Plant community drivers of ecosystem response to extreme summer drought in successional old-fields

Background/Question/Methods

As long periods of summer drought become increasingly common, creative solutions and considerations are needed for alternative management strategies that limit productivity losses in grasslands that to prevent impacts on numerous ecosystem services. While losses in plant community productivity with drought is generally expected, the magnitude of these effects remains difficult to predict and likely depends on many abiotic and biotic factors that vary among sites. To examine underlying community-based factors associated with land-use history that dictate grassland responses to drought, we implemented a three-month extreme drought via precipitation reduction (-43%) across a range of old-fields abandoned between 6 and 94 years from agricultural use. Treatments were applied using passive rainout shelters (Drought), an infrastructure control treatment with inverted slats (Inverted), and plots open to ambient precipitation (Open). Old-fields undergoing secondary succession provide a space-for-time chronosequence highlighting a range of ecosystem and community factors that are predicted to impact resistance to drought and ecosystem function more generally. This old-field chronosequence includes grasslands with contrasting plant species diversity, soil fertility and development, and shifts in competitive and facultative plant interactions, among other characteristics, at the Cedar Creek Ecosystem Science Reserve in MN.

Results/Conclusions

During the summer of 2021, our 43% precipitation reduction treatment resulted in soil water content that was 18% lower than ambient conditions. The impact of this applied drought reduced biomass by 20% overall, however the effect of drought within individual old-fields ranged from -55% (50 years abandoned) to +30 % (94 years). Importantly, time since abandonment was not a linear predictor of the drought effects, and within old-field variation in other abiotic or biotic components appears to drive drought resistance. For example, while surface soil samples (0-20 cm) from the same fields show accumulations of soil carbon with time since abandonment (0.03% per decade), the plant-based metrics such as leaf area index and tiller density did not show a clear trajectory with time. The variability in multiple co-varying ecosystem components will therefore provide additional insights as to the main predictors of productivity and the range of drought responses observed among old-fields. Moving forward, increased understanding of ecosystem components that may mitigate drought response could help land managers predict and reduce losses in productivity associated with climate change among diverse types of grassland communities.