Simulated effects of climate change, fragmentation, and inter-specific competition on tree species migration in northern Wisconsin, USA

The reproductive success, growth, and mortality rates of tree species in the northern United States will be differentially affected by projected climate change over the next century. As a consequence, the spatial distributions of tree species will expand or contract at differential rates. In addition, human fragmentation of the landscape may limit effective seed dispersal, and inter-specific competition may limit the migration of climate-adapted species, restraining the rate of tree species migration. If the northward migration of tree species adapted to a warmer climate lags behind the rate of climatic change, overall growth rates and aboveground biomass of northern forests may be significantly reduced relative to their potential. We used a spatially interactive forest landscape model, LANDIS-II, that simulates tree species establishment, growth, mortality, succession, and dis- turbance. We simulated multiple scenarios of disturbance and climatic change across a ~15 000 km 2 forested landscape in northwestern Wisconsin, USA. These simulations were used to estimate changes in aboveground live biomass and the spatial distribution of 22 tree species. We observed a reduction in aboveground live biomass relative to the potential biomass for the combined soils and changing climate. We regressed the reduction of potential aboveground biomass against a measure of fragmentation, the initial biomass for 22 tree species, and soil water holding capacity calculated at 3 spatial resolutions. We also regressed the range expansion of 3 individual tree species that are expected to expand their distributions against the same variables. Species migration and range expansion were negatively correlated with fragmentation both in total and for 2 of the 3 species examined in detail. The initial abundances of some tree species were also significant predictors of species migration and range expansion and indicate significant competition between existing species assemblages and more southerly species that are expected to migrate north. In conclusion, the above- ground biomass of northern forests may be limited by interactions among climate change, interspe- cific competition, and fragmentation.

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