The Role of Seed Banks in the Vegetation Dynamics of Prairie Glacial Marshes
The presence of viable seed in 24 substrate samples from Eagle Lake, a marsh in north- central Iowa, was tested by placing subsamples of each sample under 2 environmental conditions. One set of subsamples was placed underwater (submersed treatment). Seeds of 20 species germinated and grew in this treatment. On the average, there were 8.3 species/sample. The 2nd set of subsamples was kept moist, simulating conditions on an exposed mud flat (drawdown treatment). In the drawdown treatment, on the average, seeds of 12.9 species germinated/sample. Altogether seeds of 40 species germinated in this treatment of which only 24% were also found in the submersed treatment. By combining the results from the 2 treatments, the seed banks in the 6 vegetation types studied were estimated to range from 21,445 to 42,615 seeds/m2 on the average in the upper 5 cm of soil. Field studies at Eagle Lake (1974) and Goose Lake (1976), when these marshes had no standing water, revealed that the most abundant species whose seeds germinated on exposed mud flats were the same as the most abundant species in the experimental drawdown samples from Eagle Lake. In 1975, when Eagle Lake had standing water again, the submersed and floating species that were found were the same as those found in the experimental submersed samples from Eagle Lake. The seed-bank results and vegetation sampling reveal that there are 3 types of species present in prairie marsh seed banks: emergent species (Typha, Scirpus, Sparganium, Sagittaria) germinate on exposed mud flats or in very shallow water; submersed and free-floating species (Lemna, Spirodela, Ceratophyllum, Naias, Potamogeton) whose dormant seeds or turions can survive on exposed mud flats for a year and which germinate when there is standing water; and mud-flat species (Bidens, Cyperus, Polygonum and Rumex) which are ephemerals whose seeds can only germinate on exposed mud flats during periods when no standing water exists in the marsh because of drought or water level manipulation. When the marsh refloods, these species are eliminated from the visible marsh flora. Primarily because of fluctuating water levels and muskrat damage, prairie marshes have cyclical changes in their vegetation during which mud-flat, emergent, or submersed and free-floating species replace each other as the dominant type of species in a marsh.