Western Arctic Linkages Experiment (WALE)
Modeling the role of high latitude terrestrial ecosystems in the arctic system: A retrospective analysis of Alaska as a regional system.
The Western Arctic Linkage Experiment (WALE) was initiated to investigate the role of northern terrestrial ecosystems in the larger Arctic system response to global change through model and satellite remote sensing analyses of regional carbon, water and energy cycles (McGuire et al. overview paper below). The NTSG portion of this investigation focused on assessing annual variability and regional trends in vegetation productivity for the WALE domain of Alaska and NW Canada, and the primary mechanisms driving observed changes over the 19-year (1982 - 2000) study period. To accomplish these objectives, we applied a biome-specific production efficiency model (PEM) driven by daily surface meteorology and satellite remote sensing observations of photosynthetic leaf area. We also conducted prognostic regional simulations of terrestrial carbon budgets for the same period using two ecosystem process models, BIOME-BGC and TEM; these model simulations were used for independent assessment of satellite remote sensing derived results and to elucidate underlying mechanisms driving changes in vegetation productivity and the terrestrial carbon cycle. We find evidence of a small, but widespread, positive trend in vegetation gross and net primary production (GPP and NPP) for the region from 1982 to 2000 coinciding with summer warming of more than 1.8 °C and subsequent relaxation of cold temperature constraints to plant growth. Prognostic model simulation results were generally consistent with the remote sensing record and also indicated that an increase in soil decomposition and plant-available nitrogen with regional warming was partially responsible for the positive productivity response. Despite a positive trend in litter inputs to the soil organic carbon pool, the model results showed evidence of a decline in less labile soil organic carbon, which represents approximately 75% of total carbon storage for the region. These results indicate that the regional carbon cycle may accelerate under a warming climate by increasing the fraction of total carbon storage in vegetation biomass and more rapid turnover of the terrestrial carbon reservoir.
A second objective of our research was to clarify the role of spring thaw timing in determining annual vegetation productivity, and whether a recent advance in the northern seasonal thaw cycle is sufficient to account for the sign and magnitude of the estimated positive vegetation productivity trend for the western Arctic. To accomplish this objective we conducted a temporal change classification of daily terrestrial microwave emissions from the SSM/I time series to determine the spatial pattern and annual variability of the primary springtime thaw event for Alaska and Northwest Canada from 1988 to 2000. We compared these results with PEM and BIOME-BGC outputs across the domain to assess relations between thaw timing and spatial patterns and annual variability in vegetation structure and productivity for the region. The SSM/I derived timing of the primary springtime thaw event was well correlated with annual anomalies in maximum LAI in spring and summer (P ≤ 0.009; n = 13), and GPP and NPP (P ≤ 0.0002) for the region. Mean annual variability in springtime thaw was on the order of ±7 days, with corresponding impacts to annual productivity of approximately 1% per day. Years with relatively early seasonal thawing showed generally greater LAI and annual productivity, while years with delayed seasonal thawing showed corresponding reductions in canopy cover and productivity. The apparent sensitivity of LAI and vegetation productivity to springtime thaw indicates that a recent advance in the seasonal thaw cycle and associated lengthening of the potential period of photosynthesis in spring is sufficient to account for the sign and magnitude of an estimated positive vegetation productivity trend for the western Arctic from 1982-2000.