Background/Question/Methods
Carbon uptake by ecosystems is important in mitigating future climate change, yet responses of ecosystem carbon uptake to climate change are poorly understood. We manipulated air temperature and precipitation to study ecosystem responses to climate change along an elevation gradient in northern Arizona covering a broad range of climatic regimes. Our study sites focused on grass-dominated habitats in four vegetation types along a 1400m elevation gradient, including mixed conifers, ponderosa pine forests, pinyon-juniper woodlands, and high desert grassland. A soil downward transplant experiment was initiated in 2002 to simulate warmer temperature expected with doubled atmospheric CO₂ concentration. Because of the uncertainty of future precipitation change, two precipitations treatments (+50% and -30%) were applied to the original and transplanted soil monoliths.

Results/Conclusions
The temperature treatments raised soil temperatures by 2 ºC, and the precipitation treatments altered soil water content in the expected direction. Increased precipitation enhanced gross primary production (GPP) and respiration, and experimental warming also stimulated respiration. Positive effects of increased precipitation on GPP outweighed positive effects on respiration, leading to an increase in carbon (C) gain. Warming increased GPP more than respiration in ponderosa pine site, resulting in an increase in C gain, but the effect was reversed for the high elevation mixed conifer site. Interactions between warming and precipitation treatments varied during the growing season. Warming and reduced precipitation decreased C gain in the dry season, but enhanced C gain during monsoon season. The highest respiration was observed in response to the combination of warming and increased precipitation in the monsoon season. Our experiment demonstrated that the magnitude of effects of temperature and precipitation change on GPP and respiration determined C gain in various ecosystems. Single factor treatment could be very informative of the direct effect, but interactions between multiple factors determined the responses throughout the growing season.