Evidence of a tipping point in a southern African savanna?

• Carbon isotopes from soil profiles were used to study changes in tree abundance.
• The data show that trees have encroached on areas that were previously grass-dominated.
• This suggests that savannas and grasslands may be at a critical tipping point.
• Management action may be required to prevent conversion to forest.

Many ecosystems exhibit threshold behaviour, where periods of relative stability are punctuated by rapid transitions between alternate stable states when an ecological threshold, or tipping point, is reached. This is of concern in grass-dominated habitats, many of which appear to be on the point of conversion to more wooded vegetation assemblages. However, changes in grass-dominated ecosystems are often difficult to interpret, because it is not always clear whether grasslands are ancient or are anthropogenically derived from past deforestation. As a result, the conservation, maintenance and restoration of ancient grasslands are sometimes neglected.In this study, the history of vegetation change in the savannas of the Hluhluwe-iMfolozi Park, KwaZulu-Natal, South Africa, are investigated by analysing stable carbon isotopes (δ13C) from soil profiles. Without exception, the data show that C3 dominated thicket, forest, and densely wooded savanna now occur on sites that were previously C4 grassland or open savanna. Although the drivers of this change are not clear, there is potential for management intervention because tree density can be manipulated through fire, a natural part of this dynamic landscape. The study identified two sites which are at a threshold between C4 and C3 dominance, and highlighted them as priorities for conservation management intervention.