Stomatal sensitivity to VPD across 38 tree species reflects past drought responses but is little explained by stomatal and leaf economic traits
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Climate change is exposing trees to rising atmospheric water demand. Because stomata regulate plant water loss, understanding stomatal behaviour is critical. However, few comparative studies examine species-specific stomatal responses to atmospheric demand.
Here, we quantify stomatal behaviour in 38 temperate tree species (27 broadleaved, 11 conifers) in a German research arboretum during a warm summer without soil drought. We measured the diurnal stomatal conductance (g s ) of 787 leaves and modelled g s species-specific responses to leaf-to-air vapour pressure deficit (VPD L ). Four descriptors of the g s -VPD L relationship were derived: maximum stomatal conductance (g max ), the VPD L that maximises g s , the VPD L associated with a 50% reduction in g s , and the stomatal closure rate. We relate these metrics to drought-induced shifts of carbon isotope ratios and to leaf functional traits.
Large variation in stomatal behaviour exists, ranging from early-closure to VPD L -tolerant strategies. Species maintaining g s at higher VPD L also show weaker physiological responses to past drought. g max is partly explained by leaf economic traits; other gs-VPDL descriptors are not related to stomatal anatomy and leaf economy.
Our interspecific comparison reveals substantial variation in stomatal responses to atmospheric demand that is relevant to drought responses, but cannot be readily inferred from commonly measured leaf traits.
