Grazing Effects on the Seed Bank of Rangeland in the Coteau Region of North Dakota

Guojie Wang, Shiping Wang, Laboratory of Quantitative Vegetation Ecology, Institute of Botany, the Chinese Academy of Sciences

Bob Patton, Paul Nyren, Xuejun Dong, and Anne Nyren, NDSU Central Grassland Research Extension Center

Holly Johnson, USDA-ARS, Northern Great Plains Research Lab



Introduction


One of the main questions in managing grazing lands is how grazing effects the potential of vegetation for natural regeneration. Processes leading to the establishment of new plants are primarily controlled through the availability of seeds in the soil and of safe sites for emergence and further establishment of the new species (Harper 1977, Fowler 1988, Eriksson et al. 1992).


Digging seed bank samplesIn recent years, investigation on the size, composition, and dynamics of a plant community’s seed bank has brought a better understanding of its capacity to establish adult plants even under continual disturbance (van der Valk et al. 1989). It has also become evident that different prevailing strategies among the components of the seed bank provide important clues on the mechanisms that allow species coexistence, particularly in perennial plant systems (Grime 1989). In these communities, seed banks are generally characterized by those species tolerant of the conditions prevailing in the presence of undisturbed vegetation (Thompson et al. 1979). Conversely, species intolerant of the undisturbed environment have persistent seeds that may survive for long periods in the soil until favorable conditions occur again after disturbance (Parker et al. 1989, Milberg 1992). It has been repeatedly observed that in most heavily grazed communities dominant perennial palatable grasses without persistent seed banks are displaced and eliminated from the above-ground canopy as well as from the seed bank (George et al. 1992). These and other related phenomena stress the importance of including information on the characteristics of soil seed banks in sustainable management for rangelands.


sampling above ground biomassIn the mixed-grass plant community dominated by Kentucky bluegrass (Poa pratensis L.) in the Coteau region, the abundance and diversity of the plant community increases as grazing pressure increases (Table 4 of Vegetative response to grazing intensities on mixed-grass prairie) and different plant compositions may be associated with different grazing intensity histories. The goal of this study is to identify the differences resulting from grazing intensities and what characteristics of the seed bank will correlate with different grazing intensities. The structure of the soil seed bank could be a consequence of past grazing intensities and at the same time a frame within which future structural changes could be defined.


The objective of this study is to evaluate the effect of grazing intensity on the successional process by measuring the seed bank under light and extreme grazing intensities. (1) Is the soil seed bank under different grazing intensities and different sites (silty and overflow) similar? 2) Are there species present in the seed bank that are not represented in the mature flora and are there relationships between the seed bank and the present plant community or former plant community? (3) Does the soil seed bank change over time and why?


Methods


The experiment was conducted on the light and extreme treatments on the grazing intensity trial pastures at the CGREC. One silty and one overflow site on each pasture was sampled.


The plant community composition and structure were evaluated during the growing seasons on the two grazing intensities before the samples of soil seed banks were taken (see Vegetative response to grazing intensities on mixed-grass prairie).


seed bank germination samplesSoil samples were taken in late fall of 2004 by removing the soil from a 10 x 10 x 4 inch area. Ten samples were taken on each site. Each sample was oven dried at 86°F for two days. The samples were then screened through a 0.25 inch mesh to remove small stones, large roots, and rhizomes. Each soil sample was divided into two sub-samples. The first set of samples was placed in flats measuring 9 inches x 19 inches and is being germinated in a greenhouse at the USDA-ARS Northern Great Plains Laboratory in Mandan, ND. The temperature in the greenhouse is kept at 77°F and the flats are watered daily. At the end of two months the seedlings will be identified and counted. The second set of samples will be germinated in Spring of 2005.


Results

 

The germination experiment is still being conducted, therefore, the results will not be available until 2005.


Acknowledgments


This research was supported by CGREC. We would like to thank USDA-ARS Northern Great Plains Field Laboratory for assistance with this study.


NDSU Central Grasslands Research Extension Center
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