As usually defined, the Massenerhebung or "mountain mass elevation" effect means the occurrence of physiognomically and sometimes floristically similar vegetation types at higher altitudes on large mountain masses than on small isolated peaks, especially those in or near the sea. Although the effect was first reported in the European Alps (Schroeter, 1908) and in North America (where it is known as the "Merriam effect"; Martin, 1963), it is best known in the tropics. Perhaps its clearest expression is the occurrence of Dwarf Forest at lower altitudes on isolated peaks than on the main mountain masses, which are taken as the norm (Figure 8.1).
Some explanations of this phenomenon have involved mean temperatures and cloud formation. Cloud formation is often observed on isolated peaks at quite low altitudes, and it was shown by Hastenrath (1968)—using radiosonde balloons in Mexico—that lapse rates were somewhat steeper over lowlands than over large mountain masses. Similarly, a steep lapse rate (0.74°C per 100m) was recorded on the 735 m high island of Krakatau in Indonesia (Forster, 1982); the regional average is 0.61°C per 100m (Walker and Flenley, 1979). Presumably, the afternoon clouding seen on isolated peaks is related to this temperature regime, and also, in the case of islands, to the greater evaporation from the sea (M. Bush, pers. commun.). The clouding does have pronounced ecological effects. For instance, it increases humidity to 100% and reduces total insolation received by about 30% compared with unclouded sites in Sabah, Malaysia (Bruijnzeel et al., 1993). The difficulty comes in relating these changes to the morphological peculiarities of the vegetation. Usually, if plants are grown in high humidity and low insolation, they become etiolated (i.e., they are tall, have long internodes, and large, thin, pale green leaves). This is the exact opposite of the attributes of tropical mountain Dwarf Forest, which are stunted growth, short internodes, small, thick leaves (with a hypodermis), and often extra pigments (antho-cyanins or flavonoids).
Now, let us consider the UV-B hypothesis as an explanation for the Massenerhebung effect. UV-B light, like visible light, may experience total reflection at water or cloud surfaces. Total insolation may thus be increased by up to 70% through reflection from clouds (Figure 8.2). In the early morning, low peaks are surrounded by a sea of clouds (Figure 8.3), which will reflect sunlight strongly up onto vegetation. Islands receive similar reflection from the sea surface. Rayleigh scattering of light from air molecules in the sky will also be particularly effective in clear mornings and favours UV above all other wavelengths (Dave and Halpern, 1976).
Later in the day the clouds move uphill and envelop the upper forests, reducing them almost to darkness (Hope, 1986). This later reduction of insolation may serve to exacerbate the effects of any UV-B damage caused earlier in the day. This is because of photo-reactivation, a process by which plants repair themselves from UV damage. Photo-reactivation is strongly dependent on visible light insolation (Caldwell, 1971).
The daily tropical regime of a heavy dose of UV-B in the morning, followed by semi-darkness, may therefore be particularly harmful to plants that are not adapted to it. There have been several attempts to relate the Massenerhebung effect to soil attributes. Among the more successful of these was that by Bruijnzeel et al. (1993), who found a correlation between the occurrence of stunted forest and phenolic compounds in leaf litter. The latter were thought to cause stunting by harmful effects on plant physiology. This theory is complementary to the UV theory, however. The usual response of plants to excess UV-B is to produce protective compounds that absorb UV-B. These are usually flavonoids or alkaloids (Caldwell, 1981), or anthocyanins (Lee and Lowry, 1980b). Many of these compounds are phenolic or are likely to break down into phenolic compounds in litter. They could provide a reinforcement mechanism, exaggerating the original stunting caused by the UV-B (Bruijnzeel and Proctor, 1993).
I conclude, therefore, with the tentative hypothesis that the Massenerhebung effect could be partly the result of a high dose of UV-B due to reflection from clouds or the sea in the mornings. Obviously, further research is needed to test this idea, although there is little doubt that the full explanation will be a multivariate one. Some factors may be more important in one location, while other factors may dominate elsewhere.
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