Twentieth Century Changes of Forest Carbon Pools of a Northern Hardwood Forest with Emphasis on Soil Organic Carbon Pools

Changes in forest Soil Organic Carbon (SOC) pools are difficult to detect because 1) the long timescales involved, 2) relative differences are small compared to aboveground biomass C, 3) the number of samples may not capture spatial heterogeneity of soils in forest stands, 4) sampling methods may sacrifice accuracy for sample size and 5) uncertainty about land use changes. These challenges are addressed in this study by using detailed field data and by applying ecosystem models to simulate changes in biomass C and SOC of 22 even- aged northern hardwood stands stretched throughout the Green Mountains of Vermont. A century long "snapshot" of results reveal overall small (less than 10 percent) decreases in SOC throughout the Twentieth Century as a result of clearing the land for settlement and logging. These legacy effects, borne out only by simulations, indicate that managed forest systems may be steadily, albeit slowly, declining in SOC content. Net Ecosystem Production for even-aged stand in this region was still positive in the past century because SOC decreases were small compared to aboveground biomass increases. The potential for simulation models to estimate future responses of forest growth and changes in SOC pools was also explored and are presented. Overall, results indicate that simulation outputs are reliable and adequate at both local and regional scales given sufficient information about vegetation, soils and land-use history.