Temporal variation in stomatal sensitivity to vapour pressure deficit in western riparian forests

Increasing atmospheric vapour pressure deficit (D) can influence plant water and carbon uptake. However, growing season variation in stomatal responses to D among tree taxa has not been thoroughly quantified and therefore has not been well‐characterized in stomatal regulation models. Using sap flux data from nine riparian species spanning a 600‐m elevation gradient in semi‐arid northern Utah, USA, we fit a time‐varying empirical model of stomatal conductance to D in a hierarchical Bayesian framework. The reference conductance (Gref, conductance at D = 1 kPa) term was modelled as a function of cumulative growing season D, which varied with site elevation. Seven species exhibited Gref that varied significantly with cumulative growing season D, but the direction was not consistent among species. Two low‐elevation ring‐porous species, the invasive Tamarix ramosissima and Elaeagnus angustifolia, exhibited significantly positive correlation between Gref and cumulative D, such that standardized stomatal sensitivity (S) decreased during the season. Despite lower D at the mid‐ and high‐elevation sites, five diffuse‐porous native species exhibited progressively increasing sensitivity to D during the growing season. Stomatal strategies exhibit seasonal trends that vary by environmental conditions (D) and functional traits (wood anatomy), which complicates the prediction of plant hydraulic function under increasing atmospheric drought. In the increasingly arid western United States, the progressively less sensitive stomatal behaviour of invasive taxa may hasten their dominance in riparian forests. Read the free Plain Language Summary for this article on the Journal blog. Read the free Plain Language Summary for this article on the Journal blog.