Application of a forest gap model for prediction of browsing effects on riparian forest succession

Abstract

Selective browsing by white-tailed deer ( Odocoileus virginianus) severely impacts seedling dynamics within riparian forests on the Patuxent Research Refuge, MD. To evaluate the long-term implications of browsing for forest dynamics, field studies of seedling recruitment, seedling growth, and photosynthetic performance are used to adapt a forest gap model (ZELIG) for simulating browsing interactions with tree species life history characteristics. Primary alterations to the model include addition of small seedling demography, soil saturation effects, shade adaption of tree photosynthesis, and browsing algorithms. Simulated stands were subjected to browsing as a function of total seedling density. If a stand is browsed, then seedling species are browsed selectively based on deer feeding preferences. Model output without browsing impacts closely matches current riparian forest composition. Simulated browsing depresses total forest basal area during mid-succession by reducing regeneration and growth of early successional yellow poplar ( Liriodendron tulipifera), allowing late successional American beech ( Fagus grandifolia) to gain earlier dominance. Greater browsing intensities increase the impact on basal area and displace overall forest composition further from control conditions. Control and browsing simulations converge toward an American beech dominated forest in late succession. Thus browsing interacts with tree species life history strategies to alter the rate of succession, but not the long-term trajectory of succession. Further advances in simulating the impacts of deer browsing will benefit from incorporating landscape-scale deer population dynamics and spatially explicit distributions of forest trees.