Abstract (from http://onlinelibrary.wiley.com/doi/10.1111/pce.12790/full): Future climate change is expected to increase temperature (T) and atmospheric vapour pressure deficit (VPD) in many regions, but the effect of persistent warming on plant stomatal behaviour is highly uncertain. We investigated the effect of experimental warming of 1.9–5.1 °C and increased VPD of 0.5–1.3 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 transpiration 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 [...]