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

INTRODUCTION

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.

STUDY SITE

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%.

METHODS

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 …