Ecological effects of fire-breaks in the montane grasslands of the southern Drakensberg, South Africa
Fire-breaks, by legislation, are burnt annually before mid-winter in the southern Drakensberg, affecting 5-10% of the landscape, and resulting in marked selection for the early season flush by both livestock and wildlife. This study investigated whether this severe defoliation regime has had an effect on surface soil properties, botanical composition, plant diversity, phytomass production, and the nutrient content of phytomass by comparing paired samples of grassland and immediately adjacent fire-breaks. Fire-breaks differed quantitatively in composition from adjacent grassland although both were characterised as Themeda triandra-Tristachya leucothrix grassland. Only two grass species of lesser abundance, Brachiaria serrata and Eragrostis racemosa, were clearly more abundant on fire-breaks. Differences in plant diversity (species m-2, species 20m-2) were slight. Surface soil of fire-breaks compared with grasslands was slightly more acidic (0.2 units), lower in carbon (1% difference), and lower in total nitrogen (0.03% difference), suggesting deterioration of soil conditions on fire-breaks. Phytomass of fire-breaks and of grasslands burnt in early spring in the season sampled (biennial or greater burning frequency) were equivalent by February, indicating there has been no decline in productivity over time. Concentration of nitrogen, phosphorus, calcium, magnesium, potassium, sodium, zinc and copper in above-ground phytomass did not differ between fire-breaks and grasslands, although concentration of manganese tended to be greater in grassland. Expected seasonal trends, especially for nitrogen and phosphorus, were apparent. Fire-breaks therefore appear to be a sustainable management practice, in part because mid-winter burning allows sufficient recovery of canopy cover to mitigate against erosion by high intensity spring storms and the duration of grazing impact is alleviated by spring burning elsewhere. Grazing impact on fire-breaks may become more severe if grassland burning becomes more infrequent.