Patterns of diversity of plant communities in an urban tidal marshland
We studied the vegetation of six protected sites in the Meadowlands, a brackish tidal marshland in the New Jersey-New York metropolitan region. We distinguished plots along an elevation gradient from low marsh through adjacent upland vegetation: low marsh, low transition, high marsh, high transition, and upland. The lower marsh habitats are often submerged and the dominant plant species is the native Spartina alternifolia. The high marsh is dominated by the native species Spartina patens and Distichlis spicata and is increasingly invaded by European haplotypes of Phragmites australis. The upland habitats are dominated by woody plants. We used the method of nested multiple scale vegetation sampling (Whittaker Plot design), as well as determining vegetation cover and evaluating conservation value by applying Coefficients of Conservatism values to the analysis. Our goals were to understand community-richness structure in a human-impacted urban marsh and to determine the conservation value of varied plant communities along the gradient from low marsh to upland.
Our results show several different trends when correlating vegetation traits to the elevation and marsh gradient. Small-scale plant richness (1m2) increased linearly along the transect from low marsh to upland. Large-scale plant-richness (100m2) and beta-diversity increased in a single step from the marsh habitats to upland. Plant cover peaked at the middle of the elevation gradient, at the high marsh habitats. We observed a similar peak pattern for Conservatism values, indicating that the species growing in the high marsh are restricted to the wetland habitat. Canonical Correspondence Analysis confirms that the gradient in species richness is correlated to the elevation gradient and shows a vegetation cover gradient perpendicular to it. Within the low marsh habitat there appears to be a strong gradient of canopy cover with more open sites supporting more species. The mid-elevation habitat (high marsh) exhibits both a higher vegetation cover and more species with higher Conservatism values. Yet this is also the habitat in which the non-native Phragmites australis is at its peak. This habitat seems to have a potential for harboring marshland-specific native plants, and conservation restoration efforts should target it.