Interactive effects of chronic deer browsing and canopy gap disturbance on forest successional dynamics

The interaction of browsing by white-tailed deer (Odocoileus virginianus) and canopy gap disturbances may affect long-term tree composition and lead to significant changes in forest structure. We used an individual-based forest gap model (ZELIG) to better understand the aggregate and interactive impacts of these processes on the long-term (200 years) successional dynamics of a mesic deciduous forest. We parameterized ZELIG to: (1) simulate successional dynamics within a temperate deciduous secondary forest typical of eastern North America; (2) simulate browsing impacts by white-tailed deer; and (3) simulate gap-scale disturbance of variable size and frequency. Our estimates of browsing impacts by species were derived from a 20-year, four-hectare deer exclusion study. Model calibration matched observed tree species composition, density by size class, and total basal area (39.92 m(2) ha(-1) vs. 37.13 m(2) ha(-1)). Simulated deer browsing had little impact on total basal area over two centuries. However, deer browsing had substantial impacts on community composition, creating a less diverse understory, lower species richness, and decreased abundance of Quercus species, while retaining the dominance of Liriodendron tulipifera. Simulated gap disturbances exacerbated the impacts of chronic deer browsing and these impacts became stronger over time. Our analyses suggest that recent increases in white-tailed deer density within many forests of eastern North America will result in altered community dynamics that persist beyond the sapling level, and that any increases in overstory disturbance frequency will exacerbate these impacts.