Effects of multiple global changes on carbon dioxide and water vapor fluxes in an annual grassland

Global environmental change has the potential to alter both the timing and magnitude of growth and activity in plant species worldwide. The Jasper Ridge Global Change Experiment (JRGCE) explores the effects of elevated carbon dioxide (CO₂), warming, increased precipitation, and nitrogen deposition on a California annual grassland. These four factors have been manipulated at two levels, in a full factorial design replicated eight times, starting in the fall of 1998. Since the fall of 2005, net ecosystem exchange of carbon dioxide and water vapor has been measured in six replicates of all treatments. Open-top dynamic chambers were used to minimize pressurization and heating by the chamber, and to allow long-term chamber operation. Chambers were placed to monitor a single set of replicates at a time, and were moved between replicate blocks once every five days. Measurements were made continuously throughout the growing season, and once in the summer. Elevated CO₂ delayed both the peak and the end of the growing season, as measured by the peak in CO₂ uptake and the decline in respiration, respectively. Increased nitrogen deposition accelerated the peak in respiration early in the season, and also increased CO₂ uptake at the peak of the season by 24-163%. Warming decreased evapotranspiration by an average of 22%, a difference that was maintained throughout the growing season. Increased rainfall slightly delayed the peak of CO₂ uptake but otherwise had little effect.