Lowland tree loss threatens cloud forests

Science News, Oct 20, 2001 by S. Perkins

Changes in regional climate brought about by large-scale deforestation in the eastern lowlands of Central America are affecting weather downwind in the mountains, imperiling ecosystems there.

The so-called cloud forests of Monteverde lie along the crest of Costa Rica's Cordillera de Tilaran mountains. These habitats rely on the almost perpetual fog that forms as moisture-laden Caribbean winds rise up the eastern slopes of the mountains and pass through altitudes at which clouds condense, says Robert O. Lawton, an ecologist at the University of Alabama in Huntsville. The humidity in those breezes is enhanced by moisture expelled from the leaves of lowland forests.

By the early 1990s, more than a century of deforestation had left only 18 percent of the Costa Rican lowland forests east of the peaks untouched. The pastures that replaced forests don't humidify the winds as well as forests do and are better at warming the atmosphere. As a result, the winds off these pastures must rise farther up the Cordillera de Tilaran slopes before clouds condense.

Satellite photos of the lowlands in the dry season show that clouds are absent or sparse over deforested areas but are thicker over the forests of neighboring Nicaragua. Computer simulations of daytime cloud formation in the area support these observations, Lawton notes, and they also suggest that the altitude of the cloud base would rise about 200 m above today's height if the lowlands were completely deforested. Lawton and his colleagues report their results in the Oct. 19 SCIENCE.

The gradual shifting of bird ranges upslope and a recent population crash among frogs and toads in the Monteverde cloud forest suggest that the veil of clouds may be lifting. Scientists had already blamed the rise of the cloud base for the longer periods of mistfree conditions observed at the downwind edge of the forest. Lawton warns that in the future, the clouds may disappear from the Monteverde slopes for days at a time during the dry season--a development that could lead to collapse of the ecosystems there.

Intact forests play a large part in humidifying the air above broad inland areas such as the Amazon Basin, says Ning Zeng, an atmospheric scientist at the University of Maryland in College Park. However, the importance of forests in moisturizing winds as they pass over the narrow band of lowlands upwind of the Monteverde cloud forest remains unclear, he adds. The cloudfree areas observed by satellite could result from other factors related to deforestation, such as improved mixing of the lower atmosphere over treefree areas, he notes.