Quantification of Total Available Water in French temperate and mediterranean forests by three different methods.

During drought periods, the survival of trees and forest ecosystems strongly depends on soil water reserves. Isotopic studies show that during droughts, trees can use deep soil water resources (Carrière et al, 2020). However, the quantification of accessible water reserves to trees has so far mostly considered surface layers. A quantification of deep water resources accessible to and used by trees is missing.

In this work, we considered the total amount of water trees can access and use, defined as “TAW” (Total Available Water). TAW includes both root access to water and the ability of trees to take up this water. Our objective was to quantify TAW and to analyse its variability across four French forest sites: two temperate sites (Barbeau and Hesse), dominated respectively by Sessile Oak with Hornbeam and by Beech, and two Mediterranean sites (Puéchabon and Font-Blanche), dominated respectively by Holm oak and Aleppo Pine.

We used complementary approaches to quantify TAW: (i) the application of pedotransfer functions (Szabó et al., 2021) on soil core analyses; (ii) the analysis of soil moisture profiles obtained from sensors installed at different depths (Maysonnave et al. 2022) ; and (iii) the calculation of cumulative water deficit based on evapotranspiration measurements from flux towers (Giardina et al., 2023).

Our results show that TAW is higher in temperate forests than in Mediterranean forests. This difference is strongly linked to soil depth and to the proportion of stones in the soil. Other factors also play a role, especially the leaf water potential at the wilting point, which is lower in Mediterranean forest species. This allows these species to absorb water more efficiently during drought and increases their effective water availability. A strong intra-site variability of TAW was also observed, with coefficient of variation ranging from 8% to 60% depending on the site (for method (iii)).

Each of the three methods used in this work has its own limitations for estimating TAW. By using a combination of these three methods, we obtained complementary information and a more robust estimation of TAW and deep water reserves accessible to trees at the study sites. These approaches can contribute to mapping soil water stocks in France and to modelling the future of forests under climate change.