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Ecological effects of fire-breaks in the montane grasslands of the southern Drakensberg, South Africa
Abstract
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.
Keywords: composition, forage quality, Highland Sourveld, nutrients, ordination,
production
African Journal of Range & Forage Science 2004, 21(1):
1–9
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.
Keywords: composition, forage quality, Highland Sourveld, nutrients, ordination,
production
African Journal of Range & Forage Science 2004, 21(1):
1–9
