How do disturbances and environmental heterogeneity affect the pace of forest distribution shifts under climate change?

Although it is widely predicted that the geographic distributions of tree species and forest types will undergo substantial shifts in future, modelling approaches used to date are largely unable to project the pace at which forest distributions will respond to environmental change. The expansion and contraction of forest distributions act against considerable demographic inertia in the present composition and size-structure of forest stands as climate-induced changes in growth, mortality, and recruitment alter population dynamics through time. We aimed to better understand how shifts in forest distributions reflect long-term changes in tree demographic rates and population dynamics, and how such shifts are influenced by 1) disturbance from forest harvesting and 2) local environmental heterogeneity. Using a simple, data-constrained gap model, we simulated regional forest dynamics in the eastern United States over the next 500 yr. We then compared the geographic distributions of five different forest types through time under present and altered climatic conditions, in scenarios that variously included and excluded forest harvesting and environmental heterogeneity. Although we held climate fixed after 100 yr, it took another 160 yr after this for these forest types to collectively experience 90% of their eventual climate-related distribution gains and losses. Competition strongly affected the nature of responses to climate change. Harvesting accelerated and amplified gains by an early-successional forest type at the expense of a late-successional one, but these gains did not occur faster than those for other forest types. Environmental heterogeneity had little effect on distribution gains or losses through time. These findings indicate that forest distributions should respond quite slowly to climate change, with the leading and trailing edges of different forest types shifting over a span of centuries. Disturbances can expedite some transitions, but are unlikely to lead to wholesale changes in forest types in the coming decades.