Long-Term Ecosystem Impacts Of An Introduced Grass In The Northern Great Plains

INTRODUCTION

Introduced species often displace native species and reduce diversity (Wilson 1988, Drake et al. 1989, Wilson and Belcher 1989, Billings 1990, D'Antonio and Vitousek 1992, Burke et al. 1997). Less obvious effects of introduced species include the alteration of ecosystem functions, including the size of nutrient and energy pools (Vitousek 1990). Introduced N-fixers, for example, can significantly increase soil N (Shaughnessy 1986, Stock et al. 1995, Vitousek et al. 1996). Even more subtle differences between native and introduced species, such as in allocation to roots or litter, could also influence nutrient pool size (Mooney et al. 1987, Paustian et al. 1990, Robles and Chapin 1995). In spite of this potential, there are few examples of introduced species altering ecosystem properties (Vitousek 1990), leading to suggestions that species with similar life-forms are interchangeable with regard to their effects on ecosystems (Johnson and Mayeux 1992). Here we examine the influence of a widely introduced grass on prairie ecosystem function.

Agropyron cristatum, an introduced [C.sub.3] perennial tussock grass, has been widely planted on the northern Great Plains since the 1930s and now dominates 6-10 x [10.sup.6] ha (Lesica and DeLuca 1996). Soil C is significantly lower in areas planted with A. cristatum than in native prairie (Dormaar et al. 1990, 1995). Similarly, soil C is significantly lower under introduced pines than native South African grassland (Versfeld and van Wilgen 1986). Such differences, however, could result because species are typically introduced to soils that have already lost organic matter during cultivation (Dormaar and Smoliak 1985, Aguilar et al. 1988, Bowman et al. 1990, Davidson and Ackerman 1993, Burke et al. 1995, Bashkin and Binkley 1998). We avoided this potential problem by comparing stands of A. cristatum with stands of native grasses on soils with similar disturbance histories. Specifically, we compared abandoned fields dominated by native species with similar abandoned fields planted with A. cristatum. Comparison of one pair of abandoned fields dominated by A. cristatum and native grasses (Site 3 of Dormaar et al. 1980) showed significant differences in their effects on the chemical composition of soil organic matter.

We expected A. cristatum to alter nutrient and energy pools for several reasons. First, A. cristatum stands are stable and exclude native species for decades (Looman and Heinrichs 1973). This should be sufficient time for ecosystem-level changes to occur (Parton et al. 1987). Second, the low root:shoot ratio of A. cristatum relative to native grasses (Redente et al. 1989, Dormaar et al. 1990, 1995) should result in relatively low contributions to soil organic matter (Jenkinson 1990, Kelly et al. 1996). Third, the tissue of native grasses is typically N poor relative to introduced grasses (Wedin and Tilman 1996), including A. cristatum (Heinrichs and Carson 1956, Smoliak and Bezeau 1967), which could affect litter decomposition rates (Taylor et al. 1989). Fourth, the relatively high shoot mass of A. cristatum may increase evapotranspiration, resulting in drier soils (Trlica and Biondini 1990).

Our objective was to compare the long-term effects of native grasses and A. cristatum on soils. Undisturbed, i.e., uncultivated prairie was also studied to determine the ability of native grasses to return disturbed ecosystems to their original state. In addition to measuring soil C and N, we measured short-term resource availability (mineral N and water). In order to examine whether differences in soil were caused by differences in tissue chemistry or biomass allocation, we also compared these variables among vegetation types. Lastly, we tested whether dominance by an introduced [C.sub.3] species altered the C isotope characteristics of soil organic matter (Steuter et al. 1990), since native prairie here is dominated by the [C.sub.4] grass Bouteloua gracilis.

METHODS

We sampled a 200 [km.sup.2] area in Grasslands National Park (49 [degrees] 22[minutes] N, 107 [degrees] 53[minutes] W), [approximately]20 km north of the Montana-Saskatchewan border. The natural vegetation of the area is mixed-grass prairie dominated by blue grama grass (Bouteloua gracilis), needle-and-thread grass (Stipa comata), and spikemoss (Selaginella densa) (Looman 1963). We examined five stands in each of undisturbed prairie, successional prairie (abandoned cropland that had been allowed to undergo natural succession), and abandoned cropland planted with A. cristatum. Stands were interspersed [ILLUSTRATION FOR FIGURE 1 OMITTED]. All stands were on level, upland sites with brown Chernozemic clay loam soils (Saskatchewan Soil Survey 1992).

We used aerial photographs to identify undisturbed prairie and abandoned fields (Burke et al. 1995). Abandoned fields showed plow lines in aerial photos taken in 1955 and 1982, whereas undisturbed prairie showed none. Abandoned fields were first cultivated 19121928 and abandoned 1945-1955, according to homestead files and interviews with area residents. Stands that had been sown with A. cristatum were identified by the dominance of this species, which persists for decades (Looman and Heinrichs 1973).