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| 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 1.9–5.1 °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 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. | |
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