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1. Introduction

The present study has the following principal objectives: a) provide a detailed description of the state of knowledge of cloud forests in the humid tropics; considering and evaluating all related disciplines b) provide a complete bibliography on the topic, including all relevant references c) point out the limits and gaps in present knowledge d) propose efforts to broaden and strengthen the current state of knowledge, as well as recommend new areas for research.

Cloud forests within and outside the humid tropics have generally been studied and described from various points of view and different disciplines:

"Cloud forest" is neither a scientific term, nor does it serve as a definition within the above mentioned disciplines, and for this reason there may be confusion. (See terminology in chapter 3.) Nevertheless "cloud forest" is frequently used in scientific literature which recognizes the strong influence clouds and mist have on forest vegetation, its ecological properties and characteristics.

For the purposes of this study the following definition of "cloud forest in the humid tropics" is used:

Cloud forests include all forests in the humid tropics that are frequently covered in clouds or mist; thus receiving additional humidity, other than rainfall, through the capture and/or condensation of water droplets (horizontal precipitation), which influences the hydrological regime, radiation balance, and several other climatic, edaphic and ecological parameters.

Ellenberg (1964) provides a summarized definition, stating cloud forests as those that predominate in the zone of maximum cloud condensation.

The quantity, distribution and quality of horizontal precipitation relative to rain can vary considerably. Nevertheless, in many cases (not all), cloud forests occur in areas where clouds and mist appear in combination with heavy orographic rainfall. One feature characterizing all cloud forests in the humid tropics is the abundance of epiphytes, especially mosses and Hymenophylaceae (Walter, 1979), and in most cases the presence of Cyateacea (tree ferns) (Christ, 1910; Shreve, 1914; Kroener, 1968; Troll, 1970; Lamprecht, 1986). In the dwarf cloud forests, the mosses even cover the soil surface (Ellenberg, 1975; Ashton and Brunig, 1975; Grubb, 1977). The tropical cloud forests present optimum conditions for poikilohydric plants (Walter, 1973; Leigh, 1975).

Outside the humid tropics cloud forest research has a long history, and it is worth mentioning some of the outstanding research in this field. Although each study had distinct objectives based on priorities in each region, the main goal in many studies was the quantification of horizontal precipitation in relatively dry areas. In many cases the clouds or mist that passed through vegetation was looked upon as a possible supplementary, and in some cases even a prime water source.

  1. Due to the presence of the Humboldt current, high fog and adjective stratus clouds occur frequently in the arid coastal zones of Chile and Peru (Eidt, 1968). This phenomenon, known as "garua" in Peru and "camanchaca" in Chile, was the motive for various studies undertaken on the use of horizontal precipitation as the main source of water in a region which, due to its location and climate, is extremely arid (Knuchel, 1947; Follmann, 1963). The vegetation in this area, known as "vegetacion de loma" has been studied in detail by Koepke (1961), who divided it into seventeen different categories.
  2. Trees in plantations or large artificial fog-catchers can be the means to remove water from clouds and mist through capture and/or condensation ("fog farming") (Kummerow, 1962, 1966; Gischler, 1981). Recent experiments with fog-catchers covered with polythene screening demonstrate that these can remove up to 15 -10 litres of water per square metre of screen a day (Gischler and Fernandez, 1984).
  3. A long series of experiments (from Marloth, 1903 and 1906 until Nagel, 1956 and 1962) were carried out near Cape Town, South Africa, to quantify the horizontal precipitation of the orographic cloud known as the "table cloth" which frequently forms on the Table Mountain. This was considered as a possible contributor to Cape town's water supply. Using a system of fog-catchers, Nagel (1956) determined that the annual horizontal precipitation almost reached 170% of rainfall.
  4. The effect of coastal fog and cloud on the distribution of vegetation, especially the redwoods (Cannon, 1901; Cooper, 1917; Byers, 1930, 1953; Prat, 1953), has been the focus of much research in California.
  5. In Germany, Grunow (1952, 1955a and b, 1958, 1960a and b) carried out a long and complete sequence of studies on the quantity, distribution, intensity and structure of horizontal precipitation, including determination of droplet sizes. Several specialized instruments were designed and used in these studies, for example, the Grunow fog-catcher. Horizontal precipitation, as well as the incidence of fog as an ecological and silvicultural factor in a Bavarian mountain forest, were studied and described by Baumgartner (1957,1958a and b, 1959).
  6. In Japan, various studies were carried out to determine the interception of sea fog by trees in windbreaks. Most of these studies have been evaluated by Hori (1953).
  7. The islands of Tenerife and Hawaii are directly affected by clouds and fog and for this reason their climate and vegetation distribution has been widely studied.

The occurrence of clouds and horizontal precipitation are considered to be important hydrological and ecological factors. Regarding Tenerife, it is important to mention the following publications: Ceballos and Ortuno (1952), Garcia-Prieto, Ludlam and Sounders (1960), Kammer (1974), and Kunkel (1976). Mordy and Hurdis (1955), Ekern (1964), Duffy (1965), Juvik and Perreira (1974), Juvik and Ekern (1978), and Ekern (1979) carried out relevant studies in Hawaii.

g) In recent years various studies have been undertaken in the United States on the effect of microclimate and the vegetation structure upon the direct capture of water. They also focussed on the chemical properties of droplets captured by vegetation (Schlesinger and Reiners, 1974; Lovett, Reiners and Olson, 1982; Lovett, 1984). Merriam (1973) quantified the effects of artificial leaves on the capture of water under controlled conditions in a wind tunnel.

Figure 1 : Delimitation of the humid tropics

Shuttleworth (1977) in England, developed a theoretical estimate of the interchange of water between vegetation and mist driven by air currents.

The history of research on cloud forests within the humid tropics is rather short, since it has only been during the last fifteen to twenty years that the ecological and hydrological significance of these forests has been clearly recognized. It is for this reason that Kerfoot (1968), in an excellent review of literature titled "Mist precipitation on vegetation", was able to give only eight citations related to the humid tropics out of 156 total references. However, referring to Grubb and Whitmore (1966), he emphasized that, even though many authorities believe that mist is important only in restricted areas such as some mountain and coastal zones, it is probable that this phenomenon is of much greater importance in the tropics, where the atmosphere generally contains more water.

Kerfoot (1968) goes on to indicate, again referring to Grubb and Whitmore (1966), that in tropical mountain ranges, where water laden air masses move up against the mountains, the occurrence of clouds can be one of the most important ecological factors determining the distribution of forest types. This hypothesis has been confirmed by recent studies and research in various parts of the tropics. It is worth mentioning here the geographical areas in which principal cloud forest research in the humid tropics has been concentrated over the last twenty years.

For geographic and climatological reasons, the great majority of studies have been carried out in the American tropics. The two major areas of research have been Venezuela, where many studies have covered Andean cloud forests, and Puerto Rico, where scientists from the Institute of Tropical Forestry have done a series of studies on dwarf cloud forests (elfin woodlands). Cloud forests occur infrequently in the humid tropics of Africa. In contrast, they occur frequently in southeast Asia, except where the monsoon plays a dominant role in the hydrological and climatological regime, causing marked dry and wet seasons. Burgess (1969) reports that, in the according to Kuchler (Foeberg,Garnier and Kuchier, 1961) case of the mountains of Malaysia, cloud cover does not occur as regularly as in the Andes where it is probably significant in the distribution of different formations of vegetaion. Cloud forests do not exist in the western or central parts of the Himalayas, but are abundant in a well-defined orographic belt in the east (Schweinfurth, 1957).

Delimitation of the humid tropics

For the purpose of this study, the "humid tropics" are specified in geographic terms according to Kuchler's definition (Fosberg, Gamier and Kuchler, 1961) which delimits the humid tropics using criteria based on vegetation. The shaded areas in figure 1 represent the "more or less permanently humid" areas according to Kuchler. This area coincides to a great extent with the distribution of the "Tropical Rain Forest" according to Richards (1952). Following Vogel's recommendation (1966), the Hawaiian islands are also included within the humid tropics.

Nevertheless, several studies have been undertaken outside the humid tropics, for example, in parts of Venezuela as well as completely outside the tropics. As many of them are of particular interest to the topic or have validity within the humid tropics as well, they will also be discussed in this work.

Table 1 shows the main regions of interest in the study of cloud forests in the humid tropics and adjacent areas, and includes authors of the principal research papers and publications.

Table 1. Geographic areas and principal authors from cloud forest studies in the humid tropics
1.American Humid Tropics:

a).General: Beard (1944, 1949, 1955), Richards (1952), Lauer (1952, 1968), Troll
(1959, 1968), Ellenberg (1959, 1975), Knapp (1965), Czajka (1968), Holdridge (1971, 1982), Grubb (1971, 1977), Walter (1973, 1979), La Bastille and Pool (1978), Vareschi (1980).

b).Venezuela: Beebe and Crane (1947), Lamprecht (1954, 1958, 1976) Roth and Merida de Bifano (1971, 1979), Medina and Zelwer (1972), Veillon (1974), Steyermark (1974, 1975), Hetsch (1976), Hoheisel (1976), Huber (1976, 1978), Brun (1976), Steinhardt (1978), Bockor (1979), Vareschi (1980), Sobrevila, Ramirez and de Enrech (1983).

c).Puerto Rico: Baynton (1968,1969), Howard (1968, 1970), Gates (1969), Odum and Pidgeon (1970), Weaver (1972a, 1972b, 1975), Weaver, Byer and Bruck (1973), Byer and Weaver (1976), Brown et al. (1983), Lugo (1983), Frangi (1983).

d).Jamaica: Grubb and Tanner (1976), Tanner (1977, 1980a,1980b, 1981),Tanner and Kapos (1982).

e).Costa Rica: Holdridge (1971), Lumer (1980), Lawton and Dryer (1980), Caceres (1981), Zadroga (1981), Lawton (1982).

f).Mexico: Martin (1955), Vogelmann (1973), Lonard and Ross (1979), Puig, Bracho and Sosa (1981).

g).Colombia: Sugden and Robins (1979), Sugden (1982a, 1982b,1982c, 1983).

h).Ecuador: Grubb et al. (1963), Grubb and Whitmore (1966,1967).

2.Asian Humid Tropics:

a).General: van Steenis (1935), Richards (1952), Grubb (1971, 1977), Whitmore (1975).

b).Malaysia: Burgess (1969), Whitmore and Burnha m (1969), Flenley (1974).

c).New Guinea: Brass (1941,1956, 1959,1964), Paijchmans (1975).

d).Himalayas: Schweinfurth (1957).

3.African Humid Tropics:

General: Lebrun (1935 1960) Hedberg (1951)

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