Soil water balance performs better than climatic water variables in tree species distribution modelling

Aim Soil water is essential for the physiological processes of plant growth and fitness. Owing to the difficulty of assessing wide variations in soil water reserves, plant distribution models usually estimate available water for plants through such climatic proxies as precipitation data (P) or climatic water balance (P minus potential evapotranspiration). We evaluated the ability of simple climatic proxies and soil water balance indices to predict the ecological niches of forest tree species. Location France. Methods Soil water content and deficits were computed and mapped at a resolution of 1km x 1km throughout France. The predictive abilities of these indices were compared with those of P and climatic water balance to model the distributions of 37 of the most common European tree species. We focused on two species with contrasting water tolerance, Quercus robur and Quercus pubescens, to illustrate the differences between climatic proxies and soil water balance in species response curves and distribution maps. Results Throughout France, soil water content was poorly correlated with P and climatic water balance, because low P in the lowlands can be compensated for by water provided by deeper soils, which is not the case in most mountainous areas. Soil water balance performed better than simple climatic water variables for explaining tree species distribution, improving 82% of the models for hygrophilous, meso-hygrophilous, meso-xerophilous and xerophilous species. Main conclusions Our results showed that simple climatic values do not accurately represent available water for trees and that soil water balance indices perform better than do climatic proxies for most species. This point is crucial to avoid underestimating the importance of water in studies aimed at determine the ecological niches of plant species and their responses to climate change.