Increase in young forests, more than climate change may accelerate future colonization of temperate tree species in mixedwood boreal stands

Temperate hardwood tree species may take advantage of climate change to migrate northward tracking their optimal growth and survival niches. Other factors than climate could constrain or facilitate their establishment north of their actual range, such as competitive interactions, their ability to disperse, and forest management. The objectives of this study were to model the ability of temperate tree species to colonize boreal mixedwood stands of Eastern Canada from a few temperate tree seeders, considering the effects of climate change, competitive interactions, and specific successional stages of the receiving stands. We used the individual based forest model SORTIE-ND with adult growth forced by four different projected climate change scenarios. To mimic the natural colonization of temperate trees from marginal populations eventually established by long-distance migration, we replaced a patch in the center of the simulated stands with temperate tree species, i.e., red maple, sugar maple or yellow birch. We then performed a sensitivity analysis on the parameters determining the growth, dispersal, and mortality of temperate tree species to determine which of these processes was critical to their expansion. All three temperate tree species were able to colonize the boreal stands with higher performance in younger stands, and greater colonization skills for yellow birch. At the 2100 horizon, the impact of the climate scenarios on the final basal area of temperate tree species was minor. Processes mostly driven by competition and species auto-ecology, including dispersion, mortality, and juvenile growth parameters, were the most important for the colonization capacity. Our results suggest that the expansion of temperate tree species from already established northern marginal populations would be minimally affected by climate change, and that forest management could have a more significant impact by rejuvenating boreal mixedwood landscapes.