|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
|4. Cloud forest ecology|
The abundance of epiphyles is one of' the outstanding biotic factors within cloud forests. Many authors (Ellenberg, 1959; Grubb et al., 1963; Myers, 1969; Letouzey, 1978; Walter, 1979) mention that the cloud forest zone represents the optimal environment for epiphytes, particularly mosses, lichens, orchids and bromeliads. According to Tuckey (1970) the latter can take advantage of leaching of the upper parts of the vegetation, and Kuchler (1967) points out that epiphytes include a great variety of life forms thus introducing a new physionomic element in their host trees.
Given that epiphytes are capable of' taking direct advantage of horizontal precipitation they are frequently found in the upper levels of tree canopies (Carr, 1949; Walter, 1973; N adkarni, 1984). '['his indicates that the crowns of the dominant trees are markedly exposed to atrmospheric exchange, thus receiving high quantities of horizontal precipitation. However, in dwarf' cloud forests, Ashton and Brunig (1975) and Grubb (1977) mention that epiphytes, especially mosses, can even cover the surface of the soil, which in dwarf' forests almost invariably consists of a layer of peat.
Grubb and Whitmore (1966) indicate that the abundance of epiphytes in cloud
forests should first be linked to horizontal precipitation, and not to the high
relative Humidity that predominates at the microclimatic level within cloud
forests. Walter (1973, 1979) and Leigh (1975) mention that the cloud forest zone
represents optimal climatic conditions for poikilohydric epiphytes on vascular
* Personal communication from Jim Barborak; Head, Wildlands Program, CATIE.
However, in the upper layers of the forest canopy vascular epiphytes with xeromorphism also exist (Brass, 1956; Grubb and Whitmore, 1966), resisting dessication during cloudless periods.
The formation of xeromorphic structures and their different climatological and ecological interpretations in cloud forests were discussed in previous sections of this chapter.
Another phenomenon frequently encountered in cloud forests is endemism, whether floral or faunal. A typical case is that of the Cerro de la Neblina at the frontier between Venezuela and Brazil, where in recent years studies have been initiated on existing species. According to Begley (1984), scientists currently working in this area think that the greater majority of plant species found at the Cerro de la Neblina exist nowhere else in the world. The reason for this phenomenon is probably the biogeographical effect of isolation. Begley (1984) cited R. MacDiarmid, one of the researchers currently working at the Cerro de la Neblina, who calls the area "an island in the clouds", offering excellent conditions for biological studies, especially those relating to the evolution of species.
Many authors have mentioned endemism in cloud forests, among them Martin (1955), Myers (1969), Howard (1974), Lewis (1971) and Tanner (1977). The biogeographical "island" effect in the case of Colombian cloud forest has been dealt with in various publications by Sugden (1982a, b and c; 1983). Myers (1969) stressed that, from the biogeographical and ecological points of view, the concept of "cloud forest" as a habitat is extremely useful in the definition of problems and in data organization. Endemism can be observed in a more marked form in cloud forests directly bordering relatively dry zones.
In many cases of protected cloud forests endemic fauna, found and studied by biologists, has been an important factor in justifying and ensuring their protection for the future.
Other cases, as for example the cloud forest of Montecristo in El Salvador, represent the only remaining habitat for various mammals, which have been exterminated in the rest of the region (Daugherty, 1973).