The state of the carbon cycle in the northeastern United States and Canada 1982-present: Results from a constrained, dynamic terrestrial biosphere model

This study analyzed the carbon dynamics of terrestrial ecosystems from 1982-present in the Northeastern United States and Canada. In contrast to previous analyses using simplified biosphere models, the ED-LSM terrestrial biosphere model used in this study is a complete dynamic vegetation model capable of predicting both short-term carbon fluxes and long-term changes in above- and below-ground carbon stocks. Prior to the analysis, the model’s parameterization had been optimized against multiple data constraints, including eddy-flux measurements of fast time-scale ecosystem carbon fluxes and forest inventory measurements of long-term above-ground carbon dynamics. Subsequent regional- and site-level validation against independent datasets showed that a model that accurately predicted observed seasonal, annual and decadal-scale patterns of carbon fluxes within the Northeastern region.

The mean annual carbon flux over the region during the simulation period was uptake of 0.57 tC ha^-1 y^-1, with substantial inter-annual variability: annual carbon fluxes ranged from a spatially-averaged mean uptake of 1.3 tC ha^-1 y^-1 in 1991 to a near-neutral biosphere in 1996. The three dominant causes of interannual variability in terrestrial carbon uptake during the period, in order of importance, were: (i) summer-time precipitation anomalies, (ii) spring-time temperature anomalies, and (iii) fall temperature anomalies. Above average temperature and precipitation conditions during all 3 of these periods caused increases in net primary productivity (NPP), but drove even larger increases in soil respiration, resulting in overall negative impacts on the rate of carbon uptake by terrestrial biosphere. The analysis also indicated a significant long-term decadal-scale trend in NPP of 0.05 tC ha^-1 y^-1 resulting from an increasing difference between regional biomass growth and regional carbon losses due to mortality and forest harvesting. However, due to high degree of interannual variability in soil respiration rates, this increasing long-term trend in NPP did not translate into a significant long-term trend in net carbon uptake.