Climate changes will include gradual but chronic alterations in temperature and precipitation (‘presses’) as well as more extreme discreet climate events such as heat waves and droughts (‘pulses’). Research to date has tended to study ecological responses to presses and pulses independently, despite recognition that they are likely to interact. Altered ecosystem structure and function resulting from press or pulse changes may affect responses to future climate extremes. Understanding the nature of these interactions and legacy impacts will be important for predicting ecosystem responses to climate changes. We investigated how ecological legacies of experimentally imposed press and pulse precipitation changes alter the impact of an extreme drought on root biomass production, a key aspect of ecosystem carbon cycling with the potential to feed back to atmospheric drivers of climate change. To do this, we imposed a 2-year extreme drought (66% reduction in ambient rainfall) on two unique grassland precipitation experiments: a long-term (15-year) press experiment that chronically altered rainfall timing / intensity and a pulse experiment that imposed a previous short-term (2-year) extreme drought.
We found that extreme drought alone reduced root biomass production by approximately 12% (compared to non-droughted controls). Both press and pulse precipitation change legacies amplified the negative response of root production to extreme drought. Extreme drought following chronic alteration of precipitation patterns decreased root production by 23%, whereas extreme drought that followed a previous extreme drought decreased root production by 56%. Reductions in root production with drought were greatest in shallower soil layers. The dominant species at the site (Andropogon gerardii) showed a smaller reduction in root production with drought than other graminoid species, indicating that this dominant grass may buffer the system against precipitation alterations. Overall, our results suggest that plants will produce less root biomass with future climate changes that include chronic alterations in precipitation patterns combined with more frequent extreme drought. This decrease in plant carbon inputs to soils will have important implications for ecosystem carbon storage and may slow carbon cycling in response to forecast precipitation changes.