The relationships between water potential and water content in plants and soil have long been of interest, and there is increasing focus on understanding how these fundamental measures are linked at larger spatial and temporal scales. In this Perspective, we explore how the theory of pressure-volume (PV) relationships can be applied at ecosystem scale. We define and evaluate the concept and limitations of the ecosystem and vegetation pressure-volume curves and discuss its application using existing data. As a proof of concept, plot-scale aboveground vegetation PV curves were generated from equilibrium (e.g. predawn) water potentials and water content of the above ground biomass of nine plots including tropical rainforest, savanna, temperate forest, and a long-term Amazonian rainforest drought experiment. Initial findings suggest high consistency among sites where the steady-state water:biomass ratio is approximately 1:3, while the relative values of ecosystem hydraulic capacitance and accessible water storage (the water volume between saturation and a threshold) do not vary systematically with biomass. The ecosystem-scale PV relationship provides a thermodynamically consistent steady-state view of ecosystem form and function and a biophysically robust basis for the interpretation of remote sensing data of vegetation and soil water content, with promise for revealing useful trends across ecosystems.

We aim to identify the relative importance of vapour pressure deficit (VPD), soil water content (SWC) and photosynthetic photon flux density (PPFD) as drivers of tree canopy conductance, which is a key source of uncertainty for modelling vegetation responses under climate change. We use sap flow time series of 1858 trees in 122 sites from the SAPFLUXNET global database to obtain whole-tree canopy conductance (G). The coupling, defined as the percentage of variance (R2) of G explained by the three main hydrometeorological drivers (VPD, SWC and PPFD), was evaluated using linear mixed models. For each hydrometeorological driver we assess differences in coupling among biomes, and use multiple linear regression to explain R2 by climate, soil and vegetation structure. We found that in most areas tree canopy conductance is better explained by VPD than by SWC or PPFD. We also found that sites in drylands are less coupled to all three hydrometeorological drivers than those in other biomes. Climate, soil and vegetation structure were common controls of all three hydrometeorological couplings with G, with wetter climates, fine textured soils and tall vegetation being associated to tighter coupling. Differences across sites in the hydrometeorological coupling of tree canopy conductance may affect predictions of ecosystem dynamics under future climates, and should be accounted for explicitly in models.

Hydraulic properties control plant responses to climate and are likely to be under strong selective pressure, but their macro-evolutionary history remains poorly characterized. To fill this gap, we compiled a global dataset of hydraulic traits describing xylem conductivity (Ks), xylem resistance to embolism (P50), sapwood allocation relative to leaf area (Hv) and drought exposure (ψmin), and matched it with global seed plant phylogenies. Individually, these traits present medium to high levels of phylogenetic signal, partly related to environmental selective pressures shaping lineage evolution. Most of these traits evolved independently of each other, being co-selected by the same environmental pressures. However, the evolutionary correlations between P50 and ψmin and between Ks and Hv show signs of deeper evolutionary integration because of functional, developmental or genetic constraints, conforming to evolutionary modules. We do not detect evolutionary integration between conductivity and resistance to embolism, rejecting a hardwired trade-off for this pair of traits.