Warming trends at high altitudes

• According to model projections, by the time atmospheric carbon dioxide has doubled, temperature altitude shifts for the climatic optimum for montane ecotones could be hundreds of meters (7). Based on these predictions, cloud forests with narrow ranges could be completely replaced by lower elevation systems. Peak systems would likely be pushed into extinction (7). 

• Exacerbating the impacts of warming trends is the possibility that high altitude systems will be more affected by climate change than systems at lower altitudes (17). As ocean surface temperatures rise, large amounts of water vapour are pumped into the atmosphere. As this moisture condenses, atmospheric warming accelerates, and thermal profiles shift from a dry adiabatic lapse rate to a moist/saturated adiabatic lapse rate. A moist adiabatic ascent, in turn, causes increases in surface temperatures to be amplified with height, meaning that high altitude locations will warm more than low altitude sites (7; 17). An early-generation general circulation model (GCM) run by Still et al. (1999), predicted that a 2-degree warming of sea surface temperatures would produce an atmospheric temperature increase of 2.4 degrees at 1.5 km and a 3.53-degree increase at 4.5 km. Such a trend would be particularly harmful to high-altitude cloud forests.

Cloudiness

• Orographic clouds form the mists that envelope tropical cloud forests. This particular type of cloud is formed when wind is forced upward by topographic relief, in this case a trade wind against a mountain slope (7).  Cloud forests rely these low-lying orographic clouds in order to maintain a number of their defining characteristics. By maintaining high relative humidity levels, the clouds create supportive environments for the forests’ large number of epiphytes and diverse array of amphibian species. High relative humidity is also necessary for the integral process of horizontal precipitation. In addition, cloud immersion reduces the amount sunshine that penetrates into the forest, influencing levels of evapotranspiration, which in turn, influence the composition of flora and fauna. 

• Climate changes threaten to decrease low-level cloudiness due to increasing sea surface temperatures. These temperature changes will increase evaporation from the sea, pushing more water into the tropospheric water cycle (18). Evidence of such a trend is supported by the findings of Still et al. (1999), which concluded that decadal-scale trends of relative humidity, temperature, and specific humidity in the troposphere over the tropical western Pacific have been increasing since the 1970’s. As the moisture levels in the air rise, adiabatic lapse rates shift from dry to saturated. As a result, freezing temperatures shift upwards, as do the levels of cloud formation (5). As Pounds et al. (1999) notes, such a change might alter regional hydrology by decreasing cloud water input during the dry season. A study at Monte Verde, Costa Rica, the resulting trends of which are summarized in the figure to the right, indicate decreases dry-season precipitation and stream flow. Studies indicate that the orographic cloudbank is already rising and forest cloudiness is decreasing (18,7).

• Global climate change may not be the only influential factor affecting the cloudiness of tropical cloud forests. It is possible that rising levels of cloud formation can also be attributed to lowland deforestation (13). This is due to the fact that evapotranspiration occurs at a lower rate over deforested pasture than over forests. Additionally, sensitive heat transfers from the surface to the atmosphere increase over cleared land (13). The water budgets of a cloud forest, a montane forest, and an open pasture are compared in figure 3 (below).   These atmospheric changes decrease moisture transfer to the mountains, lifting the condensation level and the level of cloud formation upwards (13).  It is unclear whether local or global change has a greater influence on the rising cloudbank (13; 17; 7).  What is clear is that each is sure exacerbate the effect of the other, implying an even greater threat to cloud forest systems.

Disturbance Events

• There is evidence that global climate change may induce a higher incidence of intense rainfall events, typhoons, and hurricanes (8). More disturbance events would cause wind damage to trees, as well as increased soil erosion (7). Soil erosion is then likely to cause a number of problems such as nutrient leaching and drought. In addition to more disturbance events, Hulme and Viner (1998) have also documented an overall increase in the length of the dry season in the topics for all regions, (with the exceptions of east and northeast Africa as well as the Middle East into monsoon India). These same regions have also decreased in soil moisture and relative humidity. Extended dry seasons will increase water stress for the forest systems, damage vegetation, and eventually cause increases in drought and fire (7). Worsening the matter, cloud forests are known to be especially vulnerable to disturbance due to their slow growth and inability to colonize disturbed regions (7). Consequentially, the likelihood of cloud forest survival/recovery appears grim, especially when considered in conjunction with other potential impacts of climate change.