Interactive effects of elevated CO2, warming, and water additions on the timing and rate of plant development in a mixed-grass prairie ecosystem

Background/Question/Methods Global change will undoubtedly affect the mechanisms that govern natural processes. This study investigates the effects of CO₂ enrichment, warming and water additions on phenological growth stages of dominant prairie species in the west-central Great Plains. Observational data was collected on a weekly basis from mid-March through November to determine differences in the onset of certain developmental phases for Bouteloua gracilis (Bogr), Pascopyrum smithii (Pasm) and Stipa comata (Stco).  Four key growth stages were indentified: beginning of spring growth (BSG), start of anthesis (AS), seed maturation (SM) and end of season growth (ESG). An additional component of this research is to predict plant development as a function of CO₂, temperature, and soil moisture. After the model is parameterized, new long-term simulations will evaluate the possible changes in plant development under different climate change scenarios. Data collected will enhance the GPFARM-Range model (Ascough et al. 2002), a decision support tool developed by the USDA-ARS to assist ranchers with managing site-specific areas to optimize production and economic benefits while minimizing environmental impacts. 
Results/Conclusions

We observed significant overall treatment effects for Bogr at AS (p=0.017) as well as for Stco at ESG (p=0.021). To determine the effects of CO₂ enrichment, warming and irrigation had on plant phenology, we ran a pairwise comparisons test to isolate treatment effects. We found a significant CO₂ effect (p=0.047) for Stco at ESG, suggesting water savings resulting from reduced stomatal conductance permitted Stco to persist later into the growing season. Additionally, a heating effect was evident for both Bogr (p=0.032) and Stco (p=0.042) at AS, where AS was initiated earlier in the growing season. Lastly, a significant irrigation effect was detected for Bogr at AS where those individuals treated with a shallow, more frequent irrigation regime set seed earlier in the season than those exposed to less frequent, deep irrigation treatments (p=0.028). These results indicate that seed production is largely mediated by warmer temperatures and water additions. It is likely that changes in temperature and CO₂ concentrations will influence soil moisture and seedling recruitment, which may alter the structure of plant communities. Intraspecific seed production also varied according to interactive treatment effects where plants occurring in non-irrigated plots flowered less frequently than those that were exposed to either a deep or shallow irrigation regime. Shifts in species abundance may adversely affect organisms that depend on certain species (i.e. cattle) for forage.