The stomatal regulation of 38 tree species as a window into water-use strategies under climate change

Climate change is increasing the compounding of droughts and heatwaves, causing widespread tree growth decline and mortality. To improve predictions of forest vulnerability, understanding current tree water-use strategies is critical. However, few data exist that contrast more than a handful of species’ responses to the same growth conditions.

We investigate water-use in 38 temperate tree species (27 angiosperms, 11 gymnosperms) at a research arboretum in Germany (ARBOfun, Großpösna). In the summer of 2024, we measured stomatal conductance (g_s; which regulates carbon assimilation and transpiration) in three individuals per species repeatedly over diurnal cycles. The summer was hot, yet soil water availability remained sufficient. This allowed us to isolate stomatal responses to vapor pressure deficit (VPD), a key component of tree water-use strategies under atmospheric drought, and determine proxies of stomatal sensitivity to increasing atmospheric aridity, such as the inflection point of the g_s-VPD curve. Species showed a wide variation in stomatal sensitivity to VPD, ranging from early-closing to high-VPD-tolerant strategies. Ongoing analyses relate these species-specific sensitivity proxies to leaf traits and growth responses, advancing our understanding of water-use diversity under climate change.