Extreme climatic events down-regulate the grassland biomass response to elevated carbon dioxide

Terrestrial ecosystems are considered as carbon sinks that may mitigate the impacts of increased atmospheric CO₂ concentration ([CO₂]). However, it is not clear what their carbon sink capacity will be under extreme climatic conditions. In this study, we used long-term (1998-2013) data from a C3 grassland Free Air CO₂ Enrichment (FACE) experiment in Germany to study the combined effects of elevated [CO₂] and extreme climatic events (ECEs) on aboveground biomass production. CO₂ fertilization effect (CFE), which represents the promoted plant photosynthesis and water use efficiency under higher [CO₂], was quantiffied by calculating the relative differences in biomass between the plots with [CO₂] enrichment and the plots with ambient [CO₂]. Down-regulated CFEs were found when ECEs occurred during the growing season, and the CFE decreases were statistically significant with p well below 0.05 (t-test). Of all the observed ECEs, the strongest CFE decreases were associated with intensive and prolonged heat waves. These findings suggest that more frequent ECEs in the future are likely to restrict the mitigatory effects of C3 grassland ecosystems, leading to an accelerated warming trend. To reduce the uncertainties of future projections, the atmosphere-vegetation interactions, especially the ECEs effects, are emphasized and need to be better accounted.