Legacy of increased precipitation variability increases drought sensitivity of tallgrass prairie plant production belowground, but not aboveground

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Ingrid Slette and Alan K Knapp, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Global climate change is altering precipitation patterns and amounts around the world. Forecast changes include both increased intra-annual precipitation variability (e.g., fewer and larger rainfall events) and more extreme precipitation events (e.g., droughts). Though both types of change have been shown to impact ecosystem function independently, it is not known how a legacy of one may affect responses to the other. We imposed a 2-year drought (growing season precipitation reduced by 66%) on experimental plots which had received either ambient or chronically increased intra-annual precipitation variability (fewer and larger rainfall events with 50% longer dry intervals between them) for 16 years. This experiment is located in a mesic tallgrass ecosystem at the Konza Prairie Long-Term Ecological Research (LTER) site. We quantified aboveground and belowground net primary production and soil CO2 flux during and after drought, in order to assess how the legacy of an altered precipitation pattern affected key ecosystem responses to an extreme precipitation event.
Results/Conclusions
We found that 16 years of increased precipitation variability reduced shallow soil moisture and ANPP (by ~9%, p<0.001), but that the legacy of the treatment did not alter the decline in ANPP during drought (p>0.05). Indeed, we found that the legacy of increased precipitation variability only amplified drought-induced reductions in net primary production belowground (p<0.05), not aboveground. This negative legacy effect of chronically increased precipitation variability was driven entirely by the non-dominant species in the community, whose BNPP was approximately 115% lower in the former altered vs. former ambient precipitation treatment. After drought, production was higher than during drought both belowground and aboveground, regardless of past precipitation variability (p<0.05). However, rain use efficiency (g ANPP per mm precipitation) was about 36% lower than the long-term average during this year, suggesting a negative legacy effect of the experimental drought and incomplete recovery. Both during and after drought, the legacy of chronic precipitation change reduced soil CO2 flux sensitivity to rainfall but did not alter average soil CO2 flux rates. Our findings have important implications for understanding how precipitation changes might alter key aspects of ecosystem carbon cycling. Accounting for belowground dynamics and legacy effects of past precipitation change will improve ecological understanding and model representation of drought impacts.