An integrated terrestrial ecosystem carbon-budget model based on changes in disturbance, climate, and atmospheric chemistry

Disturbances (e.g. fire, insect-induced mortality, and harvest) and management practices (e.g. planting) affect the forest carbon (C) cycle, so do non-disturbance climatic and atmospheric factors (e.g. growing season length and temperature, abiotic decomposition factor, annual precipitation, atmospheric CO2 concentration, and nitrogen (N) deposition). Previous studies investigated the effects of these factors individually or in some combinations, but not their integrated effects at regional and global scales. This study describes an Integrated Terrestrial Ecosystem C-budget model (InTEC), which integrates effects of all these factors on the annual C cycle of a forest region. InTEC is based on the Farquhar's leaf photosynthesis model, the Century C cycle model, the net N mineralization model of Townsend et al. [Ecol. Appl., 6 (1996) 806] and an age–NPP relationship derived from forestry inventory-based age–biomass relationships. To integrate these existing models, which were developed for different purposes and had different spatial and temporal scales, into a coherent mechanistic model, we (1) develop a spatial and temporal up-scaling algorithm to use the instantaneous leaf-level model for a region at annual time step; and then (2) combine the upscaled results with an age–NPP relationship to obtain the annual NPP of a forest region. A historical change approach is then used to describe the regional annual C cycle, which not only improves the accuracy of its historical and present estimates, but also enables us to predict its future responses, both of which are critical in formulating mitigation and adaptation strategies for global changes. Applying InTEC to Canada's forests, we first investigate the impacts of each factor on the C cycle over the short term (i.e. in the year of perturbation) and the long term (i.e. in the years after perturbation). The short-term and long-term effects are determined by changing one of the 10 factors in year 1 since the industrialization while keeping this factor in all other years and all other factors in all years at pre-industrial levels. Integrating all these short-term and long-term effects for the actual historical data of the 10 external forcing factors, we then estimate that the annual mean NBP (=NPP — soil respiration — fire emission — forest praoducts oxidation) of Canada's forests was 40±20 Tg C per year (i.e. a sink) in 1810s, reduced to −131±66 Tg C per year (i.e. a source) in 1870s, increased thereafter to a maximum of 200±100 Tg C per year in 1930s, and decreased again to 57±27 Tg C per year in 1990s. From 1800 to 1998, the aboveground biomass of Canada's forests increased by ∼19%, while the soil C stock increased by ∼2%.