Climate extremes such as droughts are becoming more frequent and more intense. Persistent alterations in ecosystem structure and function following drought (‘drought legacies’) may affect responses to future climate extremes. Understanding the nature of these legacy impacts will be important for predicting ecosystem responses to global climate changes. We investigated how repeated extreme droughts affected net primary productivity (a key aspect of ecosystem carbon cycling with the potential to feed back to atmospheric drivers of climate change) in a mesic grassland at the Konza Prairie LTER site. We experimentally imposed two extreme droughts (66% reduction in ambient growing season precipitation for two years), separated by a recovery period (two years of average precipitation) and measured plant production aboveground and belowground during and after the droughts.
Results/Conclusions
The legacy of the first extreme drought increased ecosystem sensitivity to the second extreme drought. This was particularly true belowground. We found that root production decreased more than aboveground production during the second drought and took longer to recover after the droughts. A single extreme drought reduced root production by about 30% (compared to control plots). By the end of the second extreme drought, root production had decreased by 65%, whereas aboveground production had decreased by 57%. After each of the extreme droughts, aboveground production recovered (to control-plot levels) after just one year of average precipitation. In contrast, belowground production showed no recovery after even several years of average precipitation. Root production only recovered after an extremely wet year. Our results suggest that more extreme droughts will cause increasingly larger reductions in productivity, especially belowground. Unless these extreme dry years are followed by extreme wet years, we can expect little recovery in root production, belied by quick recovery aboveground. The difference in belowground vs. aboveground productivity patterns suggests that belowground responses cannot be reliably predicted from aboveground responses. Understanding these asymmetric drought legacies will be important in understanding ecosystem responses to a changing climate.