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22 November 2012
In the fight against desertification, small areas of modest grasses and shrubs slow the soil loss more effectively than phalanxes of trees. Nevertheless, the human factor matters a great deal
The EU’s driest region is Murcia in Spain. But it is also an area of intensive agriculture for such arid land. “In Murcia, tree planting has dried out the land, reduced stream flows and lowered water levels in springs,” explains Jean Poesen, professor of geography at the Catholic University at Leuven (KUL) in Belgium, adding: “Replacing trees with olive groves has reversed this process.”
The work done in Murcia is part of the RECONDES research project, which looked at ways of using small-scale planting to slow or reverse desertification. “We used native plants, not introduced ones, concentrating on hotspots for erosion,” project coordinator Janet Hooke, tells youris.com. Hooke is also a professor of physical geography at the University of Liverpool, UK.
The research phase of the project ran from 2004 and 2007. The group found that in the right place, a comparatively small area of low vegetation, for example along the edges of paths, or on the ramps connecting different levels of a terrace system, can slow soil loss dramatically. This approach leaves almost all the land available for agriculture.
“It is well-known that in dry regions, trees tend to die while grasses survive, and then flourish again when it rains, “Max Rietkerk, professor in the section of environmental sciences at the University of Utrecht in the Netherlands, who was not involved in the project, tells youris.com. He adds: “Also, shrubs and grasses live close to the soil and have deep roots, so they are better at catching soil than trees are.
The key, says Hooke, is “connectivity reduction.” This involves blocking pathways in the landscape along which soil can move. This means getting farmers to plant vegetation on abandoned land, not just on the area that is actively farmed.
Poesen explains that most soil erosion in a specific location often happens on less than 5% of the land area, where there is very severe damage. He says: “Farmers do not see these areas as their responsibility, but the soil that is eroded into these gullies fills up reservoirs and blocks roads, while the gullies themselves expand into what was once productive farmland.”
Jamie Skinner, team leader for water at the International Institute for Environment and Development in Edinburgh, UK, agrees with Poesen that social systems are more important than environmental ones when it comes to tackling erosion and controlling water flow. He tells youris.com: “Sometimes the runoff from rainfall is so fast that you have to use a physical barrier to slow it down, like dams and bunds [an
earth or stone wall], until you can get the vegetation going. The vegetative solution is often a good one but it cannot always be the starting point.”
“The rocket science in this is not biology or geomorphology,” Skinner adds, “it is in social organisation. For example, it is often important to reduce grazing pressure on threatened land. But that can reduce the income of the farmer whose goats graze on steep slopes. In Europe, subsidies for specific crops sometimes act against sustainable land management.”
Skinner, who has worked on these issues in West Africa and in the Mediterranean region, says “The balance between vegetation approaches and civil engineering depends very specifically on the site and on the behaviour of the people who use it.”