The dynamics of mopane woodlands in Botswana

The impact of elephants, fire, and other agents on the woody vegetation in semiarid northern Botswana was studied by Raphael Ben-Shahar during 19911993. The various plant species were affected quite differently by elephants and fire. Thus, Acacia spp. and Colophospermum mopane were utilized heavily by elephants, while others, such as Burkea africana, Brachystegia boehmii, and Dicrostachys cinerea were affected more by fire.

Ben-Shahar constructed a model of the dynamics of mopane because this was the dominant feature (30% of stems) of the vegetation and constituted an important part of the elephant's diet. The model incorporated field data on biomass of mopane trees and shrubs as well as elephant density at various study sites. Growth rates of mopane and rates of biomass production were related to rainfall isohyets for the region. The model predicted that mopane trees and shrubs could approach maximum biomass in 6 years under high rainfall and no elephant browsing, while they could attain this limit in about

10 years or a bit longer under low rainfall when growth rates averaged only 50% of the highest rates. Browsing by elephants obviously inhibited biomass production. Interestingly, the model showed that, even under the impact of 15 elephants/km2, an unusually high density, the mopane woodlands could increase their biomass over the short term (5 years) and later stabilize when rainfall conditions were optimum (fig. 6.8). At this high elephant density, a decline in mopane biomass occurred only when dry conditions depressed growth rates to below 70% of the maximum rates. When the maximum plant growth rates in different rainfall zones were achieved, the region could sustain 3-9 elephants per square kilometer. Ben-Shahar came to the overall conclusion that "there was no substantial evidence to imply that elephants will reduce the biomass of mopane woodlands below a sustainable level if their numbers are allowed to increase considerably beyond the current estimate. Elephant culling as a means to prevent woodland loss is unlikely to meet the objective" (1996, p. 514).

Chris Styles and J. D. Skinner, who studied the utilization of mopane trees in Botswana's Northern Tuli Game Reserve, came to a similar conclusion. In spite of heavy browsing by elephants during summer and by other herbivores such as eland, causing plants to remain shrubs, there was no indication that mopane was in decline.

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