When European settlers began raising cattle in South Africa around 100 years ago in open grassy landscapes with few trees, they found that the density of woody plants suddenly increased. Historical photographs show that areas now covered in bush were once grassland areas largely devoid of trees. This phenomenon, called bush encroachment in South Africa, has also been observed in Australia, Texas, Argentina and many other places. It is a big problem for cattle farmers because trees compete with grass, reducing the feed available for cattle and other grazers. Up to a fifth of the land in South Africa (12–20 million hectares) is thought to be affected by bush encroachment. Bush encroachment has historically been blamed on sustained heavy grazing in fenced-offareas, which disadvantages grass relative to trees. Sharp contrasts, easily observable across fence lines, show that some farms (or camps within farms) are more affected than others. Therefore, the direct cause of bush encroachment must be related to land management rather than climate or atmosphere, which pay no heed to fences. However, it is likely that atmospheric and climatic change are contributing or predisposing factors to bush encroachment.

Periodic natural fires used to keep the trees in check. When most of the grass, the main fuel for fire, is eaten by herbivores, hot, frequent fires are not possible. The formerly suppressed tree seedlings are then able to shoot up, out of the range of grazers and flames, and come to dominate the grasses. This further suppresses grass growth and the ecosystem becomes stuck in a bush-encroached state. Ranchers frequently extinguish natural fires to preserve their grazing, and have added to the problem by hunting out browsers such as kudu, giraffe and elephants that formerly ate the trees. It is speculated that, in the pre-colonial era, indigenous grazers and the cattle maintained by pastoralists did not lead to bush encroachment because the herds were migratory and fires were not controlled.