Global environmental change has the potential to drive major changes in both the magnitude and timing of plant growth and activity worldwide. Prediction of these changes is complicated by the large interannual differences in plant activity that occur naturally. 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. In the 2005-06 and 2006-07 growing seasons, net ecosystem exchange of carbon dioxide and water vapor was 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 both growing seasons.

Results/Conclusions

The two years differed in natural precipitation by more than a factor of two, 2005-06 being the wettest water year since 1999, and 2006-07 the driest. In contrast to expectations, biomass was much greater in the dry than the wet year. Total annual carbon exchange followed the same pattern, such that the dry year was a carbon sink and the wet year was a source. Despite these differences in total carbon uptake, the relative effects of the global change treatments were similar in the two years. All of the treatments except nitrogen caused changes in the timing of phenological events at the end of the season, when water availability declines. Although they appeared in both years, these effects were stronger in the dry year. These results highlight the importance of interannual variability in carbon dynamics, and the limitations of extrapolating from a small number of years when measuring carbon sinks.