OOS 2-1: Impacts of eastern hemlock mortality on ecosystem function: A guide to restoration strategies in the southern Appalachians
James M. Vose, Chelcy R. Ford, Jennifer D. Knoepp, and Katherine J. Elliott. USDA Forest Service Southern Research Station
Background/Question/Methods Despite the implementation of various biological and chemical controls, eastern hemlock trees (Tsuga canadensis (L.) Carr.) are declining throughout much of the eastern US due to hemlock woolly adelgid (HWA). The decline is especially rapid in the southern Appalachian region of the US. We used a combination of intensive measurements and monitoring at the Coweeta Hydrologic Laboratory in western N.C. to (1) determine the spatial distribution of hemlock in southern Appalachian ecosystems, (2) monitor the rate of spread and decline of HWA infested tress, and (3) quantify the impacts of HWA induced hemlock mortality on water, carbon, and nutrient cycling pools and processes. Our primary objective was to use our understanding of the impacts of hemlock decline on ecosystem structure and function to guide restoration strategies. Results/Conclusions Eastern hemlock is present in mixed stands within riparian zones in the southern Appalachians; highest hemlock densities occur within 50 m of streams and hemlock can occupy as much as 50% of the basal area. HWA was first detected in a few scattered trees in the Coweeta basin in 2003. However, by 2005, HWA was detected in 100% of hemlock trees across a network of vegetation plots. By 2008, average crown loss of infested trees exceeded 80%. Sapflow measurements and modeling approaches showed that annual stand level transpiration has been reduced by 10%, with greater reductions (up to 30%) in the spring and fall. Carbon cycle components were impacted within three years of infestation. We observed a rapid decrease in stem growth in hemlocks, increased growth of co-occurring hardwoods, and decreased fine root biomass and soil respiration. Changes in nutrient cycling were not detected, suggesting that some of the impacts of hemlock mortality on ecosystem processes make take several years to manifest. While we anticipate a substantial pulse of coarse woody debris (CWD) as hemlock trees fall, likely replacement species (Betula and Acer) decompose quickly. Restoring hydrologic processes, forest structure, and CWD are the highest priorities for ecological restoration. In particular, establishing evergreen tree cover will be necessary to restore soil moisture dynamics and establishing species with recalcitrant CWD will be necessary to maintain temporal CWD dynamics in the forest floor and streams.