Global climate models predict increases in average annual temperatures and changes in the amount and pattern of precipitation worldwide. Until recently, experimental studies of climate change almost universally focused on the effects of a single step changes in temperature or precipitation. It is not known whether most ecosystem and community functions respond linearly or non-linearly to temperature increases, or how their responses are altered by concurrent changes in precipitation. The Boston Area Climate Experiment (BACE) in Waltham, Massachusetts, addresses this critical question by exposing an old-field community to a factorial combination of heating and precipitation treatments in gradients. The BACE uses infrared heaters to achieve four levels of warming (up to 4 °C), with feedback control from infrared radiometers. Rainout shelters remove half of the incoming precipitation from the drought treatment and sprinkle the removed water immediately onto the rain addition treatment. The gradient factorial approach of the BACE enables tests of hypotheses that species- and ecosystem-level responses to warming are nonlinear, and that the nature of these responses is mediated by precipitation. Here, we report the initial responses of herbaceous plant growth, above- and belowground, and tree seedling photosynthesis to the experimental manipulations in the BACE.

Results/Conclusions

Precipitation and warming treatments both affected herbaceous plant growth and tree seedling photosynthesis. At the end of the first full treatment year, the total harvested herbaceous biomass was lowest in warmed, low precipitation plots and highest in warmed plots with additional precipitation. Root growth was not significantly affected by warming, but decreased precipitation resulted in greater root growth at deeper (>10cm) depths. The photosynthetic rates of the three planted tree seedling species responded differently to the treatments. A. rubrum seedlings appeared to be more sensitive to changes in temperature, while Q. rubra seedlings were more sensitive to changes in precipitation. In its first full year of operation, the BACE already indicates that interactive climate effects are likely to change biomass production and establishment of tree seedlings in old field plant communities.