In the mixed-grass prairie community dominated by Kentucky bluegrass there are complex interactions among structural and dynamic characteristics of the aboveground vegetation and the soil seed bank. These relationships are expressed as changes of the dynamic behavior of progress such as seed germination, seed persistence, and the apportionment of the seed bank among different botanical groups. At the same time, the effective grazing pressure upon plant individuals or bunches is modified by spatial heterogeneity (silty, overflow), in patches with different topographic orientation, shelter possibilities, and forage available. When grazing routines persist for a sufficient length of time, changes in others at the seed bank level, and an element of stability and persistence is thus introduced. Generalization can be made describing the main interactions and ecological meaning of the structure and dynamics of the soil seed bank.
A first generalization is that the ranking of species cover does not reflect that of the soil seed bank. For example, germinable seeds of buckbrush and prairie rose were not found even on the sites with higher shrub aboveground biomass or cover. Annual forbs, the second most abundant group in the total germinable seed bank, represented only a minor percentage of the total aboveground biomass.
A second trend refers to the extent of grazing as it affects different vegetation patches. On silty range sites, light grazing intensity increased the grass seed bank values and decreased the perennial forb seed bank values, this maybe because of water, nutrients, herbivore selection, and/or light availability. Seed losses in the overflow range site maybe a consequence of increased predation because microhabitats suitable for seed consumers are more abundant in the overflow range sites, or because of higher seed removal by flowing water and wind.
Another trend is that the different germination times produced different results. Not all viable seeds will germinate due to different germination requirements. The WS time had a higher rate of germination than did the WW and SS treatments. This may be because there were more dormant seeds in the WW treatment and more seed loss after dispersal in the SS treatment. The WW and SS treatments may have experienced a decline in germination because of the time itself, loss of viability of the seed, wind relocation and/ or predation.
This research was supported by CGREC. The authors wound like to thank Shawn Dekeyser, Rangeland Specialist in the Department of Animal and Range Sciences, NDSU, and Gordy Jensen, USDA, Mandan, for their help in recognizing weed samples. We would also like to thank Rick Bohn, Danjun Wang and Xubin Pan for their help in the greenhouse work and the field work, Amanda Hancock, Sandi Dewald, Joan Dolence, and Janet Patton, for offering constructive comments on earlier drafts of the manuscript.
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