8 March 2017

Africa has become greener the past 20 years


Deforestation in Africa has been high on the environmental agenda for decades. But a new study from Department of Geosciences and Natural Resource Management shows that the realities are more complex. The new study has just been published in the scientific journal Nature Ecology and Evolution.

By using Satellite Data the researchers have shown how climate change and people have affected the spread of trees and shrubs in Africa over the past 20 years. The data shows that there has been a decrease in green areas with a high population density, but that the continent generally has become greener.

Many earlier studies have overlooked that woody cover has actually increased over the past 20 years in 36 pct. of Africa. Decreases are found in only 11pct. of the land area. Increases are mainly located in drylands and explain the observed ‘greening’ of semi-arid areas, both north and south of the Equator.

Changes in rainfall and a growing concentration of CO2

By using an ecosystem model the researchers found that much of this increase may be explained by changes in rainfall and the growing concentration of CO2 in the atmosphere. In humid parts of Africa, trends in woody cover are more diverse. Negative trends dominate where population density is high, and often in areas with dense forests with high ecological and economic value.

Thus the findings contradict, on one hand, generally held views of loss of woody cover in drylands, e.g. in the Sahel-belt across Africa, yet on the other hand it supports the concerns for deforestation, due to agricultural expansion in more densely populated regions, and due to logging in the sparsely populated Congo basin.

The positive and negative impacts of observed trends are difficult to balance since woody cover serves many purposes and delivers a wide range of services. Seen from a global climate change perspective, the increase in carbon stocks, supposedly associated with the increase in woody cover in much of Africa, is positive. Also, lower albedo due to greater woody cover in drylands may have a positive effect on rainfall.

Further, the evidence does not support the existence of a general ‘fuelwood crisis’, threatening woody cover in much of Africa. However, livestock farmers in Southern Africa perceive encroaching woody cover as a degradation of the ecosystem, and the ecological services provided by few spreading species may be of limited use.

Moreover, the loss of forests in certain humid areas may imply serious losses of biodiversity and ecosystem services. At continental scale, it is thus impossible to draw final conclusions, and difficult to state if positive and negative effects are balanced.

Calculate the carbon content in the future

"It is the first time that we are looking at changes in woody vegetation and anthropogenic and climatic drivers at the same time," says postdoc Martin Brandt, and continues "When we know the level of biomass and how it changes over time, we can calculate the carbon content in woody vegetation and how it can increase or decrease in the future and thereby affect the climate".