The study investigated the effects of exposure to increased relative air humidity (RH) on stomatal morphology and sensitivity to stomata closure inducing stimulus (low RH) in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) coppice growing in field conditions. Artificially elevated RH reduced air vapour pressure deficit by 5%–10% and altered stomatal sensitivity; trees grown under high RH exhibited stronger stomatal response to decreasing air humidity. We found no difference in mean stomatal pore length between treatments and a small decline in stomatal density under humidification. The lack of correlation between stomatal sensitivity and morphological traits suggests that stomatal sensitivity was unaffected by stomatal morphology. In light of rising atmospheric humidity predicted for high latitudes, strict stomatal control over water loss might be beneficial for trees if drought events become more frequent in the future. However, our experiment revealed that about two-thirds of the leaf-to-air vapour pressure difference (VPDL) response curves demonstrated the opposite pattern, i.e., stomatal opening in response to increasing VPDL. Strict stomatal regulation is probably not beneficial to fast-growing aspen coppice under low RH, as this trait may restrict their carbon gain and growth rate.
Survey and synthesis of intra- and interspecific variation in stomatal sensitivity to vapour pressure deficit
