The varied topography and large elevation gradients that characterize the arid and semi-arid Southwest create a wide range of climatic conditions - and associated biomes - within relatively short distances. This creates an ideal experimental system in which to study the effects of climate on ecosystems. Such studies are critical given that the Southwestern U.S. has already experienced changes in climate that have altered precipitation patterns, and is expected to continue to experience dramatic climate change in the coming decades. Climate models currently predict a transition to a warmer, more arid climate in the Southwest.  It is uncertain what impact this change might have on regional changes in carbon storage, hydrologic partitioning and water resources.  We are using a sequence of six widespread biomes along an elevation gradient in New Mexico -- ranging from hot, arid ecosystems at low elevations to cool, mesic ecosystems at high elevation to test specific hypotheses related to how climatic controls over ecosystem processes change across this gradient. We used eddy covariance towers and associated ecological and meteorological measurements in each biome to directly measure the exchange of carbon, water and energy between the ecosystem and the atmosphere.

Results/Conclusions The desert grassland was a small carbon source in both 2007 and 2008 (mean 45 g C m^-2), and sink strength increased in the other biomes with elevation with mean sink strength for both years of -60, -125, -180,-220 and -375 g C m ^-2 in desert shrubland, juniper savanna, piñon-juniper woodland, ponderosa pine forest and spruce forest, respectively.  The temperature sensitivity of ecosystem respiration and gross primary production (GPP) was highest during the pre-monsoon period and varied by biome.  Temperature sensitivity was lowest in the low elevation and high elevation biomes and peaked in the mid-elevation juniper savanna and piñon-juniper woodland.   All of the biomes were dependent upon monsoon precipitation for a large percentage of total annual carbon uptake, but the percentage decreased with increasing elevation.  This gradient offers us a unique opportunity to test the interactive effects of temperature and soil moisture on ecosystem processes as temperature decreases and soil moisture increases markedly along the gradient and varies through time within sites.  These results have important implications for understanding the impact of various climate change scenarios on regional carbon storage.