Main versus interactive effects of global change on a constructed old-field ecosystem

Changes in global atmospheric carbon dioxide concentration, air temperature, and precipitation will occur simultaneously, although their potential to interact in their effects on complex natural ecosystems is largely unknown.  The Old-field Community Climate and Atmospheric Manipulation (OCCAM) experiment in east Tennessee was designed to determine potential interactive effects of carbon dioxide enrichment, warming, and irrigation on a constructed old-field ecosystem.  In 2004 and 2005, total aboveground production was 50% greater in wet than in dry plots, but production was not affected by warming or elevated carbon dioxide, either alone or in combination with other factors.  In 2006, production was again 50% greater in wet than in dry plots, but was also 40% greater in warmed plots and 20% greater in plots with elevated carbon dioxide; there were few interactions among factors in 2006.  During the 2004 and 2005 growing seasons, leaf area index (LAI) was greatest under elevated carbon dioxide and in wet plots; warming decreased LAI only during the 2004 growing season.  LAI in both years was seldom affected by interactions among treatments.  During the growing season in both years, shallow soil moisture (0-15 cm) was strongly affected by main, but not interactive, effects of the treatments:  elevated carbon dioxide and watering increased soil moisture, whereas warming decreased soil moisture.  Results contrast with our a priori expectation that interactions would be as important as main effects for these three important drivers of global change, at least in terms of primary production, canopy structure, and soil moisture.