Slash-and-burn agriculture fells trees and leaves them to dry, before burning them

Neil Palmer (CIAT)

[MEXICO CITY] Slash-and-burn agricultural practices, banned by governments because of the risk of uncontrolled fires, provide better growing conditions for valuable new trees than more modern methods of forest clearance, a study suggests.

Starting in 1996, researchers cleared 24 half-hectare areas of tropical forest in Quintana Roo state, in southern Mexico, using three methods: clear-felling, where most of the trees are cut down; bulldozing; and slash-and-burn, a practice common among smallholders, in which trees are felled, left to dry and then burned, to prepare the land for agriculture.

Mahogany seeds and seedlings were then planted and, after 11 years, the researchers compared the sites and found that slash-and-burn techniques had provided the best growing conditions for mahogany.

But, more interestingly, many valuable species had thrived in the slash-and-burn plots, said Laura Snook, one of the study authors and programme director at Bioversity International, which conducts research into agricultural biodiversity for the improvement of livelihoods.

In clear-felled areas, more than half of each area contained tree species of no commercial value, Snook said. In areas cleared by slash-and-burn, 60 per cent of species were commercially valuable. Additionally, the largest trees in slash-and-burn areas were 10 per cent bigger than those in bulldozed areas.

Snook was presenting the results of the study — which ended last year — at the annual conference of the International Society of Tropical Foresters, at Yale University, United States, last month (26 January).

In clear-felled plots, trees grow from trunks, and roots remain in the ground, which creates a canopy favouring shade-loving trees. These tend to have no commercial timber value.

"Many valuable timber trees require sunlight to regenerate," said Snook.

She told SciDev.Net: "The stimulus to growth [in slash-and-burn plots] is a result of the burning, which releases the nutrients in the trees that were felled, dried and burned, and makes them available to new trees".

Slash-and-burn is also cheaper than modern clearing methods and more familiar to local people. But some governments have banned this practice because of uncontrolled fires.

"Policymakers may not be aware that fire is a natural phenomenon in many forests and is important to sustain the diversity," Snook said.

Fabian Islas Gutiérrez, director of the country's National Centre for Disciplinary Research in Conservation and Improvement of Forest Ecosystems (CENID-COMEF), welcomed the research and said that the study is the first of its kind to be conducted in Mexican forests.  

"We have verified that, after a disturbance such a hurricane or a strong fire, vegetation is able to re-establish. In that sense, there is consistency with Snook's results." But he added the method may not be viable in different conditions from those in which the experiment was conducted.