|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
|4. Cloud forest ecology|
Studies of the hydrological behaviour of watersheds in temperate zones indicate that the elimination of tree cover results in an increase in runoff, due to the reduction of water loss through the high level of evapotranspiration, characteristic of forests. However, in the case of cloud forests, particularly in tropical zones, deforestation can result in substantial water loss (Zadroga, 1981). This is due to various factors, of which the most important is the additional input of water into the forest through horizontal precipitation, which can represent a considerable increase in the water balance.
Wicht (1961) emphasized that the lack of consideration for the input of water through horizontal precipitation in watersheds with cloud forest components, demonstrates a serious error in the volumetric determination of precipitation, which in turn introduces false values in the calculation of the water balance of the waterhsed.
Tosi (1974) mentions that deforestation of tropical cloud forests results in a marked reduction in runoff, which at the same time signifies a diminution in the feeding of ground-water. Budowski (1976,1980) indicates that cloud forests through their sponge effect are of considerable hydrological importance, and their disturbance could result in catastrophic consequences for valleys located downstream.
An energetic factor also enters cloud forest hydrology: a certain quantity of water deposited through horizontal precipitation on leaves corresponds with the quantity of water evaporated from the leaves during cloudless periods. This quantity of water would have been used in transpiration of soil water (McCulloch and Dagg, 1965).
Zadroga (1981) therefore summarizes three components of major importance in the evaluation of the effect of cloud forests on the hydrology of a watershed:
The increase in runoff in tropical watersheds with cloud forest components was recognized as far back as the 1960s, particularly in Hawaii. For this reason Duffy (1965) published an article with the title: "Water becomes the most important forest crop".
Zadroga (1981), in a study of the hydrological importance of cloud forests, analyzed data on precipitation and runoff in watersheds on the Atlantic and Pacific slopes of Costa Rica. Considerable differences were noted as far as the runoffl precipitation relationship was concerned. The value for the Atlantic slope, affected by the high incidence of clouds, was 102%. The Pacific slopes however, demonstrated a value of 34%. In the waterhseds on the Atlantic side, runoff exceeded the amount of rainfall over a seven month period. These seven months corresponded to the period of trade wind domination, pushing moisture-laden air masses towards the Atlantic slopes. Zadroga (1981) thus considers that an under-estimation of horizontal precipitation would be the principal explanation of this phenomenon.
Horizontal precipitation can maintain the base flow of a river even during periods of scarce rainfall. But Zadroga (1981) stresses that studies and quantitative analyses were still lacking to explain the hydrological behaviour of the cloud forests.
Pereira (1981) indicates in a work on "future trends in watershed management and land development research" referring to the tropics:
There are plenty of problems remaining such as the interesting examples of unusual situations in cloud forests.
In spite, of this, there is an increasing consciousness of the hydrological value of certain cloud forests within the tropics. It is worth mentioning here the case of the La Tigra National Park (formerly San Juancito Forest Reserve) in Honduras. This park, located barely 20 km from the capital of Honduras, Tegucigalpa, has a surface area of 7,500 ha, the majority covered in cloud forest at altitudes between 1,500 and 2,200 masl. The area provides between 30% and 50% of drinking water for Tegucigalpa (Campanella et al., 1982). During the dry months (March, April and May) the percentage of drinking water provided by the La Tigra area rises dramatically while other sources of water for Tegucigalpa drop their supply to a marked degree*. It is for this reason that many efforts to improve protection of this area have been justified with the argument of the hydrological importance of this forest. For the Honduran institutions involved, it is obvious that the supply of water to the capital could be endangered if the La Tigra area is inadequately protected.