History of chronically increased precipitation variability alters grass production allocation during extreme drought

Abstract:

Global climate change is causing chronic changes in temporal precipitation variability (e.g. fewer and larger rainfall events with longer dry intervals between them), as well as more extreme droughts. Chronically increased precipitation variability can alter ecosystem structure and function, and these alterations may affect how systems respond to future extreme climate events. Understanding how legacies of past precipitation change may alter sensitivity to future extreme droughts will be important for predicting ecosystem responses to climate change. We investigated how a history of an experimentally-imposed chronic increase in precipitation variability alters the impact of an extreme drought in a mesic grassland. To do this, we imposed a 2-year extreme drought (66% reduction in ambient growing season rainfall) in a unique long-term grassland precipitation experiment that chronically increased rainfall variability for 15 years at the Konza Prairie LTER site. We found evidence that a history of increased precipitation variability altered the aboveground vs. belowground allocation of plant production. The legacy of precipitation change increased the sensitivity of belowground production to extreme drought (by ~20% compared to plots without the legacy), but did not alter the response of aboveground production, leading to a decreased root: shoot ratio. The dominant species at the study site was less affected than other species were by the precipitation change legacy. We found that the dominant species has a deeper rooting depth than other species, which may have helped it to avoid the impacts of increased precipitation variability. Consistent with the overall decline in belowground plant production, we found that soil CO₂ flux (a major biosphere-to-atmosphere C flux) was lower during drought in plots with vs. without the legacy of precipitation change. These results emphasize the importance of considering the legacy effects of past precipitation changes when assessing the impacts of climate extremes, and of quantifying dynamics belowground as well as aboveground. The impacts of extreme droughts may be underestimated if precipitation history and/or belowground dynamics are not considered.