2012 Annual Report 1a.Objectives (from AD-416): Evaluate how warming and increasing atmospheric CO2 concentration affect the structure and functioning of Great Plains grasslands through their affects on primary production, N and C cycling, water relations, and plant community dynamics. 1b.Approach (from AD-416): Stable Isoptope Lab/University of Wyoming (SILUW) and ARS will collaborate in planning and conducting cooperative global change research at the High Plains Grasslands Research Station (HPGRS) to investigate how global change influences ecosystem functioning through evaluating the combined effects of rising atmospheric CO2 and temperature on C and N cycling, plant/soil water relations, weed invasions and plant community ecology. Research will be conducted under realistic field conditions in a native, semi-arid grassland in which ambient CO2 is increased from present-day levels of 390 ppm to 600 ppm and temperatures increased 1.5/3 C day/night. These altered environmental conditions are predicted to develop in Wyoming over the second half of the century. 3.Progress Report: Collaborators at the University of Wyoming are working with ARS to increase our understanding of how plants and soils respond to global warming, rising atmospheric carbon dioxide (CO2), and altered precipitation, as part of a comprehensive research project predicting how western rangelands will respond to climate change. We are in the 7th year of the Prairie Heating and CO2 Enrichment (PHACE) experiment, being conducted at the USDA-ARS High Plains Grassland Research Station in Cheyenne, WY. Research through this cooperative agreement on the PHACE experiment focused on three major elements:. 1)leaf carbon metabolism;. 2)ecosystem scale CO2 and H2O exchange; and. 3)plant community restoration. Light- and dark-acclimated leaf respiration for the dominant perennial grass, western wheatgrass (Pascopyrum smithii), was determined. These measurements provide important mechanistic information about how climate change influences carbon exchange in semiarid rangelands. The measurements complement ongoing studies at PHACE examining ecosystem scale respiration and photosynthesis. In addition, efforts continue to monitor CO2 exchange at the plot level to examine how climate change impacts net carbon uptake. A new subexperiment was implemented within PHACE plots to examine success of restoration under different climate conditions. The work follows emergence and establishment of grass and forb species of different biogeographic origins across the northern and southern mixed-grass prairies in North America. Three graduate students are contributing to this research. The PHACE experimental treatments will be terminated in September 2013. Above and below ground biomass and soils will be harvested for analysis and disturbed plots will be restored. To ensure accountability of funds utilized, the ADODR and his staff hold meetings every 6 months with collaborators to discuss research and site issues; staff of both groups communicates regularly on experimental protocols, site management, and presentation of results. The group continues to coordinate on modeling and data base management efforts so the results of this work can be extrapolated by modeling projects involving project scientists and potential future collaborators.