Using long‐term field data to quantify water potential regulation in response to VPD and soil moisture in a conifer tree

Summary The regulation of vascular water potential (Ψ stem ) by stomata is one of the most dynamic and important behaviours in vascular plants, playing a central role in determining gas exchange and vulnerability to drought. Yet, the species‐specific characterization of Ψ stem regulatory behaviour in response to soil or atmospheric dryness remains elusive. We hypothesize that Ψ stem regulatory behaviour can only be defined when the combination of both vapour pressure deficit (VPD) and soil water potential (Ψ soil ) effects is considered. To test this hypothesis, we collected a high‐resolution time series of Ψ stem using optical dendrometers from trees of a hardy conifer, Callitris rhomboidea , monitored across multiple highly variable growing seasons. The regulatory behaviour of Ψ stem collected over a total of 571 d could be predicted on the basis of diurnal Ψ soil and VPD ( R 2 = 0.74) using five mechanism‐aligned parameters that describe specific stomatal regulation. Our novel approach to predict species‐specific water potential variation in response to seasonal change using data from a continuous Ψ stem monitoring technique creates a new opportunity to quantitatively compare water use and climatic sensitivity between diverse species or genotypes in the field or laboratory.