Epsilon Open Archive

Abstract

Grazing, fire and selective tree cutting are major disturbances shaping species diversity and productivity in savanna-woodland ecosystems. Their effects, however, are highly variable and poorly understood. Structural and functional responses of the Sudanian savanna-woodland ecosystem to such disturbances were investigated, with particular emphasis on the herbaceous vegetation. Fuel and fire behaviour were characterized in relation to grazing, dominant grass type and wind direction. The results indicated that fuel consumption by fire was significantly reduced by moderate livestock grazing intensity; rate of spread was faster and flame height shorter in head fire than back fire. The average maximum temperature was high on: non-grazed areas; areas dominated by annual grasses; plots subjected to head fire; and at the soil surface. Residence time of temperatures above 60ºC, considered lethal for plant tissues, exhibited a similar trend to that for fire temperature. Examination of structural responses of herbaceous vegetation to disturbances revealed that grazing favoured the diversity of perennial grasses. Fire favoured annual and perennial grasses richness. Selective cutting had no effect on any of the vegetation attributes assessed. The combined effect of grazing, fire and selective cutting increased the diversity of forbs. Structural responses exhibited significant temporal variation, which could be related to inter-annual variation in rainfall, stocking rate and fire intensity. Principal Response Curves analysis provided further evidence that their effects were species-specific. Functional responses of the ecosystem were characterized by soil physico-chemical and hydrological properties, aboveground biomass production and chemical concentration. High grazing intensity reduced soil water infiltration and aboveground biomass through increased trampling and herbage removal. Annual early fire tended to reduce steady state infiltration rate due to its effect on soil organic matter that explained 34% of the steady state infiltration rate. Long-term frequent fire affected neither soil physico-chemical properties nor total aboveground herbaceous biomass. Further analysis of fire effects on biomass production and nutrient composition of four grass species indicated that it significantly reduced above-ground dry matter, crude protein, neutral detergent insoluble crude protein and Ca, Fe, and Mn concentrations. In conclusion, moderate grazing intensity could be used to modify fire behaviour, to reduce susceptibility to erosion and to maintain sufficient biomass production. For vegetation management, back fires are recommended as they are less intense. The variation in structural responses between sites highlights the importance of landscape-scale approaches for understanding the impacts of disturbances on Sudanian savanna-woodland ecosystems that seems to follow the non-equilibrium theory of disturbance dynamics.