Environmental heterogeneity is predicted to increase diversity, and diversity is predicted to increase stability in ecosystem function. By this logic, heterogeneity should promote ecosystem resilience. Precipitation patterns in the Midwest are expected to become more variable in the future, with increasing severity of drought. Grassland accounts for a large fraction of terrestrial productivity, so understanding factors influencing resilience to drought is needed to predict future ecosystem functioning. We leveraged a 20-year-old prairie restoration that contains manipulated soil nutrient availability and soil depth to test if grassland restored on heterogeneous soil is more resilient to drought relative to homogeneous control soil conditions. We predicted that more heterogeneous plant communities in heterogeneous soil would be more resilient to drought events because they contain more forb species that exploit deeper soil water. We analyzed 15 years of aboveground net primary production (ANPP) from two contrasting soil heterogeneity treatments to calculate drought sensitivity and legacy effects from two drought events in 2012 and 2018. Drought sensitivity and legacy effects were determined using the proportional change in ANPP during a drought to the relative average non-drought ANPP and after drought to the relative average non-drought ANPP, respectively.
Results/Conclusions
Plant communities in heterogeneous soil had more total and grass ANPP than the control communities overall years, but there were no overall years difference in forb ANPP between the two treatments. Forbs comprised a larger proportion of total ANPP in 2018 and 2019 for both treatments relative to 2012 and 2013. Both restored prairie communities exhibited low sensitivity to the milder 2012 drought, and positive drought legacies due to increased grass ANPP in 2013. Inversely, restored communities exhibited greater sensitivities to the more severe 2018 drought, and reduced positive drought legacies compared to the 2012 drought. These drought responses may be explained by the differences in severity of these two droughts including duration and precipitation reduction. Regardless, plant communities in heterogeneous soil had greater drought sensitivity and more positive legacy effects than communities in homogenous control soil for both droughts. Nutrient availability also appears to influence grassland resilience as heterogeneous communities with strips of enriched nutrients tended to have higher ANPP in years following droughts compared to other soil manipulations. Our findings suggest that soil resource heterogeneity can promote greater resilience of restored grasslands to interannual variability in precipitation likely due to an increase in drought tolerant forbs.