Résumé
The frequency and intensity of extreme weather events are increasing in response to climate change. Among such events, heatwaves impact both public health and ecosystem functioning. In France, in 2019, two heatwaves took place between June and August, lasting however only a few days each. While the historical temperature record was broken at the end of June 2019, the intensity and duration of those two events differed between regions of France. In forests and crops, warm temperatures, together with moderate soil drought, led to a reduction in photosynthetic activity, inducing a loss of ecosystem productivity and carbon (C) sequestration. In particular, crop management, such as irrigation, could play an important role. In peatlands, the potential effect of heatwaves is more uncertain, as they often develop in humid areas. This study aimed at (i) describing how 2019 heatwave events impacted C and water vapour (₂O) fluxes at several forest, cropland and peatland sites within the ICOS ecosystem network in France, covering several soil, climate and vegetation conditions and (ii) exploring the physiological processes affected by the heatwaves. At all sites, CO₂ and ₂O fluxes, measured by eddy-covariance according to the ICOS protocols, were analysed to infer heatwave impacts on net ecosystem exchange (NEE), gross photoynthesis (GPP), ecosystem respiration (Reco), and evapotranspiration. Canopy conductance (gc) was inferred from evapotranspiration by inverting the Penman-Monteith equation. For forest sites, the heatwave generally caused a reduction of gc, especially after midday, which indicates a stomatal closure in response to higher VPD observed in the afternoon. This was concurrent with reduced NEE. Depending on sites, the forest would switch from a C sink to a C source, or become a weaker C sink, with total or only partial recovery after the heatwave. GPP was generally lower during the heatwave, again with total or partial recovery. Ecosystem respiration appeared to be stimulated by the higher temperatures, but frequently displayed inconsistent responses, possibly due to issues with partioning algorithms. At FR-Pue, where temperatures were the highest, the heatwave caused immediate, partial leaf mortality. Peatlands displayed no noticeable change in gc during the heatwaves, possibly due to surface temperatures not being as high as in forests, except for FR-LGt, where temperatures were higher and gc was reduced. That site became a stronger source of CO₂ during the heatwave. For crop sites, the most noticeable impacts were observed at the three maize-cropped sites, among which both FR-Gri (Paris area) and FR-Mej (Brittany) exhibited reduced photosynthetic activity and stomatal conductance (both up to about 50 %) in July, as plants had reached their maximal leaf development and soil moisture availability was limited. At the FR-Lam site, where maize was irrigated, no reduction in photosynthetic activity was observed; total crop production was even larger than usual. Heatwaves will become more common under climate change. Our results indicate that they will likely diminish ecosystems C sequestration. While the measured responses were expected (stomatal closure, its impact on photosynthesis, and increased respiration), modelling such events remains a challenge.