RECONSTRUCTION AND ANALYSIS OF HISTORICAL CHANGES IN CARBON STORAGE IN ARCTIC TUNDRA

Surface air temperature in arctic regions has increased since pre-indu strial times, raising concerns that warmer and possibly drier conditio ns have increased soil decomposition rates, thereby stimulating the re lease to the atmosphere of the large stores of carbon (C) in arctic so ils. We used a model (MEL-GEM, Marine Biological Laboratory General Ec osystem Model) of ecosystem C and nitrogen (N) dynamics to predict and analyze historical (1829-1990) changes in C storage in a N-limited, t ussock-tundra ecosystem near Toolik Lake on the North Slope of Alaska. The model simulates stand-level photosynthesis and N uptake by plants , allocation of C and N to foliage, stems, and fine roots, respiration in these tissues, turnover of biomass through litterfall, and decompo sition of litter and soil organic matter. We first calibrated the mode l by deriving a single parameter set that closely simulated the respon se of tussock tundra to decade-long experimental manipulations of nutr ients, temperature, light, and atmospheric CO2. The calibrated model p redicts that historical increases in temperature and atmospheric CO2 h ave increased total ecosystem C storage. Higher temperatures increased soil and plant respiration, but those losses of C were overcompensate d by increased photosynthesis resulting from redistribution of N from soil to plants. This redistribution of N was due to increased net mine ralization and uptake of N. Increases in atmospheric CO, also increase d photosynthesis, but consequent increases in C storage were constrain ed by limits on increases in the C:N ratio of vegetation. In contrast, hypothesized historical decreases in soil moisture substantially decr eased simulated total ecosystem C storage as a result of large increas es in soil respiration. With decreased soil moisture, increases in pho tosynthesis associated with redistribution of N from soil to plants on ly partially compensated for respiratory C losses, as plant uptake of N could not keep pace with increased N mineralization rates. Consequen t losses of N from the ecosystem contributed to the declines in C stor age under drier conditions. Based on the combined effects of reconstru cted historical changes in atmospheric CO2, mean growing-season temper ature, and two alternative soil moisture scenarios, the model predicts a -5.4 to +2.3% change in ecosystem C from 1829 to 1990. These estima tes are consistent with field evidence that historically recent change s in C storage of tussock tundra have been relatively small. However, the model also predicts that relatively large transient losses of ecos ystem C (50-180 g.m(-2).yr(-1)) may frequently have occurred since the late 1800s, with one of the largest simulated C losses of the 20th ce ntury occurring from 1988 to 1990 (3-yr average = 133 g C.m(-2).yr(-1) ). These simulated losses were similar to losses measured in tussock t undra at Toolik Lake from 1983 to 1990 (e.g., 1990 losses = 156 g Cm-2 .yr(-1)) but nonetheless suggest that these measured losses may be tra nsitory in nature.