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

The regulation of vascular water potential (Ψ) 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 regulatory behaviour in response to soil or atmospheric dryness remains elusive. We hypothesize that Ψ regulatory behaviour can only be defined when the combination of both vapour pressure deficit (VPD) and soil water potential (Ψ) effects is considered. To test this hypothesis, we collected a high-resolution time series of Ψ using optical dendrometers from trees of a hardy conifer, Callitris rhomboidea, monitored across multiple highly variable growing seasons. The regulatory behaviour of Ψ collected over a total of 571 d could be predicted on the basis of diurnal Ψ and VPD (R = 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 monitoring technique creates a new opportunity to quantitatively compare water use and climatic sensitivity between diverse species or genotypes in the field or laboratory.