Trees and shrubs are encroaching on open areas over Africa

Scientists can assess how the Earth’s surface is changing using satellite imagery: Are deserts expanding? Where are sea levels rising? How much of our planet is now urban? And at what rate is forest cover declining?

That latter question we know a fair amount about: scientists have mapped continental-scale changes in the cover of woody plants – that is, trees, shrubs and lianas (woody vines). And we know that forests are – overwhelmingly and globally – in decline, a trend induced mainly by human clearing.
 

In Africa, the encroaching of woody plants into rangeland could mean less space for wild and domestic grazers. It could also mean more habitat and food for browsing animals – those that feed on high-growing, woody plants – more fuel wood and more carbon sequestration.

A less well-known and -understood phenomenon is the gradual encroachment of woody plants into non-forest areas. In Africa, the encroaching of woody plants into rangeland could mean less space for wild and domestic grazers. It could also mean more habitat and food for browsing animals – those that feed on high-growing, woody plants – more fuel wood and more carbon sequestration.

To better understand encroachment on the continent, UCT researchers used satellite data to map changes in woody plant cover across sub-Saharan Africa over the past three decades. They also used a machine-learning model to assess what factors could explain their observations.

Overall, the study showed that there has been an 8% increase in woody plant cover in non-forest areas of sub-Saharan Africa during the past 30 years. The nature and extent of this change has been variable across smaller scales: some places have seen increases, others decreases, in the cover of woody plants. But the overall trend remains one of growing woody plant cover.

Over the past three decades, 7.5 million square kilometres (55%) of non-forest biomes in sub-Saharan Africa gained woody plant cover. On the other hand, 2.2 million square kilometres saw declines in woody plant cover. The difference roughly equates to the areas of Algeria and the Democratic Republic of Congo combined. Loss of woody plant cover was prevalent in parts of the Sahel, East Africa and much of Madagascar; encroachment was dominant in the central-interior of Africa.
 

Over the past three decades, 7.5 million square kilometres (55%) of non-forest biomes in sub-Saharan Africa gained woody plant cover.

Most of the variation in African woody cover change could be explained by factors other than CO₂ levels: local changes in fire, herbivory and direct human disturbance, such as deforestation, predominate.

The widespread increase in woody plants across the continent corroborates other global trends showing growth in vegetation cover, and challenges the long-held narrative of desertification.

The UCT authors on the paper were Zander Venter (PhD candidate, Department of Biological Sciences), Associate Professor Michael Cramer (Department of Biological Sciences) and Dr Heidi-Jayne Hawkins (Department of Biological Sciences).