Applying the eco‐hydrological equilibrium hypothesis to model root distribution in water‐limited forests

Abstract

Drought is a key driver of vegetation dynamics, but plant water‐uptake patterns and consequent plant responses to drought are poorly understood at large spatial scales. The capacity of vegetation to use soil water depends on its root distribution (RD). However, RD is extremely variable in space and difficult to measure in the field, which hinders accurate predictions of water fluxes and vegetation dynamics. We propose a new method to estimate RD within water balance models, assuming that vegetation is at eco‐hydrological equilibrium (EHE). EHE conditions imply that vegetation optimizes RD such that transpiration is maximized within the limits of bearable drought stress, characterized here by species‐specific hydraulic thresholds. Optimized RD estimates were validated against RD estimates obtained by model calibration from sap flow or soil moisture from 38 forest plots in Catalonia (NE Spain). In water‐limited plots, optimized RD was similar to calibrated RD, but estimates diverged with higher water availability, suggesting that the EHE may not be assumed when water is not limiting. Thereafter, we applied the optimization procedure at the regional scale, to estimate RD for the water‐limited forests of Catalonia. Regional variations of optimum RD reproduced many expected patterns in response to climate, soil physical properties, forest structure, and species hydraulic traits. We conclude that RD optimization, based on the EHE hypothesis and a simple description of plant hydraulics, produces realistic estimates of RD that can be used for model parameterization and shows promise to improve our ability to forecast vegetation dynamics under increased drought.

Dependence of Aspen Stands on a Subsurface Water Subsidy: Implications for Climate Change Impacts

“…Ecologically, to optimize growth and minimize water stress, stands should develop over time so as to remain somewhat above this threshold (Figure ) by adjusting stand K max and hence ∑ s E pot (Figure ). This resembles Eagelson's concept of the long‐term equilibrium between plant available water and the abundance of transpiring plants (Cabon et al, ; Eagleson, ). But the concept also provides a framework for charting long‐term forest growth or dieback in response to water availability.…”

Constant theoretical conductance via changes in vessel diameter and number with height growth in Moringa oleifera

“…Deviation from the predicted distribution can even be diagnostic of forest disturbance (Coomes et al , 2003; Sellan et al , 2017). This energy equivalence expectation, supported here by constant conductance per unit leaf area, represents a potential link between hydraulics, stand productivity, and the shaping of optimal canopy height and density by microsite (Eagleson, 1982; Cabon et al , 2018). The finding of constant conductance per unit leaf area with height growth is therefore important for understanding forest function.…”

Leveraging plant hydraulics to yield predictive and dynamic plant leaf allocation in vegetation models with climate change

“…A number of other optimization studies have used the concept of ecohydrological equilibrium, where allocation to leaves and roots is assumed to be in equilibrium with water availability (Eagleson, ; Westoby et al, ). The ecohydrological equilibrium framework has successfully reproduced observed trends in A L and root distributions across environmental gradients (Cabon, Martínez‐Vilalta, Martínez de Aragón, Poyatos, & De Cáceres, ; Schymanski, Sivapalan, Roderick, Beringer, & Hutley, ; Yang, Medlyn, De Kauwe, & Duursma, ).…”