The relative importance of hydrology and substrate in the vegetation dynamics of restored freshwater wetlands

The vegetation dynamics of non-tidal freshwater wetlands have been the subject of many studies. Yet there remains a basic theoretical question: Is hydrology or substrate more important in controlling the vegetation dynamics of freshwater wetlands? The primary objective of this research was to investigate the relative importance of substrate (i.e., soil type) and hydrologic regime (i.e., timing of drawdown) in vegetation dynamics. To that end, a descriptive study of the vegetation of twelve restored wetlands of various ages and an experimental study of the growth and germination of two species, Alisma plantago-aquatica and Panicum virgatum, common at the restored sites, were conducted. Both studies indicated that hydrology, as expressed by vegetation zone (i.e., the relative elevation of subplots) is the primary controlling factor in the vegetation dynamics of these restored wetlands, across and within sites. Seasonal variations in water level strongly influenced the overall character of vegetation across restored sites and within vegetation zones of sites by determining individual species distributions. The seed bank at the restored sites appeared to be responding to differences in the timing and extent of drawdown. While the effects of hydrologic regime on vegetation dynamics were not unexpected, the effects due to substrate were the more important result. This research provides direct evidence that within a particular hydrologic regime, substrate characteristics can affect the growth and germination of wetland plants and, at the landscape level, the nature of freshwater wetland plant communities. The amount of time elapsed since restoration was also shown to affect the nature of the vegetation of these restored sites. Observations indicate that wildlife, in all likelihood, have contributed to the plant species diversity of these restored wetlands. Located in topographically low areas of agricultural fields, these wetlands were restored to control agricultural runoff and provide wildlife habitat. These restored wetlands were found to provide important habitat for both flora and fauna and significantly increase the biodiversity of this largely agricultural area.