Sapling growth in response to light and nitrogen availability in a southern New England forest

In the northeastern US, the availability of light and nitrogen (N) limits the rate of woody plant growth in the forest understory and N limits forest productivity. Interspeci®c differences in the growth responses of saplings to the availability of these resources could affect the species composition of forest communities. While many studies have examined the relationship between shade-tolerance and forest dynamics, few studies have examined the relationship between natural variation in N availability and sapling growth. The objective of this study was to compare the relative importance of natural variation in light and N availability on sapling growth in a northwestern Connecticut forest. We studied six species that differ in shade-tolerance and presumed N requirements. We randomly selected individuals growing in the understory and in singleand multiple tree-fall gaps representing a natural gradient in light and N availability. Light availability was measured using hemispherical photography. N availability was measured as the rate of N mineralization and nitri®cation in buried bag soil incubations during the mid-summer. For each species, we developed species-speci®c regression models of sapling growth as a function of light and N availability. Variation in light availability explained 21±79% of the variation in sapling growth. The six species differed in their growth rates under high light and under low light. Only red maple and sugar maple growth were signi®cantly related to N availability and N availability explained <7% of the variation in the growth of these two species. Red maple growth increased with increasing N mineralization under high light. Sugar maple growth increased with increasing nitri®cation under low light. In the four remaining species there was no signi®cant relationship between sapling growth and N availability. Despite the correlation between N availability and forest productivity regionally, our results imply that light availability is the dominant resource limiting sapling growth in this forest. # 2000 Elsevier Science B.V. All rights reserved.