Academic journal article The American Midland Naturalist

Vegetation Biomass Dynamics and Patterns of Sexual Reproduction in a Northern Mixed-Grass Prairie

Academic journal article The American Midland Naturalist

Vegetation Biomass Dynamics and Patterns of Sexual Reproduction in a Northern Mixed-Grass Prairie

Article excerpt

ABSTRACT.-The sustainability of natural grasslands is in large part dependent upon the successful ingress and propagation of alien plant species. We attempted to characterize the propagation potential of the dominant plant species of a southeastern Montana mixed-grass prairie community including two introduced grasses, Bromus japonicus and B. tectorum. Aboveground standing crops and densities of seeds were estimated on 10 dates between September 1991 and August 1993 and seedling densities were estimated on 21 dates between September 1991 and July 1993. Total aboveground standing crop ranged from about 110 to 340 g m^sup -2^ and was dominated by three grasses: Bouteloua gracilis, a warm-season, perennial; Pascopyrum smithii, a cool-season, perennial; and B. japonicus, a cool-season, annual grass. Seed production was dominated by B. japonicus (~10,400 seeds m^sup -2^). Bouteloua gracilis and P smithii seed production was negligible with maximum densities of about 120 and 25 seeds m^sup -2^, respectively. Density of seedlings was greatest for B. japonicus (-2100 seedlings m^sup -2^), Festuca octoflora (- 1100 seedlings m^sup -2^), and Plantago patagonica (-350 seedlings m^sup -^2). Maximum number of B. gracilis seedlings was 25 m^sup -2^. No P smithii seedlings were observed during the study. Temporal differences in seedling densities were closely tied to late summerearly fall precipitation patterns. Although community-level seed production and seedling establishment processes were dominated by B. japonicus, the overwhelming presence of herbage produced by the two vegetative propagating species, B. gracilis and P smithii, casts doubt on a trend in ecological succession toward an annual grass-dominated community.

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 (<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 (10550'W, 46o20'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 (Fig. 1). Daily temperatures occasionally exceed 37 C during summer whereas winter temperatures occasionally dip below -40 C. …

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