Tree species are expected to shift their distributions in response to climate change but the rate is expected to be variable and dependent on seedling establishment and growth. We investigated the processes controlling the rate of seedling growth and establishment across the boreal-deciduous ecotone (BDE) in the Green Mountains of Vermont. Four tree species were used to represent each of the two forest types in our study region; sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) represented 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 growth and establishment at different elevations, specifically the direct and interactive effects of light, temperature, browsing, and calcium availability. We investigated these questions by planting seedlings of our four focal species at four elevations extending up from core northern hardwood forest (500 m) through core boreal forest (950 m) under different environmental conditions.
Results/Conclusions
Our results suggest that while conditions are currently unsuitable for the establishment of northern hardwood species within the core boreal forest (950 m), warming by an additional 3°C will substantially increase their growth and survival and alter high elevation species dominance patterns by replacing spruce and fir with sugar maple. Current upslope expansion of northern hardwoods may also be limited by small rodent predation on seedlings. The effects of light environment on survivorship varied by species but 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. Patterns of biomass accumulation differed from survivorship for each species however, with sugar maple growing best at 800m, near its current altitudinal limit. 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.