Fine Root Responses to Soil Warming at Harvard Forest: Seasonal and Long-term Effects

Although global mean temperatures continue to increase, the responses of fine roots to climate warming, and implications for ecosystem-atmosphere-carbon dynamics, are uncertain. We initiated research at a long- term (15 year) and a more recent (3 year) soil warming study at the Harvard Forest, Massachusetts, to assess fine root (less than 3 mm in diameter and less than 1 mm in diameter) biomass responses to warming. We found that a 5-degree-C increase in soil temperature remarkably affected fine root biomass, particularly that of the smallest size class which represents the most dynamic component of the root-soil nutrient system. During both the 2006 and 2007 growing seasons, Heated plots exhibited reductions in fine root biomass of over 40% compared to Control plots. These results held for both the short-term (e.g., p=0.10, 0.06, and 0.0007 for April, June and July 2007, respectively) and the longer-term warming studies (p<0.05 in both June and July 2007). We believe the reduction in fine root biomass during the growing season results in part from the increased nitrogen mineralization rates and increased nitrogen availabilities that we observe in the Heated plots at both sites. We are currently investigating the mechanism(s) for this reduction in fine root biomass, to determine whether fewer fine roots are being produced in the Heated plots, compared to the Control plots, or whether production is similar but turnover rates are higher under warmed conditions.