Lowland tree loss threatens cloud forests

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 Tilarn 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 Tilarn 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 up-slope 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.