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The Threatened Hemlock: Managing Ecosystem Changes

HWA1The nonnative invasive insect hemlock woolly adelgid (HWA) is taking its toll on eastern hemlock trees. Once infested, with HWA hemlock trees seldom recover and die within 3 to 10 years. This evergreen tree is important across the forest landscape in the eastern U.S., but it especially important in riparian areas in the southern Appalachians. Unlike the blight that wiped out the American chestnut tree, current scientific and technological advances are allowing land managers to aggressively respond to HWA, to understand the impacts of hemlock mortality, and to implement restoration.

Why should land managers care?

A mature eastern hemlock can reach a height of 175 feet and 6 feet in diameter and can live more than 800 years. The tree’s dense, evergreen canopy creates a unique environment that serves as critical habitat for many animal species.   In addition, hemlock is an iconic tree that adds economic and social values to the southern Appalachian region. Since HWA was first inadvertently released into the U.S. in the 1950s, the insect has spread into 18 states and has been particularly destructive in Southern Appalachians. HWA is an “all-lands” problem that requires coordinated efforts among land management agencies -- sixty-percent of eastern hemlock trees occur on private lands in the Southern Appalachian region.

What can be done to slow the spread of HWA?

HWA3HWA management involves the integrated use of multiple approaches including chemical control, biological control, cultural treatments, host resistance, and host gene conservation.  Chemical control has been extremely effective at small scales, but biological control is the only viable option for controlling HWA across the landscape.  Research is continuing on effective control methods that may someday effectively stop the spread of HWA.

What are the Impacts of Hemlock Mortality?

Despite aggressive HWA control efforts, large numbers of hemlock tress in the southern Appalachian region and others are dead or in poor health.  This has important implications.  In addition to providing year round cover for wildlife, hemlock has a strong influence on riparian habitat conditions and stream health.  For example, hemlock needles and wood decompose slowly providing unique habitat for important forest floor organisms such as salamanders.  The shade cast by these majestic trees cools the water where brook trout and other stream organisms live.  With hemlock mortality, many of these important functions are changing.   For example, hemlock mortality is adding large quantities of litter to the forest floor and streams and even greater quantities will be added as standing dead trees continue to decompose.   Hemlock mortality is also changing basic ecological processes, such as the cycling of carbon, nutrient and water.  For example, some studies predict as much as 30% increase in streamflow during the winter months. Many of these short-term changes are localized and small; however, much more significant changes are expected in the coming decades as other species replace hemlock.  For example, in areas where Rhododendron is absent, red maple, sweet birch, and yellow poplar are likely replacement species.  Where Rhododendron is present, it is likely to limit recruitment of overstory species.  In either case, these new species will change habitats and ecological processes required by terrestrial and aquatic species.   The good news is that land managers have an opportunity to implement restoration activities now to prevent many of these undesirable long-term impacts.

Hemlock Plots

Progression of HWA infestation and crown loss within a 1600 ha watershed in the Southern Appalachians

What can land managers do now to minimize future impacts?

AdelgidControlling the spread and virulence of HWA is the best way to minimize future impacts;  however, where those efforts fail, dead and weakened tress will continue to degrade and land managers should anticipate the need to manage for large pulses of branches and wood to riparian areas and streams.  For example, in near-stream recreation areas, directional felling may be required to minimize hazards and stream crossings should be carefully monitored to ensure that culverts remain clear.  In order to maintain or restore the attributes provided by hemlock trees, restoration efforts may require novel approaches such as the introduction of non-native or hybridized hemlock species, facilitated movement of native species to new habitats, and aggressive management of existing undesirable species to facilitate desirable species.  In all cases, monitoring will be required to evaluate successes or failures and guide adaptive approaches.

Guiding Decision Making

Decision makers are now faced with making decisions about how to invest resources into the critical components of the HWA problem: control, detection monitoring, understanding effects of hemlock mortality, and restoration.  The following table outlines the decision options for managing HWA-Hemlock in the Southern Appalachians and the possible outcomes and implications of these decisions:

Decisions Outcomes Implications
1a.Intensify detection
and monitoring
Increased probability of early detection Enhanced control opportunities
2b.Monitor casually Lowered probability of early detection Reduced monitoring costs
2a. Apply control treatments Reduced probability of mortality

Chemical:  Must be ongoing to avoid mortality; per year costs remained relatively fixed; efficacy on individual trees is very good (e.g., 95%) when applied appropriately; costly to treat large numbers of trees.

Biological:  Treatment is not ongoing once agents establish and become self-perpetuating; high initial costs incurred in development/delivery but per year costs decrease over time after establishment; efficacy is less certain but some agents showing promise; more practical than chemical on landscape  scale.   Defers restoration activities and allows for development of new knowledge.

2b. Forego control treatments Rapid mortality (3-10 years following infestation) Sets time table of trajectory of ecological changes. May impose constraints on other management choices.
3a. Alter species composition using native species Replaces some ecosystem services (e.g., shading) Silvicultural options are not well defined. Efficacy is challenged by competition with rhododendron and other species.
3b. Alter species composition using nonnative hemlocks Replaces a broader range of ecosystem services Public acceptance on introducing nonnative species in public lands uncertain
3c. Forego species management Species composition determined by site conditions May result in undesirable forest composition with long-term implications for structural and functional attributes
4a. Post-mortality sanitation Reduces short run impacts on aquatic systems. Protects recreationists and aesthetic impacts. Reduces uncertainty about long-term impacts of standing dead hemlock, but initiates a trajectory of change.
4b. Forego sanitation treatments Short run (5-15 years) impacts on aquatic systems and recreation values. Increases uncertainty about long-term impacts of standing dead hemlock, but allows for a “wait and see” approach.


Article Authors: James M. Vose and David N. Wear, Center for Integrated Forest Science, USDA Forest Service, Southern Research Station; Albert E. Mayfield III, Insects, Diseases and Invasive Plants of Southern Forests, USDA Forest Service, Southern Research Station; and, C. Dana Nelson, Southern Institute of Forest Genetics USDA Forest Service, Southern Research Station.

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