Stomatal acclimation to vapor pressure deficit doubles transpiration of small tree seedlings with warming

Future climate change is expected to increase temperature (T) and atmospheric vapour pressure de?cit (VPD) in many re-gions, but the effect of persistent warming on plant stomatal be-haviour is highly uncertain. We investigated the effect of experimental warming of 1.95.1 C and increased VPD of 0.51.3 kPa on transpiration and stomatal conductance (gs)of tree seedlings in the temperate forest understory (Duke Forest, North Carolina, USA). We observed peaked responses of tran-spiration to VPD in all seedlings, and the optimum VPD for transpiration (Dopt) shifted proportionally with increasing chamber VPD. Warming increased mean water use of Carya by 140% and Quercus by 150%, but had no signi?cant effect on water use of Acer. Increased water use of ring-porous spe-cies was attributed to (1) higher air T and (2) stomatal acclima-tion to VPD resulting in higher gs and more sensitive stomata, and thereby less ef?cient water use. Stomatal acclimation main-tained homeostasis of leaf T and carbon gain despite increased VPD, revealing that short-term stomatal responses to VPD may not be representative of long-term exposure. Acclimation responses differ from expectations of decreasing gs with in-creasing VPD and may necessitate revision of current models based on this assumption.