Background/Question/Methods   Until recently, experimental studies of climate change focused on temperature or precipitation, but rarely both. Additionally, warming studies almost universally focused on the effects of a single step increase in temperature. It is not known whether most ecosystem and community variables respond unimodally, linearly or otherwise to temperature increases, or how these responses would be affected by accompanying changes in precipitation. The Boston-Area Climate Experiment (BACE) was constructed to address these issues. The BACE imposes a factorial combination of precipitation and temperature manipulations on old-field plots. Unlike most previous experiments, the BACE features four levels of warming, enabling tests of the hypotheses that species- and ecosystem-level responses to warming are nonlinear, and that the character of these responses depends strongly on precipitation. The experiment uses infrared heaters to achieve warming of up to 4 degrees C, with feedback control from infrared radiometers. Rainout shelters remove half of the incoming precipitation from the drought treatment. During the growing season this water is immediately sprinkled on the wet treatment, providing it with 150% of ambient rainfall. Here, we report responses of the plant community and ecosystem to the first full year of experimental manipulations in the BACE.

Results/Conclusions   Warming linearly increased plant growth in the spring, as measured by leaf area index. In the summer, responses of plant growth to warming depended on the precipitation treatment, as warming decreased plant growth rates in the drought treatment but increased plant growth in the wet treatment. Responses of soil respiration to warming depended on the season. Warming increased respiration in winter, but decreased it in summer, leading to no net effect over the course of a year. Precipitation affected total soil respiration nonlinearly: increases and decreases in precipitation both suppressed annual carbon efflux. We discuss this result in the context of measured root production and heterotrophic respiration.