A theoretical framework to quantify ecosystem pressure-volume relationships

Water potential’ is the biophysically relevant measure of water status in vegetation relating to stomatal, canopy, and hydraulic conductance, as well as mortality thresholds; yet this cannot be directly related to measured and modelled fluxes of water at plot- to landscape-scale without understanding its relationship with ‘water content’.  The capacity for detecting vegetation water content via microwave remote sensing further increases the need to understand the link between water content and ecosystem function.  In this review, we explore how the fundamental measures of water status, water potential and water content, are linked at ecosystem scale drawing on the existing theory of pressure-volume (PV) relationships. We define and evaluate the concept and limitations of applying PV relationships to ecosystems where the quantity of water can vary on short timescales with respect to plant water status, and over longer timescales and over larger areas due to structural changes in vegetation. 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 that the stored water and ecosystem capacitance scale linearly with biomass across diverse systems, while the relative values of ecosystem hydraulic capacitance and physiologically accessible water storage do not vary systematically with biomass.  The bottom-up scaling approach to ecosystem water relations identified the need to characterise the distribution of water potentials within a community; and also revealed the relevance of community-level plant tissue fractions to ecosystem water relations.  We believe that this theory will be instrumental in linking our detailed understanding of biophysical processes at tissue-scale to the scale at which land surface models operate and at which tower-based, airborne and satellite remote sensing can provide information.

Methods
The main data used in the analyses were plot-level forest inventories.  Seven of the eight sites were in Australia, where the inventories were undertaken by CSIRO. The original data sets and protocol are available on the TERN (Terrestrial Ecosystem research Network) website: https://portal.tern.org.au. The non-Australian site was in Amazonian rainforest in Floresta Nacional de Caxiuanã, Brazil, and is the site of a long-term throughfall exclusion experiment.  Both the experiment and inventory methodologies are described in da Costa et al. (2010) and Rowland et al. (2015).

The actual data are in their original raw form, although the data sets have been subsetted in some cases to represent a fraction of the original area.  For example 1 hectar out of 25 for the Robson Creek site, or only the data representing the most recent collection.

Related works:

da Costa, A. C. L., Galbraith, D., Almeida, S., Portela, B. T. T., da Costa, M., de Athaydes Silva Junior, J., Braga, A. P., de Gonçalves, P. H. L. L., de Oliveira, A. A. R., Fisher, R., Phillips, O. L., Metcalfe, D. B., Levy, P., Meir, P., Silva Junior, J. D. A., Braga, A. P., de Gonçalves, P. H. L. L., de Oliveira, A. A. R., Fisher, R., … Meir, P. (2010). Effect of 7 yr of experimental drought on vegetation dynamics and biomass storage of an eastern Amazonian rainforest. The New Phytologist, 187(3), 579–591. https://doi.org/10.1111/j.1469-8137.2010.03309.x

Rowland, L., Da Costa, A. C. L. C. L. L., Galbraith, D. R. R., Oliveira, R. S. S., Binks, O. J. J., Oliveira, A. A. R. A. R. R., Pullen, A. M. M., Doughty, C. E. E., Metcalfe, D. B. B., Vasconcelos, S. S. S., Ferreira, L. V. V., Malhi, Y., Grace, J., Mencuccini, M., & Meir, P. (2015). Death from drought in tropical forests is triggered by hydraulics not carbon starvation. Nature, 528(7580), 1–13. https://doi.org/10.1038/nature15539