Effects of Elevated CO2 and Warming on Plant Productivity, Soil Moisture, and Plant Water-Relations in a Semi-Arid Grassland

Friday, 19 December 2014: 4:30 PM
Kevin E Mueller1, Dana M Blumenthal1, Elise Pendall2, David G Williams3, Dan R LeCain1 and Jack A Morgan1, (1)United States Department of Agriculture, Agricultural Research Service, Rangeland Resources Research Unit, CO, United States, (2)University of Western Sydney, Penrith, NSW, Australia, (3)University of Wyoming, Laramie, WY, United States
In a mixed-grass prairie near Cheyenne, WY, we conducted a 7-year climate change experiment with factorial manipulations of air temperature and atmospheric carbon dioxide (dubbed the Prairie Heating and CO2 Enrichment experiment; PHACE). Carbon dioxide treatments were ambient (~390 ppm) and elevated (~600 ppm), implemented with Free-Air CO2Enrichment (FACE) technology. Temperature treatments were ambient and warmed (+1.5°C during the day and +3°C at night), maintained by a Temperature Free-Air Controlled Enhancement (T-FACE) system. Using the first three years of data, Morgan et al. (2009) reported that elevated CO2 stimulated plant production, regardless of the temperature treatment and especially in years when ambient soil moisture was low, likely due in part to the positive effect of CO2 on soil moisture that is mediated by plant physiological responses to elevated CO2 (e.g. reduced stomatal conductance). Here, we report the effects of elevated CO2, warming, and interannual weather variability (and their interactions) on plant productivity and soil moisture using the full 7-year dataset. Preliminary analyses show: 1) evidence of a persistent effect of both CO2 and warming on soil moisture (positive and negative, respectively) and indications that the effect of each treatment on soil moisture varied with soil depth and with time within the growing season, and 2) evidence that effects of both CO2 and warming on productivity are temporally dynamic, due to a combination of interannual weather variability, shifts in plant community composition, and diverse responses of different plant functional types and species to the treatments. Finally, to identify how plant physiological processes mediated the impact of CO2 and warming on plant productivity and soil moisture, we will briefly describe the response of plant traits to the treatments (e.g. leaf gas-exchange, leaf drought tolerance, depth of water uptake from soil). Collectively, our results suggest that decadal scale experiments (and longer) will be required to better understand the impacts of climate change in semi-arid grasslands and other ecosystems with inherently large interannual variability in weather, productivity, and community composition.