Region: Linyanti, Botswana
Riverine woodland, because of its proximity to the life-giving waters of rivers and its palatability during the driest times of the year, is often the first vegetation type to show the impact of the sometimes destructive feeding of elephants. Recently there has been a growing concern over the loss of trees from woodlands in certain parks and reserves of eastern and southern Africa – particularly in areas of high elephant numbers. The loss of canopy trees from the riparian woodland along the Linyanti River, together with the high elephant numbers in this region, has raised questions regarding the changes in the species composition and structure of the woodland. Therefore the study has used aerial photographs to determine the overall loss of canopy trees from the woodland lining the Linyanti between 1992 and 2001. The number of trees that appeared in the canopy during this time was also determined.
At first glance the findings of the study were perhaps not as drastic as might have been expected considering the high elephant numbers of the region. Between 1992 and 2001 the overall disappearance of large canopy trees in the riverine areas was only marginally more than 15%, effectively a rate of disappearance of less than 2% per annum and a rate that is comparable to losses in areas without high elephant numbers.
There are nonetheless causes for concern. Large canopy trees are appearing at a much lower rate than at that which they disappear, meaning that there was a 10% net loss of canopy trees over the study period. It is also clear from the study that the tree species known to be susceptible to elephant damage (camel thorn Acacia erioloba and knob thorn Acacia nigrescens), and in a size class (5-13m tall) where this is particularly so, showed the highest degree of loss, thus implicating elephants in much of the loss of large canopy trees. Evidence from the study suggests though that most of this damage occurred prior to 1992 when the rate at which large trees were lost was seemingly much higher.
The study confirmed that loss rates of large tree species vary considerably from year to year – regardless of whether this is as a result of local climatic variation or factors such as elephant browsing – but importantly also emphasises that these loss rates have high variability over longer time periods as well. Essentially what we need is more and longer-term studies in the region to understand exactly what impact the elephants have on the system.