Quantification of the 2018 drought for European forests and impacts of stomatal and non stomatal limitation of photosynthesis

In order to assess the impact of the european 2018 drought on CO2 and H2O fluxes and compare the behaviour of different forest ecosystems, a normalized drought index based on edaphic water amounts is highly needed. In this study, we propose to use the relative extractible water (REW) which is a relative index based on soil water content, root depth and soil properties. This index was used in this study to assess the impact of edaphic drought on the dynamics of relevant parameters related to stomatal and non stomatal regulations of gross primary productivity (GPP), canopy transpiration and water use efficiency. To do so, we used a big leaf framework in which the vegetation is represented as a single uniform layer to infer some bulk ecosystem characteristics from eddy covariance CO2 and heat flux measurements at a daily time scale. These parameters were used to better understand which mechanisn were responsible for GPP and evapotranspiration regulation during the drought. The obtained parametres are the canopy surface conductance (gs), the maximum carboxylation rate (Vcmax) and the slope parameter linking canopy conductance to gross primary productivity (g1) which is directly link to the ecosystem water use efficiency. In this study, we show different behaviour for different kind of ecoystems in how these parametres were affected by the dourght. First, we show that canopy transpiration, GPP and gs were signtificantly reduced when REW decreases under a given REW threshold. In parallel, we show that some ecosystems were able to decrease g1 during the drought which indicates an increase of the vegetation’s water use efficiency while other did not. Added to that, we show that, at most sites, the observed decrease of GPP could not entirely be explained by decreases of canopy conductance. Indeed, Vcmax also started to decrease when REW was very low thereby indicating non stomatal limitation of GPP. During the presentation, we will illustrate the different observed behaviour and highlight the different perspectives offered by this work.