Recruitment from the Seed Bank and the Development of Zonation of Emergent Vegetation During a Drawdown in a Prairie Wetland

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Abstract
(1) Patterns of recruitment of five emergent species from the seed-bank along a height gradient in an experimental wetland complex during a period with no standing water (drawdown), are described in relation to the development of zonation. The drawdown was preceded by two years in which water levels were maintained at 1 m higher than normal which destroyed most of the emergent vegetation. (2) Zonation of established, adult emergents was shown by the separation of peak frequences of different species along the height gradient and the occurrence only one species in 64% of quadrats sampled. (3) For four emergent species, the distribution of seedlings along the height gradient during drawdown were similar to the pre-flooding distributions of adult plants. Scirpus lacustris and Typha spp. both had peak abundances of seedlings and adults at lower heights than those of Scolochloa festucacea and Carex atherodes. The seedlings of the fifth species, Phargmites autralis, reached their maximum density at a height well below that at which adult plants were most frequently encountered prior to deep-flooding. (4) Seedling of Scripus lacustris, Typha spp. and Phragmites australis all had maximum densities at the same height. Seedlings of Scolochloa festucacea and Carex atherodes also reached maximum densities at about the same height. Seedlings of more than one species were found in 81% of the permanent quadrats sampled. (5) For a given species, the difference in heights between permanent quadrats where maximum densities of seedling occurred and seed-bank samples with maximum densities of germinable seeds was generally no more than 20 cm; most distributions were unimodal. Along the height gradient, differences in environmental conditions seem to have had less impact than the distribution of seeds on the distribution of seedlings. (6) The differences in distributions along the height gradient and frequencies of monodominant stands between seedlings and adult plants imply that post-recruitment processes play a major role in the development of zonation patterns in the wetland studied.