Short-term effects of fire frequency on vegetation composition and biomass in mixed prairie in south-western Manitoba

The net effects of one, two, and three spring burns in consecutive years on the aboveground biomass, species composition, and soil variables were assessed in two different mixed-grass prairie sites in south-western Manitoba. Precipitation in the first year was greater than the 30-year average but lower during the next three years. The first site (Area 6) was characterized by Bouteloua gracilis, Stipa spartea, Selaginella densa and lichens, while the second, somewhat drier site (Area 10) was dominated by B. gracilis and Carex spp. Each burn treatment was applied to 15 plots in a random block design. Vegetation and soil data collected following the third burn are presented. In general, the effects of repeated burning were more pronounced in the drier Area 10, where litter was significantly reduced with each additional burn. In Area 6, litter was significantly reduced only after three years of burning. In both sites, the cumulative effect of fire had no significant effect on total standing crop, even after three consecutive years of burning. In the drier Area 10, however, the biomass of several components shifted such that B. gracilis biomass increased significantly and forb biomass decreased significantly after three burns. Plant cover was also affected more in the drier Area 10. The cover of B. gracilis, the dominant C-4 grass, significantly increased and Carex spp. cover decreased after two burns in the drier site. In Area 6, the most significant effect of fire was a reduction in Selaginella and lichen cover. Bare ground increased in both sites as the number of burns increased. There was little change in soil nutrients with burning, but soil moisture somewhat decreased and surface temperature significantly increased after three burns.