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

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 (R² = 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.