XVI International Botanical Congess

Comparisons of Australian temporary upland wetlands, where flooding and drying occur unpredictably, with a North American tidal freshwater marsh, where water levels vary diurnally and substrate is always moist, show some similarities but many differences in seed and seed bank characteristics. Hydrologic regime provides the over- arching selective feature. Oxygen availability, temperature, and light, related to water level, vary spatially and temporally and affect germination, germination rates, and establishment. Seed size, species tolerance to drying, and germination cues differ between the two wetlands. Functional groups (e.g., submerged, amphibious) are more diverse in the temporary wetland where all species tolerate drying. We believe the amphibious zone, with its hydrologic gradient, continues to be the site of selection pressure determining establishment from seed.