Spatial Distribution of Chesapeake Bay Riparian Hemlock Forests Threatened by Hemlock Woolly Adelgid

Landscape-scale maps of tree species densities are important tools for managing ecosystems threatened by forest pests. Eastern hemlock dominates riparian forests throughout its range. As a conifer in a deciduous landscape, hemlock plays an ecohydrological role, especially when other species are dormant. The nonnative, hemlock woolly adelgid has caused widespread hemlock decline and mortality. We used two existing basal area raster layers first to identify Chesapeake Bay subwatersheds with ≥6 percent hemlock basal area and second to quantify hemlock basal area densities within fixed-width riparian buffers of 50 m, 100 m, 250 m, and 500 m. Hemlock densities were higher in riparian zones compared with entire subwatersheds. In five subwatersheds, 50 m and 100 m zones had higher percentages of pixels with ≥25 percent hemlock basal area. We produced maps identifying hemlock riparian densities in the Pine Creek Watershed, which managers can use to prioritize sites for supplemental conifer planting under anticipated hemlock decline. Study Implications: Forest inventory and satellite data were used to map riparian hemlock stands in the Pine Creek Watershed (Pennsylvania). Pine Creek is a subwatershed of the Chesapeake Bay and an important tributary of West Branch Susquehanna River. Pine Creek headwaters are a brook trout refuge, and hemlock shading along streams stabilizes water temperature. These fisheries provide recreational value and economic support to local communities. Hemlock woolly adelgid, an invasive insect, has recently entered the watershed and will cause hemlock decline and mortality. Our maps assist the Pine Creek Watershed Council in identifying riparian areas for supplemental planting of alternative conifer seedlings.