No abstract

Climate change is imposing drier atmospheric and edaphic conditions on temperate forests. Here, we investigated how subsoil (down to 300 cm) water extraction contributed to the provision of water in the Fontainebleau-Barbeau temperate oak forest over two years, including the 2018 record drought. Deep water provision was key to sustain canopy transpiration during drought, with layers below 150 cm contributing up to 60% of the transpired water in August 2018, despite their very low density of fine roots. We further showed that soil databases used to parameterize ecosystem models clearly underestimated the amount of water extractable from the soil by trees. The consensus database established for France gave an estimate of 207 mm for the soil water holding capacity (SWHC) at our study site, when our estimate based on the analysis of soil water content measurements was 1.9 times higher, reaching 390+/-17 mm. Running the CASTANEA forest model with the database-derived SWHC yielded a 350 gC m-2 y-1 average underestimation of annual gross primary productivity under Global Change Biology For Review Only current climate, reaching up to 700 gC m-2 y-1 under climate change scenario RCP8.5. Correctly estimating SWHC is a challenge for accurate simulations of the carbon cycle in a changing climate and we showed that subsoil SWC measurements are needed for this.