End-of-season senescence in grassland species can be traced to leaf temperature during preceding summer drought

In natural multi-species communities, drought extremes elicit complex, though seldom measured, ecophysiological responses triggered by divergent drought coping strategies and plant-plant interactions. This raises the question whether the whole-season impact of such events is in any way predictable in such systems from stress measurements during the drought. Here, we experimentally induce local variation in soil moisture in a humid, mull-species, temperate grassland in summer and test whether any of the stress indicators (stomatal conductance, net photosynthetic rate and midday leaf surface temperature) measured on the seven most abundant species is a good early warning signal of end-of-season senescence. We found that, across species, plants exposed to lower soil water content experienced similar elevation of leaf surface temperature, and that plants with warmer leaves during the drought extreme were consistently more senescent at the end of the growing season two months later, averaging 0.7% surplus leaf senescence with every additional 1 degrees C. We also observed links between lagged effects in the weeks after the drought, which were weakly negative on stomatal conductance, but strongly positive on photosynthesis in some species, and end-of-season senescence. Part of the damage might thus be ascribed to these drought legacies. To conclude, even in complex field settings, local leaf surface temperature measured at an early stage can be a powerful and species-specific indicator of the whole-season impact of drought extremes. This opens perspectives to estimate where in the landscape such events will be most detrimental.