Understanding and predicting the dynamics of ecological systems has always been central to Ecology. For community and ecosystems ecologists, one of the earliest conceptual foci in this area was based on interest in ecological dynamics driven by disturbance. Today, ecologists recognize that in addition to natural and human-caused disturbances, a fundamentally different type of change is also occurring – one driven at the global scale by the combined and cumulative effects of anthropogenic activities affecting earth’s climate and biogeochemical cycles. This type of change is historically unprecedented in magnitude, and as a consequence, such alterations are leading to trajectories of ecological responses that differ radically from those observed in the past. Through both short- and long-term experiments, we have been trying to better understand the mechanisms and consequences of ecological change in grassland ecosystems likely to result from global change drivers. We have manipulated key resources (water) and modulators of resources (temperature) in field experiments that vary from 1-17 years in duration, and used even longer-term monitoring data from the Konza Prairie LTER program to assess how grassland communities and ecosystems will respond to changes in resources.

Results/Conclusions

Trajectories of change in aboveground net primary production (ANPP) in sites subjected to 17 years of soil water augmentation were strongly non-linear with a marked increase in the stimulation of ANPP after year 8 (from 25% to 65%). Lags in alterations in grassland community composition are posited to be responsible for the form of this trajectory of change. In contrast, responses in ANPP to chronic increases in soil moisture variability appear to have decreased over a 10-yr period of manipulation. The loss of sensitivity to increased resource variability was not reflected in adjacent plots where precipitation was manipulated for only a single year. This suggests that this grassland has acclimated to this chronic change in variability. Five years of warming in this grassland has altered soil moisture, but has yet to reveal any definitive temporal trajectory of change in ANPP. Finally, 20 years of climate and ANPP data suggest that ANPP is increasing in this grassland without any increase in precipitation. This increasing trajectory in ANPP suggests that either other resources, or modulators of resource availability, are being altered.