Seed Banks and Propagule Dispersal in Crested-Wheatgrass Stands

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Abstract
(1) Crested wheatgrass (Agropyron cristatum and A. desertorum) are well adapted to semi-arid regions and are often sown in pure stands for land reclamation. Stands dominated by crested wheatgrass are exceptionally stable. This study was designed to investigate reasons for this stability. (2) Four areas previously sown with crested wheatgrass were compared with adjacent native plant communities. Cover of the existing vegetation and seed banks in soils were examined. The distribution of propagules of Elymus elymoides, a species with high potential for wind dispersal, was also studied. (3) Crested wheatgrass strongly dominated the seed banks where it occurred in pure stands and was co-dominant with Artemisia tridentata in the seed banks of the seeded areas where native species were becoming re-established. Seedling emergence data indicated that the rain of native propagules into seeded areas is sparse. Stand stability may be largely a consequence of dominance of the seed bank by crested wheatgrass. (4) For many species, including the dominants, the abundance of viable propagules in the soil was closely associated with the distribution of mature plants. Cover, seedling emergence, and propagule dispersal for Elymus elymoides were closely intercorrelated. Dispersal of this species into crested-wheatgrass stands was very sparse depsite an abundant seed source. (5) The relative abundances of species represented in the soil seed bank were significantly correlated with the composition of the vegetation for three of the four native communities. No such correlation existed within seeded areas. At all sites, species that were dominant in the vegetation were usually also dominant in the seed bank of associated soils. (6) Seeding disturbed areas of shrub-steppe with crested wheatgrass usually retards the development of a diverse plant community. If an increase in species'' diversity is desired, existing crested-wheatgrass plants and their propagules in the soil must be destroyed and other species deliberately introduced.

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