Evidence for a threshold in ecosystem functioning during an extreme drought in a semi-arid grassland

Climate change has intensified the severity and duration of droughts in grasslands globally, increasing the risk of extreme multiyear droughts.  While most grasslands are considered to be resilient to short-term drought, extreme multiyear droughts can have longer-lasting consequences. For instance, full recovery of ecosystem structure after the 1930s central US Dust Bowl drought required 20 years. Due to the rarity of such multiyear droughts, we know little about the influence of drought duration on ecosystem recovery post-drought or the mechanisms influencing recovery. Here, we evaluated the recovery dynamics of carbon uptake (estimated via aboveground net primary productivity, ANPP) in a semi-arid shortgrass steppe ecosystem after 4 vs. 5-years of experimental drought (66% precipitation reduction). We also assessed the influence of post-drought precipitation amounts on recovery by implementing treatments equivalent to 150% of long-term average (LTA) for this site after the drought ended. Non-droughted plots experienced similar treatments.  We observed dramatic differences in recovery in the 150% LTA treatments after four vs. five years of drought, and thus identified a clear threshold in drought duration impacts in this grassland. After four years of drought, C4 grass productivity increased substantially in the first year and fully recovered by the second. In contrast, there has been little to no recovery of C4 grasses after five years of drought. Plant communities in the 5-year drought treatment shifted to dominance by annual (weedy) forbs, with 3-4 times greater ANPP compared to non-droughted plots. This increase in ANPP was likely due to a 20-fold increase in available soil nitrogen in the recovery period. Our results demonstrate the existence of an abrupt threshold in response to drought duration in this grassland. Once this drought duration threshold is crossed, catastrophic changes in vegetation structure, carbon dynamics, and ecosystem recovery ensue.