The Twin Limit Marsh Model: A Non-equilibrium Approach to Predicting Marsh Vegetation on Shorelines and in Floodplains.

Vast areas of wetland occur on shorelines of ponds, lakes, and rivers. These wetlands are divided into vegetation zones, including aquatic vegetation, marsh, and swamp. Here, we provide a simple, mechanistic, and non-equilibrium model that explains the occurrence of marsh as a function of past flood pulses. Marshes are sandwiched between two limits, both of which fluctuate with time. The lower limit is set by the tolerance of marsh plants to continuous flooding. The upper limit is set by competion with woody plants, which are killed by extreme high water events. The twin limit marsh model (TLMM) requires long-term water-level records and two biological inputs: duration of flooding required to drown marsh plants (f) and the duration of dewatering required for woody plants to reinvade once water levels drop (s). In the temperate zone, we suggest that f is ~4 yr and s is ~30 yr. We illustrate the model for the marshes of Lakes Erie and Ontario. High water years that kill woody plants, followed by low water years, produce large expanses of marsh. The regulation of lakes and rivers generally has negative effects on marsh area and diversity. The TLMM can be calibrated for other climates or ecoregions. [ABSTRACT FROM AUTHOR]