Jack Morgan, USDA-ARS Crops Research Laboratory, 1701 Center Avenue, Fort Collins, CO 80526, Daniel Milchunas, Colorado State University, Forest Rangeland Watersherd Stewadship, 211 Natural Resources, Fort Collins, CO 80523, Elise Pendall, Univ ersity of Wyoming, 1000 E University Ave #3165, Laramie, WY 82071, William J. Parton, NREL/CSU, Fort Collins, CO 80523, and Justin Derner, USDA ARS Rangeland Resources Research Unit, High Plains Grasslands Research Station, 8408 Hildreth Road, Cheyenne, WY 82009-8899.
Rising atmospheric carbon dioxide (CO2) is known to be an important contributor to radiative forcing of Earth’s climate and is projected to be leading to global warming. Direct effects of this gas on photosynthesis, plant water relations and net primary productivity (NPP) have been underway for more than a century, and likely have already contributed to important changes in rangelands. Research conducted in native and semi-natural rangelands exposed to elevated CO2 atmospheres suggests that CO2-enhanced water relations is the primary mechanism leading to increased NPP in these ecosystems. Although higher productivity can be viewed as a potentially beneficial response in typically water-limited rangelands, this response may also have negative implications, especially for land managers. Plant species differ in their sensitivities to CO2, with winners and losers emerging in experiments designed to evaluate species responses to CO2. Research conducted on the Shortgrass Steppe of Colorado showed CO2-induced changes in species composition that resulted in less desirable forage species for domestic livestock. Further, the inability of soil N release to keep pace with enhanced NPP at elevated CO2 can result in a lowering of forage quality sufficient to impair performance of both domestic livestock and native grazers. Herein we discuss these research findings in light of their implications for future management of semi-arid rangelands.