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

Abstract:

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.