AN ASSESSMENT OF THE ECOHYDROLOGICAL ROLE OF EASTERN HEMLOCK (TSUGA CANADENSIS) AND ITS PENDING DECLINE IN CHESAPEAKE BAY WATERSHEDS

Forest cover in the Chesapeake Bay Watershed (CBW) accounts for approximately 55% of the total land area. Forests play a critical role in regulating hydrologic processes by moderating the timing and magnitude of streamflow. Eastern hemlock is distributed widely throughout the northeastern USA and the higher elevations of the southern Appalachians. Hemlock is a long-lived, shade tolerant species 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 this region. Hemlock serves several distinct ecohydrological roles: as an evergreen it maintains year-round transpiration rates; the dense evergreen canopy increases interception rates and effects stream temperatures.

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. Since its introduction in the 1950s, the non-native hemlock woolly adelgid (Adelges tsugae) has spread to infest hemlock in at least 18 eastern states and caused widespread decline and mortality, typically within 4-10 years. Mortality results in permanent reductions in winter transpiration rates 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, demonstrated permanent reductions in yield and transient increases in peakflow during large-flow events. In this poster, we will describe the process and results of determining the amount and change in hemlock basal area, at two points in time up to 2014, in the CBW. We will use the most recent data from the USDA Forest Service Forest Inventory and Analysis plots located within the CBW boundary to estimate the basal area and location of hemlock stands. We will also focus on identifying areas with high concentrations of hemlock near headwater streams, especially those demonstrating high mortality, for future field monitoring of hydrologic processes as anticipated hemlock decline increases.