, guidelines for retention of genetic diversity, delineation of seed and
breeding zones, and movement limitations to retain adaptive capacity. To achieve deployment of resistant
planting stock, investments in infrastructure will be required for operational level production and
plantings. Establishment of genetics and production capacities should be a collaborative effort with states,
universities and non-profit institutions to promote coordinated efforts, increasing efficiency and
accelerating successful development and deployment of resistant planting stock. Historically, successful
tree improvement programs are those with long-term support and a cohesive planting program12.
Integration of genomics tools with traditional phenotyping and breeding approaches can further reduce
the time required to achieve operational deployment of resistance.
References: 1Leak 2006, NJAF 23(2): 141-143; 2Koch et al., 2010, Can J For Res 40:265-272; 3Iverson et al., 2016, Ecosystems doi:
10.1007/s10021-015-9929-y; 4Clark et al., 2014, J For 112(5):502-512; 5Hebard 2012, Gen Tech Rep PSW-GTR-240: 221-234; 6Saielli et al.,
2014, For Sci 60(6): 1068-1076; 7Duan et al., 2012, Environ Entomol 41:245-354; 8Villari et al., 2016 New Phytol 209: 63-79; 9Koch et al., 2012,
Gen Tech Rep PSW-GTR-240: 235-239; 10Noakes et al., 2014, Cons Gen Res 6:969-970; 11Koch et al., 2015, New For 46(5): 995-1011;
12
Wheeler et al., 2015, J For 113(5): 500-511.