Modelling carbon responses of tundra ecosystems to historical and projected climate: sensitivity of pan-Arctic carbon storage to temporal and spatialvariation in climate

Historical and projected climate trends for high latitudes show substantial temporal and spatial variability. To identify uncertainties in simulating carbon (C) dynamics for pan-Arctic tundra, we compare the historical and pr ojected responses of tundra C storage from 1921 to 2100 between simulations by the Terrestrial Ecosystem Model (TEM) for the pan-Arctic and the Kuparu k River Basin, which was the focus of an integrated study of C dynamics fro m 1994 to 1996. In the historical period from 1921 to 1994, the responses o f net primary production (NPP) and heterotrophic respiration (R-H) Simulate d for the Kuparuk River Basin and the pan-Arctic are correlated with the sa me factors; NPP is positively correlated with net nitrogen mineralization ( NMIN) and R-H is negatively correlated with mean annual soil moisture. In c omparison to the historical period, the spatially aggregated responses of N PP and R-H for the Kuparuk River Basin and the pan-Arctic in our simulation s for the projected period have different sensitivities to temperature, soi l moisture and NMIN. In addition to being sensitive to soil moisture during the projected period, R-H is also sensitive to temperature and there is a significant correlation between R-H and NMIN. We interpret the increases in NPP during the projected period as being driven primarily by increases in NMIN, and that the correlation between NPP and temperature in the projected period is a result primarily of the causal linkage between temperature, R- H, and NMIN. Although similar factors appear to be controlling simulated re gional-and biome-scale C dynamics, simulated C dynamics at the two scales d iffer in magnitude with higher increases in C storage simulated for the Kup aruk River Basin than for the pan-Arctic at the end of the historical perio d and throughout the projected period. Also, the results of the simulations indicate that responses of C storage show different climate sensitivities at regional and pan-Arctic spatial scales and that these sensitivities chan ge across the temporal scope of the simulations. The results of the TEM sim ulations indicate that the scaling of C dynamics to a region of arctic tund ra may not represent C dynamics of pan-Arctic tundra because of the limited spatial variation in climate and vegetation within a region relative to th e pan-Arctic. For reducing uncertainties, our analyses highlight the import ance of incorporating the understanding gained from process-level studies o f C dynamics in a region of arctic tundra into process-based models that si mulate C dynamics in a spatially explicit fashion across the spatial domain of pan-Arctic tundra. Also, efforts to improve gridded datasets of histori cal climate for the pan-Arctic would advance the ability to assess the resp onses of C dynamics for pan-Arctic tundra in a more realistic fashion. A ma jor challenge will be to incorporate topographic controls over soil moistur e in assessing the response of C storage for pan-Arctic tundra.