Fighting with Fire: Restoring Montane Grasslands and Controlling Melilotus in Rocky Mountain National Park

Abstract

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.