Pattern and process of a floodplain forest, Vermont, USA: predicted responses of vegetation to perturbation

We evaluated the potential diversity of vegetation in a lowland floodplain forest as a way of predicting the range of possible communities that might develop when a natural system is subjected to flood control or other common perturbations. The potential floras (newly fallen seeds, seeds stored in soil, seeds transported by water) were quantified and compared with the standing flora at different distances from the stream. The species composition of newly fallen seeds was similar to that of the standing flora, but concordance of the standing and potential floras was otherwise low. The composition of seeds in flood water was most different from the standing flora: only 14 of the 40 flood water species were present in the standing flora. Overall, only 25 of the 73 species that germinated from the combined potential floras were present in the standing flora. Concordance of species composition among the potential floras was also low. For example, the density of seeds germinating from flood water collections was related to distance from stream, with forb density highest near the stream, but no similar trend was noted for seedlings emerging from soil collections. Equal numbers of annual and non-native species germinated from the soil and flood water but the species composition was different. Seeds stored in the soil that were subjected to hydric germinating conditions generated a somewhat different complement of species and densities than replicate samples subjected to mesic germinating conditions. A total of 48 species germinated from the soil collections (hydric & mesic combined), but only 29 species were common to both. The composition and distribution of the standing vegetation at different distances from the stream appeared to be defined mainly by the flooding regime, but selective disturbance agents such as beaver activity and Dutch elm disease probably also affected the spatial distribution of species within the forest. Collectively, these factors generated a highly diverse mosaic of possible floras. Flood control can be expected to homogenize this mosaic and reduce the forest's potential diversity, thereby compromising the value of floodplain forests as storage banks of biotic diversity.