The Seedling Regeneration Niche: Habitat Structure of Tree Seedlings in an Oak-Pine Forest

Abstract

The physical environment to which a seedling is subjected affects its probability of survival and recruitment into a population. Aspects of the physical and biotic environment form components of the plant habitat and regeneration niches defined by Grubb. The importance of different variables may change during the life cycle of a long-lived plant. We measured nine variables that characterize the habitat of six species of one year old tree seedlings in an oak-pine forest in the New Jersey Pine Barrens [USA]. Variables were measured for 25 randomly located individuals of each species as well as at 25 random points in the same stand as the seedlings and in two other stands. Principal components analysis (PCA) of seedling plus random point data produced two habitat gradients: Axis I was a gradient from canopy cover to moss and lichen cover and higher light intensity. Axis II was a gradient from high total cover and shallow litter to habitats with less cover and deeper litter layers. Random points were concentrated in areas and deep litter and low light whereas most seedlings grew in areas with more light and less litter. Discriminant functions analysis indicated that seedling habitats breadth was larger but that the habitat of Pinus echinata could be distinguished from that of Quercus coccinea and Sassafras albidum. Seedling density differed among the three stands. Analysis by PCA of the random points from the three stands produced similar habitat gradients as in the PCA derived from tree seedling habitats. In the stand where seedling density was lowest, litter layers were significantly deeper, shrub density was greater and light was lower than in the other stands. These trends were confirmed by discriminant functions analysis. The multivariate analysis of seedling habitat and regeneration niches can be used to explain, in part, seedling density in the ground layer of different forest stands.