If you're a tropical mountain cloud forest, having your head in the clouds is a good thing. The constant cloak of protective moisture creates a special environment that harbors a diverse set of plants and animals, including many species unique to the forests. Now, a new study in the 19 October issue of the international journal Science suggests that the loss of nearby lowland forests may rob these mountain refuges of critical cloud cover.
Cloud formation in these areas might be restored by breaking up the currently deforested land into a mosaic of uses, including fruit groves, forest corridors along rivers, pasture, and cropland, says Science author Robert O. Lawton.
Lawton, U.S. Nair, and Ron Welch of the University of Alabama in Huntsville explored the link between deforestation and cloud formation for the well-known Monteverde cloud forests of Costa Rica, a center of research, conservation, and ecotourism for nearly three decades. The Monteverde forests are home to thousands of animal and plant species, including the resplendent quetzal bird, golden toads, the jaguar, 5000 moth species, and 400 orchid species.
Cloud forests in the Caribbean form where mountain peaks force trade winds up past the point where clouds condense, leaving the windward side of the mountains immersed in a nearly perpetual bank of clouds. The cloud bank provides moisture in the form of mist and cloud droplets and reduces moisture loss by plants. In this wet and sheltered environment, plants called epiphytes can root themselves on trees and other plants, rather than in the soil.
"This type of ecosystem depends on the forest staying constantly wet, and consistently foggy. Any changes in this sodden state can produce dramatic changes for the flora and fauna," says Lawton.
Recent hints from Monteverde, including a crash in frog and toad populations and changes in bird ranges, suggest that the base of its cloud bank may be rising, lifting the nourishing veil that allows the forests to flourish.
To further examine this phenomenon, the Science study authors looked to lowland areas upwind of the mountain forests--the place where clouds first form. As Caribbean winds pass over these areas, the amount of moisture and heat transferred from the lowland surface to the atmosphere affects both the overall number of clouds formed and the height at which clouds form, says Lawton.
Previous observational data and computer modeling suggest that deforestation, and conversion of forest to pasture or cropland, alters the land surface in ways that can affect moisture and heat transfer. Deforestation can increase the reflective brightness of the surface, increase its soil compaction, reduce the overall amount of water released by plants, and allow more surface heat to escape.
In general, all these changes decrease the amount of moisture and increase the heat traveling from the surface to the atmosphere, which means that air over deforested areas has to be lifted higher than usual to form clouds.
"If it has to be lifted too high, you've got a problem," says Lawton.
Using satellite imaging, the research team monitored the appearance of cumulus clouds over lowlands upwind of Monteverde, in forested areas in neighboring Nicaragua and deforested areas in Costa Rica. The images show that cumulus clouds are absent or poorly developed over deforested areas, compared to forested areas. A prominent zone of reduced cloudiness lies directly upwind of Monteverde.
The Science researchers' detailed computer simulations of dawn-to-dusk cloud formation in the lowlands support the visual evidence that deforestation has a significant impact on cloud formation. The simulations also show that average cloud base height over deforested areas would rise higher than the peaks that contain Monteverde by the late morning part of the cycle, leaving the current cloud forest exposed.
It's not clear yet how the rate of deforestation might affect the rate of cloud forest change, or what other kinds of climate conditions could exacerbate the problem of disappearing cloud cover.
"Although regional trends like El Niño--or global warming--are clearly a matter of some concern, I think our work shows that local climate effects can have significant consequences for these ecosystems," says Lawton.
Encroaching human populations are beginning to stress cloud forests as well, according to Lawton.
"These areas were thought to be mostly inhospitable, and they were left behind as waves of humans moved by. But as population density builds, people are nibbling away at habitats that they wouldn't have touched four decades ago."
The other members of the research team include R.A. Pielke Sr. at Colorado State University in Fort Collins. This research was supported in part by the NASA Earth Observing System (EOS), Clouds and the Earth's Radiant Energy System (CERES), and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER).
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