Microtopography and centimeter-scale patterns of plant community structure in fens of the New Jersey Pine Barrens

Microtopography is one of the primary factors structuring vegetation within wetlands. In riverine fens of the New Jersey Pine Barrens, centimeter-scale variation in elevation influences vegetation patterns but it is not known whether these patterns are based on shared habitat preferences or the individualistic responses of particular species. We studied the relationship between elevation and the distribution of vascular plants at both the scale of an individual plant's neighborhood (ca. 7 cm2) and a hummock-hollow scale (ca. 0.1 m2) in several fens. At the plant neighborhood scale, stem density and species density both increase significantly with elevation, although elevation generally explains less than one third of the variation in these measures. The richest neighborhoods supported up to nine species in a 7 cm2 plot. Thirty-one species occurred within our neighborhood scale sampling grids, with species showing individualistic distribution patterns along elevation gradients: some show no relationship, some increase gradually, and some increase abruptly with increasing elevation. No species occurred more frequently at low elevations. A cluster analysis revealed no separation of communities along the elevation gradient, further supporting the individualistic nature of community structure in these wetlands. Similar results were found using larger plots to selectively sample hummocks or hollows: higher microtopographic positions support greater above-ground biomass and higher species density, up to 23 species in plots < 0.1 m2. The results show that at neighborhood and small-patch scales, microtopographic relief influences the distribution and abundance of individual species, but is a relatively poor predictor of community composition.