Putting the pieces together: woody plant encroachment across a precipitation gradient in southern Africa

Abstract

Woody plant encroachment is one of the most widespread land-cover changes in many countries. We put the data together along a precipitation gradient within southern Africa from 150 mm/annum (Namibia) to 1 500 mm/annum (eastern KwaZulu-Natal, South Africa). We found that extreme rainfall was the most important factor causing woody plant encroachment in semi-arid environments. In one mesic environment, transit through an animal, dung, simulated grazing and fire had important effects on encroachment. In the humid environment, we found that, without grass competition, shade was the most important factor. Grass competition was important in all environments. We found that there was a negative correlation between the change in soil carbon from open grasslands to paired encroached grasslands along the rainfall gradient, indicating that more carbon occurred in the soil at lower rainfall. In our conservation study, we found that the palatable tree species were being removed by herbivores and some of them were extirpated, and the unpalatable species were increasing in density. In sum, grass competition is a key factor across the gradient, but it is highly likely that global climate change (especially elevated CO₂ and warming interactions) is the most important factor for woody plant encroachment. Key factors to study are facultative resprouting and plant–soil feedbacks.