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The indigenous flora of the Great Plains region of North America evolved under widely fluctuating climatic conditions and chronic herbivory (Coupland, 1958; Dix, 1962; Wells, 1970; Milchunas et al., 1988; Lauenroth et al., 1994). Currently, florae in this region are evolving with the additional influence of recently ([less than]100 years) introduced alien species. One of the most ubiquitous introduced species is Bromus japonicus,(2) a cool-season, annual grass, that has successfully invaded large portions of grazed as well as ungrazed North American Great Plains, mixed-grass prairie (e.g., see Whisenant, 1990; Haferkamp et al., 1993; Heitschmidt et al., 1987, 1995).
We know from general observation that Bromus japonicus can modify the physiognomy of mixed-grass prairie communities. We know also that the invasion of Bromus tectorum, another introduced cool-season, annual grass, has resulted in a successional conversion of many western North America sagebrush-perennial bunchgrass-dominated communities to B. tectorum-dominated communities (Mack, 1981; Mack and Pyke, 1984). However, we are uncertain as to the potential of B. japonicus to be a keystone species in altering the rate, direction and extent of ecological succession in the North American mixed-grass prairie region. Such knowledge is critical if we intend to manage these grasslands for long-term sustainability. Thus, our objective in this study was to characterize potential plant propagation processes by quantifying vegetation growth dynamics and sexual reproduction patterns of a northern mixed grass prairie community containing an abundance of B. japonicus.
The study site was the Fort Keogh Livestock and Range Research Laboratory located near Miles City, Montana (105[degrees]50[minutes]W, 46[degrees]20[minutes]N). Regional topography ranges from rolling hills to broken badlands with small intersecting streams flowing intermittently into large permanent rivers located in broad, nearly level valleys. Indigenous vegetation on the 22,500-ha station is a Bouteloua-Stipa-Agropyron mixed-grass dominant (Kuchler, 1964). Long-term annual precipitation averages 338 mm with about 60% received from April through August [ILLUSTRATION FOR FIGURE 1 OMITTED]. Daily temperatures occasionally exceed 37 C during summer whereas winter temperatures occasionally dip below -40 C. The average frost-free period is 150 d.
The 3-ha study area was located within a 30-ha livestock grazing exclosure established in 1985. The indigenous plant community was dominated by Bouteloua gracilis, a warm-season, perennial grass, and Pascopyrum smithii, a cool-season, perennial grass. Other important species were Bromus japonicus, a cool-season, annual grass; Poa secunda, a cool-season, perennial grass; Sporobolus cryptandrus, a warm-season, perennial grass; Festuca octoflora, a cool-season, annual grass; Plantago patagonica, a cool-season, annual forb; and Opuntia polyacantha, a warm-season, succulent. Soils were fine, montmorillonitic Borollic Camborthids of the Kobar series. A-horizon soil texture graded from silty loam to silty clay loam. Slope was [less than]3%.
Three 0.025 ha (10 by 25 m) sample sites were selected for study from within the 3 ha study area. Site selection was based on a visual assessment of the communities' emulation of a "typical" Northern Great Plains indigenous plant community. Each site consisted of 20 proximate, 10-m long transects alternating between a width of 1 m (sample area) and 1.5 m (sample collection access area). Each 1-m wide sample belt was subdivided into ten 1-[m.sup.2] plots that were then …