|Cloud Forests in the Humid Tropics: A Bibliographic Review (UNU, 1987, 81 pages)|
|4. Cloud forest ecology|
Climatic Elements and Factors
Hydrological Characteristics at Watershed Level
The present chapter attempts to deal with those ecological factors which condition and affect cloud forests. Within these factors, climatic elements per definition take priority and will be discussed in more detail; then edaphic and hydrologic parameters will be dealt with; and finally mention will be made of other ecological factors, particularly those of a biotic nature.
Horizontal precipitation and its measurement. It has already been necessary on various occasions to point out the influence of climate and its parameters on the presence, distribution and structure of cloud forests. Holdridge (1971) summarizes the climatic conditions necessary for cloud forests, naming them "special atmospheric conditions":
Wherever fogs and mists occur with great frequency, as they do on windward mountain slopes in the condensation or "cloud belt"... they may constitute a significant source of additional moisture. Fog-borne moisture, dew, and heavy mists may condense upon exposed vegetational surfaces, and drip, or run down stems, to the ground. Such moisture, however, is not recorded in properly installed standard rain gauges, and its quantity is known to be highly dependent upon both the successional stage and foliage characteristics of the dominant vegetation. Hence, it does not enter into the computation of mean and annual precipitation for the determination of the Life Zone itself. Rather, it is considered to be a wet-atmospheric factor entering the classification at the secondary or association level.
Many authors consider that the additional supply of water, in combination with lower temperatures in mountainous zones, is an important ecological factor affecting the different types of cloud forests. This input of precipitation, in addition to rain, has been variously named by different authors (Kittredge, 1948; Geiger, 1961; Lamb, 1965; Kerfoot, 1968; Holdridge, 1971; Whitmore, 1975; Caceres, 1981):
In the present study, the entry of water into the ecosystem conditioned by the condensation process of the humidity of clouds or fog on vegetational surfaces, or by means of direct contact of cloud droplets with the vegetation, will be called horizontal precipitation and signifies an input of additional water to rainfall. The term "horizontal interception" used in different publications could be misleading in that "interception" as a micrometeorological and hydrological parameter, is generally understood to mean water intercepted by vegetation and then evaporated from the plant surfaces; in other words, water leaving the ecosystem.
The quantity of horizontal precipitation depends as much on inherent vegetational factors as climatic factors and elements. These will be discussed later, when the different climatological scales (macro, meso and micro) will be analysed.
The inherent vegetational characteristics, are roughly the following:
a) collect total precipitation beneath the canopy by means of troughs or a large number of rain gauges, subtracting from this the rainfall above the forest or in a nearby unforested area.
b) collect cloud droplets by means of artificial apparatuses known as "fog catchers".
The first method gives real values for net precipitation, being ecologically and hydrologically relevant for the ecosystem under study. Considering the interception (water intercepted by vegetation and then evaporated) the behaviour of gross rainfall (measured outside or above the forest) together with other climatic elements, this method allows the calculation of the actual contribution of horizontal precipitation to net precipitation. Measurement techniques require a large quantity of equipment, and are thus costly and present a difficulty in that the location of the collection troughs or rain gauges does not give readings that are representative of the ecosystem as a whole.
The second method that has been used by many scientists outside the tropics (see chapter 1) and by Ekern (1964), Baynton (1969), Vogelmann (1973), Juvik and Ekern (1978) and Caceres (1981) in humid tropical zones, determines the quantity of water that can theoretically be extracted from clouds by condensation processes and through capture by artificial obstacles. The main difficulty in using fog catchers is finding a design which gives accurate and representative readings of the horizontal precipitation within the ecosystem under study (Baynton, 1969). Another problem relating to the installation of the fog catcher is the selection of the level above the ground at which it is located (Ekern, 1964).
Table 2 shows, in a summarized form, the results of horizontal precipitation calculations with fog catchers in different cloud forests within the tropics. The values indicated within the table show a great variability at the absolute level, as well as the relative level compared to rainfall.
Absolute values vary between 325 mm/y and 941 mm/y; and the relative values between 7.2% and 158.5% of rainfall. Relative values for extremely rainy climates or seasons remain quite low (between 7.2% and 18% of rainfall equivalent).
On the other hand, it is important to note that during the dry seasons (Vogelmann, 1973) the relative values of horizontal precipitation are extremely high and can even exceed those of rain Juvik and Ekern (1978) mention a reading of 181.9 mm of horizontal precipitation in 3 dry months with only 14.5 mm of rainfall in a site located near a peak, which is the equivalent of 1,254% of rainfall. It is very probable that the relatively high quantity of water received through horizontal precipitation during the "dry" periods plays an important role in cloud forest ecology.
Juvik and Ekern's (1978) results additionally show that windward sites generally receive a great deal more horizontal precipitation than leeward ones, in absolute as well as relative values. However, in many cloud forests, where not only the slopes but also the peaks are swathed in clouds carried by winds, it is possible to observe what is known as the "spill-over effect" immediately on the leeward side of the crest where horizontal precipitation can reach very high levels. See data from Honulalai of Juvik and Ekern (1978) in table 2.
Table 2. Contribution of horizontal precipitation according to different studies undertaken in specific of the humid tropics
precipitation(% of rainfall)
(Pico del Oeste)
|elfin cloud |
|oak cloud |
|(14 weeks) |
|1898 mast; |
|Juvik and |
|Closed forest |
|Costa Rica |
|1300 masl. |
Caceres (1981) also reports, in addition to the readings of fog catchers, interesting values for net precipitation (throughfall and stem flow). Net precipitation varies between 82% and 99%, with an average of 92%, of the rain measured outside the forest. In a broad study carried out on the hydrological cycle of an Andean cloud forest, Steinhardt (1978) measured gross precipitation (outside the forest) and the net precipitation. The latter represents 90% of the gross precipitation. In both studies stem flow did not amount to even 1% of precipitation.
In comparison with other studies carried out on tropical forest interception, mentioned by Baumgartner and Brunig (1978), the readings cited by Steinhardt (1978) and Caceres (1981) for net precipitation are extremely high, which indicates a significant and effective contribution of horizontal precipitation to net precipitation.
Another study, not included in table 2, is that of Dohrenwend (n.d.) that determined, through the use of fog catchers, the contribution of horizontal precipitation to sub-alpine tropical vegetation resulting in approximately 20% of additional rainfall equivalent.
It is worthwhile mentioning here that solitary trees collect a great deal more horizontal precipitation per surface area than forests of the same species (Ekern, 1964; Vogelmann, 1973). According to Kammer (1974) this is due to the efficiency of the vegetation to collect and condense cloud moisture through exposure to wind. Merriam (1973), who studied horizontal interception with artificial leaves in controlled conditions, in a wind tunnel, concluded that:
Little is known of the effect of form and size of the leaves on horizontal interception, however, Went (1955) and Vogelmann (1973) consider that conifers are more efficient than broadleaved species.
Macroclimatic aspects. 'I'he relation.between macroclimate and cloud forests and their occurrence has been discussed in chapter 2. In general, macroclimatic parameters affecting cloud forests are:
Huber (1976) came across these three types over a horizontal distance of two kilometres. It is likely that, apart from altitudinal differences, other climatic effects at the mesoclimatic level, especially the density and frequency of clouds and wind velocity modified by the topography, would contribute to these vegetational differences.
Troll (1968) mentioned two topoclimatic effects which could influence cloud forests:
In addition, topography can significantly modify wind action in certain mountain locations, increasing wind velocity and thus speeding up the exchange between atmosphere and vegetation. This can augment the quantity of water entering the ecosystem by means of horizontal precipitation, or strongly alter the structure of the forests conditioning elfin woodlands at the topoclimatic level (wind forests, according to Ashton et al., 1978).
Microclimatic aspects. Richards (1952) gives an extensive summary of tropical rain forest microclimate in general. However only a few limited studies exist of microclimates within cloud forests.
Beebe and Crane (1947), as well as Huber (19763, included some simple microclimatic measurements (mainly temperature, humidity and light) in their ecological studies of the cloud forest at Rancho Grande (Venezuela). In addition Huber (1976, 1978) determined the light compensation point of 54 species resulting in a very large variability. Baynton (1968, 1969) investigated the microclimate of a dwarf cloud forest in Puerto Rico, including the establishment of wind profiles. Lotschert (1959) included evaporation measurements within his microclimatic studies of a cloud forest in El Salvador.
Apart from the general characteristics of microclimate within humid tropical forests (Richards, 1952) it is possible to summarize the microclimatic properties of cloud forests as follows:
The principal microclimatic parameter affecting cloud forests is the high relative humidity of the air in combination with horizontal precipitation. These two elements, very often associated with rather low temperatures, keep cloud forests permanently humid. This in turn facilitates the presence of epiphytes (mosses and lichens) which are able to keep the microclimate humid even when, at the macroclimatic level, the relative humidity has dropped (Leigh, 1975; Grubb, 1977; Tanner, 1980b). In elfin woodlands the reduction in transpiration below the canopy, due to the abundance of mosses, can result in a complete obstruction of development of undergrowth or any other vegetation on the soil away from the mosses (Leigh, 1975).
Wind generally favours transpiration by reducing the external resistance to transpiration. However, extremely humid air, particularly that charged with water droplets, can block transpiration with a permanent layer of water on the leaves. This effect and its ecological importance within cloud forests has been discussed in chapter 3 (Other Language Terms). The hydrological aspects of reduced transpiration in cloud forests at watershed level is discussed below. Crubb (1977) indicated that the high humidity favourized the "invasion of lichens and bryophytes" and, referring to Berrie and Eze (1975), mentions the damage which can be caused by such invasions; not only by covering and obscuring the leaves, but also through the destructive effects on the cuticula. Crubb (1977) considers that "infections" of these epiphyllic organisms, conditioned by the extremely humid microclimatic environment, cause the most serious damage to the vegetation. It therefore appears strange that drip tips* are lacking in cloud forests. Crubb (1977) supposes that these are only effective in climatic conditions when heavy rain (or storms) are interspersed with periods of sunshine, but are ineffective in permanently cloudy conditions. Ellenberg (1975), who also mentions the lack of drip tips in cloud forests, on the other hand, actually questions the teleological interpretation of this phenomenon.
Another relevant microclimatic phenomenon that has attracted the attention of various authors is the presence of xeromorphism within cloud forests in the humid tropics. Walter (1979) considered that the leaves on the trees, even in the most humid tropical zones, were exposed to solar radiation for several hours thus resulting in a heating up of the leaves by 10 K above air temperature. Ellenberg (1959) mentions xeromorphism of epiphytes, giving the same explanation as Walter. Leigh (1975), on the other hand, considers that there is still no satisfactory explanation of the phenomenon that, according to him, assumes a considerable importance in the formation of a thick layer of undecomposed organic matter frequently found in cloud forests. Crubb (1977) rejects the climatic explanation of xeromorphism in cloud forests; for him physiological factors and nutritional effects determine the xeromorphism of leaves in cloud forests.
The formation of xeromorphic structures in cloud forests can also be interpreted as a protective mechanism for plant surfaces against the chemical impacts of horizontal precipitation which, according to Lovett, Reiners and Olson (1982) have different chemical properties from the rainfall, tending to be much more acid (Falconer and Falconer, 1980; Schrimpff et al., 1984). According to Falconer and Falconer (1980), the acidity of cloud moisture droplets is more pronounced in humid tropical air masses with dew point > 15°C.
It is worth mentioning here that a great deal of microclimatic research remains to be carried out in different types of cloud forests to increase knowledge of its ecological and hydrological importance. To date only dwarf cloud forests have been the object of more detailed microclimatic studies and considerations in that their unusual appearance has attracted the attention of various scientists (see chapter 3, Terminology -Elfin Woodlands).
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).
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.
The abundance of epiphyles is one of' the outstanding biotic factors within cloud forests. Many authors (Ellenberg, 1959; Grubb et al., 1963; Myers, 1969; Letouzey, 1978; Walter, 1979) mention that the cloud forest zone represents the optimal environment for epiphytes, particularly mosses, lichens, orchids and bromeliads. According to Tuckey (1970) the latter can take advantage of leaching of the upper parts of the vegetation, and Kuchler (1967) points out that epiphytes include a great variety of life forms thus introducing a new physionomic element in their host trees.
Given that epiphytes are capable of' taking direct advantage of horizontal precipitation they are frequently found in the upper levels of tree canopies (Carr, 1949; Walter, 1973; N adkarni, 1984). '['his indicates that the crowns of the dominant trees are markedly exposed to atrmospheric exchange, thus receiving high quantities of horizontal precipitation. However, in dwarf' cloud forests, Ashton and Brunig (1975) and Grubb (1977) mention that epiphytes, especially mosses, can even cover the surface of the soil, which in dwarf' forests almost invariably consists of a layer of peat.
Grubb and Whitmore (1966) indicate that the abundance of epiphytes in cloud
forests should first be linked to horizontal precipitation, and not to the high
relative Humidity that predominates at the microclimatic level within cloud
forests. Walter (1973, 1979) and Leigh (1975) mention that the cloud forest zone
represents optimal climatic conditions for poikilohydric epiphytes on vascular
* Personal communication from Jim Barborak; Head, Wildlands Program, CATIE.
However, in the upper layers of the forest canopy vascular epiphytes with xeromorphism also exist (Brass, 1956; Grubb and Whitmore, 1966), resisting dessication during cloudless periods.
The formation of xeromorphic structures and their different climatological and ecological interpretations in cloud forests were discussed in previous sections of this chapter.
Another phenomenon frequently encountered in cloud forests is endemism, whether floral or faunal. A typical case is that of the Cerro de la Neblina at the frontier between Venezuela and Brazil, where in recent years studies have been initiated on existing species. According to Begley (1984), scientists currently working in this area think that the greater majority of plant species found at the Cerro de la Neblina exist nowhere else in the world. The reason for this phenomenon is probably the biogeographical effect of isolation. Begley (1984) cited R. MacDiarmid, one of the researchers currently working at the Cerro de la Neblina, who calls the area "an island in the clouds", offering excellent conditions for biological studies, especially those relating to the evolution of species.
Many authors have mentioned endemism in cloud forests, among them Martin (1955), Myers (1969), Howard (1974), Lewis (1971) and Tanner (1977). The biogeographical "island" effect in the case of Colombian cloud forest has been dealt with in various publications by Sugden (1982a, b and c; 1983). Myers (1969) stressed that, from the biogeographical and ecological points of view, the concept of "cloud forest" as a habitat is extremely useful in the definition of problems and in data organization. Endemism can be observed in a more marked form in cloud forests directly bordering relatively dry zones.
In many cases of protected cloud forests endemic fauna, found and studied by biologists, has been an important factor in justifying and ensuring their protection for the future.
Other cases, as for example the cloud forest of Montecristo in El Salvador, represent the only remaining habitat for various mammals, which have been exterminated in the rest of the region (Daugherty, 1973).