How important is CO2 in the responses of a northern mixed-grass prairie to temperature?

Background/Question/Methods

In addition to being sensitive to climate, plants can respond directly to changes in ambient CO₂ through two important mechanisms.  Increases in CO₂ can stimulate photosynthesis, especially in C₃ species whose photosynthetic metabolism is not CO₂- saturated at present atmospheric concentrations. Secondly, CO₂ induces stomatal closure in almost all herbaceous species.  In many cases, plant responses to CO₂ and temperature are oppositional.  For instance, while warmer temperatures would presumably favor warm-season, C₄ species , rising CO₂ should benefit C₃ species due to their greater photosynthetic sensitivity to CO₂.  Or, while rising temperature leads to desiccation, rising atmospheric CO₂ has the opposite effect of improving soil water content through reduced canopy transpiration.  The Prairie Heating and CO₂ Enrichment (PHACE) experiment evaluates the interactive effects of elevated temperature ([1.5/3.0 ºC warmer day/night]) and rising CO₂ (ambient [385 ppm], and  [600 ppm] CO₂)  on the ecology of the semiarid northern mixed grass prairie near Cheyenne, WY, USA.  Aboveground net primary production (ANPP) was estimated from biomass harvests conducted near the time of peak biomass in late July each year, and root biomass was determined at the same time from soil cores.  Soil water content was monitored continuously to 80 cm depth  (EnviroSMART probe, Sentek Sensor Technologies, Stepney, Australia).  Results are presented for 2006 (CO₂ enrichment alone), and 2007-2009 (CO₂ enrichment plus warming). 

Results/Conclusions

As predicted, warming resulted in significant reductions in soil water content while CO₂ had the opposite effect of enhancing soil water, relative to that observed in control plots.  ANPP was unaffected by warming, but warming enhanced C₄ perennial grass production, most notably by 23% in 2009 when precipitation was 20% above average.  In contrast, CO₂ consistently enhanced ANPP an average 34% in 2006-2008, but was insensitive to CO₂ in 2009.  Aboveground biomass of C₃ grasses responded more to CO₂ than C₄ grasses.  Few CO₂ by temperature interactions were observed within years except for root biomass in the top 5 cm of the soil profile, where warming decreased root biomass under present ambient CO₂, but significantly increased it at 600 ppm CO₂.   Different treatment sensitivities between wet and dry years suggest that plant responses to CO₂ and temperature were strongly influenced by soil water content.  The results illustrate the need to consider the counteractive effects of CO₂ in warming experiments as well as in paleoecological understanding of plant community responses to climate change.