Degree Type


Date of Award


Degree Name

Doctor of Philosophy




Several plant communities in the Delta Marsh were analyzed with respect to the distribution, abundance, diversity, and temporal variation in buried seed populations and annual seed rain. Numbers of germinable seeds were measured by placing substrate and seed trap soil samples under two simulated environmental conditions (drawdown and shallow-flooded) in an outdoor shelter;From the seed bank, four times as many seedlings and twice as many species occurred in substrate samples under the drawdown treatment as compared to samples under the shallow-flooded treatment. Many of the common species in the seed bank (Scirpus validus), (Scripus maritimus), Rumex maritimus, Aster brachyactis), were not represented in the present vegetation. Seed numbers varied widely among plant communities, however, seed densities of Carex atherodes, Cirsium arvense, Scirpus validus, Typha spp., and Zannichellia palustris were significantly higher in certain communities. High soil salinity levels were negatively correlated with germinable seed numbers of most species, although seeds of Scolochloa festucacea, Atriplex patula, and Aster brachyactis were more numerous in soil samples from saline (> 5 mmhos/cm) areas. Germinable seed densities were related to elevation with lowest seed densities in open water areas (1039 seeds m('-2)) and highest densities (6596 seeds m('-2)) in shoreline zones. Dispersal events (wave action, sedimentation, wind) have concentrated seeds into shoreline zones, with little seed accumulation occurring in open water areas. Temporal and spatial heterogeneity of seed numbers in the seed bank was very high because of local seed production, dispersal events, and differential seed longevity among species;Reduction in the size and diversity of the seed bank in the surface soil layers reflected recent changes in the vegetation of the Delta Marsh. Those vegetation changes were a result of stabilization of water levels (since 1961), and under that environmental regime, reproduction by seed became less important than reproduction by vegetative means. This has led to fewer "disturbance" species in the contemporary flora.



Digital Repository @ Iowa State University,

Copyright Owner

Roger Lynn Pederson



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104 pages