The effects of microtopography on environmental conditions, plant performance, and plant community structure in fens of the New Jersey Pinelands

This research presents a series of studies examining centimeter- to decameter-scale variation of microtopography and vegetation within wetlands in the New Jersey Pinelands. The wetland community studied, the Pinelands fen or sedge savanna, shows patchiness in the vegetation at multiple scales and offers the opportunity to study how small-scale variation influences patterns at larger scales. These fens support many rare and endangered plant species and are among the highest conservation priorities in the region.; In Pinelands fens, small hummocks about 25 cm diameter and 5-10 cm tall support a greater diversity of plants than adjacent hollows. In one study, I demonstrate significant differences in hydrologic and soil conditions between microenvironments separated by less than ten centimeters elevation. Growth and reproduction are greater on hummocks than in hollows for most of the dominant plants in the community. In a second study, I examined the relationship between microelevation and community structure at multiple scales. At a plant neighborhood scale (7 cm2), stem density and species density both increase significantly with increasing microelevation. Species show individualistic distribution responses along microelevation gradients but generally share a preference for the hummocks. In a third study, I tested the relationship between heterogeneity and diversity, which is frequently proposed as a mechanism for the maintenance of diversity. I found that heterogeneity (measured here as microtopographic variance) was not significantly related to diversity. I attribute this to the shared preference of most species for the highest microtopographic positions. Without sufficient specialization within the species pool for different physical conditions, heterogeneity does not facilitate diversity.; Overall, this research demonstrates that microtopography can generate a mosaic of microhabitats with a significant impact on community structure. At the centimeter-scale, hummocks support extremely high plant diversity, apparently as a result of slight variations in environmental conditions that support higher plant densities. At larger spatial scales, microtopographic heterogeneity does not significantly affect plant community structure. This indicates that (1) the factors which structure vegetation at fine-scales do not necessarily influence pattern at larger scales and (2) that environmental heterogeneity will not facilitate increased diversity in communities where the organisms share similar habitat preferences.