Contribution of deep soil layers to the transpiration of a temperate deciduous forest: quantification and implications for the modelling of productivity

Climate change is imposing drier atmospheric and edaphic conditions on temperate forests. Here, we investigated how deep soil (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 largely underestimated the amount of water extractable from the soil by trees, due to a considerable underestimation of the tree rooting depth. The consensus database established for France gave an estimate of 207 mm for the soil water holding capacity (SWHC) at Fontainebleau-Barbeau, when our estimate based on the analysis of soil water content measurements was 1.9 times as high, 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 current climate, reaching up to 700 gC m−2 y−1 under climate change scenario RCP8.5. It is likely that the strong underestimation of SWHC that we show at our site is not a special case, and concerns a large number of forest sites. Thus, we argue for a generalisation of deep soil water content measurements in forests, in order to improve the estimation of SWHC and the simulation of the forest carbon cycle in the current context of climate change.HighlightsForest-atmosphere carbon exchanges remained insensitive to record drought.Deep soil (150-300 cm) provisioned up to 60% of the water transpired by the forest during drought.Soil databases were underestimating soil water holding capacity by a factor of two.Simulated forest productivity is strongly sensitive to soil water holding capacity parameter.Deep soil water content measurements are urgently needed to correctly estimate the soil water holding capacity.