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Modeling the relative importance of land use history and CO2 fertilization in forest carbon dynamics of the eastern U.S.
Albani, Marco*,1, Moorcroft, Paul1, 1 Harvard University, Cambridge, MA
ABSTRACT- Atmospheric measurements and inventory data suggest that since the 1980's the forests of North America are storing between 0.15 and 0.30 Pg C year-1, a substantial component of the continental carbon sink. Two fundamental mechanisms for the accumulation of carbon are 1) the recovery of forest ecosystem from previous land uses; and 2) the enhancement of growth by CO2 fertilization. Assessing the magnitude of C accumulation that is due to each has fundamental implications for the long-term future of the North American carbon sink. In this study we use a mechanistic individual-based terrestrial ecosystem model, the Ecosystem Demography (ED) model, to estimate the magnitude of the two terms. Unlike conventional global vegetation models, ED is capable of mechanistically account for both the effect of CO2 fertilization on plant growth and the impact of land use changes on plant demography, stand composition and size structure, stand age structure, and ecosystem productivity, all variables that can be directly compared to inventory data. We use the ED model to simulate the historical pattern of land cover and carbon accumulation in the eastern portion of the conterminous U.S at 1° resolution, forcing the ecosystem model with an historical dataset of land use change and atmospheric CO2 concentration. We obtain for each grid cell different scenarios of current rates of Above-ground Net Ecosystem Productivity (ANEP) and current levels of Aboveground Biomass (AGB) as a function of forest stand age. Modeling results are compared to the data from the US Forest Inventory and Analysis, and we assess the ability of each mechanism to predict the observed patterns of ANEP and AGB variation with stand age. While land use history affects the age-class distribution within each simulation cell, CO2 fertilization modifies the historical rates of ANEP and thus directly impacts the current relationship between AGB and stand age, enabling us to assess the magnitude of carbon accumulation due to each mechanism.
Key words: carbon fluxes, ecosystem model, forest inventory, North America