In Africa, a fight is happening. On one side natural forces are making the continent greener, and on the other, people are removing trees and bushes from the continent.
In densely populated regions, people are cutting down trees and forests, but elsewhere, where human populations are more thinly spread, bushes and scrub vegetation are thriving.
Now, scientists have quantified for the first time how vegetation across the continent has changed in the past 20 years.
Thirty six per cent of the continent has become greener, while 11 per cent is becoming less green.
The results show that not all is lost for Africa’s nature, say the scientists behind the new research.
“Our results are both positive and negative. Of course it’s not good that humans have had a negative influence on the distribution of trees and bushes in 11 per cent of Africa in the last 20 years, but it doesn’t come as a complete surprise,” says co-author Martin Brandt from the Department of Geosciences and Natural Resource Management at the University of Copenhagen, Denmark.
“On the other hand it’s not all negative as an area—three times larger than the area where trees and bushes are disappearing—is becoming greener, which is positive, at least from a climate point of view,” he says.
The new study is published in the scientific journal Nature Ecology and Evolution.
Challenges the general view of Africa
The study challenges the view that Africa is undergoing a sustained loss of trees and bushes, says Professor Henrik Balslev from the Department of Bioscience at Aarhus University, Denmark. Balslev was not involved in the study.
The new study offers a nuanced picture of how population growth in Africa influences vegetation in different ways.
“The study gives a much more nuanced picture of people’s influence on vegetation in Africa, south of the Sahara, than we had before. The study will have significant impacts on how we evaluate people’s influence on African nature in the future, as the expected population grows dramatically,” he says.
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Abstract (bold mine)
The rapidly growing human population in sub-Saharan Africa generates increasing demand for agricultural land and forest products, which presumably leads to deforestation. Conversely, a greening of African drylands has been reported, but this has been difficult to associate with changes in woody vegetation. There is thus an incomplete understanding of how woody vegetation responds to socio-economic and environmental change. Here we used a passive microwave Earth observation data set to document two different trends in land area with woody cover for 1992–2011: 36% of the land area (6,870,000 km2) had an increase in woody cover largely in drylands, and 11% had a decrease (2,150,000 km2), mostly in humid zones. Increases in woody cover were associated with low population growth, and were driven by increases in CO2 in the humid zones and by increases in precipitation in drylands, whereas decreases in woody cover were associated with high population growth. The spatially distinct pattern of these opposing trends reflects, first, the natural response of vegetation to precipitation and atmospheric CO2, and second, deforestation in humid areas, minor in size but important for ecosystem services, such as biodiversity and carbon stocks. This nuanced picture of changes in woody cover challenges widely held views of a general and ongoing reduction of the woody vegetation in Africa.
Africa’s human population has increased from about 230 million in 1950 to over 1,000 million in 2010 and is expected to grow to as high as 5,700 million by the end of the twenty-first century1. This growth has led to the expansion of agricultural land and the reduction of natural forests and other woody vegetation2,3,4, affecting biodiversity and carbon storage3. Severe droughts in recent decades have also had an adverse impact on humid and sub-humid forested areas5. In contrast, studies of drylands have shown an increase in vegetation productivity over the past 30 years6,7,8, also highlighting the importance of drylands for global carbon variability and as a land CO2 sink9. Whether this increase in vegetation productivity is driven by the growth of woody vegetation and/or by an increase in productivity of herbaceous vegetation is not clear6,7,8. This is because the scattered nature of woody plants in drylands is very different from forests with closed canopies and is challenging to detect with optical satellite imagery at regional to continental scales10,11. Previous studies have used vegetation indices as proxies for net primary productivity, but these indices measure the photosynthetically active part of the vegetation, and most studies do not distinguish between woody and herbaceous vegetation12,13. Furthermore, studies of deforestation in humid areas traditionally report the presence or absence of forests3 and do not assess gradual changes in forest biomass within existing forests (for example forest degradation). They are also based on temporal snapshots of satellite imagery at a higher spatial resolution and only capture forests based on given definitions, such as tree height and canopy cover percentage3,14, which substantially underestimate shrubs and scattered trees in drylands10. Consequently, little quantitative information is available about the state, rate and drivers of change in the cover of woody vegetation at the scale of the African continent. This information is crucial for ensuring that the design of natural resource management in relation to deforestation and desertification is based on observations rather than based on narratives.
