Applying the Concept of Ecohydrological Equilibrium to Predict Steady State Leaf Area Index

Leaf area index (LAI) is a key variable in modeling terrestrial vegetation because it has a major impact on carbon and water fluxes. However, several recent intercomparisons have shown that modeled LAI differs significantly among models and between models and satellite-derived estimates. Empirical studies show that LAI is strongly related to precipitation. This observation is predicted by the ecohydrological equilibrium theory, which provides an alternative means to predict steady state LAI. We implemented this theory in a simple optimization model. We hypothesized that, when water availability is limited, plants should adjust steady state LAI and stomatal behavior to maximize net canopy carbon export, under the constraint that canopy transpiration is a fixed fraction of total precipitation. We evaluated the predicted LAI (L_opt) for Australia against ground-based observations of LAI at 135 sites and continental-scale satellite-derived estimates. For the site-level data, the root-mean-square error of predicted L_opt was 1.07 m² m⁻², similar to the root-mean-square error of a comparison of the data against 9-year mean satellite-derived LAI (L_sat) at those sites. Continentally, L_opt had an R² of 0.7 when compared to L_sat. The predicted L_opt increased continental-wide with rising atmospheric [CO₂] over 1982–2010, which agreed with satellite-derived estimations, while the predicted stomatal behavior responded differently in dry and wet regions. Our results indicate that long-term equilibrium LAI can be successfully predicted from a simple application of ecohydrological theory. We suggest that this theory could be usefully incorporated into terrestrial vegetation models to improve their predictions of LAI.