Responses of North American Great Plains grasslands to global change are uncertain, due in part to differing physiologies of dominant C₃ and C₄ grasses.  Warmer temperatures would presumably favor warm-season, C₄ grasses, while rising CO₂ should benefit C₃ species, whose photosynthetic apparatus is CO₂-unsaturated at present-day levels of CO₂.  The Prairie Heating and CO₂ Enrichment (PHACE) experiment tests the interactive effects of rising CO₂ (present ambient [380 ppm], and elevated [600 ppm] CO₂) and elevated temperature ([1.5/3.0 ºC warmer day/night]) on the ecology of the northern mixed grass prairie (n=5).  Aboveground plant biomass (AGB) was measured in late July each year, and plant/litter/ground cover measured with photography throughout the growing season.  Soil water content was monitored continuously in each plot with Sentek soil moisture probes.  Results are presented for 2006 (CO₂ enrichment alone), and 2007 and 2008 (CO₂ enrichment plus warming). 

Results/Conclusions

Elevated CO₂ enhanced total mid-season AGB (2006-2008), although warming eliminated this response in 2008.  Aboveground biomass of both C₃ and C₄ grasses were enhanced under CO₂-enrichment (2006), although warming eliminated the CO₂-induced AGB response of C₃ grasses in 2008.  In contrast, the enhancement of C₄ AGB by CO₂ enrichment was more evident in the warmed treatment.  Seasonal measurements of plant cover showed similar response patterns as AGB.  Repeated measurements within each growing season showed that seasonal soil water dynamics and differences in plant phenology affected the relative expression and timing of cover responses between the C₃ and C₄ grasses.  Soil water content was often enhanced under elevated CO₂, reduced under warming, and intermediate in the ambient CO₂/non-warmed and elevated CO₂/warmed treatments.  The results suggest that long-term changes in productivity and species composition in mixed-grass prairie will be determined by a complicated interaction involving direct responses to CO₂ and temperature and indirect responses mediated by changes in soil water dynamics.