Modelling fine root distribution and its role to increase tree resilience to drought in two Mediterranean forests

The role and importance of deep roots for tree survival during drought is an intuitive expectation. Numerous studies have examined root distribution, rooting depth, and the use of deep water resources. However, the impact of deep roots systems on the amount of deep water uptake and their contribution to tree survival during drought is still understudied.In this study, we first measured predawn leaf water potential and the isotopic signatures of sap water and potential water sources. These isotopic measurements allowed us to estimate the proportion of deep water contributing to the total volume of water transpired by trees. Measurements were conducted on three tree species (Quercus ilex, Fagus Sylvatica and Abies alba) over two summer seasons (2014 and 2015) at two study sites in Mediterranean regions of France. We then used the plant hydraulic model SurEau-Ecos to infer the mobilisation of deep water reserves by trees, fitting the model to observed leaf water potential . We used ecophysiological trait databases for initial model parameterisation. Finally, we adjusted a single parameter defining root distribution to fit the model to the observations.We obtained a root distribution that satisfactorily reproduced both leaf water potential and deep water use. This allowed us to quantify temporal variations in deep water use for each species. During periods of water stress, trees uptaked 102 mm of water from the deep soil reservoir. Without this resource, trees would likely have experienced hydraulic failure. Over the study period, deep water contributed on average between 8.5 and 37 % of total tree water use, depending on species.To further investigate the role of fine root distribution in survival under extreme drought, we analysed the model sensitivity to the root development parameter and to deep-water reserves in terms of hydraulic failure risk. These analyses showed that survival time could vary by up to 100 days depending on the proportion of the root system located in deep soil. We also identified an optimal degree of deep root development that maximised tree survival. Overall, the methodology we developed will help better quantify the role of deep water uptake. They appear essential for both groundwater recharge assessments and vegetation drought response analyses.