The more the better? A limited proportion of deep roots maximizes tree survival under drought in Mediterranean forest

Abstract. Deep roots are assumed to enhance tree survival during drought by providing access to water stored at depth. However, the respective roles of deep soil water storage (SWC_Deep) and fine-root distribution (FineRoot_Distrib), which jointly determine access to deep water during drought, remain poorly understood. We assessed their relative importance during drought using the mechanistic hydraulic model SurEau-Ecos. The model was calibrated with leaf water potential and isotopic data collected in 2014 and 2015 for three species (Quercus ilex L., Fagus sylvatica L., and Abies alba Mill.) across two Mediterranean sites in France. Our results show trees predominantly rely on shallow water under non-limiting conditions, whereas SWC_Deep mobilisation becomes essential during drought, accounting for 11–36 % of annual transpiration. Sensitivity analyses reveal a “rare-root strategy”, whereby a limited proportion of deep roots maximizes survival by enabling access to deep water while preventing rapid depletion of deep reserves. These findings highlight that fine root distribution, through its control on the rate of SWC_Deep depletion, is a key driver of drought resistance, with an influence comparable to other major ecohydrological traits. Accounting for FineRoot_Distrib is therefore essential when assessing vulnerability to drought.