Contents - Previous - Next

This is the old United Nations University website. Visit the new site at http://unu.edu

2. Cloud forest distribution in the humid tropics

Occurrence in Terms of Altitude (Altitudinal Belts)
Area of Distribution (Horizontal Extension)

Occurrence in Terms of Altitude (Altitudinal Belts)

The term "cloud forest" has always been used to describe and generally define forests that are influenced by the frequent presence of clouds or mist. All of the authors who apply the term recognize this influence within the microclimate, ecology, structure, and hydrologic behaviour.

For this reason, there are many classifications of tropical mountain vegetation that even assign altitudinal belts to cloud forest formations.

Here are a few examples:

• Troll (1956) developed a schematic profile of humid vegetation by altitudinal belts covering the globe from the north to the south pole. This profile shows an altitudinal belt for cloud forests in the humid tropics (figure 2)
• Mann (1968), in a study on South American ecosystems attributes large areas, based on altitude, to the "silva nebula" (figures 3a and 3b)
• Lauer (1968) described the different attitudinal vegetation belts in Central America, including Panama, and in parts of Mexico and Colombia (figure 4). Using the geoclimatic classification first applied by Alexander von Humboldt in 1811, Lauer attributes cloud forests to a large portion of the "sierra fria" belt extending southwards from Nicaragua
• Troll (1959), considering topoclimatic phenomena, drafted the different attitudinal vegetation belts and landscape elements in the area of the "altiplano", La Paz and Las Yungas in Bolivia where cloud forests cover a large part of the mountain sides (figure 5).

Figure 2: A schematic profile of the world's humid vegetation from the north to the south pole mm emphasis an the tropics, according to Troll (1956), modified

The four examples mentioned indicate the influence of climate and orography on the distribution of cloud forests. Figure 2 shows, in a very general way, the boundaries of the cloud forest belt in the tropics. Depending on latitude, the lower limit varies between 1,500 and 2,500 masl, and the upper limit lies between 2,400 and 3,300 mast, indicating the existence of an altitudinal belt of approximately 800 to 1,000 metres at all latitudes. This, of course, is based on a very small scale graphic, giving only an approximate and generalized global overview.

Figures 3a and 3b show major variations in the distribution and extent of cloud forests in South America in relation to exposure.

Finally, figure 5 illustrates that climatic and geographic factors, on a local or regional level, may additionally influence formation and elevation of clouds, as well as their water content, thickness, and dynamics. For this reason, there are several authors who describe cloud forests even below 1,000 masl (Richards, 1952; Laserre, 1961; Weaver, Byer and Bruck, 1973; Walter, 1979; IUCN, 1982). Knapp (1965), referring to Beard (1949) mentions the presence of cloud forests at 300 masl in exposed areas of the Lesser Antilles.

Figure 3a: The cloud forest zone (silvo nebula) on the western Andean slope, occord ing to Mann (1968)

Figure 3b: The cloud forest zone (silvo nebula) on the eastern Andeon slope,occording to Mann (1969)

Figure 4 The altitudinal vegetation belts from Mexico to Colombia, according to Louer

Figure 5: A schematic profile of vagetation on the eastern Andean slops, from Lo Paz to the Yungos, according to Troll (1959), modified

On the other hand, according to Hedberg (1951), cloud forest reaches 3,500 masl on Mount Kenya. Also Troll (1959, 1968) found cloud forests ("bosques de ceja") in the eastern Andes at 3,500 masl. Hueck (1978) emphasizes that in some cases these forests can even reach 3,900 masl.

Ellenberg (19641, in a description of mountain vegetation and its productivity in Peru, points out that there generally exist three different levels of cloud condensation, each having its respective type of vegetation influenced by clouds or fog:

• cloud forests between 2,000 and 3,000 masl in the eastern Andes (the humid region)
• semi-evergreen scrub vegetation between 3,000 and 3,500 masl. on the western slope of the Andes
• "loma" vegetation caused by the "garua" phenomenon, existing from sea level to approximately 1,000 masl. along the arid Pacific coast.

All the previously mentioned examples indicate that there are various climatic and geographical factors which intervene and influence the elevation limits of the cloud forest bell in the humid tropics:

• mean moisture content of the atmosphere (Kerfool, 1968)
• cloud formation by convective or advective processes
• effect of the trade wind inversion and its variation on cloud formations (Riehl, 1954,1979; Dohrenwend, 1972)
• direction and velocity of the prevailing winds (Kammer, 1974)
• the mass elevation effect ("Massenerhebungseffekt") (Richards, 1952; Grubb, 1971,1977)*
• size and orientation of the principal mountain ranges (macro-relief)
• micro-relief within mountains which can have important topoclimatic effects (Troll, 1968)
• mean distance to the nearest sea as a function of prevailing winds
• surface temperature and prevailing water currents of the nearest sea.

For these reasons, it is not possible to define universally valid elevation limits for the occurrence of cloud forests in the humid tropics. Furthermore, the width of the altitude belt of the cloud forest as a function of latitude cannot readily be determined. But, the following general conclusions may be made:

The band of dense cloud cover in the humid tropics is generally found between 1,200 and 2,500 masl but in many cases it can reach more than 3,000 masl or begin below 1,000 masl. These limits depend on the structure of the troposphere, temperature and hum city conditions, the behaviour of the trade wind inversion, and on landmass elevation. Independent of the structure of the troposphere, but in accordance with the air temperature and relative humidity, orographic clouds occur frequently in mountain areas as a result of congestion and ascent of air masses. In coastal areas, gradual transitions between coastal fogs and orographic clouds may result (Flemming, 1971).

The adiabatic ascent of moist air causes condensation at certain elevations and thus produces clouds. Together with strong winds, the vegetation in contact with the clouds can capture a considerable amount of water in addition to the orographic rainfall that is often produced in these zones. 'I'he presence of cloud forests apparently requires that the clouds occur with a certain frequency, regularity, or periodicity and in combination with winds that permit a more intensive exchange between vegetation and the atmosphere.

According to Holdridge's (1967, 1982) life zone system, cloud forests as "wet atmospheric association" predominate in the life zones from moist forest to rain forest in the premontane and lower monotone belts, although Myers (1969) points out that cloud forests can also occur in other life zones or orographic belts.

Grubb (1971, 1977) emphasizes that dense and frequent clouds can decrease the lower elevation limits of different tropical forest formations such as the "Upper montane rain forest" and the "Lower montane rain forest".

------------------------------------
* This effect causes the occurrence of a determined species or vegetation formation on isolated and exposed mountains at much lower elevations than in extensive areas of high mountains, e.g. the mountain moss forest formation is found al 500 masl in the Seychelles, at 1,000 masl in the Philippines, at 2,400 masl on Mt. Kaindi, and at 3,100 masl on Mt. Wilhelm in New Guinea, according to Jeffrey, Brown, and McVean (In: Flenley, 1974).

Figure 6: Cloud forest distribution in the humid tropics and adjacent areas

Area of Distribution (Horizontal Extension)

The following is an attempt to determine the extent of the principal areas of cloud forests in the humid tropics within the zone defined in the introduction (see figure 1). Boekor (1979) estimates the total surface of cloud forests in the humid tropics at 500,000 km2 which would be equivalent to approximately 10% of the existing "tropical rain forest" according to Persson (1974) and would correspond, for example, to the total area of Kenya or Thailand.

Given that the occurrence of cloud forests is strongly linked to geographic and climatic factors, different sources are used to define the distribution of the cloud forests and indicate their limits in a general fashion in figure 6. The principal references used are: Martin (1955), Schweinfurth (1957), Lauer (1968), Wiebecke (1971), Paijchmans (1975) and IUCN (1982).

It is important to mention here that figure 6 is only an attempt to give a general idea of the geographic distribution of cloud forests in the humid tropics, due to considerations of scale and to lack of sufficient secondary information for the African and Asian tropics. For this reason figure 7, which shows the contiguous distribution of the Cyatheaceae according to Kroener (1968), is added for comparative purposes and as an additional indicator. As already mentioned in chapter 1, various authors indicated that the presence of Cyatheaceae was typical of tropical cloud forests. Comparing the two maps (figures 6 and 7), one can observe that in the humid tropics the distribution of cloud forests coincides, to a very great degree, with the contiguous distribution of the Cyatheaeeae.

Figure 7: Continous distribution of the Cyatheaceos (tree) in the tropics and adiacent areas according to Kramer (1968)

Contents - Previous - Next