RECONSTRUCTION AND ANALYSIS OF HISTORICAL CHANGES IN CARBON STORAGE IN ARCTIC TUNDRA
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Surface air temperature in arctic regions has increased since pre-industrial times, raising concerns that warmer and possibly drier conditions have increased soil decomposition rates, thereby stimulating the release to the atmosphere of the large stores of carbon (C) in arctic soils. We used a model (MEL-GEM, Marine Biological Laboratory General Ecosystem Model) of ecosystem C and nitrogen (N) dynamics to predict and analyze historical (1829-1990) changes in C storage in a N-limited, tussock-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 decomposition of litter and soil organic matter. We first calibrated the model by deriving a single parameter set that closely simulated the response of tussock tundra to decade-long experimental manipulations of nutrients, temperature, light, and atmospheric CO2. The calibrated model predicts that historical increases in temperature and atmospheric CO2 have increased total ecosystem C storage. Higher temperatures increased soil and plant respiration, but those losses of C were overcompensated by increased photosynthesis resulting from redistribution of N from soil to plants. This redistribution of N was due to increased net mineralization and uptake of N. Increases in atmospheric CO, also increased photosynthesis, but consequent increases in C storage were constrained by limits on increases in the C:N ratio of vegetation. In contrast, hypothesized historical decreases in soil moisture substantially decreased simulated total ecosystem C storage as a result of large increases in soil respiration. With decreased soil moisture, increases in photosynthesis associated with redistribution of N from soil to plants only partially compensated for respiratory C losses, as plant uptake of N could not keep pace with increased N mineralization rates. Consequent losses of N from the ecosystem contributed to the declines in C storage under drier conditions. Based on the combined effects of reconstructed historical changes in atmospheric CO2, mean growing-season temperature, and two alternative soil moisture scenarios, the model predicts a -5.4 to +2.3% change in ecosystem C from 1829 to 1990. These estimates are consistent with field evidence that historically recent changes in C storage of tussock tundra have been relatively small. However, the model also predicts that relatively large transient losses of ecosystem 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 century 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 tundra 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 transitory in nature.