Non-selective defoliation by mowing and burning has long-term selective effects on mesic grassland in South Africa

Abstract

Burning and mowing are non-selective defoliations that remove end-of-season growth, reduce uneven grazing, and homogenise forage for the new season. We predicted that annual dormant-season mowing would impact mesic grassland less than burning because of its lower intensity. This hypothesis was tested using winter and spring burning and mowing, combined with different summer mowing regimes, in a long-term experiment in Pietermaritzburg, established in 1950. The interactive effects of summer mowing and dormant-season defoliation on grass abundance, species composition, and diversity were analysed using ANOVA and PERMANOVA. Burning and dormant-season mowing differed significantly in their effects on species composition, but not diversity, while summer mowing exerted an additional influence. Annual dormant-season mowing without summer mows caused the largest shifts in dominant and subdominant grasses, whereas annual burning plus two summer mows had the least effect. Themeda triandra and Heteropogon contortus were favoured by the high-light conditions created by burning and mowing, whereas Aristida junciformis and Tristachya leucothrix thrived under mowing, where litter reduces light. Annual burning largely maintained the original dominant and subdominant grasses, whereas mowing shifted composition, particularly promoting A. junciformis. We conclude that non-selective defoliation can have selective effects on mesic grassland, and mowing cannot substitute for burning.