|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
The term "rain forest", which attempts to classify a certain type of tropical vegetation, has frequently been the object of discussion due to varying interpretations or having been substituted by other equivalent, partly equivalent or more specific terms (Richards, 1952; Odum and Pidgeon, 1970; Letouzey, 1978). Furthermore, according to different authors, terminologies, and classification systems, there are various types of forests which may or may not belong to the category "rain forest".
Equally, the term "cloud forest", as frequently used in publications, lacks precision, has been interpreted, defined and used in different ways for distinct types of vegetation. In spite of this, many forests merit the appellation because the frequent presence of clouds plays a predominant ecological role (Huber 1976). According to the type, frequency and periodicity of the clouds, together with the other climatic and geographic factors one can observe the occurrence of different types of cloud forests.
On the other hand, there exist types of vegetation sometimes called "cloud forests" which would better be called otherwise, due to other predominating factors. Ashton et al. (1978) proposed the term "wind forest" for forests in very exposed sites even though in these places wind frequently occurs together with dense, rapidly moving clouds thus depositing a great quantity of horizontal precipitation. These forests which, due to their form and structure, are also known as dwarf forests, elfin woodlands, mountain thickets, mossy forest and frequently even cloud forests
(Howard, 1970), are treated separately at the end of this chapter.
The term "bosque nublado" is the most frequent and general denomination in Spanish to descibe forests under the strong influence of clouds (Lamprecht, 1954). Veillon (1974) uses the term "selva nublada" which he considers equivalent to the "bosque muy humedo montano bajo" according to Holdridge's (1967, 1982) classification. Other authors, e.g. Caceres (1981) mention cloud forests in the "bosque muy humedo premontano" life zone.
Beard (1944) in his system of classification equates the term Montane Rain Forest with the Spanish "selva nublada". This latter term was first applied by Pittier (1937) as a sub-category of the "selves ombr6Mas". In Central America, especially in El Salvador, the term "bosque nebuloso" is frequently used (Serrano, 1977). Czajka (1968), in a study of vegetation profiles of the mountains between Alaska and Tierra del Fuego, utilizes the term "bosque montano nebuloso" to describe the cloud forests in the American humid tropics. The term "bosque montano nebuloso" is a literal translation of the German "Gebirgs-Nebelwald" used by Lamprecht (1954).
Other terms used to describe forests and vegetation of frequently clouded areas ot the eastern Andes of Peru and Bolivia are "bosques de ceja" or "ceja de montana" (Weberbauer, 1911; Troll, 1968; Hueck, 1978). These terms, particularly "ceja de montaria" as used by Weberbauer, frequently describe dwarf forests ["mist forests" or "elfin woodlands" (Richards, 1952)] that occupy the altitudinal belt below the "paramo" or "puna". However, high, dense forests are sometimes also included in the "bosques de ceja" types.
Acosta-Solis (1968) on the other hand distinguishes within the eastern range of the Ecuadorian Andes, between "selva mesotermica higrofitica nublada" (800 masl) and "ceja andina" (2,800 - 3,200 masl), the latter falling within the category "selves sub-mesotermicas".
Veillon (1955) in a study of Andean forests of Venezuela, mentioned "bosques nublados andinos" which possess characteristics of high, dense, evergreen, species rich forests which are not equivalent to "bosques de ceja".
"Cloud forest" is the most common term used worldwide to describe forest types under the influence of clouds. (Beebe and Crane, 1947; Beard, 1955; Drewes and Drewes, 1957; Ellenberg, 1964; Troll, 1968; Roth and Merida de Bifano, 1971, 1979; Lawton and Dryer, 1980; Sugden, 1983). However, considerable numbers of authors have created other terms to describe such forests. The majority of these terms are those included in tropical vegetation classifications systems, e.g. Montane rain forest (Beard, 1944, 1949, 1955). v
Richards (1952) cited various authors of local studies that created different terms for the denomination of cloud forests: Exel (1944) named the vegetation of Sao Tome above 1,400 masl. "mist forest". Lane-Poole (1925) described two types of cloud forests in the mountains of New Guinea: "mid-mountain forest" which coincides with the presence of cloud cover, and "mossy forests" that are dwarf forests with a large amount of epiphytes found in the persistent mist zone. The terms "submontane rain forest" and "montane rain forest" used by van Steenis (1935) in Malaysia correspond with the terms "mid-mountain forest" and "mossy forest" of Lane-Pool (1925) in New Guinea.
Holdridge (1967) in his life zone system, developed the term "climatic association" within azonal climates. One of these associations is "wet atmospheric association of lower montane wet or rain forest" to which Holdridge assigned the cloud forests. However, Myers (1969) indicated that cloud forests also occur in other life zones. Zadroga (1981), in a study of the hydrological importance of a cloud forest, applied the term "montane cloud forest" emphasizing orographic factors and the role of cloud forests in high altitude watersheds within the tropics.
One of the best known and most widely used classifications of tropical vegetation is that of Beard (1944, 1949, 1955). His term, which corresponds with cloud forest or "selva nublada" is the term "montane rain forest" (Beard, 1944). In his presentation of improved classification, Beard (1955) compared "montane rain forest" with "cloud forest". Lamprecht (1977) confirmed the synonomy of the two terms.
However, above the "montane rain forest" two further types exist which are also subjected to the influence of the mist belt (Beard, 1955), but have not been classified by Beard as cloud forest: "montane thicket" and "elfin woodland" or "mossy forest". Probably, for these two types, Beard attributed other ecological or climatological factors as predominant ones (e.g. exposure to strong winds). Myers (1969) considered "montane thicket" and "elfin woodland" as particular types of cloud forest in his studies in Panama.
Generally speaking, the terms "cloud forest" and "montane rain forest" are synonymous, although Ellenberg (1964) used "montane rain forest" to describe forests occurring below the cloud forest belt. Ellenberg's "montane rain forest" is probably a literal translation of the German term "Montaner Regenwald".
Although Beard (1944, 1955) has dwarf forests (elfin woodlands, mossy forests) as special types outside the montane rain forest, some authors include dwarf (cloud) forests within the montane rain forest (Richards, 1952; I.eigh, 1975; Lawton, 1982).
The most frequently used term in German literature to describe cloud forest is the term "Nebelwald" (Lauer, 1952; Schweinfurth, 1957;Troll, 1959; Ellenberg, 1959, 1964; Knapp, 1965; Walter, 1973, 1979; Brun, ]976). The term "Nebelwald" signifies fog forest. In recent years the majority of authors use the term "Wolkenwald" (Lamprecht, 1972, 1976, 1978; Hoheisel, 1976; Steinhardt, 1978;
Bockor, 1979) in order to represent the translation of the most widely used term at the international level, "cloud forest", more accurately.
Ellenberg (1975) created the term "Orealer Wolkenwald" emphasizing the orographic influence on the occurrence of cloud forest.
Huber (1976) indicated that from the meteorological point of view, there is no difference between clouds and fog other than that fog is in direct contact with the earth's surface. In the case of mountains swathed in clouds, this difference has already been eliminated. Therefore, "Nebelwald" and Wolkenwald can be considered synonymous. Nonetheless, many authors now prefer the term Wolkenwald which more accurately reflects the climatic conditions than "Nebelwald" (Bockor, 1979).
Vareschi (1980), on the other hand, makes a strict difference between "Nebelwald" and "Wolkenwald" attributing to each distinct climatological and ecological properties. According to Vareschi the type "Nebelwald" or "Bergnebelwald" is very common in all mountainous zones within the tropics and corresponds with the general cloud condensation level. But the "Wolkenwald" type, according to Vareschi, is conditioned and caused by a rare and particular combination of climatic factors which permit cloud condensation in very warm areas at low altitude. In conformity with Vareschi's studies, the "Wolkenwald" type represents the global optimum for the vegetation and at the same time receives the highest diversity parameter according to Vareschi's classification. The "Wolkenwald" type (sensu Vareschi) is extremely rare and not synonymous with cloud forests or dwarf forests that occur at relatively low altitudes in the Antilles and which were described by Beard (1949).
Knapp (1965) using the term "Nebelwald" distinguishes between "montaner Nebelwald" (mountain cloud forest) and "Krummholz-Nebelwald" (dwarf cloud forest), a most useful differentiation of the two categories which probably corresponds to the montane rain forest and elfin woodland (mossy forest) types of Beard (1955).
Another German term synonymous with cloud forest, created by Lamprecht (1954) and also used by Huber (1976), is "Gebirgs-Nebelwald" which means mountain cloud forest.
Other Language Terms
In French the most frequently used terms to define cloud forests are "fores de nuage" and "fores nepheliphile" (Letouzey, 1978). Lebrun (1975) described cloud forests in the Congo creating the term "fores mesophile de montagne". Another term mentioned by Letouzey (1978) is "sylve a lichens" which was used to describe mossy forests in Madagascar. Aubreville (1949) described cloud forests with the term "fores tropical humide de hautes altitudes".
Beard (1944) presents the Portuguese equivalents for montane rain forest (mate nublada) and elfin woodland (bosque anao).
Mann (1968) used a Latin term "silva nebula" to describe large altitudinal levels of vegetation under the influence of clouds (see figures 3a and 3b).
Terminology - Elfin Woodlands
Woody vegetation, which frequently forms the upper limit of tropical mountain forests (Letouzey, 1978) or covers the summits and ridges of isolated and exposed mountains, has been the object of many studies. This type of vegetation is generally characterized by its stunted stature, twisted trunks with many branches and by a large number of epiphytes (especially mosses) that can completely cover the trunks, branches and ground (Howard, 1968; Weaver, Byer and Bruck, 1973). Beard (1944) in his classification system, referred to this type of vegetation by its physionomy, as elfin woodlands, which has been translated into Spanish as "bosques enanos".
One can generalize in saying that elfin woodlands occur at altitudes above those of cloud forests. Nevertheless, many authors include elfin woodlands within cloud forests (Richards, 1952; Troll, 1968) while others have a tendency to exclude them (Beard, 1944 and 1955; Holdridge, 1982).
Given that elfin woodlands frequently form the upper limit of cloud forests, their occurrence in tropical mountain zones varies likewise: in the Lesser Antilles, Beard (1949) describes elfin woodlands below 1,000 masl. and in special cases even as low as 500 masl. On the other hand, in the Peruvian eastern Andes, Hueck (1978) mentions that elfin woodlands ("bosques de ceja") in some cases reach 3,900 masl. This disparity is due to various factors, among which the most significant is the socalled mass elevation effect ("Massenerhebungseffekt") (Richards, 1952). Grubb (1971) tied this effect and its variations to the frequency of clouds or fog.
A large number of different terms exist to describe elfin woodlands, particularly in English works. The oldest and most widely used term is elfin woodlands. Beard (1955), although continuing to use the term, recommends more correct terms such as "montane woodland" or "elfin thicket". Another term frequently used is "mossy forest" (Richards, 1952).
Whitmore (1975) indicates that mossy forests can be found as an element within types of upper montane rain forest.
Other terms describing elfin woodlands due to the high incidence of mosses are "mountain moss forest" (Flenley, 1974) and "mossy montane forest" (Richards, 1952). Weaver (1972a, 1975) and Byer and Weaver (1976) preferred to use the terms "dwarf forest" and "dwarf cloud forest". Troll (1959), Baynton (1968, 1969) and Gates (1969) applied the term "elfin forest". Beard (1955) in his classification of tropical American vegetation excludes "montane thicket" from "elfin woodlands", while other authors include it e.g Howard, (1970)].
Other terms also refer to the characteristics of elfin woodlands subjected to strong winds that supposedly occur in sites in which elfin woodlands predominate. Ashton et al. (1978) use the term "wind forest", while Holdridge (1982) classifies elfin woodlands as "climatic association of strong winds."
Lugo(1983) and Frangi (1983) propose the inclusion of certain elfin woodlands within wetlands and calls them mountain wetlands or "sierras pantanosas de la montanan", supposing that the hydrological environment is the principal factor in these ecosystems, which would indeed correspond with many elfin woodlands according to the two authors.
Knapp (1965) used the term "Krummholz-Nebelwald" for a vegetational map of the Antilles, which also included wetlands without trees. Beard (1944) translates elfin woodland into Portuguese as "bosque anao" and Letouzey (1978) mentions that elfin woodlands in Madagascar were referred to by the French term "sylve a lichens".
Each one of the above mentioned terms attempts to describe elfin woodlands, placing emphasis on certain characteristics: be they altitude and form of vegetation ("bosque enano", dwarf forest, elfin woodland, "Krummholz-Nebelwald"), density (montage thicket), environmental factors such as clouds (dwarf cloud forest), winds (wind forest, climatic association of strong winds), humidity (wetlands, "sierras pantanosas de la montana") or the abundance of epiphytes, particularly mosses (mossy forest, mountain moss forest).
Each one of these descriptions can represent an element of reality, but the great variety of terms indicates that knowledge and research is still lacking. However, many authors have speculated over the reasons for the small size and structure of elfin woodlands. Weaver, Byer and Bruck (1973) summarize with the following list of possible causes:
Leigh (1975), in order to explain the stunted growth of dwarf forests, supports the theory of reduced transpiration due to high air humidity, and accepts a certain influence of wind in exposed sites. He attributed the stunted growth of the trees and their branches to thin, nutrient-poor soils.
Grubb (1977) on the other hand, rejects the theory of reduced transpiration which, in his view, does not effect the absorption of nutrients. He attributes the stunted growth of the trees to low temperatures in combination with reduction of photosynthetically active radiation. He accepts the effect of strong winds, particularly in regions dominated by trade winds.
Jaffe (1980) showed that the wind's bending effect can cause reduced growth. He incorporates this effect within the thigmomorphogenesis phenomenon.
Lawton (1982), describing elfin forests at an altitude of 1,750 masl. in the continental divide of Costa Rica, also considers strong winds as a principal cause of the low and twisted stature of these forests. He additionally mentions lightning as another destructive factor on mountain ridges, which does not permit normal tree development.
In this present study elfin woodlands are included within cloud forests, since it is proved that the majority is subjected to the presence and influence of dense and frequent clouds, which, apart from other factors, play an important ecological role.