Variability of Tree Transpiration Along a Soil Moisture Gradient in a Southeastern U.S. Piedmont Watershed

Quantifying species-specific transpiration at different soil moisture levels is important for understanding the effects of environmental change including drought, floods, and urbanization on ecosystem responses. We measured sap flux density (J_s) during a wet and dry year in a 12-ha forested watershed dominated by chestnut oak (Quercus Prinus), loblolly pine (Pinus taeda), northern red oak (Quercus rubra), red maple (Acer rubrum), sweetgum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera), Virginia pine (Pinus virginiana), and white oak (Quercus alba). The tree transpiration (T_s) and watershed-level transpiration (T_w) was determined in three zones (riparian buffer, mid-hillslope, and upland-hillslope) of contrasting topography and soil moisture. In loblolly pine, Virginia pine, and sweetgum, J_s were significantly higher in the riparian buffer zone when compared to other zones, whereas J_s in tulip poplar and red maple were significantly lower in the riparian buffer than in the mid-hillslope. During the dry period, J_s in loblolly pine, white oak, sweetgum, and Virginia pine was less responsive to VPD on upland-hillslopes compared to the riparian buffer. In contrast, J_s in tulip poplar and red maple on the mid-hillslope were more responsive to VPD than on the riparian buffer, suggesting there was sufficient energy and soil water to maintain transpiration for these species. The average annual calculated T_w based on T_s data from the riparian buffer was 447 mm, mid-hillslope was 377 mm, and upland-hillslope was 340 mm. We conclude that there is a large spatial variability in tree and forest transpiration due to differences in the soil moisture regime and in the forest tree species composition.