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The Drying of Costa Rican Tropical Montane Cloud Forests
Volume 9, Number 47: 22 November 2006

Ray et al. (2006) report that "tropical montane cloud forests occupy approximately 0.4% of the earth's surface, and yet they support about 20% and 16% of the planet's plant and vertebrate diversity, respectively." In the case of the Monteverde cloud forest of Costa Rica, they further note there has been "an increase in the dry season (February-April) mist free days in the Monteverde preserve region since the early 1970s," and that "anuran population crashes and bird migrations to higher elevations during the same time period have been attributed to the reduction in the crucial moisture input from dry season mist."

Originally, Pounds et al. (1999) and Still et al. (1999) attributed these biological and climatic changes to increasing sea surface temperatures caused by CO2-induced global warming, which they suggested should lead to a reduction in orographic cloud formation and a raising of the cloud base, especially during the dry season when the Monteverde forest relies heavily on moisture it receives directly from clouds. At the same time, they noted an increase in what they termed the "warmth index," which they took to imply a greater demand for evapotranspiration; and it was the combination of these two factors - a reduction in the amount of cloud contact with the mountain-top forest and the forest's increased need for water - that led them to claim that CO2-induced global warming was the ultimate culprit behind the changes in the environmental conditions they contended were responsible for the observed decimations and migrations of animal life in the forest preserve.

A few years later, however, the responsibility for this sad scenario was shifted to another phenomenon by the impressive work of Lawton et al. (2001) and Nair et al. (2003), who convincingly demonstrated that the reduced evapotranspiration that followed on the heels of prior and ongoing deforestation upwind of the Monteverde cloud forest decreased the moisture contents of the air masses that ultimately reached the tropical preserve, while regional atmospheric model simulations they conducted indicated there should also have been reduced cloud formation and higher cloud bases over these areas than there were before the deforestation began.

More recently, in a study that extends the work of Lawton et al. and Nair et al. while exploring in more detail the impact of deforestation in Costa Rican lowland and premontane regions on orographic cloud formation during the dry season month of March, Ray et al. (2006) used the mesoscale numerical model of Colorado State University's Regional Atmospheric Modeling System to derive high-spatial-resolution simulations that were "constrained by a variety of ground based and remotely sensed observations," in order to "examine the sensitivity of orographic cloud formation in the Monteverde region to three different land use scenarios in the adjacent lowland and premontane regions," namely, "pristine forests, current conditions and future deforestation."

This observation-constrained modeling work revealed, in the researchers' words, that historic "deforestation has decreased the cloud forest area covered with fog in the montane regions by around 5-13% and raised the orographic cloud bases by about 25-75 meters in the afternoon." In addition, they say it suggests that "further deforestation in the lowland and premontane regions would lead to around [a] 15% decrease in the cloud forest area covered with fog and also raise the orographic cloud base heights by up to 125 meters in the afternoon." These findings clearly relieve anthropogenic CO2 emissions of any blame whatsoever for the decreases in frog and toad populations that have been experienced in the highland forests of Monteverde, Costa Rica, while placing the blame squarely on the shoulders of those responsible for the felling of the adjacent lowland forests.

Sherwood, Keith and Craig Idso

References
Lawton, R.O., Nair, U.S., Pielke Sr., R.A. and Welch, R.M. 2001. Climatic impact of tropical lowland deforestation on nearby montane cloud forests. Science 294: 584-587.

Nair, U.S., Lawton, R.O., Welch, R.M. and Pielke Sr., R.A. 2003. Impact of land use on Costa Rican tropical montane cloud forests: Sensitivity of cumulus cloud field characteristics to lowland deforestation. Journal of Geophysical Research 108: 10.1029/2001JD001135.

Pounds, J.A., Fogden, M.P.L. and Campbell, J.H. 1999. Biological response to climate change on a tropical mountain. Nature 398: 611-615.

Ray, D.K., Nair, U.S., Lawton, R.O., Welch, R.M. and Pielke Sr., R.A. 2006. Impact of land use on Costa Rican tropical montane cloud forests: Sensitivity of orographic cloud formation to deforestation in the plains. Journal of Geophysical Research 111: 10.1029/2005JD006096.

Still, C.J., Foster, P.N. and Schneider, S.H. 1999. Simulating the effects of climate change on tropical montane cloud forests. Nature 398: 608-610.