|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
The history of cloud forest research in the humid tropics is extremely short considering that it has only been over the last fifteen to twenty years that the value and importance of these forests has been recognized, from the ecological as well as the hydrological viewpoints.
The first individuals to draw attention to the particular value of certain cloud forests were forest ecologists and biologists that discovered special formations and vegetational structures in addition to endemic species of flora and fauna.
It has only been within recent years that interest has been raised in (micro-) climatic processes and the hydrological importance of cloud forests. In spite of the fact that the term "cloud forest" has been used to describe a large number of different vegetation types, it is a term placing emphasis on climatological and hydrometeorological phenomena, which have been thoroughly dealt with in previous chapters of this study.
Cloud forests can occur at extremely different altitudes above sea level. Depending on the influence of a number of climatic and geographical factors, as for example the mass elevation effect, cloud forests have been described at elevations between 500 and 3,900 masl. in the tropics (Beard, 1949; Hueck, 1978). The majority of cloud forests, the extent of which has been estimated by Bockor (1979) at 500,000 km2 in the humid tropics, are found between 1,200 and 2,500 masl. This indicates that cloud forests are situated in upper and middle-level watersheds and thus play an essential hydrological role.
As shown in figure 6, cloud forests extend the length of large parts of mountain ranges and mountains within the tropics, many slopes of which are subjected to rapidly accelerating deforestation and spontaneous colonization. Figure 6, referring to the tropics throughout the world, can only give a broad idea of the geographic distribution of cloud forests. For the preparation of more detailed maps at regional and country level, it is recommended that consideration be given to likely locations of cloud forests, those life zones extending from Moist Forest to Rain Forest in the Premontane and Lower Montane belts where, according to Holdridge (1982), the wet atmospheric association dominates, this being equivalent to cloud forest.
The vegetation classification systems and climate zoning in tropical mountainous areas should consider the cloud belt as a special unit (Baumgartner and Brunig, 1978), since the continual cloud cover, its direct contact with vegetation and the process of horizontal precipitation, directly and indirectly influence the water and energy balance, physiological processes, soil properties and the vegetation ecology
Although studies exist that attempt the quantification of horizontal precipitation through fog catchers in a specific location within the forest, there is no clear idea of the contribution of horizontal precipitation in more extensive areas. Further research is necessary in a series of selected cloud forest sites, related to the different types of precipitation: gross precipiation, throughfall and stem flow, with emphasis on the interception process. The data supplied by such research, together with standard meteorological observations at each site, would allow an accurate calculation of the role played by cloud forests in the quantity and distribution of net precipitation in which horizontal precipitation can be of great significance.
Microclimatic and hydrometeorological research undertaken to date in cloud forests has been concentrated on elfin woodlands (dwarf forests) in the analysis and interpretation of physiological processes (e.g. reduction of transpiration) and the explanation of its structure and ecology.
The majority of detailed studies being undertaken in certain specific cloud forests are generally of the ecology, composition and endemic species. Without depreciating the great value of these studies, which have frequently played a key role in the protection of cloud forests, it is proposed here that future research should also place emphasis on the analysis and quantification of the hydrological importance of cloud forest as components of watersheds in the humid tropics.
These research efforts should not only focus on the process and quantity of horizontal precipitation, but also study the total hydrological impact of the cloud forest, as well as considering the hydrological importance of the large quantity of epiphytes that predominate in cloud forests, together with the thick peat layer which frequently covers the soil.
Bearing these considerations in mind, two distinct types of cloud forest, affected by different climatic conditions (of particular hydrological importance), could be selected: a) an area of cloud forest where the frequency of cloud cover occurs in combination with heavy and sometimes persistent orographic rainfall, with high annual precipitation; b) an area of cloud forest with a regime or seasons that are "dryer" and where horizontal precipitation could represent a relatively high component of total precipitation.
According to the current state of knowledge of the process and quantity of horizontal precipitation in different tropical cloud forests (see chapter 4, Climatic Elements and Factors) and their hydrological characteristics at watershed level (see chapter 4, Hydrological Characteristics at Watershed Level), the following hypothesis provides working guidelines for research into the two above mentioned eases:
Case a) those cloud forests in areas of heavy orographic rainfall, with their distinctive canopy strata, richness of epiphytes, and thick peat layer functioning as a sponge mechanism with a high capacity for water retention and runoff control, being able to mitigate the impact of torrential rains;
Case b) in areas or during very "dry" seasons, horizontal precipitation captured by cloud forests, represents a considerable increase in precipitation and runoff (as indicated, for example, in the case of La Tigra National Park in Honduras).
In both cases cloud forest destruction would have serious consequences for the lower watersheds. In the first case disastrous results could be expected in the form of floods combined with accelerated erosion. The second case would result in a marked reduction of river flow during the "dry" seasons and with the possibility of high water problems during the rainy periods.
The greater majority of tropical cloud forests are considered as extremely fragile ecosystems playing an important hydrological and ecological role. These are rapidly being converted into one of the ecosystems most threatened by human colonization (Zadroga, 1981). Many institutions and decision makers remain unaware of the serious consequences of cloud forest destruction.
Daugherty (1973) demonstrated that deforestation of cloud forests can trigger off quite catastrophic erosion processes. Apart from the danger of erosion and the hydrological function of cloud forests, there are various further arguments in favour of total protection (see chapter 5). Nonetheless, some special cases exist which allow the consideration of and research into sustainable and cautious forest management for the future. Such management practices must also be based on previous detailed research into the ecology, structure and dynamics of the forest and ensure all its secondary [unctions. In such eases, sound and sustainable forest management at the technical level can prove to be the best tool for forest protection.
The initiation, support and promotion of activities for the protection of tropical cloud forests is an important task for the future. Not only should their ecological value and significance as a special ecosystem be recognized, but also their important hydrological role as water regulators in watersheds, in combination with their soil conservation capacities on mountain slopes.