Quantifying structural and physiological controls on variation in canopy transpiration among planted pine and hardwood species in the southern Appalachians
Recent studies have shown that planted pine stands exhibit higher evapotranspiration (ET) and are more sensitive to climatic conditions compared with hardwood stands. Whether this is due to management and stand effects, biological effects or their interaction is poorly understood. We estimated growing season canopy- and sap flux-scaled leaf-level transpiration (Ec and EL) in five major overstory species over 3 years. Four hardwood species, Liriodendron tulipifera, Carya spp., Quercus rubra and Quercus prinus, were measured in an unmanaged watershed. Pinus strobus was measured in an adjacent planted pine watershed. We hypothesized that (1) species would differ in EL and stomatal conductance (GS), and their relationship with vapour pressure deficit (D); and (2) differences in growing season ET between the stands would result primarily from the differences in interception (Ei). Growing season ET in the planted pine stand exceeded hardwood ET by twofold during all 3 years. Transpiration and Ei contributed similarly to the ET difference, suggesting that physiological differences were equally as important as structural factors to the overall difference in ET. Among species, mean EL and GS differed significantly, as did their relationship with D. EL and GS of oaks and hickories were least responsive to changing D, while L. tulipifera and P. strobus were most responsive. This species-level understanding of variation in EL and GS and their interactions with climatic driving variables has important implications for predicting watershed-level responses to stand management, species invasion and loss, and climate variability.