The climate change hypothesis

One of the more interesting explanations for the decline of a woodland was provided by David Western and C. van Praet in 1973. The fever tree Acacia xanthophloea at Amboseli in Kenya was declining through the 1950s and the 1960s. This species was heavily utilized by elephants; in fact, over 83% of trees examined during the study showed signs of debarking, although very few trees had been pushed over.

A casual observer would have blamed the elephant for the decline of fever trees, but Western and van Praet took a more holistic approach to the problem. Analysis of soil samples revealed that tree stands in advanced stage of collapse were found in highly saline soils, while healthy stands grew in non- or mildly saline soils (fig. 6.6). Further investigations showed that groundwater levels had risen 3-4 m between 1961 and 1964. It seemed obvious that the rising water table had introduced a high level of soluble salts to the rooting layer of fever trees, creating a physiological drought. For once, the elephant was clearly not to blame for decline of a tree. Long-term climatic cycles, not well understood, were driving the vegetation dynamics of the Amboseli basin.

The Amboseli phenomenon of changing water tables was possibly a special case that did not apply to most other regions where trees had declined. Ambo-seli continued to witness an overall decline in tree canopy through the 1980s in the face of an expanding elephant population. David Western later conceded that elephants, indeed, were also responsible for this continuing vegetation change.

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