|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
|4. Cloud forest ecology|
According to Whitmore (1975) cloud forests, and particularly those of the upper mountane rain forest type, show a thick and widespread layer of undecomposed
* Thin extension at the apex of the leaves which supposedly facilitates the trickling of water. organic matter ("peat"). Brass (1941), Reynders (1964), Grubb and Whitmore (1966) and various other authors, indicate that peaty mountain areas correspond with dense and persistent cloud zones. Soil formation is particularly affected by the large amount of water entering the ecosystem. Following Whitmore (1975), the different consequences of this phenomenon are:
Frangi (1983) emphasized that the low deficit of atmospheric saturation in cloud forests results in a reduction of the pumping of water from the soil to the atmosphere, thus favouring wetland conditions even in areas of high permeability and inclination.
In addition the predominating low temperatures due to high altitude reduce the biological activity in the soil and the chemical meteorization in many cloud forests. The soils are usually highly acidic (pH 3.0-3.5) even in those originating from calcareous bedrock (Reynders, 1964) since there is permanent leaching.
Grubb and Tanner (1976) and Tanner (1977) studied soils in tour different cloud forests (mountane rain forests) in Jamaica and established four different categories:
In the case of mor ridge forest soils the organic layer is substantial (20-50 cm) and less in the other types.
Lotschert (1959) mentions a layer of organic material more than a metre deep in a cloud forest in El Salvador. Brewer-Carias (1973) reported that the soil of the Cerro de la Neblina cloud forest in Venezuela was covered with a thick layer of raw humus that in certain localities reached more than four metros in depth.
Leigh (1975) quoting Burgess (1969) emphasized that hardpans were frequently found between the peat and the mineral horizon. This phenomenon could lead to blocking plant access to the nutrients within the mineral layer. According to Schuylenborg (1958) podsolization within the tropics is generally more common in more humid and cold environments, although podsols are also frequently found in the humid tropic lowlands.
Peat formation can also be conditioned by the type of foliage that in certain cloud forests exhibits a high degree of xeromorphism, making its decomposition difficult (Whitmore and Burnham, 1969).
Hetsch (1976), who investigated relationships between precipitation and soil formation in the Venezuelan Andes, found that in cloud forest zones, in spite of the stability and structure of the soils, the high permeability and great capacity for infiltration and a permanent percolation, the soils were practically always saturated with water.
According to Hetsch (1976) the humus content of the soil and the C/N relationship increased with precipitation, reaching its highest level in cloud forest areas and diminishing above the cloud forest belt. However, it appears that various other factors interfere with this phenomenon in addition to the quantity of water entering the ecosystem (complex interplay of factors).
Folster and Fassbender (1978), in a soil study of Colombian and Venezuelan Andean forests, mentioned an abrupt change in soils at the level dominated by cloud forests: the colour of the soils changes from reddish to yellow with a lowering of the pH due to high precipitation and the associated change in humus dynamics. Although Hetsch (1976) supposes that the H+ ions of the organic soil matter, which suffers from decomposition difficulties, is the principal source of acidity, it is important to stress here that horizontal precipitation tends to have a lower pH than rain (Falconer and Falconer, 1980; Schrimpif et al., 1984). This could considerably affect the ecosystem, not only at soil level, but also at the plant surface level and particularly the leaves.
Another factor which could influence soil acidity of cloud forest soils is the high level of leaching as a result of abundant rainfall (Tuckey, 1970). Given that horizontal precipitation is extremely high in many cloud forests, and more acid than rainfall, two possible consequences are:
Both cases promote acidification of the soil and create favourable conditions for the formation of a peat layer.
Generally, the presence of peat is most pronounced in those parts of cloud forests of the upper montane rain forest type where Grubb (1971) believes that there is a reduced level of phosphorous, nitrogen and oxygen. Nonetheless, in all cloud forests of restricted growth a fairly thick layer of practically un-decomposed organic matter is noted. On the other hand, the soils in cloud forests with vigorous growth and high trees do not show this phenomenon (13 uber, 1976).
With respect to the distribution and nutrient cycling within cloud forests, an extremely extensive and complex subject, it is worth mentioning that the principal research and publications are to be attributed to Grubb and Whitmore (1966), Grubb (1977) and Steinhardt (1978).