Non-stomatal processes reduce gross primary productivity in temperate forest ecosystems during severe edaphic drought

Abstract

Severe drought events are known to cause important reductions of gross primary productivity (<i>GPP</i>) in forest ecosystems. However, it is still unclear whether this reduction originates from stomatal closure (Stomatal Origin Limitation) and/or non-stomatal limitations (Non-SOL). In this study, we investigated the impact of edaphic drought in 2018 on <i>GPP</i> and its origin (SOL, NSOL) using a dataset of 10 European forest ecosystem flux towers. In all stations where <i>GPP</i> reductions were observed during the drought, these were largely explained by declines in the maximum apparent canopy scale carboxylation rate <i>V</i>_CMAX,APP (NSOL) when the soil relative extractable water content dropped below around 0.4. Concurrently, we found that the stomatal slope parameter (<i>G₁</i>, related to SOL) of the Medlyn <i>et al</i>. unified optimization model linking vegetation conductance and <i>GPP</i> remained relatively constant. These results strengthen the increasing evidence that NSOL should be included in stomatal conductance/photosynthesis models to faithfully simulate both <i>GPP</i> and water fluxes in forest ecosystems during severe drought. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'.