Anthropogenic climate change is a primary cause of species range shifts worldwide and will continue to impact species distributions as global temperatures continue to rise. The rate at which species are expected to respond to climate change is variable, dependent, in part, on recruitment rates along ecotones. We investigated the processes controlling the rate of seedling establishment of high elevation boreal and low elevation northern hardwood species along the boreal-deciduous ecotone (BDE) in the Green Mountains of Vermont. We focused on four abundant canopy species occurring in boreal and deciduous forests in our study region; sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) representing the low elevation northern hardwood forest, and red spruce (Picea rubens) and balsam fir (Abies balsamea) are dominant species of the high elevation boreal forest. Our objectives were to identify the mechanisms controlling seedling establishment at different elevations, specifically: What mechanisms limited establishment of northern hardwood species in higher elevation boreal forest and conversely, what limited boreal species from lower elevation deciduous forest? We investigated these questions by planting seedlings of our four focal species at four locations extending up from core northern hardwood forest (500m) through core boreal forest (950m) under different environmental conditions.  

Results/Conclusions

We examined the direct and interactive effects of variation in light, temperature, predation and calcium availability, which produced differences in seedling growth and survivorship. Our results indicate that while conditions are currently unsuitable for the establishment of northern hardwood species within the core boreal forest (950 m), continued warming will substantially increase their survival. Upslope expansion of northern hardwoods was also limited by small rodent predation on seedlings. The effects of light environment on survivorship varied by species but also exhibited an interactive affect with elevation, likely the result of differences in soil moisture holding capacity. Overall survivorship for every species was highest in the lowest elevation site (500 m), suggesting that boreal species distributions are controlled by competitive interactions at their downslope range limit rather than direct climatic constraints. Our results suggest that boreal species exhibit an asymptotic rather than parabolic growth response to climate variation within our observed temperature range. The results of this study can help in conservation efforts by informing dynamic vegetation models and identifying environmental conditions that either facilitate or inhibit species migration.