Steep Slopes and Cliff Edges Are Browsing Refugia for Woody Species within Temperate Deciduous Forest

Throughout the Eastern Deciduous Forest Biome, overbrowsing by white-tailed deer (Odocoileus virginianus) has reduced plant diversity and impoverished forest regeneration. However, some geomorphic features may provide refuge from browsing. For example, browse-sensitive species are often abundant on the tops of tall boulders and treefall tip-up mounds. If refugia are common, they might serve as viable seed-source populations for recovery of vulnerable plant species that have been locally or regionally extirpated. Here, we test the hypothesis that cliff edges and steep slopes provide refuge for browse-sensitive woody species. We quantified stem density or individual patch size, or both, of four woody species at six steep exposures across the Hemlock-Northern Hardwoods Association. Level areas adjacent to steep exposures served as deer-accessible reference sites. The combined density of our focal species was 16 times higher on cliff edges and steep slopes versus reference areas, and combined mean patch size was nearly six times larger. We also conducted a remotely sensed spatial analysis of slopes between 35° and 75° throughout the State of Pennsylvania to quantify the abundance and spatial aggregation of these potential refugia. Steep slopes occurred in all 67 Pennsylvania counties, representing up to 3% of land surface in rugged counties. Steep slopes were also often clustered or large, suggesting locally high gene flow may maintain genetically diverse and vigorous seed-source populations of browse-sensitive species. Our study is the first to quantitatively demonstrate that cliff edges and steep slopes have high conservation value by serving as browsing refugia for browse-sensitive woody plants in temperate forests.