PARENT SESSION
Poster Session # 9: Herbivory.
Tuesday, August 5 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.
Bare ground and bovines: Effects of grazing history and intensity on savanna soils and vegetation.
Feral, Christie*,1, Epstein, Howard1, 1 Environmental Sciences Department, University of Virginia, Charlottesville, Virginia, USA
ABSTRACT- Reductions in interspecific competition due to grass removal and shifts in nutrient availability are key factors in plant community changes linked to cattle grazing in savannas. We studied the effects of continual and discontinued cattle grazing on soil properties and vegetation composition along grazing intensity gradients. Plots (100 m2) established at increasing distances (up to 3 km) from a central borehole ran along grazing intensity gradients at two farms with nearly opposite grazing histories near Ghanzi, Botswana: a cattle farm grazed until 1994 by wildlife only and a converted cattle farm supporting only wildlife since 1995. Soil properties and vegetation composition were measured along three 10-meter lines in each plot. 1) Soil pH is significantly lower at the cattle farm (ANOVA, p<.00001), possibly due to differences in soil parent materials and patterns of urine and dung deposition. Soil pH decreases with distance at the game farm (r2=0.18, p<.0001)and may be related surface calcrete near the borehole. 2) Mean soil crust strength increases with distance at both sites (r2=0.31-0.62, p<.0001) and is greater overall at the game farm (ANOVA, p<.0001) except at a distance of 50m from the borehole. 3) At the cattle farm, a negative relationship between distance from the borehole and the along-line proportional cover of bare soil (r2 = 0.77, p<0.001) appears correlated with a similar relationship between distance and grazing intensity. 4) At both sites, grass density is positively related to distance from the borehole (r2=0.69-0.86; p<0.001), while forb density decreases with distance (r2=0.23-0.43, p<0.01). Mean forb and woody plant density are greater at the game farm (ANOVA, p<.05-.0001). 5) Acacia mellifera is a species associated with bush encroachment. Juvenile A. mellifera density is higher at the cattle farm (ANOVA, p<0.05). Seventy-two percent of juveniles grew beneath Grewia flava canopies. These results suggest that grazing intensity predominantly controls proportional cover by bare ground and grass, as well as soil crust strength. Recovery from cattle grazing pressures may be dominated by forbs and shrubs, while increases in A. mellifera may be tied to the presence of facilitating shrubs.
Key words: soil properties, grazing impacts, vegetation cover, vegetation change