Juvenile response to canopy structure: Population and community-level implications in a subalpine forest of the northeastern U.S

In closed-canopy forests, changes in the relative abundance of tree species due to major disturbance or to its absence are thought to be governed principally by differences among component species in the performance of juveniles. However, few studies have examined the performance (particularly survivorship) of saplings and seedlings across a range of light environments in the field. Thus, for most forest systems, we know little about differences among tree species in their response to light. I examined growth and survivorship, in conditions ranging from large gaps to full shade, for three tree species that comprise 95% of a subalpine spruce-fir forest at Mt. Moosilauke, N.H.: balsam fir ( Abies balsamed); mountain paper birch (Betula cordifolia ); and red spruce (Picea rubens). I then determined the implications of these results for the dynamics of this subalpine forest.; I examined differences among species using two methods: censuses of 2415 individuals over 2 years, which provided direct estimates of growth and mortality rates; and the use of dendro-ecological techniques to reconstruct the performance of current adults when juveniles, from the growth patterns preserved in xylem rings.; These methods indicated that there was a trade-off in high-light growth and low-light survivorship between mountain paper birch, the most pioneer-Eke species, and the two conifers. In contrast, very little difference was detected between balsam fir and red spruce (species considered to have contrasting life-history strategies) although red spruce did appear to be able to persist for longer periods in low light than balsam fir.; The implications of these differences for community dynamics was assessed using matrix population models. By simulating changes in the disturbance regime, it was found that all population parameters, not just juvenile growth and mortality, had a great effect in determining each species' response to changes in the disturbance regime. Species differences in juvenile performance in high- and low-light were not great enough to alter species composition at any simulated disturbance regime. Rather, it appeared that other demographic rates, particularly seedling recruitment, played a more important role in determining the differences in species' population growth rates and ultimately the species composition of the forest.