The fire hypothesis

Several ecologists studying landscape change recognized that fire may act syn-ergistically with elephants and other herbivores in the decline of woodlands (fig. 6.5). As elephants felled large trees, the increased light stimulated the growth of grasses, which now provided more fuel for dry season fires. The more fierce and frequent fires affected both large trees and young saplings. Thus, Buechner and Dawkins recorded that the stripping of bark of even large trees by elephants at Murchison exposed the inner tissues to fire damage and

Figure 6.5

A fire-burned patch of forest in Bandipur National Park, India. In the short term, fire reduces the availability of forage for elephants and other herbivores, although the flush of grass after the rains arrive attracts these grazing mammals.

Figure 6.5

A fire-burned patch of forest in Bandipur National Park, India. In the short term, fire reduces the availability of forage for elephants and other herbivores, although the flush of grass after the rains arrive attracts these grazing mammals.

death. P. J. Thomson observed, in the Chizarira Reserve, that the burning of grass and woody vegetation in the lower stratum removed this food resource for elephants, which then turned to mature trees for palatable food in the Brachystegia woodlands. The presence of thick rootstocks with numerous fire scars in these woodlands also indicated that regeneration had been suppressed by fires in the past.

The synergistic effect of fire and elephants in the decline of woodlands was also stressed by Peter Guy for the Sengwa region in Zimbabwe. A game fence surrounds a wildlife research area that has a much higher concentration of elephants than the communal lands outside. Fire and elephants have an impact on woodlands in the research area, while fire is the main agent of impact on the woodlands outside. While the biomass of shrubs was the same in both these strata, the biomass of trees was clearly lower in the research area (8.5 t /ha) compared to outside (26.2 t /ha). Both these were lower than a theoretically computed biomass of 40.3 t /ha for undamaged woodland.

None of the above studies, however, factored in the regeneration and recruitment potential of woody species in the absence of fire. In other words, an important question is whether a woodland affected by elephants can maintain a stable biomass through its natural regeneration capacity if fire is absent. The

Serengeti, where several studies have been carried out since the late 1960s, provides some answers.

In the Seronera region of the Serengeti, Harvey Croze made detailed estimates of elephant damage to and regeneration potential of the tree acacias, particularly Acacia tortilis during 1968-1971. He argued that the regeneration potential of this species was adequate to replace large trees killed by elephants, but that the saplings had to be protected from fire. There was a possibility of cycles in the recruitment and dynamics of acacias, but Croze was categorical that the large acacias would not disappear because of elephants, but perhaps as a result of fire. Others, such as Mike Norton-Griffiths, were even more emphatic in pinning the blame on fire for the decline of woodlands. Using large-scale time series maps of fires in the Serengeti during 1963-1972 and superimposing these, along with spatial data on elephant numbers and climatic parameters, on maps of woodland changes, Norton-Griffiths showed statistically that fire was the most important variable in explaining the change in tree cover. Robin Pellew added another dimension to the problem of declining acacia woodlands. Browsing by giraffes on recruitment-size acacia trees was not only checking their growth into mature trees, but also was making them highly susceptible to mortality from fire.

The Acacia tortilis woodlands in the Serengeti were reassessed in 1982 by R. W. Ruess and F. L. Halter. By this time, the elephant population of the Serengeti had declined substantially due to hunting. In spite of this, there was a greater than 70% reduction in the abundance of A. tortilis trees over 5 m compared to the 1971 situation. This was attributed to two factors: the browsing pressure from giraffes, which arrested the trees to below 3 or 4 m in height, and the occurrence of fire, although less frequent now, which killed the small trees. The longer-term assessment thus provided broad support to the indictment of fire by Croze and Norton-Griffiths and to Pellew's observations of the interaction between giraffe and fire in causing the eventual decline of mature acacias.

Fire could also manipulate the vegetation and elephant distribution in a manner that has a bearing on damage to woodland. By removing grass cover and woody undergrowth, fire could influence the movement of elephants and their feeding on trees. At Kasungu National Park in Malawi, Richard Bell and Hugo Jachmann observed, in experimental fire plots established in Brachystegia woodland, that elephants browsed on trees to a significantly lower extent in early-burn plots (21.7% of trees) compared to unburned plots (29.2%). Further, from aerial surveys during 1978-1982 of the park, they also observed that elephants were preferentially distributed during 4 of these 5 years in un-burned areas. From these patterns of elephant distribution and feeding in burned versus unburned areas, they concluded that pressure on woodlands would be lower in burned areas, and that fire could be used to achieve management goals. Obviously, the pressure of elephants on trees in unburned areas would correspondingly increase.

In Zambia's Luangwa Valley, Dale Lewis made similar observations of lower browsing pressure on burned mopane woodland. Lewis observed that this was achieved in spite of elephants preferring to feed on mopane branches scorched by fire compared to unburned trees.

Measurements of feeding patterns on burned and unburned areas of mo-pane woodland in Kruger National Park, South Africa, by Andrew Kennedy, however, found the opposite to be true. The elephants here distinctly preferred the fibrous stems of unburned trees over the charred shoots of mopane. Thus, the African observers differed in their views as to the precise role played by fire in vegetation change.

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