COMPETITION, FIRE, AND NUTRIENTS IN A MIXED-GRASS PRAIRIE

Competition intensity is predicted to increase with soil fertility and decrease with disturbance, causing variation in the abundances of competitively subordinate species. Variation in competition intensity was examined in an experiment in which fire and fertilizer were applied to mixed-grass prairie in Manitoba. Treatments consisted of annual burns, a single burn, an untreated control, and fertilization. Fire reduced standing crop and litter, and increased the frequency of bare ground. Nutrient addition increased standing crop and decreased bare ground. Species composition and diversity were largely unaffected by fire. Two common species, Bouteloua gracilis and Carex obtusata, were significantly more abundant in fertilized vegetation than in controls; species diversity decreased in fertilized plots. Two tussocks each of Festuca ovina and Stipa spartea were chosen in each plot. One plant of each pair had all neighbors within 30 cm clipped and removed during 2 growing seasons. Each species grew larger and faster in the absence of neighbors, but the magnitude of this effect did not vary with fire, fertilizer or standing crop. Fire and nutrient addition produced significant changes in the community structure of mixed-grass prairie but did not alter competition intensity in the manner predicted. -from Authors