Vegetation patterns and species diversity in floodplain ponds of the Nashua River, north central Massachusetts

A literature review and synthesis of floodplain pond ecology indicates that these habitats share many characteristics that set them apart from other wetland and aquatic ecosystems. Floodplain ponds are highly dynamic, non-equilibrium systems characterized by strong spatial and temporal turnover in flora associated with flood-mediated disturbance effects and periodic drought. Fifteen floodplain ponds on the Nashua River in central Massachusetts were studied over a three-year period. Plant communities were classified. Spatial and temporal patterns in species richness and composition were documented. There were few widespread species and many narrowly distributed species. Floristic turnover within ponds averaged 52% per year. Jaccard similarity between ponds averaged 34%. Ponds fell into two distinct richness classes: poor ponds with 46 or fewer species, and rich ponds with 89 or more species. Species richness was dependent on the frequency and duration of growing-season water level fluctuations as controlled by the presence or absence of a channel connection to the river. Summer flooding of periodically flooded shoreline habitats appears to act as a disturbance. Long duration flooding in isolated ponds may be sufficiently intense to reduce diversity, whereas short duration flooding in connected ponds may act as an intermediate intensity disturbance thereby promoting diversity. A Monte Carlo simulation of Jaccard similarity coefficients and species frequencies was used to develop a null model for evaluating observed similarities between ponds and the distribution of low-presence species. Species-poor ponds were more similar to each other than expected, whereas comparisons between rich and poor ponds were not different from the null expectation. Floras of species-rich ponds had a disproportionate number of low-presence species. All ponds accumulate core floras from a large shared species pool, but species distribution from the pool (i.e. the identities of the species in a specified pond) appears to be stochastic. Rich ponds have their core floras augmented from a pool of species not available to poor ponds