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The re-greening of the Sahel: natural cyclicity or human-induced change?

The area covered by barren land in the West African Sahel has been fluctuating for decades giving rise to debate about whether the process is attributable to natural processes or human activities. A paper published in Land contributes to the ongoing debate by analysing remotely-sensed and rainfall data from 1975–2011 and reviewing information provided by farmers and land management professionals.

Source: Agroforestry World

The Sahel, that transition zone between the arid Sahara in the north and the sub-humid tropical savannahs in the south, has been the focus of scientific interest in environmental-human dynamics and interactions since the 1990s. Following the devastating drought and famine that occurred there in the ’70s and ’80s, researchers investigated the trend of the environmental change, in particular, vegetation cover. There is a consensus that the Sahel, in some areas, is gaining in vegetation cover as compared to the 1980s, but there are speculations about the drivers of the changes. Can the re-greening process be attributed to natural processes or human activities?

Previous studies have established correlations between vegetation cover and rainfall increase; and between derived soil moisture, rainfall and vegetation cover. Other studies argue that the state of greenness as observed from remotely sensed data could not have been reached without additional human factors such as agricultural practices; or attribute the greening trend to the shifting of certain tree species toward a more arid-tolerant flora. Some authors have come to the firm conclusion that Sahelian farmers have ingeniously modified their agroforestry, water and soil management practices to attain more crops and permanent vegetation.

Although all mentioned studies have sound scientific evidence, they still lack site-specific evaluation at the landscape level. This study attempts to fill that gap by examining barren land dynamics from 1975 to 2011 and their interaction with rainfall variability. The study was carried out in the south-Sahelian phyto-geographical zone in the north of Burkina Faso, West Africa, in an area characterized by a low relief. Land use is dominated by agricultural practices and grazing. The vegetation is dominated by a mixture of grassland, shrubs and thorny trees. The last decades have seen intense seasonal and permanent out-migration due to environmental degradation and the population was about 875,000 inhabitants in 2010.

Barren land is defined as the partially crusted hard soil with little or no vegetation cover—almost impermeable and at risk of being swept away by harsh winds. Barren lands are widely open areas, easily detectable from satellite images taken during dry seasons. Landsat imagery over 36 years (1975 to 2011) was used to detect the extent of the barren land in the study area during dry seasons (November to April).

Discussions with farmers and land management staff shed light on anthropogenic efforts toward soil restoration to enable the subsistence farming agriculture. Informal meetings with local land management staff and farmers as well as field observations helped to identify soil management practices in the area. Several field visits/informal meetings were held with local soil and water conservation leaders in the north of Burkina Faso.

Results revealed that during the study period (1975–2011), the area of barren land showed alternating decreases and increases. The findings pointed clearly to cyclic fluctuation of the area of barren land and rainfall variability during the 36-year period. Increased rainfall does explain a decrease in area of barren land and, a decrease in rainfall predicts an increase in the area of barren land. Anthropogenic factors such as land use practices may also have contributed to the change in the area of barren land.

Other conclusions and implications for future research:

  • Although it is evident that the observed dynamics of barren land is the result of combined natural and human factors, most of these factors need to be understood better.
  • Biophysical and socio-economic variables must be analysed to understand the driving force of the changes.
  • Some hydrological modelling techniques could be tested to understand the role of the soil and water conservation techniques on restoration of barren land.
  • Predicted trends of the Sahel ecosystems could be tested using appropriate models to guide policy-makers for efficient management.
  • Agricultural water management techniques that combine appropriate tree species should be encouraged for sustainable soil conservation.

Read the full paper here.

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