Climate change is expected to cause local ecosystem state changes, population extirpations, shifts in geographic distributions, and altered demographic rates. Most climate models for forested systems in western North America predict more frequent and more severe fires. These predictions collectively indicate increased extirpation probabilities for slow-growing tree species. Because of long generation times for most tree species, these predictions are difficult to test, but models are useful for exploring potential population-level consequences. We present a density-dependent, stage-based projection model to investigate the effects of climate-driven changes to fire regimes on the probability of and time to extirpation for a metapopulation of whitebark pine (Pinus albicaulis), a long-lived species of management concern in western North America. We parameterized the model using demographic data collected between 1990 to 2017 from whitebark pine communities in areas recovering from the 1988 Yellowstone fires and contiguous unburned areas, and from other data within the Greater Yellowstone Ecosystem. We used the model to explore the effects of reducing fire return intervals from >200 years to <30 years over a 100-year time horizon by quantifying population extirpation risk and elasticity of the stochastic growth rate. We did not consider whitebark pine mortality from the spread of exotic disease or native insect outbreaks.
Results/Conclusions
In preliminary model runs for one subpopulation, whitebark pine extirpation occurred in only 6% of iterations, indicating the species is likely to persist despite decreasing fire return intervals. Stochastic elasticity estimates indicate that reproductively mature individuals have the highest influence on growth rate. Although preliminary results suggest that whitebark pine persistence is likely despite changing fire regimes, they assume that nearby seed sources are available. Incorporation of metapopulation dynamics will provide direct assessment of increased risk to the seed source.