Effects of Invasive Hemlock Woolly Adelgid on Northeastern Forests

Hemlock is a "foundation" tree species in eastern forests and its presence defines the properties of a unique ecosystem that is presently declining due to the introduction and spread of an invasive insect, the hemlock woolly adelgid (HWA), which currently covers 25% of hemlock's geographic range. However, the impacts of HWA are highly variable, causing rapid, stand-wide hemlock removal in some regions and little visible impact in others.

At sites where hemlock trees succumb to HWA, ecosystem function, including water, carbon and nutrient cycles, are dramatically altered. Dr. Shannon LaDeau and colleagues, Drs. Gary Lovett and Jennifer Fraterrigo (University of Illinois) are investigating the biological mechanisms that generate spatial variability in hemlock resistance to HWA impacts across New York and the broader northeastern hemlock forest.

The relationship among winter temperatures, HWA populations, and hemlock impact is generally assumed to be temperature-driven. However, in many parts of the HWA range, sites with similar climates experience diverse HWA-related impacts, ranging from apparent coexistence of HWA with hemlock for over a decade to complete collapse of the hemlock ecosystem in under five years.

This research integrates experimental and field data with Bayesian data-model assimilation techniques and GIS to evaluate how multiple interacting forces, including climate, air pollution and other insect pests, generate spatio-temporal heterogeneity in hemlock forest decline.

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