Erosion in the shade of trees

1From 1946 to 1948, Algeria and Madagascar experienced significant soil erosion problems, including the formation of gullies, which was a new phenomenon. Following in the footsteps of the Americans, Orstom researchers undertook to study these processes in Africa, and later in Latin America and Asia. In the 1950s, they began by measuring the loss of matter on calibrated parcels. In 1973, these results were summarised to tailor the “universal soil loss equation” to African conditions and highlight some of its limitations. This equation, proposed by the Americans in 1959, is only universal in name, as it is only indicative of the impact of water erosion on the scale of the parcel, and in a US context.

Erosion gully, Mexico.

2In addition, these material loss measurements revealed the effective resistance of tropical soil and the relevance of maintaining plant cover rather than terracing. This was enough to revolutionise soil conservation, notably in Africa which saw a rapid transition from an approach based on terrace cultivation to erosion control using agro-ecological techniques, i.e. plant cover management, the use of crop residues, rotations, minimum tillage and fallows.

••• Orstom has long advocated for agro-ecological approaches to soil conservation in the tropical belt •••

Uprooted olive tree, Tunisia.

3In the late 1970s, researchers attempted a more experimental approach: drawing inspiration from a US model, they built a large-scale rainfall simulator that they used from the tropical rainforest to the Sahel. As this was a very heavy device, they designed a smaller, easily transported version which has since been used on four continents. In the late 1990s, they also installed the device in water catchments to quantify erosion due to gullies and landslides, showing for example that the soil of a forest with no undergrowth degrades rapidly because raindrops expand as they fall on leaves and then hit the ground hard. Contrary to popular belief, tree plantations do not necessarily protect soils better than scarcer vegetation.

“As a national partner of IRD, I have witnessed the numerous impacts of multi-scale studies on erosion in Laos: they have helped raise the awareness of policy-makers while also resulting in direct recommendations to farmers on how to manage their land, emphasising the effects of inappropriate practices on erosion, land fertility and the quality of downstream water.” 
Oloth Sengtaheuanghoung, soil scientist, Department of Agricultural Land Management, Laos