Changes in ecosystem carbon storage over 40 years on an old-field forest landscape in east-central Minnesota

Organic carbon (C) storage in complex landscapes and its temporal change can be important in the global C budget. Change in C storage between 1938 and 1977 was estimated for a 2224 ha old-field/forest landscape in east-central Minnesota by coupling change in area of seven vegetation types (five forest and two non-forest) with vegetation-specific C densities (mg ha-1). Carbon densities were based on sampling carried out between 1974 and 1990, Areas of vegetation types in 1938 and 1977 were determined from aerial photographs. Carbon density was greatest in forest overstory (60-100 Mg ha(-1)) and organic and mineral soil (30-100 Mg ha(-1) to 25 cm depth). Ecosystem C storage was approximately 212 000 Mg in 1938 and 225 000 Mg in 1977, an increase of ca, 13 000 Mg across the study area. This was due largely to an increase in upland forest at the expense of non-forest area. The largest proportional increase in C storage was in trees (a 13% gain), while mineral soil gained 4% and herbs gained 6%. C storage in O horizon and shrubs remained constant. For the 20% of the landscape originally occupied by cultivated fields, an empirical model based on chronosequence studies indicated a 40% increase in C storage over 40 years; C increased in mineral soil, O horizon and trees as both herbaceous succession and forest encroachment occurred. Uncertainties of the estimates, based on propagation of standard errors, were 5% to 19% for C storage and 6% to 1000% for change in C storage, Uncertainty was due primarily to sample variability, but included uncertainty in biomass equations and GIS processing. This uncertainty demonstrates the difficulties associated with expanding from point sample data to landscape-scale estimates of C storage.