Interactive effects of atmospheric and climate change on aboveground production in a constructed old-field ecosystem

The Old-Field Community Climate and Atmospheric Manipulation (OCCAM) experiment at Oak Ridge National Laboratory, Tennessee was established to investigate interactive effects of elevated [CO₂], warming, and soil drying on a multi-species old-field community.  This constructed old-field ecosystem, planted in open-top chambers, consists of C₃ and C₄ grasses, herbaceous dicots, and legumes.  Total aboveground biomass was measured at the end of the growing season in 2005 and 2006.  In both years, biomass was approximately 50% higher in wet relative to dry treatments.  This pattern was dominated by the response of Lespedeza cuneata, with 397 and 171 g/m² (2005) and with 426 and 172 g/m² (2006) in wet and dry treatments, respectively.  In 2006, warming increased biomass by 40% and elevated [CO₂] increased biomass by 20%.  In 2006, the effect of warming was driven by Festuca pratensis with 179 and 79 g/m² in warmed and ambient temperatures, but no individual species contributed to the pattern in elevated [CO₂].  In both years, the few interactions among factors were variable and detected only at the individual species level.  Correlation analysis, performed with biomass and resource availability (soil nutrients, water, and temperature), revealed that biomass was positively correlated with nitrogen availability during the 2005 growing season (April-Sept.), whereas biomass was positively correlated with soil temperatures during the 2006 growing season.  Results from this experiment indicate that the response of total aboveground biomass to global change may be dictated by species-specific and community level responses to changes in resource availability.