SPATIAL DISTRIBUTION OF RIPARIAN HEMLOCK FORESTS IN THE CHESAPEAKE BAY WATERSHED: THREATS OF AN INVASIVE INSECT ON ECOHYDROLOGY

Hydrologic processes in forests are particularly sensitive to disturbances that reduce tree vigor and reduce transpiration. Few studies have examined how insect outbreaks affect landscape-level hydrologic processes. Eastern hemlock (Tsuga canadensis) is a long-lived, shade tolerant tree that is considered a foundation species, especially in riparian corridors and cove habitats, where trees can be highly concentrated. Hemlock stands are characterized by a dense, evergreen canopy that creates a unique microenvironment within a broader forest landscape, which is dominated by deciduous species in the northeastern USA. Hemlock serves distinct ecohydrological roles: as an evergreen it maintains year-round transpiration rates; the dense evergreen canopy increases interception rates and affects stream temperatures.

Since the 1950s, the non-native hemlock woolly adelgid (Adelges tsugae) (HWA) has spread to infest hemlock in at least 18 states and caused widespread decline and mortality. Permanent reductions in winter transpiration rates occur but eventual increases during the growing season as hemlock is replaced by deciduous species. A study in the southern Appalachians documented that hemlock loss from watersheds during large flow events, had permanent reductions in yield and transient increases in peakflow.

We used tree data from USDA Forest Service Forest Inventory and Analysis (FIA) field plots, located within sub-watersheds of the Chesapeake Bay, and interpolated hemlock distribution layer to estimate hemlock basal area at multiple scales. Watersheds where hemlock basal area was at least 6% of the total forest cover were further examined at two geographic scales: streams buffered at 250m and 500 m. These watersheds are located in northern PA and southern NY, where HWA has had less impact to date. Hemlock was more concentrated in the riparian buffer zones, compared to the sub-watershed scale, and basal area was higher in the 250 m zone. We have used this information to produce watershed maps of forests at different landscape scales, identifying high concentrations of hemlock in riparian buffers near headwater streams. This information can be used to prioritize potential sites for field monitoring of hydrologic processes as anticipated hemlock decline increases.