NITROGEN AVAILABILITY INFLUENCES REGENERATION OF TEMPERATE TREE SPECIES IN THE UNDERSTORY SEEDLING BANK

Abstract

While we have a good mechanistic understanding of effects of light availability on tree seedling regeneration in forests, we know little of the processes controlling seedling responses to changes in nutrient availability in the understory (low light). To examine how nitrogen availability may influence patterns of seedling regeneration in mixed temperate forests in eastern North America, we investigated nitrogen impacts on the dynamics of both coniferous and broad-leaved tree species in the understory. In addition, we compared these regeneration responses across coniferous (eastern hemlock) and broad-leaved (red oak) stand types. We applied nitrogen (0, 2.5, or 7.5 g·m−2·yr−1) to replicated understory plots in three hemlock and three red oak dominated stands, and examined seedling survival and growth for three coniferous and three broad-leaved species over two years. Nitrogen addition influenced different demographic stages in contrasting ways, leading to a complex series of seedling responses to increased nitrogen availability in the understory. Hemlock was the only species to show a positive survival response to nitrogen addition, and these effects only became apparent after the first growing season. In contrast, a suite of midsuccessional species (red maple, white pine, and red spruce) underwent nitrogen-induced declines in survival, particularly at early stages of regeneration. Both successional position and species' habitat preferences emerged as better determinants of species' responses to nitrogen deposition in the understory than did leaf habit (coniferous vs. broad-leaved). Effects were often, but not always, more marked in hemlock stands (characterized by very low light availability), suggesting nitrogen-induced declines in seedling abundance were likely due to an imbalance between above- and belowground resources. In the present study, we have clearly demonstrated that nitrogen availability can be an important determinant of understory seedling bank dynamics in mixed temperate forests in eastern North America. Nitrogen-induced changes in seedling performance in the understory will ultimately influence the spatial and temporal patterns of regeneration of these mixed forests.