Abstract:

A three-year study was conducted to investigate the role of elephants, fire and frost in Kalahari sand vegetation dynamics in Matabeleland North province, Zimbabwe. Over 3000 individual trees of 10 common species were mapped, tagged and visited annually to determine rates of elephant, frost, and fire damage, as well as mortality, growth, and recruitment. Statistical models describing these processes were developed using maximum likelihood methods, and incorporated into a simulation model to investigate how vegetation structure changes as a function of the disturbance regime.

Elephant damage differed strongly and consistently among species. Fire and frost damage declined as a function of tree diameter, and the impact of these factors was mitigated by the protective effects of surrounding trees. Fire damage was amplified by frost events, as expected, but trees with prior damage were found to be more resilient to new disturbances than undamaged trees, contrary to expectations. Tree growth was highly variable, and strongly affected by disturbance. Tree neighborhoods did not affect growth rates, although there was indirect evidence for competition in some species.

Elephants are the primary drivers of woodland decline in this system, and their impacts are magnified by the effects of fire and frost. Frost, though weak when acting independently, appears to be an important abetting factor in the presence of elephants and/or fire. Woodland and scrub represent alternative stable states in Kalahari sands, but this stability depends on the relative dominance of unpalatable species. Scrub is also highly resistant to conversion into grassland, due to the ability of coppiced trees to weather successive disturbance events, and to the high resprouting potential of trees in this system.

Even barring further increases in population density, much of the Kalahari sand vegetation of western Zimbabwe will be driven into a terminal scrub phase, with the notable exception of Baikiaea woodlands, which will most likely remain in their current state or decline very gradually under the combined effects of elephants, fire, and frost. Even drastic reductions in elephant population density are not projected to alter this outcome, due to the nonlinear relationship between elephant population density and vegetation structure.