Responses of evapotranspiration to experimental warming and elevated CO2 in a semi-arid grassland

Background/Question/Methods

Global climate change is expected to alter seasonal patterns and rates of evapotranspiration, particularly in arid regions. Evapotranspiration and associated [CO₂] fluxes are important components of ecosystem feedbacks to climate. While climate change will involve elevated [CO₂] and increased temperatures, independently these factors will have different impacts on ecosystem water cycling due to their opposing effects on transpiration. Ecosystem responses will additionally be controlled by community properties such as leaf area and species or functional group composition. We used canopy gas exchange chambers to quantify evapotranspiration (ET) in a semi-arid grassland altered by elevated [CO₂] and warming at the Prairie Heating and [CO₂] Enrichment (PHACE) experiment near Cheyenne, WY. The chamber system used a fast-response infrared gas analyzer (LI-7500) mounted inside a clear acrylic chamber, with simultaneous measurements of light, air temperature, relative humidity, and soil temperature and moisture content. Our factorial experimental allowed us to distinguish effects of elevated [CO₂] from warming and reveal the driving factors controlling ecosystem-level water fluxes at different times throughout a growing season.

Results/Conclusions

The canopy gas exchange chamber allowed us to measure water and [CO₂] fluxes within less than one minute of chamber closure, before water vapor and air temperature changed significantly. Midday rates of ET across treatments during the 2010 growing season ranged from 1.4 to 7.4 mmol m^-2 s^-1, similar to rates measured in this ecosystem type using non-chambered techniques. The greatest differences in ET among treatments occurred during the middle of the growing season; plots exposed to elevated [CO₂] had the highest ET. At the end of the growing season, when ET was generally decreasing, the combined warming and elevated [CO₂] treatment had the highest ET. Peak season aboveground biomass was stimulated by warming at ambient and elevated [CO₂]. The influences of environmental drivers, including soil moisture, air temperature, and photosynthetically active radiation, as well as biotic drivers, including plant cover and biomass, on ET fluxes are being investigated. We expect that these results will improve predictive understanding of water cycling in semiarid grasslands in future climate conditions.