The Ecosystem Pressure-Volume Curve

The ecosystem pressure-volume curve (EPV) is the relationship between vegetation water content and a representative value of water potential applied on a ground-area basis. The EPV attempts to reconcile our detailed and physically rigorous understanding of small-scale field-measureable processes to the spatial scale applicable to ecosystem and climate science. Successfully bridging that gap in scale potentially allows us to use field measurements to interpret remote sensing data, and then remote sensing data to inform our understanding of vegetation-climate interactions. Here we clearly define the idea of the EPV, evaluate the limitations of applying values of water content and water potential to ecosystems on a ground area basis, and discuss practical ways to construct the EPV with existing data. We also present the first EPVs based on data from nine different plots, including tropical rainforest, savanna, temperate forest, and a long-term drought experiment in Amazonian rainforest (Caxiuana, State of Para, Brazil). The initial findings suggest high levels of consistency among sites. In particular, the ratio of water to biomass across ecosystems appears to be constrained to around 1:3. Seven of nine sites had closely converging relative maximum water storage (the proportion of total stored water than can be lost before an ecosystem succumbs to physiological damage) at 9.1% +/-1.8 standard deviation. Relative ecosystem capacitance may increase with site biomass (P = 0.091), but varied little across sites with a mean of 0.068 MPa-1 +/-0.029 standard deviation. These first estimates suggest that the EPV idea may reveal useful trends across ecosystems, potentially paving the way to increasing the ecophysiological significance of remote sensing data, and enabling an alternative method for modelling long-term ecosystem-climate feedbacks based on equilibrium thermodynamics.