Vulnerability of Landscape Carbon Fluxes to Future Climate and Fire in the Greater Yellowstone Ecosystem

Under climate change, an increase in fires would modify carbon stocks by decreasing the amount of carbon (C) stored in soil and biomass. Smithwick and team wish to explore under what conditions of future climate and what threshold of fire frequency would shift the Greater Yellowstone Ecosystem (GYE) from a C sink to a C source. They created downscaled climate projections for three general circulation models and used them in a dynamic ecosystem process model (CENTURY version 4.5). They also simulated C storage to 2100 for individual forest stands under three fire pathways ¾fires at every 30, 60, or 90 years¾and a control simulation (no fire) under the future and downscaled climate scenarios. Their results reveal that fire intervals less than about 90 years will cause lodgepole pine forest stands to move from a net C sink to a net C source (Smithwick et al. 2011). Under all future climate scenarios, their results show that a decreases in fire-return interval will likely reduce the ability of the GYE landscape to store C. –Loren Stutts
Smithwick, E.A.H., A. Westerling, M.Turner, W.H. Romme, and M.G. Ryan 2011. Vulnerability of Landscape Carbon Fluxes to Future Climate and Fire in the Greater Yellowstone Ecosystem. Proceedings of the 10th Biennial Scientific Conference on the Greater Yellowstone Ecosystem; October 11-13; Yellowstone National Park.

            Forests of the western United States are facing increasing fires and managers and scientists of these forests must anticipate any consequences of this trend. An increase in fire frequency will trigger significant ecological changes, in particular, carbon source-sink dynamics may be susceptible to changing fire regimes. Smithwick and team (2011) wish to further scientific understanding of the landscape-scale susceptibilities of key forest types as a result of climate change. Their goal was to identify the specific fire frequency at which conifer forests become sources of C to the atmosphere.
            To reach this goal, Smithwick and team utilized a dynamic ecosystem model to project future C stocks under different fire regimes and climate scenarios. They ran the ecosystem model (CENTURY version 4.5) aspatially for the dominant vegetation communities in the GYE and a range of estimated fire-return intervals and future and current climate conditions to identify how great a change in climate and fire regime would shift vegetation from C source to C sink. Their modeling focused on lodgepole pine, a representative forest type in the GYE, and to capture its variation in recovery, they modeled both a fast and slow recovery pathway. To estimate current and future climate conditions, Smithwick and team used historical climate data and a general circulation model (GCM) runs downscaled to the North American land Data Assimilation system. They used three GCMs (CCSM 3.0, CNRM CM 3.0, and GFDL CM 2.1) to generate a set of possible climate futures for the western US. They used climate data from the grid centered on the Yellowstone Lake climate station for simulations of lodgepole pine forest. Mortality, post-fire recovery and productivity were parameterized in CENTURY for lodgepole pine and warm-dry conifer trees based on empirical data. They assumed a C3 grass parameterization in CENTURY for all simulations. Fire return intervals used in CENTURY and landscape C modeling were based understanding of the canopy seed bank and its influence on post fire regeneration. To include the rapid and variable trend in development of a canopy seed bank, they used a 30-year fire interval. Forest C recovery time under both current and future climate scenarios was determined by comparing the time to recovery of pre-1988 C stocks of mature forest stands to that of future periods.
Smithwick and team’s simulation of the large 1988 fire resulted in 12 percent reduction of total C stocks from pre-fire levels. Assuming no fire in the post-1988 period, across the climate scenarios C stocks continued to increase through the end of the simulation.  For scenarios with fire return intervals less than 90 years, total C stocks did not recover and total ecosystem C storage declined through the future simulation period.  Yet for the 90-year fire return interval scenario, C stocks were within 5 percent of the pre-fire stocks. Their results suggest that fires would need to be separated by 90 years or longer for recovery of C stocks whereas closer more frequent fires lead to C losses.
Smithwick and team findings suggest that the threshold in which C stocks do not return to their pre fire levels for lodgepole pine forests of the GYE is at a fire return interval of about 90 years. If the fire return interval is below this threshold, forests re-burn before they re-accumulate the C lost in the previous fire. As a result, forests could become C sources in the global C cycle which may exacerbate climate change. The magnitude of the shift in C balance as a result of shorter fire intervals monitors the ability of the forests to store C. The extent of the shift in C balance will depend on future distribution of forest and nonforest ecosystems across the landscape. For example if future forests do not regenerate at all, then the balance between C sink to C source may prove to be more dramatic than Smithwick and team’s model predicts.
Overall Smithwick and team’s findings reveal that more frequent fire produces a shift in lodgepole forest from a C sink to a C source. Their results show that lodgepole forests are vulnerable to climate change and the associated increase in burning, specifically, the C storage of GYE forests is extremely sensitive to projected future fire regimes.

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