Differences in forest structure and composition along a hillslope gradient have been attributed to the variation in environmental conditions influenced by topography. The interaction of climatic variability with soil moisture at different landscape positions may affect vegetation water use and growth, with topography mitigating or intensifying drought effects. This study examined the interaction of topography, soil moisture and transpiration in low-elevation, mixed-hardwood forest in the southern Appalachian Mountains. The effects of meteorological variation (wet and dry years) and topographic position (upslope and cove) were tested on daily soil moisture amplitude and recession; and plot and species-specific transpiration.
The cove plot had taller trees with greater sapwood area than the upslope plot. Lower soil moisture recession following rainfall events, greater daily soil moisture amplitude and plot transpiration at the cove plot, even in dry years, suggests that lower slope positions may have been buffered against moderate drought. Transpiration was reduced by 51% in the dry year in the upslope plot only, and transpiration of Quercus rubra and Carya spp. in the plot reflected this pattern, suggesting water stress in dry years may be exacerbated by landscape position. Transpiration of Liriodendron tulipifera in the cove plot was similar between dry and wet years despite a small decrease in stomatal conductance in the former, indicating its sensitivity to increased vapor pressure deficit. Lower slope positions may provide a refuge from water stress for Quercus and Carya spp., while L. tulipifera may not benefit as they are more sensitive to increased atmospheric demand of water. With drought predicted to increase for these systems, the different drought responses of species, in addition to topographic effects, may lead to complex shifts in species composition.