Severe drought events are known to cause important reductions of gross primary productivity (GPP) 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 GPP and its origin (SOL, NSOL) using a dataset of 10 European forest ecosystem flux towers. In all stations where GPP reductions were observed during the drought, these were largely explained by declines in the maximum apparent canopy scale carboxylation rate VCMAX,APP (NSOL) when the soil relative extractable water content dropped below around 0.4. Concurrently, we found that the stomatal slope parameter (G1, related to SOL) of the Medlyn et al. unified optimization model linking vegetation conductance and GPP remained relatively constant. These results strengthen the increasing evidence that NSOL should be included in stomatal conductance/photosynthesis models to faithfully simulate both GPP 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'.

Bioclimatology University of Goettingen Büsgenweg 2 37077 Goettingen Germany

Department of Environmental Engineering Technical University of Denmark Bygningstorvet 115 2800 Lyngby Denmark

Earth and Life Institute UCLouvain Louvain la Neuve Belgium

European Commission Joint Research Centre Ispra Italy

Global Change Research Institute CAS Brno Czech Republic

INRA UMR ISPA Villenave d'Ornon 33140 France

Institute for Nature and Forest Research INBO Havenlaan 88 Box 73 1000 Brussels Belgium

Plants and Ecosystems University of Antwerp Universiteitsplein 1 2610 Wilrijk Belgium

Terra Teaching and Research Center University of Liège Gembloux Agro Bio Tech 5030 Gembloux Belgium

Université de Lorraine AgroParisTech INRA UMR Silva 54000 Nancy France

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Long-term daily hydrometeorological drought indices, soil moisture, and evapotranspiration for ICOS sites

Altered energy partitioning across terrestrial ecosystems in the European drought year 2018

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