Vegetation Analysis of North and South Edges in a Mature Oak‐Hickory Forest

The north and south edges of a mature oak—hickory forest on the New Jersey Piedmont are characterized by local—scale vegetation gradients. These gradients are determined by both exposure—induced microclimatic influences and historical factors. The north edge has been relatively undisturbed since establishment. A narrow vegetative transition is associated with the relatively narrow microclimatic transition within the edge. A low thicket of tree growth (Cornus florida, Prunus avium, Viburnum prunifolium) mingled with woody vines and herbs is found exterior to the original edge. The old edge ends about 10 m within the forest and is characterized by increased densities and basal areas (compared to center) for Quercus spp. and Prunus avium. The original south edge is found about 20 m within the present forest. It is characterized by increased densities and basal areas of Quercus spp. and Viburnum prunifolium. Agricultural abandonment about 1900 accounts for reforestation in front of the old edge. The area immediately in front of the original edge is an open stand in which both Prunus avium and Viburnum prunifolium have increased densities. Exterior to this is a dense strip of medium—sized trees which is characterized by increased densities of Quercus spp. and Prunus avium. The remaining area is composed of small trees (Quercus velutina, Prunus avium) with clumps of woody vines between and upon them. The wide microclimatic transition at one time within the original south edge was transferred to the present edge with the development of a closed canopy within the reforested strip. Few edge effects are found now beyond 20 m within the stand. The segregation of species within and between the edges is more subtle than that between north and south slopes in the region. Species characterized by shade intolerance or good vegetative reproduction, or both, are prevalent at the edges. Many of these species also are associated with secondary succession in forest gaps, but edge responses are stronger than gap responses for most species. These include Fraxinus americana, Prunus avium, P. serotina, Sassafras albidum, and Viburnum prunifolium. Among canopy dominants Quercus velutina shows a stronger edge response than Q. alba or Q. rubra. Carya ovalis shows no significant edge response, whereas C. ovata displays a strong one. Relatively tolerant species such as Fagus grandifolia, Acer saccharum, and A. platanoides display no edge response. Most tree distributions are more localized than those of their reproductive size classes, presumably as a result of environmental selection acting upon seedlings and saplings within different forest microhabitats such as edges, gaps, and well—stratified areas of the stand. Although the techniques are different, this study represents a local—scale example of direct gradient analysis in that plots (amenable to analysis of variance) are used as micro—locational indices of the environmental complex and are matched to known microclimatic gradients.