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In the montane grasslands of Rocky Mountain National Park, Colorado, non-native plant species are affecting the distribution and cover of native plant species. Two exotic nitrogen-fixing Eurasian sweetclover species (Melilotus officinalis and M. alba) occur in patches that have less available nitrogen (N) and a species composition different from surrounding grassland patches. I tested whether burning sweetclover-invaded patches would increase native grass and perennial diversity and available soil nitrogen at this elevation. I burned 24 sweetclover-invaded and noninvaded field plots with a propane torch in May 1999. Burning increased species richness and ground cover. Specifically, one year later native, grass, and perennial species increased in previously invaded patches and sweetclover or other exotic species associated with sweetclover did not increase. Soil ammonium (NH4+) levels increased immediately after the burn in May, and nitrate (NO3-) levels increased later in July and August. New species appeared only in burned plots. While other methods to reduce sweetclover might remove resources, fire increases light, space, and nutrient availability for native species that, in turn, suppress the exotic species. [ABSTRACT FROM AUTHOR]

Invasion of nitrogen-fixing non-native plant species may alter soil resources and impact native plant communities. Altered soils may be the driving mechanism that provides a suitable environment to facilitate future invasions and decrease native biodiversity. We hypothesized that Melilotus invasion would increase nitrogen availability and produce soil microclimate and biochemical changes, which could in turn alter plant species composition in a montane grassland community. Our research addressed the effects of white and yellow sweet clover ( Melilotus officinalis and M. alba) invasion on soil characteristics and nitrogen processes in the montane grasslands in Rocky Mountain National Park. We sampled soil in replicate sites of Melilotus-invaded and control (non-invaded) patches within disturbed areas in montane grassland habitats. Soil composites were analysed for available nitrogen, net nitrogen mineralization, moisture, carbon/nitrogen (C : N ratio), texture, organic matter and pH. Data were recorded at three sample dates during the growing seasons of 1998 and 1999. Contrary to our expectations, we observed lower nitrogen availability and mineralization in invaded patches, and differences in soil moisture content and soil C : N. Soil C : N ratios were higher in invaded plots, in spite of the fact that Melilotus had the lowest C : N ratios of other plant tissue analysed in this study. These findings provide land managers of natural areas with a better perspective on the possibilities of nitrogen-fixing species impact on soil nutrient levels. [ABSTRACT FROM AUTHOR]

Exotic (nonnative) invasion, often fostered by anthropogenic disturbances, can be detrimental to the biodiversity of indigenous plant communities. We examined the impact of two exotic species, Melilotus officinalis and M. alba (yellow and white sweet clover) in the montane grassland community in Rocky Mountain National Park to determine (1) whether native and exotic plant diversity differs between patches within grasslands that are invaded by Melilotus species and patches that are not invaded (control patches) and (2) whether the presence of Melilotus species are associated with specific exotic species. Variables that were recorded included species richness, species composition, and percent cover in invaded and control patches at multiple spatial scales. The results showed that more exotic, annual/biennial, and forb species occurred within patches invaded by Melilotus species, while more native, perennial, and grass species occurred in nearby control patches. Moreover, the invaded patches had several exotic species that were not found in control patches, whereas certain native species were only found in control patches. In some instances, the presence of Melilotus in the native community appeared unrelated to disturbance; some colonies were in seemingly undisturbed meadows well beyond disturbance edges. The data also showed that Melilotus species spread beyond original mapped boundaries in a span of two seasons. [ABSTRACT FROM AUTHOR]