Altitudinal Variations

On tropical mountains, there are marked altitudinal changes of vegetation (Troll, 1959). The concept of an actual zonation is rather too strict to be useful, but there is no doubt that as one ascends a mountain there are gradual changes. Most tree species have an ecological amplitude in terms of mean annual temperature (MAT), of about 6°C (van Steenis, 1934-36). There are some conspicuous exceptions to this, but it applies to many species. There is thus a continuum of change seen as the climber ascends to cooler altitudes. In New Guinea, for example, lowland trees start to be replaced around 1,000 m by tropical oaks (Lithocarpus spp. and Castanopsis spp.), and at a higher altitude by tropical beeches (Nothofagus sect. brassospora spp.) which continue up to about 2,800 m on the main ranges. Above the "beech" forest we find the Dwarf Forest. This is also known as the "Mossy Forest", "Cloud Forest", or "Upper Montane Forest", but the term "Dwarf Forest" is adopted in this chapter to avoid assumptions about causation. It is characterized not only by abundance of epiphytic bryophytes, but also by morphological peculiarities: stunted tree growth; small, thick leaves with a hypodermis; and presence of extra pigments (Grubb, 1977). Above this we approach the altitudinal forest limit, usually marked by subalpine forest or shrubbery, over a vertical extent of c. 200 m, before the "Alpine Grassland" begins (Corlett, 1984). These changes all tend to be lower on isolated peaks near the sea or on islands (the Massenerhebung effect, Figure 8.1).

What are the controlling factors of these variations? It has always been assumed that temperature must exert overall control. Mean annual values decline at about 0.6°C per 100 m, with increasing altitude, and the Alpine Grassland begins at about 6°C mean annual temperature (Walker and Flenley, 1979). Diurnal variation is extreme, so that the grassland experiences nightly frost and high temperatures in the day. Even the Massenerhebung effect may partly be explicable by temperature, for temperature lapse rates are steeper on isolated mountains near the sea than on large ranges which provide their own geothermal heat (Hastenrath, 1968; Flenley and Richards, 1982). How temperature operates other than by frost action is not well-understood. Various workers (e.g., Brass, 1941, 1964; Grubb and Whitmore, 1966) have suggested that the Dwarf Forest is associated with cloudiness. It has also been suggested that temperature could operate via the soil (Grubb, 1977), lower temperatures leading to greater accumulation of organic matter and to changes in nutrient status.

It may well be that the extreme diurnal temperature variation is partly responsible. Many tropical mountains, although cloud-covered in the afternoons, lose their cloud cover during the night and early morning. Given the low atmospheric pressure

Figure 8.1. The Massenerhebung (mass elevation) effect illustrated by the occurrence of Dwarf Forest on mountains in Indonesia. From left to right: Mount Tinggi (Bawean), Mount Ranai (Natura Island), Mount Salak (W. Java) and Mount Pangerango (W. Java) (after van Steenis, 1972).

Massenerhebung

Figure 8.1. The Massenerhebung (mass elevation) effect illustrated by the occurrence of Dwarf Forest on mountains in Indonesia. From left to right: Mount Tinggi (Bawean), Mount Ranai (Natura Island), Mount Salak (W. Java) and Mount Pangerango (W. Java) (after van Steenis, 1972).

Approx. distance from coast (km)

Approx. distance from coast (km)

resulting from the elevation, out-radiation is very high, resulting in extreme day/night temperature differentials (DIF). This thermo-periodicity has been studied in a number of cultivated plant species (Atwell et al., 1999). In general, a small positive DIF (i.e., day temperature a few degrees higher than night temperature) encourages growth. For instance, in Chrysanthemum there is a strong positive correlation between stem elongation and positive DIF (Carvalho et al., 2002). In Begonia, however, growing the plants with a day temperature of 22°C and a night temperature of 16°C, and adding in a 2-hour temperature drop to 12°C after sunset led to inhibition of total plant height and width (Son et al., 2002). In general, a DIF of 20°C in several species can discourage growth, leading to stunting (Atwell et al., 1999), which is a feature of the trees of the Dwarf Forest. As it is likely that few, if any, species from this forest have been investigated in this regard so far, further consideration of the role of DIF must await research. It remains, however, a distinct possibility that it is a factor of importance on tropical high mountains, where the "summer every day and winter every night" environment is normal (Troll, 1959). It seems surprising that modelers of tropical climate (e.g., Farrera et al., 1999) have chosen to use mean annual temperature and mean temperature of the coldest month, indicating seasonality, as parameters in their models while apparently ignoring DIF.

Most of the explanations of altitudinal variation so far offered are based mainly on association. Experimental evidence of causation is usually scanty, or rather inconclusive. Alternative possible causative factors—such as the decline in atmospheric pressure with altitude, or the increase in ultraviolet light with altitude—have rarely been considered.

This section investigates the hypothesis that the variation of vegetation on the upper parts of tropical high mountains (above c. 2,000 m) is related to insolation by ultraviolet light. This hypothesis was advanced by the late Francis Merton (pers. commun.) in 1973, but there was no serious consideration given to it at the time because of lack of evidence. Since then, however, discoveries have justified a revival of the hypothesis. These data relate to the effects of UV-B on plant growth.

Controlled experiments which simulate natural conditions are difficult with UV-B, for artificial light sources do not correctly reproduce the solar spectrum, and species differ widely in their response to individual wavelengths. Nevertheless, it has been possible to experiment on a range of plants (Lindoo and Caldwell, 1978; Teramura, 1983; Murali and Teramura, 1986a, b; Caldwell et al., 1995). In general, the plants became stunted and developed small, thick leaves with a hypodermis: precisely the characteristics of upper montane and subalpine forests. They also developed extra flavonoid pigments, which is also a common characteristic of the Dwarf Forest and shrubbery. In fact, the puna of Peru (a subalpine scrub) has a distinct yellowish appearance possibly caused by such pigments.

The correspondence between the features induced in crop plants, and the features present in the Dwarf Forest and shrubbery, is rather striking. It must be remembered, however, that the former is phenotypic and the latter (presumably) genotypic. This need not be an insuperable difficulty. Probably, genetic fixation of an initially induced feature would happen by natural selection. Is it possible, therefore, that the upper woody vegetation of tropical mountains is genetically adapted for resistance to UV-B?

It would be good to test this idea by considering the Massenerhebung effect, since this involves the occurrence of Dwarf Forest at anomalously low altitudes.

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