Chapter Nine
INTRODUCTION
Sumatra has a large number of mountains, some, such as most of those in the Barisan Range, formed by uplift of sedimentary deposits and others, such as Mt. Kerinci, Mt. Sinabung, Mt. Merapi and Mt. Singgalang, formed by volcanic action. The physical environment changes up a mountain, thus providing a gradual change of conditions against which the fauna and flora can be examined. These differences between the montane ecosystems and the luxuriant, hot and humid lowland forest ecosystems are stimulating subjects for study.
The mountain about which most is known, Mt. Kerinci (3, 800 m), is also the highest. It was the object of a large collecting expedition in 1914 and about 30 papers were published on different groups of animals and plants in the Journal of the Federated Malay States Museum between 1918 and 1931 (Pendelbury 1936). These papers are most useful for taxonomists, but for understanding the changes occurring with altitude, more informative papers are available on Mt. ni Telong (Frey-Wyssling 1931), the mountains in the Mt. Leuser National Park (van Steenis 1934a), Mt. Singgalang (Docters van Leeuwen) and Mt. Kerinci (Frey-Wyssling 1933; Jacobs 1958; Ohsawa 1979). A CRES team climbed Mt. Kemiri (3, 340 m) in the Mt. Leuser National Park and Mt. Kerinci as part of the preparation for this book.
CLIMATE
Temperature
There is a mountainward current of air blowing during the day which is caused by air warming in the lowlands and thus expanding and rising. As any gas rises and expands in response to lower pressure, so its temperature falls because the wider-spaced gas molecules, which move about in response to energy from the sun, collide less frequently and therefore create less heat. The montane environment does not warm up in the same manner as the lowlands because of the reduced air pressure (caused by the shorter and therefore lighter column of air above the mountain), the low density of water vapour, and the generally clearer air. These three factors result in less retention of infra-red (warming) radiation in the mountain air, even though the intensity of radiation is greater on mountains for the same reasons as above. Similarly, during the night, heat is lost quickly and the daily change of temperature at about 3, 000 m can be as much as 15° to 20°C, but this depends on cloud cover.
The rate of temperature decrease is generally about 0.6°C per 100 m but this varies from place to place, with season, time of day, water vapour content of the air, etc. Readings of minimum temperature were taken of five consecutive nights during the ascent of Mt. Kemiri and the results are shown in figure 9.1. The four uncircled points, recorded when the sky was cloud-covered early in the morning, indicate a lapse rate of 0.63°C per 100 m and suggest that snow would settle at 4, 200 m if such a mountain existed. The permanent snow line would be still higher than this and occurs at about 4, 700 m in Irian Jaya. The circled reading was recorded after a totally cloudless night. The regression coefficient of the four uncircled points is 0.99 but this may be artificially high because other writers have found two lapse rates, the first up to the cloud or condensation level (about 2, 500 m) of 0.6°C/100 m, and the second above that of 0.5°C (van Steenis 1962, 1972; Whitmore 1984). Interestingly, however, the regression coefficient calculated from the many temperature readings made by van Beek (1982) up a number of mountains in the Mt. Leuser National Park was much higher when all the points were taken together than when they were divided at the cloud level.
Of considerable importance to understanding some aspects of mountain ecology is the Massenerhebung effect which was first noticed in the European Alps. It describes the phenomenon that vegetation zones on a large mountain or the central parts of large mountain ranges (such as the Barisan Range) are higher than they are on a small mountain or outlying spurs (Grubb 1971). This must be remembered when reading about mountains in, for example, Peninsular Malaya (highest peak 2, 200 m) or Irian Jaya (highest peak 5, 300 m) because zones of vegetation and fauna will be lower in the former and higher in the latter compared with the majority of mountains in Sumatra.
Relative Humidity
The percentage saturation of a mass of air increases as its temperature falls. The dew point (the temperature at which condensation occurs and clouds or drops of dew form) at different altitudes depends therefore on the temperature and the initial moisture content of the air. The forests of higher altitudes experience a very high relative humidity, particularly at night when the temperature falls, and the dew point is frequently passed so that water condenses on the leaves. During dry spells at altitudes above the main cloud layer, however, the relative humidity can be quite low during the day, leading to wide daily extremes.
Figure 9.1. Minimum temperature readings on successive nights during the ascent of Mt. Kemiri by a CRES team.
Clouds
Clouds originate where ascending air reaches its dew point (see relative humidity). Once water droplets have been formed they tend to act as 'seeds', and more water condenses on them. During the wettest months, the slopes and peaks of mountains can be enveloped in clouds for days on end, but during the drier months, when the air is not saturated with water vapour, it is common for a belt of clouds to form around a mountain, often at about 2, 000 m. As experienced mountain climbers know, a low, even continuous, layer of cloud at 2, 000 m does not always mean that any cloud is present at 3, 000 m or above.
Rainfall
Rainfall is generally greater on the windward (western) slopes of the Barisan Range and other mountains than on the lee (eastern) slopes (p. 14). There is, however, considerable local variation and there seems to be no guiding principle relating rainfall to elevation (McVean 1974). Rainfall on mountain slopes up to about 2, 000 m will generally be higher than on the surrounding lowlands. At the cloud level, rainfall measurements are not particularly useful ecologically because water droplets in the air are utilised by the plants. Above the main cloud level, it seems reasonable to suggest that rainfall is less than below it because of the lower incidence of cloud cover.
Frost
Although the peaks of Sumatran mountains are not covered with permanent ice, they almost certainly occasionally experience frost in the hours before sunrise. Radiation of heat from the ground occurs both by day and by night but at night it is not compensated for by radiation from the sun. Thus as warm surfaces of plants, soil, rock, etc., cool so the thin layer of air immediately around them cools. Cold air is heavier than warm air and if there is not a slope for it to flow down or a wind to push it away, it will become progressively colder. Since heat loss from the earth is hampered by dust, fog and clouds the lowest temperatures will be reached on clear, dry nights. Maximum cooling will be from nonconductive surfaces such as dead twigs or grasses and dry sandy soil rather than from conductive surfaces such as rocks, water and living vegetation. Thus frost is most likely to occur on calm, dry, clear nights in flat hollows (van Steenis 1962). Such places are called frost pockets and can occur in silted-up small lakes or in the glaciated topography at the tops of the major mountains in the northern Barisan Range.
Ultraviolet Radiation
It has been suggested that ultraviolet radiation on tropical mountains is probably more intense than on mountains in any other region on Earth (Lee and Lowry 1980). This is due to the amount of ozone, which absorbs ultraviolet radiation in the stratosphere, being appreciably less near the equator, and to the atmosphere at low altitudes being more turbid and dense and thus more capable of absorbing or reflecting this radiation. Climbers must therefore protect their skins or rapidly suffer the effects of sunburn.
SOILS
The considerable changes in climate with altitude are reflected in the soils. Unfortunately the difficulty of identifying a consistent pattern parallels the confused pattern in rainfall which is influenced by local topography. Further variation obviously occurs due to the nature of the parent material. However, it is generally observed that the soils up to 1, 000 m are more or less the same as the lowland soils (i.e., latosols and red-yellow podzolic soil). Between 1, 000 m and the cloud zone at about 2, 000 m (on the higher mountains), the increase in water supply leads to leaching, podzolization or water-logging. In addition, chemical weathering and biological activity are retarded by the lower temperatures. On the CRES expedition up Mt. Kemiri no termites were seen above about 1, 500 m and earthworms apparently become scarcer as well (p. 303).
These features are accentuated in the cloud zone where the more or less continuously saturated air causes peat to be formed in blankets up to 1 m deep. The pH is often very low (pH 2.5-4.0). Above the cloud level, the peat layer remains but it is less thick and the soil is generally less wet (Burnham 1974, 1975; Whitmore and Burnham 1969). On Mt. Kemiri the percentage of carbon and the carbon: nitrogen ratio rose from 2.8% to 42.6% and from 10 to 24.6, respectively, from 880 m to 3, 310 m. Mountain soils are frequently very poor in calcium and on Mt. Kemiri most of the soil samples collected by the CRES team had levels of only 0.11 m.eq/100 g or less. Similarly, many of the mountain soils examined by van Beek (1982) had no or very little calcium. These are indeed low compared with figures of 0.9 and 1.8 m.eq/100 g for the top 15 cm of lowland soils less than 100m above sea level in North Sumatra. Soils on young volcanic debris are clearly very poor. A soil sample collected on the outer rim of the fuming Mt. Sibayak was, as expected, very low in bases (table 9.1). The important influence of soil nutrients on the growth of mountain plants is discussed on page 299 and the cycling of minerals is described on page 296. Volcano soils are discussed on page 301.
VEGETATION
Introduction
A greater variety of plant communities is encountered on most tropical mountains than in any other area of comparable size in the world. Indeed, the vegetation changes observed when climbing a tropical mountain mimic the changes seen when travelling from the equator towards the poles. The tops of some Sumatran mountains are really very similar to, for example, Scottish moors in general appearance and some of the plants are even in the same genus. In fact, the plants of the high regions of tropical mountains have closer taxonomic links with plants of distant temperate regions than with the 'sea' of lowland forest plants that surrounds them.
The forest on the lower slopes of a Sumatran mountain seems indistinguishable from 'normal' lowland forest but then, with increasing altitude, the trees become shorter, and epiphytes, climbers and other features change in abundance. This zone is called lower montane forest. There then occurs a much more striking change when the tree canopy becomes even, the trees are even shorter, squat and gnarled, the leaves are thick and small, and mossy epiphytes are common. This zone is upper montane forest. Beyond this lies the subalpine forest, a complex of grass, heath and boggy areas. The grassy mountaintops of the subalpine zone have probably been caused by occasional fires, possibly lit by men for generations in their attempt to hunt animals such as deer and pig (van Steenis 1972). Analyses of soil samples taken in the subalpine zone of Mt. Kemiri by the CRES team revealed levels of potassium as high as 1.29 m.eq/100 g, levels which, in such generally nutrient-poor places, could have been caused only by fires. It has been suggested that the grassy slopes and valleys are caused by soil factors. In the wet grassy valleys near the summit this is more or less true because there is no tree species in the whole floral area of Malesia which can stand permanent inundation at high altitude (C.G.G.J. van Steenis, pers. comm.). The soil of the grassy slopes is, however, indistinguishable from the soil under the forest (unburnt) adjacent to it (C.G.G.J. van Steenis, pers. comm.). The question of regeneration of mountain vegetation after fire is discussed on page 379.
The general characteristics of the forest types on mountains is shown in table 9.2. During the ascent of Mt. Kemiri by the CRES team, the frequency of different forms of plant life were recorded at various altitudes and these are shown in table 9.3. In both these tables, the gradual decrease in vegetation height (illustrated in fig. 9.2) is recorded. This is not, however, accompanied by a proportional decrease in bole diameter which tends to increase with altitude for a given height of tree (fig. 9.3). The changes in vegetation structure up Mt. Kerinci have been described in general terms (Frey-Wyssling 1931; Jacobs 1958; Ohsawa 1979).
The division of montane vegetation into forest types is based on both floristics (van Steenis 1972) and on general structure. Many plant species cross the ecotones (transition areas between two types of forest or other ecosystems), but occur in different forms. For example, in the subalpine zone at 3, 250 m on Mt. Kemiri, the CRES team found a flowering Leptospermum flavescens which was only 15 cm high, but 300 m lower down, the same species was a common tree about 4 m high. The dwarfed specimen had a massive root and this illustrates the fact that the measure of biomass called root-to-shoot ratio (ratio of dry weight of below-ground parts to above-ground parts) tends to increase with elevation. In the lower montane forest studied by Edwards the ratio was 0.1 (Edwards and Grubb 1982), but in harsh, cold habitats such as the subalpine zone the figure is often 3.0-4.0 (Ricklefs 1979).
After Whitmore 1984 and Grubb 1977
Figure 9.2. Cross-sections of: a - lowland forest; b - lower montane forest (low levels); c - lower montane forest (high levels); d - upper montane forest, to show decrease in tree height and simplification of structure.
Adapted from Robbins and Wyatt-Smith 1964
Figure 9.3. Relationship between tree height and trunk diameter at breast height in four montane forest types on Mt. Kemiri. a - lower montane, b and c - upper montane (two sites), d - subalpine.
Data collected by a CRES team
Leaf size and thickness change between lowland forest and subalpine forest. The CRES team measured the thickness of leaves from 10 species of trees/shrubs from the subalpine zone at 3, 000 m and the mean thickness was 0.5 mm (range 0.3-0.85). The equivalent figure for a sample of leaves from lowland forest (Ketambe Research Station) was 0.2 mm (range 0.1-0.25). The thicker leaves of montane forests have been called 'pachyphylls' by Grubb (1974, 1977), which differ from other leaves chiefly in the thickness of their palisade layer (the long, photosynthetic cells of the leaf mesophyll) and the high frequency of a hypodermis (a layer immediately beneath the epidermis, often strengthened, forming an extra protective layer). See also the paper by Tanner and Kapos (1982).
The average heights of the various forest zones in Sumatra are as follows:
|
0 -1, 200 m |
lowland forest |
|
1, 200-2, 100 m |
lower montane forest |
|
2, 100 - 3, 000 m |
upper montane forest |
|
3, 000 m + |
subalpine forest |
Temperature and cloud level are the major determinants of these forest zones (p. 299) but the vegetation zones are compressed on smaller mountains (the Massenerhebung effect, fig. 9.4). The presence of a montane plant does not necessarily correlate with altitude for reasons other than the Massenerhebung effect and this is represented by wavy lines in figure 9.4. From the locations where a particular species thrives, dispersion to other areas is achieved by upward and downward movement of its fruits, seeds or spores. Conditions of higher altitudes are mimicked by frost pockets (p. 280), beside mountain streams, on bare soil and near waterfalls. In the lowest parts of a species' range the individual plants are usually sterile. The population of a particular species in such an area therefore depends on seeds or spores being carried or blown down from above for its maintenance. A similar mechanism operates up the mountain. Thus it had been proposed by van Steenis (1962, 1972) that there is a zone of permanent occurrence which 'feeds' the populations of the upper and lower zones of temporary localities (fig. 9.5). Thus one mountain may lack a species of plant even though at the same height on a higher mountain (with a zone of permanent occurrence) that plant may be present.
Characteristic Plants
Whereas lowland forests are often characterised by trees of the family Dipterocarpaceae (p. 198), the lower montane forests are characterised by the occurrence of the families Fagaceae (oaks) and Lauraceae (laurels). Both these families are also present at lower levels and their apparent dominance is caused largely by dipterocarps becoming increasingly uncommon. Fagaceae are forest trees and are recognisable by their fruit which consists of a hard nut (containing the seed) sitting in an open or closed, usually spiky, cup. The fruit of various species of Lithocarpus, one of the commonest genera found in Sumatra, are shown in figure 9.6. Lauraceae (the family containing cinnamon Cinnamomum burmansea and avocado pear Persea americana) is represented in the mountain forest primarily by the genus Litsea. This genus has fruits which also sit in a cup, but the cup is not woody and the fruit is soft and pulpy.
Figure 9.4. Vegetation zones on five mountains to show the compression found on smaller mountains.
Figure 9.5. The zone of permanent occurrence on mountains and its significance for the distribution of species. Shaded areas represent localities where a hypothetical species might be found.
Figure 9.6. Fruits from trees of the genus Lithocarpus (Fagaceae) found in Sumatra: a-L. conocarpus, b-L. hystrix, c-L. lamponga, 6-L. rassa, e-L. blumeana, f-L. encleisacarpa.
After Corner 1952
Tree ferns are common in lower montane forests. All but one of the tree fern species in Sumatra belong to the genus Cyathea (the exception being a montane species of Dicksonia) and the identification of species is based on the form of reproductive parts and on the fine brown scales found covering young coiled leaves and the bases of older leaves (Holttum 1977). A simplified identification key to some of the species found in Sumatra is given by Holttum and Allen (1963). A recent demographic and growth study of Cyathea tree ferns in Jamaica has shown that a trunk can grow about 1 m per 15 years. A tree fern 1 m high will, however, be considerably more than 15 years old because many years elapse between the first growth of a young fern and the stage at which upward growth in the form of a trunk begins (C.G.GJ. van Steenis, pers. comm).
The upper montane forest is characterized by the order Coniferae (the pines and related trees), particularly Dacrycarpus imbricatus (de Lauben-fels 1969), and the families Ericaceae (e.g., bilberries Vaccinium, Rhododendron) and Myrtaceae (the family to which Eucalyptus, clove Eugenia aromatica and paperbark Melaleuca belong), particularly Leptospermum flavescens. These trees are often quite low, crooked and gnarled, and because of their mysterious shapes this part of the zone has earned the name of elfin forest. On ridge tops and in the upper regions of upper montane forest the conditions are quite dry and the commonest and most conspicuous epiphytes here are the beards of the bright yellow-green lichen Usnea. In the valleys and in the lower regions of this same forest more or less the same tree community is found but virtually every available space on the trees and on the ground is covered with thick carpets of liverwort and moss. This is commonly called moss forest and coincides with the most frequent level at which clouds form (p. 279).
The subalpine zone is characterized by dwarf specimens of upper montane forest species and by grasses (e.g., Agrostis, Festuca), rushes and sedges (Juncus of juncaceae, Carex, Scirpus and Cyperus of Cyperaceae), and small herbs often with colourful flowers. Many subalpine plants form rosettes of leaves just above the ground. These rosettes are caused by the distance between successive leaves being extremely short, and only the flower stalk rises any distance above the ground, presumably to decrease water loss and to facilitate pollination or seed dispersal. The leaf under-surface of some species with rosette leaves and some other species are covered with dense, silky, white hairs (see p. 290).
It has been suggested that the high levels of ultraviolet radiation received by subalpine and alpine zones on high tropical mountains (p. 280), together with altitudinal shifts in vegetation zones during the Pleistocene (p. 17), may have led to accelerated rates of mutation and speciation (Lee and Lowry 1980). This may be a cause of high levels of endemism found in tropical subalpine and alpine plant communities. Figures have not been calculated for Sumatran mountains but 37% of the plant species on the higher parts of Irian Jaya mountains are not found in Papua New Guinea (Smith 1975), and 40% of the subalpine plants on Mt. Kinabalu, Sabah, grow only on that mountain (Smith 1970).
Repeated freezing of dead plant parts is thought by van Steenis (1972) to result in mummification. The dead material becomes an ashy-grey colour and when touched is easily reduced into fine grey dust. This was observed on Mt. Kerinci by Jacobs (1958) and by the CRES team on Mt. Kemiri.
Leaf Adaptations to Temperature and Radiation
In theory, structures lose heat by convection most rapidly at their edges where wind currents disrupt insulating boundary layers of still air; the more edges (or smaller), the cooler the leaf and the lesser the water loss (Ricklefs 1979). Although there are exceptions and effects opposing this theory, it explains the general pattern found in the shrubs and small trees of the subalpine and upper montane zones where midday temperatures can be high and where water availability is low. The same plants, however, need to cope with night-time temperatures approaching zero. The cooling effect of small leaves appears to be balanced by their close arrangement on the end of twigs, their dark colour and compact crown structure. Thus a compromise structure seems to have developed whereby the leaves will warm up quickly in the early morning sun so that efficient photosynthesis can begin as soon as possible before either clouds form or the solar radiation becomes too intense. The closeness and often vertical position of these small leaves have also been interpreted as a means of 'fog-stripping'. Water-saturated air or fog would be temporarily caught within the leaves on the end of a twig with a greater likelihood of water condensing on the plant and falling to the ground. Leaves in subalpine and dry areas of the upper montane zones probably experience longer periods of lower relative humidity than do leaves in the cloudy upper montane forest and so will run less risk of encouraging epiphyllae to grow on their leaf surfaces. Many leaves of the upper montane and subalpine zone plants have characteristics normally found in the areas subject to periodic drought: that is, xeromorphic characters. In fact, however, the mountain plants are not particularly efficient in restricting water loss (Buckley et al. 1980).
The woolly hairs of some subalpine plants have been variously attributed with the ability to protect against high temperatures (Lee and Lowry 1980), intense ultraviolet radiation (Mani 1980a) and frost/freezing (Smith 1979b; van Steenis 1972). Maybe they play a role in all three. For plants with leaf rosettes on the ground such as the silverweed Potentilla borneensis, however, the woolly hairs are on the lower surface and petioles only. This would make sense as frost protection but not against ultraviolet radiation or high temperatures. Other plants provide their growing buds and perennial parts with thermal insulation by retaining old leaves, and by having tufted branches and persistent scales. Such protection is necessary because the 'cost' to a subalpine plant of replacing a leaf is much greater than for plants growing in the more hospitable environments lower down the mountain or in the lowlands. Lee and Lowry (1980) list the adaptations of leaves to intense ultraviolet radiation as thick cuticles, wax deposits, and high concentrations of red, protective leaf pigments called anthocyanins in young leaves.
Origin and Dispersal of the Flora
The mountain flora of Sumatra is derived from two sources: those which originated locally (autochthonous) and those for which the centre of origin is outside the area concerned (allochthonous) (van Steenis 1972). The local source species are themselves divisible into groups: those which are characteristic of equatorial lowland forest, such as members of the families Dipterocarpaceae, Bombacaceae (e.g., durian) or the genus Ficus (figs), and those which have a wide latitudinal distribution throughout the globe and a wide altitudinal distribution in the tropics, such as Pinaceae (e.g., Pinus), Cruciferae (e.g., mustard), Theaceae (e.g., tea) and tree ferns. Of the first group very few are found over 2, 000 m and most are found below 1, 000 m. Genera and families of the second group, however, have wide tolerances of temperature and have both low- and high-temperature adapted (microtherm and megatherm) species. The low-temperature adapted species could possibly have originated outside the tropics and migrated towards Sumatra but they are equally likely to have evolved in or near Sumatra. None of the autochonous plants are therefore very useful in deciding how Sumatra gained its mountain flora.
The allochthonous flora, however, although a minority of the total mountain flora, do allow hypotheses regarding their origin to be formed. This part of the flora belongs to genera whose species are only found in cold climates (i.e., microtherm species), and in the tropics they are never found below 1, 000 m and thus are generally found only in the subalpine forests on mountains 2, 000-2, 500 m high (see principle of the zone of permanent occurrence, p. 287). These genera, such as Rhododendron, the grass Deschampsia and the coloured herbs Gentiana and Primula, are found in many tropical and subtropical countries, yet none can tolerate a hot climate.
Wallace (1869) worked out more or less how the mountains of the Sunda Region had received their flora - that is, by dispersal at lower levels during cooler glacial periods when the Sunda Region was a single land-mass. Van Steenis (1934a, b, 1962, 1972) made exhaustive studies of the region's microtherm allochthonous flora and revealed numerous interesting patterns of distribution. Plants must have extended their ranges, hopping by one means or another along mountain chains. Some of the 'chains' of passage are still quite complete. For example, the prostrate, branched herb with small white flower called Haloragis micrantha is found from New Zealand to Japan to the top of the Bay of Bengal, as well as on the top of Mt. Papandayan in West Java. It does not seem to have crossed over the relatively short gap to Sumatra (figure 9.7). Conversely, the stately herb with up to five layers of bright yellow flowers called Primula prolifera shows an enormous gap in its distribution (figure 9.8). Soils seem have little or no influence on the distributions since a single species will be found on soils originating from igneous, sedimentary or recent volcanic parent material. The age of the rocks seem to have an effect, though,
with some species not being present on recent volcanic soils (van Steenis 1972) (p. 301).
Figure 9.7. The distribution of Haloragis micrantha
After van Steenis 1972
Figure 9.8. The distribution of Primula prolifera.
After van Steenis 1972
Figure 9.9. The three tracks by which microtherm allochthonous flora reached Sumatra.
After van Steenis 1972
From his analysis of the distribution of about 900 cold-adapted mountain species, van Steenis (1972) concluded the existence of three tracks by which plants arrived in Sumatra and other parts of the Sunda Region during some period or periods in the geological past. These three tracks are shown in figure 9.9. Continuous ranges of high mountains do not, of course, exist along the entire lengths of these tracks. During the coldest times of the Pleistocene the mean temperature dropped only about 2°C (p. 12) which, with rates of temperature change of 0.5-0.6°C/100 m (not necessarily applicable at that period), is equivalent to a drop in the levels of the forest zones of 350-400 m. The locations of mountains over 2, 500 m high, and of mountains 2, 000-2, 500 m high (fig. 9.10), illustrates how the number of viable 'stepping stones' would have increased for microtherm plants during the coldest periods of the Pleistocene.
There seem to be no easy answers to the problem of how plants actually dispersed (and currently disperse) themselves between their small islands of suitable habitat. There are four main ways in which plants disperse their fruit and seeds, or, in the case of ferns, mosses, fungi, etc., their spores:
a) by water;
b) by wind;
c) by sticking to, being eaten but not destroyed, or being removed for later ingestion but then forgotten by terrestrial mammals;
d) by sticking to or being eaten and not destroyed by birds or bats.
Figure 9.10. Location of mountains of 2, 500 m and over (circles) and of 2, 000-2, 490 m (squares) on Sumatra.
The first and third of these are clearly of no significance in explaining the distribution of microtherm plants. Water only flows downstream to unfavourable habitats, and the distances between high mountaintops are too great for terrestrial mammals to complete before either the seed is excreted or is rubbed off.
Wind seems a likely candidate for the dispersion of the minute, dust-like seeds of orchids, spores, or the light, parachute-like, plumed seeds (achenes) of the many montane species of Asteraceae (formerly Compositae). Wind must indeed transport such seeds over quite long distances but plumes only open in low humidity, and van Steenis (1972) points out that the expected wide distributions of such species is not always found.
Many of the shrubs and small trees in the upper montane and subalpine forests bear berries which, if not exactly tasty, are quite edible and form an important source of food for birds and mammals. Many of the seeds in these juicy berries pass through bird intestines without harm. However, since the time to pass through the intestine is little more than an hour, there is very little possibility of transferring seeds to mountaintops more than about 30 km away and, in addition, birds of mountaintops are mostly sedentary. Sticky or adherent seeds or fruits may attach themselves to feathers or legs, but before a long flight birds will generally preen themselves thoroughly so that their feathers give maximum flying efficiency.
If one assumes, however, that occasional, unusual dispersion of seeds occurs between neighbouring mountains by wind or by birds, it would be natural to expect that the percentage of plant species dispersed in these ways would increase up mountains in comparison with species whose seeds have no obvious dispersal device or are too heavy for wind dispersal. Although this has not been investigated in Sumatra, it has been investigated for Mt. Kinabalu in Sabah (van Steenis 1972) and Mt. Pangrango in West Java (Docters van Leeuwen 1920). The results show that the opposite is true.
The question of dispersal is further complicated by the observation that communities of plants having a variety of dispersal mechanisms have exactly overlapping ranges on different mountains as though means of dispersal was immaterial. In addition, limited experiments were conducted on Javan mountains to introduce a species not found on a particular mountain to grow there. They met with singular failure. Even if a species grew from a seed and survived, its own seeds did not survive (van Steenis 1972).
As stated above, the lowering of forest zones during the Pleistocene would have made chance dispersal easier if only because more mountain-tops would have become suitable habitat. But even then, large gaps would have existed and it is clear that the means by which plants or groups of plants effected their dispersal is not yet fully understood.
Biomass and Productivity
There appear to be no estimates of biomass or productivity in montane forests of the Sunda Region, but Grubb (1974) has summarised the trends he found from examining data for montane forests in all parts of the tropics. Thus:
a) biomass decreases proportionally less than height on passing from lowland forest to upper montane forest (see fig. 9.3, p. 285);
b) production of woody parts declines from lowland forest (3-6t/ha/yr) to upper montane forest (± 1 t/ha/yr);
c) production of litter, particularly leaf litter, decreases proportionally much less than biomass or, it seems, production of woody parts;
d) the standing crop of leaves declines proportionally much less than the overall biomass;
e) the mean life span of leaves seems to lengthen only slightly from lowland forest to upper montane forest;
f) the leaf area index (m2 of leaves per m2 of ground) decreases proportionally much more than leaf standing crop because the leaves become thicker and harder. Thus the area of leaf per gram decreases from 90-130 cm2/g in lowland forest, to ± 80 cm2/g in upper montane forest.
It appears then, that as total production falls the montane plants invest relatively more of their production in making leaves, and so it is important to understand what advantages accrue to a plant from having thick, 'expensive' leaves which may last no longer than the thinner, 'cheap' leaves of the lowlands.
Mineral Cycling
In any type of forest there are two types of mineral turnover:
a) the rapid cycling in the small litter (leaves and twigs) and in throughfall (rain reaching the forest floor), and
b) the much slower cycling in the large woody parts of trees.
As stated above, moving from lowland to montane forests, the production of woody parts decreases proportionately more than does the production of leaves. The concentration of nutrients in leaves from upper montane forest are roughly half the concentration found in leaves in low-land forests. This reduction is compounded because trees of upper montane forests seem to absorb half of the minerals from their leaves before shedding them, compared to about a quarter for trees of lower montane forests (Tanner and Kapos 1982). Thus, lower quantities of minerals cycle through leaves in montane forests than in lowland forest, and much lower quantities through woody parts.
Figure 9.11. Percentage of rain falling over a lower montane forest that actually reaches the forest floor.
After Edwards 1982
The most detailed study of mineral cycling in montane forest close to the Sunda Region is by Edwards in Papua New Guinea (Edwards 1982; Edwards and Grubb 1977, 1982; Grubb and Edwards 1982). In the final paper of the series, Edwards (1982) summarised the major findings of the preceding papers. The major external input of minerals into the montane forest was from the rain, and these minerals may have been released into the atmosphere from fires (Ungemach 1969). Not all the rain reaches the forest floor as a certain percentage soaks into bark, evaporates, or gets trapped. The percentage which reaches the ground, the throughfall, varies with rain intensity, being greater when the rainfall is heavier (fig. 9.11). This throughfall contains minerals leached from the leaf and bark surface and from decomposition products so that the mineral concentration in rain reaching the forest floor is considerably higher than that reaching, say, a neighbouring cultivated field.
In addition to the throughfall, the other source of minerals falling to the forest floor is the litterfall. The proportions of minerals in the litterfall is affected somewhat by the withdrawal of minerals from leaves before abscission (i.e., the parting of leaf and twig). Edwards compared mineral concentrations in healthy leaves with leaves in the litterfall, and found that calcium and magnesium did not seem to be absorbed, but that nitrogen and phosphorus were respectively 15% and 31% lower in the litterfall (Edwards 1982). The recycling of these two minerals may indicate that they were limiting in the soil (see p. 299). Potassium is very mobile in soil and quickly resorbed by plants so its retention is less important. The calcium levels in the top soil layers of the forest examined were extremely high (16-40 m.eq./100 g) (Edwards and Grubb 1982) compared to the levels in many mountain soils in Sumatra (p. 281) and elsewhere in the western Sunda Region. It would be interesting to determine whether calcium is absorbed before abscission in Sumatran mountain plants, and thus gain an indication of the limiting role of calcium in the mountain forest.
Figure 9.12. Summary of mineral cycling in a lower montane forest. Data in boxes are expressed in kg/ha. Complete arrows represent major pathways of mineral transfer, and data are expressed in kg/ha/yr. Dashed lines represent presumed pathways for which no data are available.
After Edwards 1982
A summary of mineral cycling in the Papua New Guinea montane forest is shown in figure 9.12.
Limitations on Forest Distribution and Growth
The lower limit of a particular type of montane forest is probably set, not by temperature, but by the ability of its species to compete with species of the lower formation. The upper limit of a forest formation, however, is set by some factor related to temperature (Grubb 1977). The Massenerhebung effect (p. 278) appears to act on plants through the height at which cloud habitually settles and this is clearly related to temperature. The most important effect of cloud is that it prevents bright sunlight from raising the temperature of plant leaves. Those not adapted to living in the cloud zone may not reach their optimum temperature for photosynthesis. The amount of radiation available for photosynthesis is also reduced by cloud (Grubb 1974). The decrease with altitude of biomass, productivity, number of life forms and number of species can also be attributed to these factors.
Although temperature and radiation are the factors primarily responsible for limiting montane forest distribution and growth, the mineral supply is also highly important. Without conducting soil analyses or productivity studies, it can be seen that:
a) the crooked and gnarled growth of many montane trees is similar to the growth of trees on infertile lowland soils or to the miniature bonsai trees deliberately grown in nutrient-poor soils;
b) the slow rate of litter decay and the subsequent humus mineralisation may be assumed to lock up plant nutrients (especially nitrogen and phosphorus) in forms unavailable for plant growth;
c) forest types are found at lower levels and forest growth is less on sites most likely to be poor in nutrients (i.e., ridges);
d) the most dwarfed forests are found on peaty soils of very low pH which are likely to be too infertile for most plants;
e) some of the species found on peat soils in upper montane forest are found also in the similarly acid, infertile heath forests of the lowlands;
f) the insectivorous pitcher plants Nepenthes (p. 260) are often abundant in peaty upper montane forest (Grubb 1977).
Quantities of nutrients in leaves (see p. 258) reflect to some degree the nutrients available in the soil and so analysis of leaf nutrients is standard practice in investigative agriculture. Grubb examined both his own and other data on nutrient levels in leaves of montane species and found considerable variation within the forest types, but was able to conclude that the supply of phosphorus seemed particularly deficient in peaty upper montane forest. This supports the suggestion that phosphorus is the factor underlying the observations d), e), and f) above (Grubb 1977). Interestingly, Grubb (1974) also analysed the nutrients in leaves of pitcher plants Nepenthes to determine how similar they were to the surrounding vegetation that was not able to catch or digest insects. The results showed that nearly three times as much phosphorus and almost twice as much potassium was present in Nepenthes leaves compared with nearby trees and shrubs.
The major limitation on leaf life in the upper montane subalpine forests is probably not, as in most other forests, the destruction by insects, but the high humidity. This allows invasion by fungi and bacteria, and by epiphyllous lichens and mosses which are able to puncture the leaf's cuticle. The very thick outer walls of pachyphylls may be an adaptation to minimise such unwanted penetration (Grubb 1977).
On page 193 it was argued that drip-tips on leaves allow water to flow rapidly off the leaves and so prevent, or at least slow down, the growth of epi-phylls. If that is so, then why do montane forest plants invest in much more expensive thick leaves? Grubb (1977) has suggested that whereas drip-tips are effective under conditions of alternating heavy rains and sun, they would not be so effective where the dampness of leaf surfaces is caused by very high humidity and very fine water droplets from clouds or fog.
Similarities between Upper Montane Forest and Heath Forest
The impoverished, acid soils of some higher mountain zones might lead to an expectation that upper montane and heath forests (p. 253) might be somewhat similar. The structure of the forests are indeed rather similar with both of their canopies being even and appearing rather pale (high albedo). The trees tend to have dense crowns, usually with microphyll leaves (p. 283) which are often held obliquely vertical and closely placed on the twigs. Large woody climbers are more or less absent and biomass and productivity also seem to be low (p. 296).
They also share some tree and shrub species which do not (or only very rarely) exist in other types of forests. Examples are sesapu Baeckia frutescens (p. 256), and species of bilberry Vaccinium and Rhododendron.
Volcanoes
Near volcanic craters the ground surface is generally rocky, dry, sterile, acid, exposed, lacking in organic matter, and often warm or hot from the gases beneath. The air contains such potentially toxic gases as sulphur dioxide (SO2) and hydrogen sulphide (H2S) (the smell of rotten eggs) as well as carbon monoxide (CO), nitrous oxide (NO), and chlorine (Cl2) (van Steenis 1972). Water flowing out of craters is commonly little more than a stream of sulphuric acid. These are hardly conditions that favour plant growth, but van Steenis (1972) reports finding the tussock-forming sedge-like Xyris capensis (a species ranging from Central Africa to Australia) in a North Sumatran crater stream, the pH of which was 2.9. Blue-green algae are also commonly found in hot, sulphurous acid water.
Ash produced by eruptions occurs in screes on the tops of volcanoes, such as Mt. Kerinci, which have been active in the recent past. They are barren, pervious, sterile and unstable, moving downhill, particularly during heavy rains. The only plants able to colonise such ground must have long, deep roots for anchorage and to find moisture. Grasses (e.g., Agrostis), sedges (e.g., Carex) and the composite Senecio sumatrana are among the first pioneers on Mt. Kerinci, but the highest-growing plant of all is a small fern (Frey-Wyssling 1933). Melastoma and pandans are common on the sulphorous screes of Mt. Sibayak and Mt. Sinabung, Tanah Karo, but they are absent from the non-sulphorous screes on Mt. ni Telong, Central Aceh (Frey-Wyssling 1931).
The magnificent edelweiss Anaphalis javanica is one of the very few plants confined to volcanoes and is known only from central and southern Sumatra. It is a long-lived pioneer of volcanic ash screes and crater soils and is frequently gregarious. It can grow to about 4 m (sometimes to 8 m) with its stem as thick as a man's wrist (van Steenis 1972), but 1 m specimens are now rare. The stem has fine, velvety white hair and the abundant flowers appear white, too, except for a central yellow disk. Most mountain plants have become dwarfed but the edelweiss is one of the giant, white-haired members of the Asteraceae which, in height at least, dominate the generally short subalpine vegetation on tropical mountains. Its equivalent on east African mountains are giant species of Senecio and in the Andes the 9 m tall Espeletia (Mani 1980a). The edelweiss has traditionally been regarded as a gift from heaven in some areas and climbers have taken small pieces with them when they descend. The habit has turned to vandalism on some mountains, such as Mt. Singgalang and Mt. Kerinci, where this strikingly beautiful plant does not simply have a portion removed, but is dug up completely in the naive hope that it might grow in the lowlands.
Figure 9.13. Gleichenia and Dipteris ferns of the higher mountain zones.
After Holttum 1977
On volcanic scree such as on Mt. Kerinci (Jacobs 1958) the fern Gleichenia arachnoidea forms dense, prickly thickets. This and the common mountain fern Dipteris (fig. 9.13) are of interest because fossils of seemingly identical plants have been found in Greenland and Britain dating from the Cretaceous period, 65-135 million years ago (Holttum 1977).
Mountain Lakes
Mountain lakes in Sumatra are generally of two types: lakes in extinct volcanic craters such as Lake Maninjau (± 600 m a.s.l) and Lake Tujuh (1, 996 m a.s.l) (de Wulf et al. 1981), and marshy lakes in small depressions formed by glacial action (see p. 12).
After volcanic activity ceases, water begins to accumulate in the craters. At first the sulphurous content of the water prevents colonisation by most plants, but in time this is either leached away or incorporated into sediments, and marshy plants can begin to grow. The depth of a crater lake obviously depends on the height above the lake floor of an outlet, but is usually quite considerable. The flora and fauna of high, deep, cold, olig-otropic (nutrient-poor) lakes is impoverished and, since the sides are generally steep, there is little or no characteristic fringe habitat.
Of more interest, however, are the marshy fringes of shallow lakes in glacial hollows such as the one at 3, 000 m on Mt. Kemiri. The rushes, sedges and grass such as Juncus, Carex, and Scirpus mucronatus form clumped mats, while nearer the water's edge the white, round-flowered Eriocaulon browniana is commonly found in hummocks. The most characteristic plant, however, is the moss Sphagnum which is sometimes also found in moss forest (Chasen and Hoogerwerf 1941). This genus has a worldwide distribution and is generally found in shallow, cold, usually acid, nutrient-poor water or in similar waterlogged soil where few other plant species thrive. Sphagnum productivity varies between 100-600 g/m2/year depending on environmental conditions (Clymo and Hayward 1982). Sumatran Sphagnum is very poorly known, but Johnson (1960) reports one species, S.junghuh-nianum from mountaintops in Peninsular Malaysia and from China, Japan and Taiwan, which is also found on Sumatran mountains.
Sphagnum grows like other mosses but the dead parts decay very slowly (even though the leaves are only one cell thick) and form the basis of much of the peat in temperate regions. The reasons for this slow decay are perhaps twofold: the very low nitrogen content (± 1% of dry weight) makes it unattractive to decomposing organisms, and the acid conditions and the low oxygen levels in the water enclosed within the carpets of the moss, also create unfavourable conditions for breakdown. Sphagnum may be just a lowly moss but it is extremely successful: virtually nothing eats it, it covers at least 1% of the earth's surface, and the sum of its living and decomposing parts is probably greater than that of any other plant genus in the world (Clymo and Hayward 1982).
The fauna associated with Sphagnum-dominated lakes is poor and at the lake on Mt. Kemiri the only animals found were: a few bloodworms (larvae of chironomid midges) which feed on detritus; hunting spiders which can walk on the water surface between clumps of Sphagnum and probably prey on emerging midges; and predatory larvae of libellullid dragonflies.
ANIMALS AND THEIR ZONATION
Invertebrates
The number of invertebrates declines with altitude. On Mt. Mulu, Sarawak, for example, the decrease in soil macroinvertebrates was accounted for largely by the gradual reduction in the abundance of ants and termites. Biomass, however, reached a maximum in the peaty soils of the taller parts of upper montane forest where beetle larvae took over from termites as the major detritivores. At higher altitudes centipedes and spiders took over from ants as the major predators (Collins 1979, 1980). A disjunction in the distribution of beetles was found up the mountain such that no beetle species found below 500 m (lowland forest) was found above 1, 500 m (upper montane forest) (Hammond 1979; Hanski 1983). The highland beetles were thus a distinct association of species. A study of large moths in Papuan New Guinea showed that the total number of species dropped from 774 at 2, 200 m (lower montane forest) to 379 at 2, 800 m (upper montane forest) (Hebert 1980).
Whereas in the lowlands the most conspicuous bees are the carpenter bees (Xylopodidae), these are rare above 1, 500 m. During sunny periods in the subalpine forest bumble bees Bombus sp. (Apidae) are a common sight as they visit flowers for pollen and for nectar. It must be remembered that a visit to a flower by a potential pollinator (bird, bat, moth, butterfly, wasp, bee, ant, fly or beetle) does not necessarily constitute actual pollination. Notes have been made on mountain plant pollination by van Steenis (1972), and for detailed, fascinating information on pollination ecology see Faegri and van der Pijl (1979).
It is well known to entomologists that moths and many other insects such as winged ants, beetles and flies exhibit the phenomenon of summit seeking, in which they fly up mountains and sometimes congregate in large numbers. The basic factors underlying this behaviour are not known and a bewildering variety of explanations have been proposed (Mani 1980b).
Reptiles and Amphibians
Nothing seems to be known about the altitudinal limits of reptiles and amphibians of Sumatran mountains but these were investigated up Mt. Benom, an isolated, granitic mountain in Peninsular Malaysia. No lizards were found above 1, 000 m but five snakes were. Only one of these was found above 2, 000 m. Similarly, seven species of frogs or toads were found above 1, 000 m but only one was found above 2, 000 m (Grandison 1972). None of the montane species found on Mt. Benom are known from Sumatra but a similar type of reduction in species would be expected on Sumatran mountains.
Birds
As part of their report on a collection of birds from Aceh, Chasen and Hoogerwerf (1941) list the species found in different habitats and at different altitudes. Thus, in the primary and dense secondary growth between 300 and 1, 200 m, 134 species were found. In the upper montane forest they found only nine, but eight of these were also found lower down. In the subalpine zone, however, 11 species were found, but only two of these were found in either the lowlands or in the upper montane forest. This illustrates that the montane avifauna is also not just an impoverished selection of hardy species but is a distinct community of species.
Similarly, on Mt. Benom, only 3% of the lowland birds reached 1, 200 m (the lower region of Fagaceae/Lauraceae forest in lower montane forest) and the species of the upper parts of lower montane forest and of upper montane forest were quite distinct from the lowland species (Medway 1972b). Similar results were found for the birds of Mt. Mulu (Croxall 1997; Wells et al. 1979). Characteristic and conspicuous birds of Sumatran mountaintops are the medium-size (± 30 cm) Sunda whistling thrush Myiophoneus glaucinus, shiny whistling thrush Myiophoneus melanurus, scaly thrush Zoothera dauma and island thrush Turdus poliocephalus (King et al. 1975; van Strein 1977). Perhaps because humans are so rarely seen on high mountains, many of these birds are very tame, and can easily be watched eating Vaccinium berries and similar foods. Above the summit of Mt. Kemiri, the CRES team observed a crested serpent eagle Spilornis cheela and a high-flying swift Apus sp. Large mountains clearly are no barriers to these master flyers.
In Papuan New Guinea, Kikkawa and Williams (1971) found that a distinct discontinuity of bird species occurred at about 1, 500-2, 200 m, which corresponded to the change from lower to upper montane forest. They continued their analysis to examine the differences in the niche occupancy of birds at high altitudes. Thus, with increasing altitude:
• the proportion of tree-nesting insectivorous species increased;
• the proportion of tree-nesting frugivorous species decreased;
• the proportion of tree-nesting omnivorous species stayed roughly the same;
• the proportion of predatory birds decreased; and
• the proportion of ground-living birds increased.
Mammals
At least 11 species of mammals found in Sumatra are more or less restricted to the mountains:
a) Grey shrew Crocidura attenuata, known from subalpine zones of Mt. Leuser, Mt. Kerinci and one caught by the CRES team on Mt. Kemiri. Also known from the highest mountain in Peninsular Malaya, Mt. Gede in West Java, and lowland areas of northern Indochina and southern China (Chasen 1940; Jenkins 1982; Lekagul and McNeely 1977).
b) Grey fruit bat Aethalops alecto, known only from mountain forests above 1, 000 m in Sumatra, Peninsular Malaysia and Kalimantan (van der Zon 1979).
c) Sumatran rabbit Nesolagus netscheri, known only from Mt. Kerinci and the Padang Highlands (Jacobson 1921; Jacobson and Kloss 1919) and Mt. Leuser National Park where local people sometimes eat it (fig. 9.14).
d) Volcano mouse Mus crociduroides, known only from 3, 000 m on Mt. Kerinci and 2, 400 m on Mt. Gede in West Java (Chasen 1940; Marshall 1977b; Robinson and Kloss 1918). Other localities may yet be discovered.
Figure 9.14. The Sumatran rabbit Nesolagus netscherii has not been seen by scientists since 1937.
e) Giant Sumatran rat Sundamys infraluteus, known from the length of the Barisan Mountains between 700 m and 2, 500 m and also on Mt. Kinabalu, Sabah. This species is known on Sumatra from only six specimens (Musser and Newcomb 1983).
f) Edward's rat Leopoldamys edwardsi, which is widely distributed and rarely found below 1, 000 m (Marshall 1977; Musser and New-comb 1983).
g) Hoogerwerf's rat Rattus hoogerwerfi, known only from 800 m near Blangkejeren, Aceh (Chasen 1940; Musser and Newcomb 1983).
h) Kerinci rat Maxomys hylomyoides, known only from 2, 200 m on Mt. Kerinci (van der Zon 1979; Musser et al. 1979; Musser and Newcomb 1983; Robinson and Kloss 1918).
i) Kerinci rat Maxomys inflatus, known only from 1, 400 m on Mt. Kerinci (van der Zon 1979; Musser et al. 1979; Musser and Newcomb 1983; Robinson and Kloss 1918).
j) Kinabalu rat Rattus baluensis, known only from 2, 200 m on Mt. Kerinci and from 2, 500 m on Mt. Kinabalu, Sabah (van der Zon 1979; Musser et al. 1979; Musser and Newcomb 1983; Robinson and Kloss 1918).
Figure 9.15. Distribution of two Sumatran rat species.
After Marshall 1977
k) Mountain spiny rat Niviventer rapit, known from 1, 400 m on Mt. Kerinci and between 1, 000 m and 2, 600 m on widely separated peaks in northern and southern Thailand, Peninsular Malaysia and Sabah (Marshall 1977; Medway 1977b; Musser and New-comb 1983). The distribution of these last two species (shown in figure 9.15) is particularly interesting. More localities in Sumatra may yet be discovered.
l) Serow, or Mountain goat Capricornis sumatraensis, known from 200 m to the vegetated summits of mountains in Sumatra, Peninsular Malaysia and north to the Himalayas of India. In Sumatra this agile animal also lives on forested limestone hills with nearly vertical sides, even though the hills may surrounded by cultivation. It leaves characteristic piles of faeces in small caves or the entrances to larger caves (Jacobson 1918; Lekagul and McNeely 1977).
The mammal fauna of Mt. Benom became markedly impoverished above 1, 200 m (the lower level of Fagaceae/Lauraceae forest) (Medway 1972b). This same depauperisation has been recorded for Sumatran mountains by Robinson and Kloss (1918), and van Strien (1978), and by the CRES expedition to Mt. Kemiri. Those mammals with a wide altitudinal range tend to be large - thus, above 2, 500 m in the subalpine and upper montane zones on Mt. Kemiri, signs (tracks or faeces) were found of Sumatran rhino Dicerorhinus sumatrensis, tiger Panthera tigris, bear Helarctos malayanus, pig Sus sp., serow Capricornis sumatraensis, and barking deer Muntiacus muntjak.
Figure 9.16. Grey shrew on Mt. Kemiri.
The most surprising mountain mammal is probably the grey shrew. This is one of the smallest mammals on Sumatra, having a body length of only about 8 cm, and a cold mountaintop is the least likely habitat to find such an animal (fig. 9.16). Small mammals lose body heat very quickly (a problem exacerbated in cold regions) because the surface area: body weight ratio is very high and the heat loss must be compensated by a high energy intake in food. Shrews are almost entirely carnivorous and eat small insects, snails, earthworms, millipedes, centipedes and spiders, resources which are largely scattered, not storable and often unpredictable. Shrews therefore have to spend a great deal of time searching for food, which itself uses up energy. It is not surprising then that shrews eat about their own body weight in food each day and are active at most times of day although they rest briefly every few hours. If more than a few hours pass without a meal, a shrew will starve to death. Shrews make underground burrows in which they build a nest, and during the cold nights the temperature there is probably significantly higher than outside.
Figure 9.17. Changes in biomass of four primate species up Mt. Benom. a- white-handed gibbon, b- siamang, c- banded leaf monkey, d- dusky leaf monkey.
After Caldecott 1980
The volcano mouse experiences similar conditions but its diet is primarily vegetable, the sources of which are generally quite large (for a mouse) and storable. Mice will eat small invertebrates when they can catch them.
The only detailed study that seems to have been conducted on altitu-dinal effects on mammals in the Sunda Region is by Caldecott (1980) and concerns the altitudinal limits of monkeys and gibbons on Mt. Benom (2, 108 m). From his repeated censuses conducted from 150 m to the summit he found that the changes in biomass of the six species present varied up the mountain (fig. 9.17). He asked two questions:
• why are the species limited by altitude?
• why are the species limited at different altitudes?
The answer to the first question lies in a combination of energetics and food supply. As described on page 282, with increasing altitude the forest becomes shorter and more crooked and gnarled. There are also few large boughs. Thus, at higher altitudes, the gibbons, which normally travel by swinging below boughs and branches, would experience increasing difficulty and thus greater energy costs in moving through the tree canopy. In addition, because productivity levels are lower at higher altitudes (p. 296), the young leaves and fruit eaten by leaf monkeys and gibbons would not be available in the same quantity as in the lowlands. Also, few tree species with leaves or fruit eaten by primates in the lowlands are found in the higher zones. Although no data exists it may be that leaves of the higher forest types (on acid, nutrient-poor soils) contain high concentrations of defence compounds (pp. 175, 230).
The answer to the second question (why are species limited at different altitudes?) is a combination of food preferences, body size and morphology. The altitudinal limit for white-handed gibbons, which are primarily frugivorous, is lower than for siamang which, although still mainly frugivorous, eat a larger proportion of leaves. This represents the differences in altitude at which these two species could reliably collect the different types of food they require to support the energy expended during food collection. The larger body size of the siamang (10-20 kg compared with 5-6 kg for the white-handed gibbon) and its thicker fur would also decrease the relative energetic costs of keeping warm at higher (colder) altitudes (Caldecott 1980). The dark-handed gibbon Hylobates agilis, found south of Lake Toba, is ecologically equivalent to the white-handed gibbon (Gittins 1979; Gittins and Raemaekers 1980), and so the ecological separation from the siamang described above would doubtless also occur in southern Sumatra.
The two leaf monkeys weigh about the same (about 7 kg) (Chivers and Davies 1978), but during a study of their comparative ecology, Curtin (1980) recorded 137 food species for the banded leaf monkeys and only 87 food species for the dusky leaf monkey. The apparently greater dietary versatility of the banded leaf monkey may be one reason it is able to live at higher altitudes than the dusky leaf monkey. Another possible reason is that the morphology and muscular anatomy of the banded leaf monkey are more adapted to the use of smaller supports and travelling on the ground (both of advantage in montane forest) than that of the dusky leaf monkey which seems to depend on larger branches in the forest canopy (Fleagle 1978, 1980). Similar reasoning can be applied to the two macaque species - the long-tailed macaque Macaca fascicularis and the pig-tailed macaque Macaca nemestrina. The former is an arboreal frugivore which is generally found beside rivers below 300 m a.s.l. (Aldrich-Blake 1980) whereas the pig-tailed macaque eats a varied diet including many terrestrial arthropods, and its morphology is dog-like and thus suitable for travel on the ground (Caldecott 1980).
Finally, there are many traditions which hold that mammals climb mountains to die; Mt. Leuser in the Gajo language means 'the mountain where animals go to die'. Jacobson (1921) and van Steenis (1938) report often finding dead or dying flying squirrels on barren mountain summits and Gisius (1930) and van der Bosch (1938) report similar observations for other animals. There are no data to negate the hypothesis but it should be remembered that:
• all animals (dead or alive) are far more conspicuous on bare mountains than in forest;
• scavenging beetles, etc., are rare at high altitudes; and
• rates of microbial decay are much slower at low temperatures and so corpses will remain for a longer time.