Chapter Five
INTRODUCTION
Peat is a soil type with a very high organic content. It is defined, for the purposes of this chapter, as a soil at least 50 cm deep which, when dried and burned, will lose more than 65% of its mass (Driessen 1977), and is therefore more than 65% organic matter. Certain international agricultural-oriented criteria now define peat as a soil with more than 30% organic matter1 (Driessen and Soepraptohardjo 1974), but peatswamp forests do not develop on such soils (Anderson 1964). It is estimated that the natural area of peat soils in Sumatra was once 7.3-9.7 million ha (Andriesse 1974), or about one-quarter of all tropical peat lands (Driessen 1977).
Peat deposits can be of two forms:
• ombrogenous peat: the most common type which has its surface above the surrounding land. Plants that grow on this peat only obtain nutrients from within the peat, directly from the rain, or from the plants themselves. There are no nutrients entering the system from the mineral soil below the peat or from rain water running into it. This type of peat is generally found near the east coast behind the mangroves and is often very deep (± 20 m). The peat and its drainage water are very acid and poor in nutrients (oligotrophic), particularly calcium.
• topogenous peat: a less common type formed in topographic depressions. Plants growing on this peat extract their nutrients from mineral subsoil, river water, plant remains and rain. Topogenous peat can be found behind coastal sand bars and inland where free drainage is hindered, such as in mountain depressions and extinct craters. The peat is usually found in a relatively thin layer (± 4 m), but a 9 m core was taken from the bottom of a pond on the peaty plateau west of Siborong-borong, south of Lake Toba (p. 18). The peat and drainage water are slightly acid (pH 5.0) with relatively abundant nutrients (mesotrophic). The surface of the peat is a fibrous crust, either hard and compacted or soft, overlying a semi-liquid interior which contains large pieces of wood and other vegetable remains.
Much of the fundamental work on peat soils by Polak (1933) was conducted in Sumatra, and early vegetation studies concentrated on the potential of peatswamp forests for forestry (van Bodegom 1929; Boon 1936; Endert 1920; Rakoen 1955; Sewandono 1938). More recently, studies have been conducted on vegetation ecology and soils of peatswamp forest in Riau (Anderson 1976; Driessen 1977); Ahmad (1978) classified coastal swamps (probably topogenous) in West Sumatra, Yamada and Soekardjo (1980) studied peatswamp forest in South Sumatra, and Whitten (1982c) investigated a topogenous peatswamp on Siberut Island, West Sumatra. Teams from the Environmental Studies Centre at IPB (Bogor) have conducted various studies on peat in the Upang delta and Banyuasin area of South Sumatra (PSL-IPB 1977). In addition, Morley et al. (1973) examined small peaty lakes in central Sumatra.
A CRES team visited the peatswamp forest around Lake Pulau Besar, Bengkalis, in the Caltex oil concession in order to collect some data for this chapter.
PEATSWAMP FORMATION
Formation of Ombrogenous Peatswamp
It was shown on page 84 that mangrove forest colonises areas of recently deposited alluvium along the coast. As the coast, and therefore the mangrove forest, advances seawards, so other plant communities develop in its wake (Anderson 1964). These are no longer inundated by tides and so coastal decomposers such as crabs are rare or absent. Decomposer microorganisms are unable to thrive in the high sulphide and salt conditions of the soil and so a layer of undecomposed vegetable matter (peat) begins to form over the mangrove clay soil.
This general scenario has been confirmed by analyses of pollen from cores of peat taken from a swamp in Sarawak (Wilford 1960). At 13 m below the present surface, a mangrove clay was found overlain by remains of mangrove vegetation. Above this, pollen from the various vegetation types typical of peatswamp forest were found in order of succession. The whole successional sequence had taken 4,500 years (Wilford 1960), indicating the period over which coastlines have been advancing by deposition of alluvium rather than being flooded by the rising sea (p. 12) (fig. 5.1). Although the average rate of peat accumulation was 0.3 m/100 years, the rate during the early stages was much higher (0.475 m/100 years for 12-10 m below the present surface). More recently the rate of accumulation has fallen (0.223 m/100 years for the top 5 m) (Anderson 1964) as nutrients became progressively more scarce. This difference explains the convex shape of the peatswamp surface. As swamps replace the mangrove and as the distance to the sea increases, so rivers deposit alluvium along their banks, forming levees which are raised above the level of the original swamp. This is the cause of the saucer-shaped base of peatswamps. A cross-section of one edge of a peatswamp in Riau is shown in figure 5.2.
Figure 5.1. Hypothetical formation of a coastal peatswamp forest. A - an estuary after the final rise in sea level (about 8,000 years ago). B-H -deposition of alluvium, colonisation by mangrove forests, a - mangrove pioneers (e.g., Avicennia), b - late mangrove species (e.g., Bruguiera), c - peatswamp forest pioneers on thin peat and slightly brackish soils, d - mixed peatswamp forest on thicker peat soils above the level of adjacent rivers, e - dwarfed 'padang' forest on thick peat.
Figure 5.2. Vertical section through a peatswamp near Muara Tolam, near the River Kampar, Riau. Note that the vertical and horizontal scales are different, a - peat layer (a matted crust above, looser below); b - peat remains and clay deposited beneath mangrove forest during an earlier time; c - clay deposited beneath the sea; d - clay deposited beneath river.
After Driessen 1977
Ombrogenous peat can form over freshwater swamp if conditions are suitable (Morley 1981) but this seems to be relatively unusual.
Formation of Topogenous Peatswamp
Topogenous peatswamp is formed in a small lake or behind a barrier to drainage such as a sand ridge, which results in waterlogged conditions. The soil pH is not entirely unfavourable to decomposer microorganisms, and many of the plants are able to root in the mineral silt and clay below the peat. This means that peat accumulation is slow, great depths of peat are not formed, and that the brown humic acids characteristic of ombrogenous peat are not so pronounced.
The area of topogenous peatswamp in Sumatra is not large, and small lakes only occur in coastal western Sumatra and certain locations inland (Hansell 1981). In the classification by Ahmad (1978), topogenous peatswamp is described as shallow freshwater swamp and fringe brackish swamp. There is virtually nothing known about the vegetation or ecology of topogenous peatswamps in Sumatra.
DRAINAGE AND DRAINAGE WATER
Although the surface of an ombrogenous peatswamp is raised above the surrounding areas, rivulets form on its surface. This is probably so because the heavily compacted mass of woody peat, and the saucer-shaped mineral base of the swamp, prevent lateral drainage (Anderson 1964). Water flowing out of such a peatswamp is generally clear but appears tea-coloured by transmitted light, and opaque black by reflected light in the same way as does Coca-Cola. Such waters are known as blackwater rivers and are generally very acidic (pH 3-4.5). They contain many fewer inorganic ions than do clear, white or muddy waters - even in the same drainage basin - have low concentrations of dissolved oxygen, and high concentrations of humic acids (Janzen 1974a; Mizuno and Mori 1970; Mohr and van Baren 1954).
The low nutrient content of blackwater rivers is partially explained by the nature of the soil, for which the only source of nutrients is rain water (p. 167). The humic acids are chelating agents for inorganic ions, binding them into larger molecules, thus preventing their uptake by plants. The generally low oxygen levels are probably due to the scarcity of aquatic plants. The high acidity of the water is due to the humic acids.
Humic acids are phenolic compounds, which are one of the many groups of plant defence (or secondary2) compounds and have been found to be very important in plant-animal interactions. Phenolic compounds are generally toxic to animals and decomposers because they bind with protein (pp. 106 and 231) and are difficult to degrade. They therefore persist to a greater degree than other chemicals in plant debris. Humic acids would therefore be expected to have detrimental effects on organisms attempting to live in blackwater rivers.
It seems that the water draining into the blackwater rivers has high concentrations of phenolic compounds (and possibly other toxic compounds) because:
• the leachate from the living vegetation and decomposing litter in the peatswamp soils is exceptionally rich in phenols and other defence compounds (but see the paper by St. John and Anderson [1982]), and because
• the soil leads indirectly, and high input of phenols leads directly, to a soil and litter decomposition community (of bacteria, fungi, etc.) which is unable to break down these compounds (Janzen 1974a).
The effect of phenolic compounds on the soils, aquatic and forest communities, will be discussed further on page 177 after a description of peatswamp vegetation.
VEGETATION
Composition
Because the surface of an ombrogenous peatswamp is out of the reach of flood water and because the only nutrient input comes from the nutrient-poor rain, there is a decrease towards the centre of a swamp in the amounts of mineral nutrients in the soil, and this is particularly marked for phosphorus and potassium (Muller 1972). The top 15 cm of a peatswamp soil, where a mat of roots is formed, generally has more nutrients than deeper down. This trend of increasing infertility towards the centre of a peatswamp seems to be reflected in the vegetation by the:
• decreasing canopy height,
• decreasing total biomass per unit area,
• increasing leaf-thickness (an adaptation to poor soils) (see pp. 257 and 286), and
• decreasing average girth of certain tree species (Whitmore 1984).
Thus there is no single type of peatswamp forest. Instead, in response to the changes in nutrients, depth of peat, etc., a series of forest types develop which intergrade with each other. Anderson (1963) has described the composition of forest types from certain peatswamp areas in Sarawak in great detail but considerable differences exist in species composition of equivalent forest types (1976), and not all forest types were found in each area.
In general, however, the sequential pattern of forest types (fig. 5.3) represents a change from:
a) a high forest with an uneven canopy which is similar in general appearance to other Sumatran lowland forests, but with fewer species and stems per unit area, and a lower canopy (36-42 m); to:
b) a dense forest with a relatively even canopy, but stunted and with various xeromorphic characters; to:
c) a very dense 'pole forest' with a canopy barely 20 m high and trunks with a diameter rarely wider than 30 cm; to, in some areas:
d) an open, savanna forest (padang) with few trees over 15 m high (Anderson 1964; Whitmore 1984).
Anderson (1976) examined trees of 30 cm girth (±10 cm diameter) at breast height and over in three areas of peatswamp in Riau at Teluk Kiambang, Muara Tolam and near the Siak Kecil River.
Figure 5.3. Sections through four types of peatswamp forest, a - high-canopy mixed forest; b - lower canopy forest; c - pole forest; d - padang forest.
After Anderson 1961
Although he had recognised six forest types in Sarawak forest, he divided these areas of Sumatran forest into just two3. The three locations had markedly different species compositions (table 5.1), indicating that no generalisations should be made about the species present from this small sample. Further variation is indicated by the dwarf forest dominated by Tristania found by Polak (1933) in the central region of the Pareh Peninsula, and the dominance of somewhat stunted Tristania obovata and Ploiarium alternifolium found in peatswamp forest in the Musi delta region of South Sumatra by Endert (1920). Tristania is easily recognised by its smooth, peeling bark which varies from pale-grey to orange in colour. It is a member of the family Myrtaceae, and like its relatives Eucalyptus, Melaleuca (paperbark tree) and Eugenia (e.g., rose apple, clove), its leaves have an aromatic smell when crushed.
* = dominant species
From Anderson 1976
Yamada and Soekardjo (1980) found that in their 0.01 ha central peatswamp plot, Polyalthia glauca was the most common tree. The topogenous peatswamp examined on Siberut Island was dominated by Stemonurus secundiflorus (Icacinaceae) and Radermachera gigantea (Bignoniaceae), which together made up nearly 50% of the trees (Whitten 1982c).
Palms are not common in peatswamp forest but a few species are more or less confined to this ecosystem. Both Endert (1920) and Yamada and Soekardjo (1980) mention Salacca conferta, a close relative of the edible salak Salacca zallaca. Salak is a stemless palm with long (6 m) leaves armed along the midrib with fans of long white spines (Whitmore 1977). Its habit of growing in dense clumps makes salak one of the most formidable natural barriers in Sumatran forest. The tall serdang palms Livistona hasseltii of Sumatran peatswamp forest often emerge above the forest canopy (Drans-field 1974), and their crowns of fan-shaped leaves can be seen clearly from the air. A total of 22 palm species have been found in the peatswamp Berbak Reserve in Jambi, making it the most palm-rich peatswamp forest yet known (Dransfield 1974). The forest around Lake Pulau Besar examined by a CRES team had large numbers of the bright-red sealing-wax palm Cyrtostachys lakka.
The only epiphytes found in the pole forest and peat padang are those with some means of obtaining mineral nutrients other than just from mineral uptake from their substrate and the rain. Almost all these epiphytes provide shelter for small, non-aggressive ants Iriodomyrmex, and in return receive nutrients in the form of discarded food, dead ant remains and ant waste products (Huxley 1978; Janzen 1974b).
Muller (1972) investigated fossil pollen in coal (fossilised peat), and marine and coastal sediments in Sarawak. He found that some of the tree genera present in peatswamps there today have been present since the Oligocene (26 million years ago) and that most have been present since at least the mid-Miocene (± 18 million years ago). The striking similarities between the ancient peatswamp and those of today indicates that the climatic changes of the Pleistocene did not influence the composition of the peatswamp communities to any great degree.
Structure
Structural and other data taken from peatswamps by Anderson are shown in table 5.2. For some variables, such as number of trees, there is considerable variation between plots in the same forest type and so the averages should be treated with caution. The number of species per plot is very similar between the forest types. This is not the pattern generally found in Kalimantan (Anderson 1964, 1976), where pole forest is much poorer in species and may indicate that the soils of the Sumatran pole forests are not as nutrient-poor as those in Kalimantan. Other areas of Sumatran pole forest in peatswamp may well have a lower species diversity.
The number of trees, as expected, increases from mixed swamp forest to pole forest, but the average girth decreases. The total basal areas4 (of the trees measured) were generally higher in mixed swamp forest than in pole forest, as was the average basal area (total basal area divided by number of trees). The single 0.01 ha plot investigated by Yamada and Soekardjo (1980) had a total basal area of 1.56 m2 (3.1 m2/0.2 ha), one-third of which was contributed by a single large Alstonia angustiloba.
ECOLOGICAL CONSEQUENCES OF LOW NUTRIENT LEVELS
The low nutrient levels of the soils in mature peatswamp forest (p. 171) almost certainly limit primary productivity. Although studies have yet to be conducted to investigate this, the low total basal area of pole forest (table 5.2) and the doubtless even lower total basal area of stunted peat padang, indicate that their primary productivity is low. In such a habitat, but where climatic conditions are favourable for animal life, plants would be expected to defend their leaves and other edible parts as fiercely as possible against potential herbivores. The three main reasons for this are:
From Anderson 1976
• The loss of a leaf or other part to a herbivore would be proportionately more serious for a plant growing in peatswamp than for a plant growing in a more fertile habitat. This is because of the greater 'cost' of replacing the eaten or damaged leaf part in infertile habitats. Thus it would be expected that proportionately more of the energy and nutrient resources of a peatswamp forest plant were used for defence.
• In a habitat with relatively low productivity, the plant should produce better-protected leaves in order to increase the life of each leaf. Leaves have a finite life, usually related to the amount of damage they sustain, and so it is advantageous to a plant to ensure that this damage occurs as slowly as possible. It is possible that the relatively thick cuticle may reduce the leaching of nutrients from living leaves.
• In habitats with low productivity and containing vegetation whose seeding strategies involve anti-predator5 measures such as extreme toxicity or gregarious fruiting (p. 221), there is usually a low species richness of trees and noticeable grouping of species (Janzen 1974a).
The trees over 30 cm girth at breast height found by Anderson (1961, 1963) in Sarawak peatswamp belong to families known to be particularly rich in defence compounds such as latex, essential oils, resins, tannins, and other phenolic or terpenoid compounds (table 5.3). This indicates that most leaf-eating animals from lowland forest on good soils would find it difficult to live in peatswamp forest.
As stated above (p. 172), many of the plants growing in the inner part of peatswamp forests have thick leaves. When these are crushed there is usually a resinous, acrid or aromatic odour or taste. This is most atypical of tropical vegetation but common for plants growing in poor conditions (Janzen 1974a), particularly where nitrogen and phosphorus are in short supply (Beadle 1966). Thick leaves are probably not only a defence against harsh environmental conditions but also a means of deterring herbivores (p. 300).
From Anderson 1963
The lack of ant-free epiphytes mentioned on page 174 has been interpreted by Janzen (1974a) as a consequence of low productivity due to a low rate of inorganic ion input to tree crowns from bird faeces, insects, and falling leaves and twigs.
ECOLOGICAL CONSEQUENCES OF HIGH LEVELS OF SECONDARY COMPOUNDS
Microorganisms
Some litter microorganisms can degrade phenolic compounds in soil and water (Alexander 1964; McConnell 1968) but they do so slowly, requiring a well-oxygenated substrate and accessory energy sources (Burges 1965). It is not surprising that microorganisms have difficulty in degrading phenolic compounds because such chemical defences are the result of selection specifically favouring compounds which are difficult for microorganisms and higher animals to digest. As a result, a leaf from a peatswamp forest will probably lie on the ground for many weeks, unattacked by decomposers, until most of the defence compounds have been leached out by rain water. It is not unexpected, then, to find that phenolic compounds have negative effects on mycorrhizae, other fungi, bacteria, roots, vertebrates, insects and worms (for references see the paper by Janzen [1974a]).
The effects of these defence compounds on microorganisms will be enhanced in the pole forest and peat padang soil by:
• the initial low nutrient quality of the soil (p. 171) making the microorganisms even more dependent on the litter itself for nutrients;
• the high acidity of the soil;
• the low nutrient quality of the litter because of selection favouring withdrawal of as many nutrients as possible from the leaves before they drop, especially on low-nutrient soils;
• a low rate of litter input because of low primary productivity.
Aquatic Animals
Data from Peninsular Malaysia suggest that blackwater rivers have an impoverished and distinct fauna. Malayan blackwater rivers have only about 10% of the fish fauna of other rivers (Johnson 1967a). Cladocera (water fleas), annelid worms, rotifers, nematodes and protozoans are rare; algae are generally rare except for a few species which are locally abundant, and macrophytes may be absent (Johnson 1968). Of the 15 fish species found in blackwater rivers, nine were air-breathers or lived near the surface (Johnson 1967b) and almost all the insects were air-breathers (Johnson 1967a). This may be due solely to the difficulties of existing at low oxygen levels but it could also be that the protein-binding defence compounds, such as tannins, deleteriously affect animals' gills. Thus the aquatic larvae of biting insects, such as mosquitoes, are rare in peatswamp forest. The productivity of animals and plants in these rivers is low and the biomass of fish may be only 0.5 g/m2. Bishop estimated the fish biomass of a 'normal' small Malayan river to be about 18 g/m2 (Bishop 1973).
What factor is responsible for this distinct aquatic ecosystem? Low nutrient levels have been suggested, but Johnson (1967b) has shown that calcium levels (for example) in blackwater rivers are generally higher than in other types of river. The low pH cannot be the only factor because those blackwater rivers in Malaya which later flow over limestone have a pH over 6.0; these streams have snails, more insects and fish, but the fish are restricted to blackwater rivers, rarely occurring in other rivers with a 'normal' pH (Johnson 1968). The blackness of the water and hence low light penetration might be thought to be an important factor but this would not explain why there are so few animals and plants at the water surface, nor why such animals and plants exist in very muddy water where light penetration is even less. So it would seem that the main factor influencing this ecosystem is the high level of phenolic compounds.
Terrestrial Animals
Peatswamp forests do not support an abundance of terrestrial wildlife. Merton (1962) said of an inland peatswamp in Peninsular Malaysia "it was the absence of animals rather than their presence which was so striking". Janzen (1974a) described an animal collecting trip in peatswamp forest in Sarawak as "generally a waste of time". An investigation of primate densities in peatswamps of Peninsular Malaysia found that away from rivers, in the species-poor forests, few primate species existed and those species present lived at densities of less than three groups per km2 (Marsh 1981). Typical figures for lowland forest would be 10 groups per km2 (Marsh and Wilson 1981). Similarly, in a study of one group of Mentawai gibbons on Siberut Island, it was found that these animals consistently under-used the area of inland peatswamp in their home range in comparison with the other forest types available to them (Whitten 1982c,d). This behaviour is understandable because primates depend on tree and vine fruit for most of their diet and the generally low productivity of peatswamp forests would result in fruit being available only rarely.