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

Abstract

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. Interspecific 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 single- and 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 nitrification in buried bag soil incubations during the mid-summer. For each species, we developed species-specific 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 significantly 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 nitrification under low light. In the four remaining species there was no significant 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.