MONITORING THE EFFECTS OF AN INVASIVE INSECT (ADELGES TSUGAE) ON FOREST DECLINE AND LANDSCAPE-LEVEL HYDROLOGIC PROCESSES

<span">Hydrologic processes in forests are 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 and cove habitats, where trees can be highly concentrated along riparian corridors. 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.

<span">Since its introduction in 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. Mortality results in permanent reductions in winter transpiration rates but eventual increases during the growing season as hemlock is replaced by deciduous species. In the southern Appalachians, hemlock loss from watersheds demonstrated permanent reductions in yield and transient increases in peakflow.

<span">We used the most recent data from the USDA Forest Service Forest Inventory and Analysis (FIA) plots located within the Chesapeake Bay Watershed boundary to estimate the amount and change in basal area in hemlock stands from 2008 to 2014. Forests with high concentrations of hemlock near headwater streams, were also identified as potential sites for field monitoring of hydrologic processes as anticipated hemlock decline increases. The hemlock forest and several major streams on a 600,000 acre watershed in a Pennsylvania state forest are being monitored in collaboration with the USGS. Forest change in the Pine Creek watershed will be monitored as HWA enters the watershed. FIA data indicates that hemlock mortality is not yet evident but adjacent watersheds show increases in mortality and decreases in abundance. Future analyses will combine data from FIA, Tioga State Forest and USGS stream monitoring in order to quantify the impacts of hemlock losses on hydrologic processes.