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Northern Research Station
11 Campus Blvd., Suite 200
Newtown Square, PA 19073
(610) 557-4017
(610) 557-4132 TTY/TDD

Hemlock Woolly Adelgid

Variability in Hemlock Decline Rates Following Infestation: Field Studies to GIS Susceptibility Modeling

[image:] A spatially continuous, comprehensive map of relative hemlock vulnerability to HWA-induced decline in the Catskills, NY area (left) summarizes the cumulative effect of key variables identified in the statistical modeling.  Masking non-hemlock pixels (right) using an AVIRIS-derived coverage of percent hemlock basal area produces a hemlock-only risk map.  This allows land managers to direct management efforts by identifying hemlock dominated stands and their vulnerability to HWA.

Research Issue

The hemlock woolly adelgid (Adelges tsugae Annand) (HWA) is an invasive insect pest that is causing widespread mortality of eastern hemlock (Tsuga canadensis). However, some stands remain living more than a decade after infestation. The ability to target management efforts in locations where hemlock is most likely to tolerate prolonged HWA infestation is critical to successful integrated pest management programs.

Our Research

We set out to examine the relationships between site, stand, soil, and foliar characteristics to identify the key variables in determining the rate of hemlock decline following HWA infestation across the Northeast. These field based results were then used to build a landscape-scale hemlock risk model for the Catskills region of New York.

Expected Outcomes

Understanding the potential drivers in hemlock decline will aid in management efforts aimed at ultimately controlling HWA. Because of the unique ecological niche occupied by hemlock stands, it is important to identify the stands that are most likely to tolerate HWA infestations so that hemlock can be preserved as a component of forest habitats in the region. At the same time, stands likely to suffer high rates of mortality can be evaluated for integrated pest management activities or conversion to other species.

Research Results

Comparisons of resistant and susceptible hemlock species indicate higher concentrations of Phosphorus (P) and lower concentrations of nitrogen (N) in resistant species. On experimentally colonized hemlocks, the numbers of live sistens present after two A. tsugae generations were correlated with higher Potassium (K) and lower P concentrations. A regional T. canadensis monitoring effort showed that concentrations of Calcium (Ca), K, N, and P were most strongly correlated with A. tsugae densities, which was the driving factor in hemlock decline. From the results of this study, we hypothesize that higher N and K concentrations may enhance hemlock palatability, thereby increasing A. tsugae population levels, whereas higher concentrations of Ca and P may deter more severe infestations.

Foliar chemistry alone can explain more than one-half of the variability in hemlock decline witnessed at 45 monitoring plots across the northeastern United States. Combining chemistry and traditional site factors, we could predict an 11-class decline rating with 98 percent 1-class tolerance accuracy on an independent validation set.

Based on these results, we tested the field-based relationships on a landscape scale using GIS modeling techniques. The initial landscape variables-only model was able to predict the rate of overall decline following HWA infestation for 21 plots across the Northeast with R2 = 0.35, p = 0.027. Adding foliar N concentration to our model improved results to R2 = 0.79, p = 0.0009. An AVIRIS-derived hemlock abundance coverage was then used to define the hemlock resource and its relative vulnerability to rapid decline. These results indicate that the inclusion of both landscape and chemical variables is critical to predicting hemlock vulnerability to HWA and that landscape-scale modeling in a GIS platform is possible with the addition of hyperspectral remote sensing coverages. While the resulting risk map covers only the Catskills region of New York, the relationships established here should be applicable to HWA infestation across the range of eastern hemlock, providing a basis for forest land management agencies to make informed management decisions.

Pontius, J.; Hallett, R.; Martin, M.; L. Plourde. 2008. A landscape scale GIS tool to assess eastern hemlock susceptibility to hemlock wooly adelgid infestation. Online Encyclopedia of Forest Environmental Threats.

Pontius, J.; Hallett, R. A.; Martin, M. E. 2006. Foliar chemistry linked to hemlock woolly adelgid success and hemlock susceptibility. Environmental Entomology, 35(1):112-120.

Pontius, J.; Hallett, R.; Martin, M. 2005. Using AVIRIS to assess hemlock abundance and early decline in the Catskills, New York. Remote Sensing of Environment, 97:163-173 .

Research Participants

Principal Investigator

  • Jennifer Pontius, Research Assistant Professor, University of Vermont, Rubenstein School of Environment and Natural Resources and Cooperating Scientist, U.S. Forest Service, Northern Research Station
  • Richard Hallett, Research Ecologist. U.S. Forest Service, Northern Research Station


Last Modified: 10/21/2010