Vegetation biomass dynamics and patterns of sexual reproduction in a northern mixed-grass prairie

The sustainability of natural grasslands is in large part dependent upon th e successful ingress and propagation of alien plant species. We attempted t o characterize the propagation potential of the dominant plant species of a southeastern Montana mixed-grass prairie community including two introduce d grasses, Bromus japonicus and B. tectorum. Aboveground standing crops and densities of seeds were estimated on 10 dates between September 1991 and A ugust 1993 and seedling densities were estimated on 21 dates between Septem ber 1991 and July 1993. Total aboveground standing crop ranged from about 1 10 to 340 g m(-2) and was dominated by three grasses: Bouteloua gracilis, a warm-season, perennial; Pascopyrum smithii, a fool-season, perennial; and B. japonicus, a cool-season, annual grass. Seed production was dominated by B. japonicus (similar to 10,400 seeds m(-2)). Bouteloua gracilis and P. sm ithii seed production was negligible with maximum densities of about 120 an d 25 seeds m(-2), respectively. Density of seedlings was greatest for B. ja ponicus (similar to 2100 seedlings m(-2)), Festuca octoflora (similar to 11 00 seedlings m(-2)), and Plantago patagonica (similar to 350 seedlings m(-2 )). Maximum number of B. gracilis seedlings was 25 m(-2). No P. smithii see dlings were observed during the study. Temporal differences in seedling den sities were closely tied to late summer-early fall precipitation patterns. Although community-level seed production and seedling establishment process es were dominated by B. japonicus, the overwhelming presence of herbage pro duced by the two vegetative propagating species, B. gracilis and P. smithii , casts doubt on a trend in ecological succession toward an annual grass-do minated community.