Drought offsets the positive effect of summer heat waves on the canopy greenness of mountain grasslands

Corona-Lozada, M. C. ; Morin, Samuel ; Choler, P.

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<p align="justify">Heat waves are becoming more frequent in a warming climate and understanding the impacts of these extreme events on terrestrial ecosystems remains a major challenge. Seasonally snow-covered mountain grasslands are temperature-limited ecosystems and one may question whether the summer heat waves there have the same negative effect on primary productivity as they have on lowland ecosystems. Here, we examined the remotely-sensed phenology of mountain grasslands in the French Alps with a particular focus on the four unusually warm summers that occurred in 2003, 2012, 2015, and 2016. Our results showed that an early and rapid senescence and a lack of late-season regrowth were the most significant responses in 2003 and 2012 when heat waves coincided with climatic water deficit, i.e. negative anomalies of precipitation minus reference evapotranspiration. Mountain grasslands located below 2350 m were the most affected. In contrast, we found low senescence and moderate to high regrowth in 2015 and 2016 which were years unaffected by severe water deficit. When water was non-limiting, warm and even very warm summers have a positive effect on the canopy greenness of the highest grasslands, particularly by extending the green period in late summer and early fall. Our study highlights the pivotal role that summer water balance has in the phenological response of mountain grasslands exposed to heat waves.</p>
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