The impacts of co-occurring hydroclimatic extremes on grassland production and carbon cycling: Can a deluge negate a drought?

David L. Hoover, Rangeland Resources and Systems Research Unit, USDA-ARS, Fort Collins, CO, Olivia Hajek, Kate Wilkins, Melinda Smith and Alan K Knapp, Department of Biology, Colorado State University, Fort Collins, CO, Olivia Hajek, Kate Wilkins, Melinda Smith and Alan K Knapp, Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Climate change is predicted to increase the frequency, intensity, and duration of extreme events such as droughts, heatwaves, and extreme precipitation or “deluges”, with potential large and long-lasting ecological impacts. While many studies focus on a single-type event (e.g. a heatwave), there is a growing need to understand the impacts of co-occurring or compound extreme climatic events (e.g. a heatwave during drought). The effects of co-occurring extremes are often greater than either extreme in isolation, however some extremes, such as a deluge during a drought, may have offsetting ecological effects. To examine this, we experimentally imposed a deluge during an extreme drought in a semi-arid grassland, and measured the impacts on soil moisture, phenology, carbon cycling, as well as above- and below-ground productivity. Precipitation treatments consisted of a control (precipitation amount and pattern of an average growing season), a drought (-78% of control), a deluge with drought (“deluge.big”; drought + 60 mm event mid-summer) and a drought with increased late summer precipitation (“deluge.small”; drought + 60 mm added over 9 events late summer). We predicted that the deluge.big treatment would ‘rescue’ ecosystem function from drought by causing rapid re-greening, and stimulating carbon cycling and productivity relative to the drought and deluge.small treatments.
Results/Conclusions
The precipitation treatments influenced soil water availability, which had clear effects on phenology and carbon fluxes. Prior to the deluge, the treatments receiving drought (i.e. drought, deluge.big, deluge.small) all experienced a large drop in greenness and reduction in net ecosystem exchange (NEE). The deluge.big treatment had rapid responses to the mid-summer deluge. Re-greening started within 3 days and lasted for roughly one month. Following deluge, there was also an immediate increase in ecosystem respiration (ER) but eventually gross primary productivity (GPP) increased with re-greening, leading to a net uptake of carbon. The deluge affected carbon fluxes for roughly one month and averaged over the growing season, the deluge.big treatment had greater carbon uptake than the drought but was not significantly different than the control or deluge small treatments. Aboveground net primary production in the deluge.big treatment was intermediate to the control and drought treatments, but not significantly different than the deluge.small treatment. Overall, these results suggest that an extreme deluge during an extreme drought may have large but short-term effects on grassland production and carbon cycling. However, contrary to predictions, the extreme deluge imposed did not fully negate the overwhelming ecological effects of the extreme drought.