Compound hydroclimatic extremes in a semi-arid grassland: Drought, deluge, and the carbon cycle

Authors: Hoover, David; HAJEK, OLIVIA - Colorado State University; SMITH, MELINDA - Colorado State University; WILKINS, KATE - Colorado State University; SLETTE, INGRID - Colorado State University; KNAPP, ALAN - Colorado State University

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2022
Publication Date: 1/23/2022
Publication URL: https://handle.nal.usda.gov/10113/7704441
Citation: Hoover, D.L., Hajek, O.L., Smith, M.D., Wilkins, K., Slette, I.J., Knapp, A.K. 2022. Compound hydroclimatic extremes in a semi-arid grassland: Drought, deluge, and the carbon cycle. Global Change Biology. 28:2611-2621. https://doi.org/10.1111/gcb.16081.
DOI: https://doi.org/10.1111/gcb.16081

Interpretive Summary: Climate change is predicted to increase extreme events, such as heat waves and floods. When extremes co-occur, such as a heat wave during a drought, the effects are often greater than either extreme alone. However some extremes may have potentially offsetting effects, such as a deluge during a drought, rather than additive effects. In this study we experimentally imposed a deluge during a drought and examined the impacts on a grassland ecosystem. Overall we found that the deluge during drought had large but short-term effects on key ecological processes but was not able to compensate for the long-term effects of drought. These results suggest that while a deluge may provide some relief during a drought, the relief will likely be short-lived.

Technical Abstract: Climate change is predicted to increase the frequency and intensity of extreme events including droughts and large precipitation events or “deluges”. While many studies focus on individual events (e.g., a heat wave), there is growing recognition that when extreme events co-occur as compound extremes, (e.g., a heatwave during a drought), the additive effects on ecosystems are often greater than either extremes alone. In this study, we assessed a unique type of extreme – a contrasting compound extreme – where the extremes may have offsetting rather than additive ecological effects, by examining how a deluge during a drought impacts productivity and carbon cycling in a semi-arid grassland. The experiment consisted of four treatments: a control (average precipitation), a drought (<1st percentile), an extreme drought interrupted by a single deluge (>98th percentile), or an extreme drought interrupted by the equivalent amount of precipitation added in several smaller events. We highlight three key results. First as expected, extreme drought resulted in early senescence, reduced carbon uptake, and a decline in net primary productivity relative to the control treatment. Second, the deluge imposed during extreme drought stimulated C fluxes and plant growth well above the levels of both the control and the drought treatment with several smaller rainfall events, emphasizing the importance of precipitation amount and event size/timing. Third, while the deluge’s positive effects to C flux and plant growth persisted for one month, the deluge did not completely offset the negative effects of extreme drought on end-of-season productivity. Thus, in the case of these contrasting hydroclimatic extremes, a deluge during a drought can stimulate temporally dynamic ecosystem processes (e.g., net ecosystem exchange) while only partially compensating for reductions in ecosystem functions over longer time scales (e.g., aboveground net primary productivity).