Converting rainforest to cropland in Africa reduces rainfall

September 19, 2011

Converting West African rainforests into cropland reduces rainforest in adjacent forest areas, reports research published in Geophysical Research Letters.

The study, based on a computer model used to simulate rainfall under different land-use conditions, found that cutting down tropical forests in West Africa reduces precipitation over neighboring forest areas by about 50 percent due to increased temperatures over cropland areas. Higher temperatures affect the formation of rain clouds.

"Rainfall was four to six times higher over warm areas (cropland) than when no deforestation has occurred, while rainfall over the remaining forest was half or less," stated a press release from the American Geophysical Union, which publishes Geophysical Research Letters. "The difference in rainfall is caused by the temperature change between cropland and forest, which produces winds that converge over the crop area and form clouds."

Deforestation in Gabon, Central Africa. Photo by Rhett A. Butler
The researchers say their work, while applied to a small region, could have implications elsewhere.

"We already know from satellite observations that changes in land use can have a big impact on local weather patterns," said lead author Luis Garcia-Carreras with the University of Leeds School of Earth and Environment. "Here we have been able to show why this happens."

"Our findings suggest that it's not just the number of trees removed that threatens the stability of the world's rainforests. The pattern of deforestation is also important."

Research elsewhere has shown that patterns of deforestation can have a signifiant impact on rainfall. A 2006 study found that deforestation which follows a "fishbone" pattern may be less damaging relative to traditional clearing. It said fishbone deforestation patterns may create conditions that increase precipitation levels which help cleared vegetation recover quicker.

Different forms of deforestation in Brazil, courtesy of Google Earth. Fishbone deforestation can also speed forest recovery relative to clear-cutting by creating a larger surface area between deforested areas and remaining forest. This enables seed-dispersers like birds, insects, and mammals to help tree species recolonize cleared areas.
The authors of the latest study say their results could help planners reduce the impact of deforestation.

"This has implications for planners in terms of how deforestation is managed," said study co- author Doug Parker of the University of Leeds. "If forest must be removed to create cropland, we need to think about what are the shapes and distributions of deforestation that will be least damaging to the adjacent forests and national parks."

The findings are particularly important in West Africa, which has less rainfall than other tropical forest regions.

"African rainforests already have the lowest rainfall of any rainforest ecosystem on Earth, which could make them particularly sensitive to changes in local weather patterns," said Garcia-Carreras. "If rainfall is reduced even further as a result of deforestation, it could threaten the survival of the remaining forest by increasing the trees' sensitivity to drought."

Luis Garcia-Carreras and Douglas J. Parker. "How Does Local Tropical Deforestation Affect Rainfall?" Geophysical Research Letters

mongabay.com (September 19, 2011).

Converting rainforest to cropland in Africa reduces rainfall.