Timing of drought in the growing season and strong legacy effects determine the annual productivity of temperate grasses in a changing climate

Abstract. The frequency of extreme weather events, such as droughts, is assumed to increase leading to alterations of ecosystem productivity and thus the terrestrial carbon cycle. Although grasslands typically show reduced productivity in response to drought, their effects have been shown to vary quite dramatically. Here we tested in a two-year field experiment, if the resistance and the resilience of grasses towards drought varies throughout a growing season and if the timing of drought, thus, influences drought-induced reductions in annual net primary production (NPP) of grasses. For the experiment we grew six temperate and perennial C3 grass species in a field as monocultures. The grasses were cut six times during the growing season and subject to 10-week drought treatments that occurred either in the spring, the summer or the fall. Across all species, drought-induced losses of productivity were smaller (−20 % to −51 %) than in summer and fall (−77 % to −87 %). This suggests a higher resistance to drought in spring when the productivity of the grasses is the highest and plants are in their reproductive stage. After the release of drought, we found no prolonged suppression of growth. In contrast, post-drought growth rates of formerly drought-stressed swards outperformed the growth rates of the control swards. In 2014, the overcompensation after drought release was similar in all seasons, but differed in 2015. The strong overcompensation of growth after drought release resulted in relatively small overall drought-induced losses of annual ANPP that ranged between −4 % to −14 % and were not affected by the timing of the drought event. Our results show that (i) the resistance of growth rates in grasses to drought varies across the season and is positively correlated with growth rates in the control, (ii) that positive legacy effects of drought indicate a high resilience of temperate grasses to drought, and (iii) that the high resilience can compensate immediate drought effects on total annual biomass production to a large extent.