2006 Annual Report 4d.Progress report. This report documents research conducted under Specific Cooperative Agreement 5409-11000-003-05S between ARS and the University of Wyoming, Department of Renewable Resources, entitled "Implications of global change for rangeland management." Additional details of research can be found in the parent project 5409-11000-003-00D, Global Change: Responses and Management Strategies for Semi-Arid Rangelands. This past year was spent in the final installation of 30 rings for a combined CO2 enrichment (Free Air CO2 Enrichment, or FACE) and warming experiment, including installing instrumentation of Sentek soil moisture tubes, minirhizotrons, soil and air thermocouples, and taking measurements of site vegetation and soils. Several students from the University of Wyoming are involved in this project. A MS student in Rangeland Ecology and Watershed Management at UW will have completed a full field season on the project by late August 2006 collecting seasonal data on leaf gas exchange responses in ambient and elevated CO2 plots. She is finding significant enhancement of leaf photosynthetic rates and predawn leaf water potentials in elevated CO2 plots for Artemisia frigida, Pascopyrum smithii, and Bouteloua gracilis compared to ambient plots. She is also collecting seasonal leaf gas exchange and water relations data on naturally occurring Linaria dalmatica at the HPGCE site, but outside the experimental plots. These data reveal that invasive Linaria is able to sustain much higher predawn leaf water potentials and leaf-level gas exchange rates through dry periods compared to native prairie species (e.g., Artemisia, Pascopyrum and Bouteloua). Finally, in summer 2006 she surveyed more then 30 vegetation transects near the HPGCE site where Linaria occurs, collecting information on species diversity and cover. She will identify associations, both positive and negative, between cover of Linaria and native plant taxa. A PhD student in Ecology at UW joined the HPGCE this summer and is now developing a dissertation proposal to continue work on the project. He led two major data collection campaigns at the HPGCE focusing on gas exchange responses to pulsed precipitation inputs. His work will demonstrate how elevated CO2 alters the patterns of leaf gas exchange responses to rainfall events. He is collaborating with the University's PI to develop a manuscript this Fall 2006 based on data collected this summer. Another MS student in Rangeland Ecology and Watershed Management at UW is joining the HPGCE this Fall 2006. She will continue with the leaf gas exchange and water relations studies at the HPGCE after another student completes her thesis work in spring 2007. An undergraduate in engineering at UW worked as a field technician during the summer 2006 field season at the HPGCE. She assisted other students with gas exchange and water relations studies. The UW Stable Isotope Facility continues to be closely involved with the HPGCE. Biomass samples collected in summer 2006 will be analyzed for 13C, %C and %N content at the UWSIF. The UWSIF also has been analyzing the irrigation water for delta 18 O and delta 2 H content and the fumigation CO2 for delta 13 C content. Isotope analyses are a central aspect of the studies being conducted by the lab groups. Measurements conducted during the growing season of 2006 by the Ecosystem Ecology group focused on canopy-scale net ecosystem exchange of carbon dioxide (CO2) and water, soil CO2 concentrations and efflux rate, plant cover by species, and normalized difference vegetation index (NDVI). Soil samples were collected in late July, and will be subjected to analysis of organic C and N content, and C isotopic composition in particulate and mineral-associated phases; inorganic carbon, pH, electrical conductivity, and potassium chloride extracts for nitrate and ammonium. Incubations for mineralization potential will also be conducted. The standing crop of root biomass will be measured, and root in-growth bags were installed for an index of belowground productivity. A poster will be presented at the Soil Science Society of America Annual Meeting in November, 2006, titled "Patterns in Net Ecosystem Exchange: First Year Responses to Elevated CO2 and Irrigation from the High Plains Global Change Experiment."