Chapter Six
INTRODUCTION
Freshwater-swamp forest and related vegetation grow on soils where there is occasional inundation of mineral-rich fresh water of pH 6 and above and where the water level fluctuates so that periodic drying of the soil surface occurs. The floods may originate from rain or from river water backing up in response to high tides. The major physical differences between this and peatswamp forest are the lack of deep peat (a few centimetres thickness is sometimes found but this seems to have little effect on tree species composition), and the sources of water being rivers and rain water (Whitmore 1984). Although freshwater-swamp forest normally forms on riverine alluvium, it can also be expected, under natural conditions, to form on alluvium deposited by larger lakes where wave action prevents the formation of peat, such as in 'Lake' Bento, Kerinci.
The high agricultural potential of their soils has meant that the freshwater-swamp forests of Sumatra have suffered considerably from human disturbance. In 1982 only 22% remained of the original area of freshwater-swamp forest in Sumatra and very little of this was in an undisturbed state. The area now is even less.
SOILS
Freshwater-swamp forest grows on recent freshwater alluvial soils. Such soils are usually more fertile than the soils on the surrounding slopes and, after draining, have obvious potential for agriculture, but they are less fertile than some of the soils on recent marine alluvium or volcanic ash (Burnham 1975). Recent alluvium from rivers and lakes has had relatively little time for soil formation, and the absence of clear soil horizons is one of its characteristics. It is further characterized by its low position, close to the water table. The actual content of the soil varies greatly with the nature of the parent material but it is usually composed of relatively fine particles, the last to be deposited by a flooding river. Recent, waterlogged alluvial soils are usually grey (or mottled-grey and orange where the soil occasionally dries) as a result of 'gleying'. The main processes of gleying are the reduction of iron compounds to their ferrous forms and their partial re-oxidation and precipitation (Young 1976). The grey colour of these soils seems to arise from the lack of mixing of the often peaty soil surface with the lower layers. This is because soil animals that would normally take organic matter into the soil (such as termites and earthworms) are not tolerant of the frequently anaerobic conditions caused by waterlogging such as occur in swamp soils. Gley soils are amongst the most poorly understood yet agriculturally most important tropical soils (Young 1976).
VEGETATION
Composition and Structure
The vegetation of freshwater swamps varies considerably in response to the wide variation in its soils. In some areas grassy marshes may be the natural vegetation, in others some form of palm- or pandan-dominated forest, and in yet others forest which is similar in structure and species composition to lowland rainforest. Under certain conditions, long, winding tree buttresses, stilt roots and pneumatophores are common. Few plant species are restricted to the freshwater-swamp ecosystem but the tree species found there tend to be gregarious and to form species-poor associations.
Very little information appears to be available on the floristics of freshwater-swamp forests in Sumatra (Kartawinata 1974) but much can be learned from the highly readable account by Corner (1978) of freshwater-swamp forests in South Johore and Singapore. In an enumeration of various forest types on Siberut Island, it was found that freshwater-swamp forest was more similar in species composition to neighbouring flat and non-swampy areas of lowland forest than it was to nearby peatswamp forest (Whitten 1982c). The total basal area, median basal area and height were not exceptional in comparison with other forest types but the trees tended to be smaller. The area examined was dominated by the extremely gregarious Alangium ridleyi, a medium-sized tree with sweet fruit, which accounted for 20% of the trees. Lake Bento, which lies south of Mt. Tujuh and northwest of Lake Kerinci, is surrounded by the highest (in elevation) forest-clad marsh in West Malesia (FAO/de Wulf et al. 1981).
Unfortunately, the primary freshwater-swamp forest in Way Kambas Reserve, set up to protect the swamp ecosystems, has been extensively logged (FAO/Wind et al. 1979). Fragments remain elsewhere but it is likely that it will never now be possible to know exactly how such forests functioned in Sumatra.
Adaptations to Floods
Trees of freshwater-swamp forest often have to endure prolonged periods of flooding. Inundation with water will flood air spaces in the soil which, in a normal, moist topsoil, can account for about 60% of the soil volume (Bridges 1970). With these spaces filled, the soil soon becomes anaerobic and plants have difficulty in obtaining sufficient oxygen through their roots.
Very little is known about trees' tolerance of, and responses to, flooding. Where logging roads have dammed small rivers in forests, it is clear from the many surrounding tree skeletons that flood tolerance is not inherent to all trees. Information that does exist concerning the effects of flooding in tropical forests comes from South America. The flood tolerance of the swamp tupelo Nyssa sylvalica, for instance, depended on its capacity to accelerate anaerobic respiration and to oxidise the 'rhizosphere' (the immediate surroundings of the roots) (Hook et al. 1971; Keeley 1978, 1979; Keeley and Franz 1979). Another tree, Sebastiana klotzchyana, commonly found in South American freshwater-swamp forests, was found to continue growing during flood conditions, whereas growth of shoots from young trees of other species found in dry areas was inhibited during artificial flood conditions. After one month of flooded conditions the roots of S. klotzchyana showed a significant decrease in uptake of oxygen (indicating they had partially 'switched-over' to anaerobic respiration), whereas roots of trees from dry areas showed a significant increase (as though they were making up for a period of reduced respiration). Rates of anaerobic respiration and the levels of metabolic products such as ethanol, malate, lactate and succlanate, varied between trees from flood areas and dry areas, and showed that a range of responses are to be found even among flood-tolerant trees (Joly and Crawford 1982).
If the roots of a plant are under water, oxygen must reach them from the exposed parts of the plant. Some trees in freshwater swamps have strongly lenticellate bark; the bark has large lenticels or holes through which diffusion of gases can occur. The submerged roots and pneu-matophores of aquatic or semi-aquatic plants are usually very aerenchymatous (they have large channels for air movement). To test whether a plant has aerenchymatous tissue, blow down one end of a cut section of root and watch for bubbles at the other end while it is held under water (Corner 1978). Bubbles will be seen if the root is aerenchymatous. It should be emphasised that since the above characters are not common to all swamp species, there is clearly more than one way to adapt to anaerobic soil conditions.
Figure 6.1. Five types of pneumatophores found in freshwater-swamp forest.
After Corner 1978
In a freshwater-swamp forest in Johore, Peninsular Malaysia, Corner (1978) found 36 species (in 19 families) with pneumatophores and these pneumatophores were of five forms (fig. 6.1):
• erect conical pegs (similar to Sonneratia, p. 77);
• erect plank buttresses degenerating into squat knee-roots where there was shallow tidal flooding (similar to Bruguiera, p. 77);
• slender loop-roots formed by erect pneumatophores bending down into the soil;
• thick loop roots formed in the same way as the previous form;
• roots formed by a descending root growing laterally from a slanting ascending root. The top section may later die.
FAUNA
The occasional high floods caused by heavy rain can create rather peculiar conditions. Corner (1978) described graphically the state of the animal life in freshwater-swamp forest during a 6 m flood of the Sedili Besar River in Johore, Peninsular Malaysia.
During the three days at Danau we paddled in a canoe through the flooded forest. The force of the flood was lost among the trees... Whenever we touched leaf, twig, trunk of floating log, showers of insects tumbled into the canoe. Everything that could, had climbed above the water. Ants ran over everything that could, had climbed above the water, even scorpions, centipedes and frogs... Lizards clung to the trunks; earthworms wriggled in the water... I realised the importance of the hillocks in and around the swamp forest to animal life, for anything that could escape the flood must have fled there. We met no corpses. Pig, deer, tapir, rat, porcupine, leopard, tiger, monitor lizard and snakes must have congregated on those hillocks in disquieting proximity.
The fauna of freshwater-swamp forests is both much more diverse and more dense than the fauna of peatswamp forests, and is similar to that in lowland dry-land forests. Nothing is known in detail about the species inhabiting Sumatran (or even other Southeast Asian) freshwater swamps but Wind et al. (1979) report siamang, dark-handed gibbons, monkeys, rusa, barking deer, pigs, elephant, tapir and tiger from Way Kambas Reserve, Lampung. The areas of swamp grassland also appear to be utilised by most of these species.
The swamp grassland of the southern part of Way Kambas is used by herons, egrets (see fig. 2.30), terns, bitterns, pond-herons, whistling ducks, pygmy geese, lesser adjudants, milky storks (fig. 6.2) and occasionally the rare white-winged wood duck (Wind et al. 1979). This species (fig. 6.3) is considered 'endangered' by the International Union for the Conservation of Nature and Natural Resources (IUCN) and has been reported in Sumatra only from the eastern side of Lampung in and around freshwater-swamp and lowland forest (Holmes 1976, 1977a). It may possibly live in peatswamp areas but no information is available. Its range used to include the swampy areas adjacent to lowland forest from Java to Burma but it seems to be extinct from all that area except for parts of Assam (northeast India), possibly northern Burma, and southern Sumatra. In the northern parts of its range it appears to require dense forest, but Holmes (1977a) found the ducks in the much more open and disturbed forest of Lampung. This ability to adapt to disturbed areas encourages some optimism regarding the security of its future.
During the wetter seasons these ducks can live almost entirely within forest, but when the slow-running streams they frequent begin to dry up they venture into areas of swamp or even rice fields. They fly to these areas just before dusk and, if not disturbed, may remain there until two or three hours after dawn. White-winged wood ducks are omnivorous, eating floating aquatic plants, seeds, insects, worms, molluscs, frogs and even small snakes and fish (MacKenzie and Kear 1976). They are usually seen in pairs but when the young are old enough to fly, family groups may be seen. One of the best ways to locate these birds is to listen for the evening light call which is described by MacKenzie and Kear (1976) as a "prolonged, vibrant, wailing honk sometimes breaking to a nasal whistle at the end". This call is never, it seems, given by birds flying alone. White-winged wood ducks nest in the holes of swamp trees, particularly 'rengas' (probably Melanorrhoea), and their breeding season lasts from December to March or April (Holmes 1977a).
Figure 6.2. Birds commonly found in grassy swamp a - tern Sterna hirundo, b - Java tree duck Dendrocygna javanica, c - Java heron Ardeola speciosa, d - Yellow bittern Ixobrychus sinensis, e - pygmy goose Nettapus coromandelianus, f - long-billed stork Leptoptilus javanicus, g - milky stork Ibis cinereus.
After King et al. 1975
Figure 6.3. White-winged wood duck Cairina scutulata.
After MacKenzie and Kear 1976
The freshwater-swamp areas used to have large populations of the estu-arine crocodile Crocodylus porosus and of the smaller false ghavial Tomistomus schlegeli. The false ghavial, which never exceeds 5 m in length, resembles the true ghavials in having a snout which is much longer and more slender than the snouts of most crocodiles. The crocodiles are the last remaining members of the Archosaurs, the group of reptiles including dinosaurs that dominated the earth during the Mesozoic. They are the largest inhabitants of freshwater swamp and their movements inhibit the encroachment of aquatic plants into waterways. In areas with a prolonged dry season they maintain residual water holes which serve as restocking reservoirs for smaller aquatic organisms which would otherwise perish. Crocodiles enrich the nutrient content of the water by converting prey into waterborne faecal particles which are food for a host of invertebrates and fish. Crocodiles have been mercilessly hunted, partly through fear (unjustified in the case of the false ghavial which rarely eats anything but fish) and unbridled greed for their skins. Luckily, the remaining small populations of estuarine crocodiles are now protected throughout Indonesia.
Like all animals that can live in both seawater and fresh water, the estuarine crocodile has physiological adaptations which enable it to control the concentration of its plasma. In a marine environment, where fresh water is scarce, the crocodile passes very concentrated urine, reabsorbing water in the kidneys, and very dry faeces by reabsorbing water in its intestines. These two measures enable it to conserve water. Calcium, magnesium, potassium, ammonium, uric acid, and bicarbonates are excreted in the urine. Sodium chloride, however, does not pass through into the urine (Grigg 1981), but is excreted through the crocodile's external nasal glands near the nostrils and lachrymal glands in the corners of the eyes.
When a female crocodile is ready to lay eggs, she seeks a shady location on land where she builds up a dome-shaped nest of leaves or tall grass (Greer 1971). Up to 50 or more eggs are laid in the middle of this nest, where they remain damp and are protected from direct sunlight. The heat generated by the decomposition of vegetable matter may help the incubation but if the mother senses that the eggs are becoming too hot she will spray urine over the nest to cool it down.
Just before the eggs hatch, the young crocodiles make high-pitched croaks which are audible even outside the nest. The mother responds by scratching away the now-hardened surface of nest material and as the young crocodiles wrestle their way out of their shells, she (and sometimes the father too) picks them up gently in her mouth and carries them away to a secluded 'nursery' area in a swampy bank. They stay there for a month or two, catching large insects and small vertebrates such as fish and frogs, while their parents guard them closely.
Figure 6.4. Large estuarine crocodiles are now extremely rare in Sumatra, but reports of smaller individuals in fresh- and peatswamp areas persist.
A.J. Whitten
Perhaps the most fascinating aspect of crocodile nesting behaviour is that the sex of the young animals is determined by the temperature of incubation. This has been known for various groups of reptiles (Vogt and Bull 1982) and has recently been determined for the Mississippi alligator of northern America (Ferguson and Joanen 1982), although experiments have yet to be conducted on Asian crocodiles. Field and laboratory experiments have shown that alligator sex is fully determined and irreversible by the time of hatching and that incubation temperatures of up to and including 30°C produced all females, whereas 34°C and above produced all males. Since nests on exposed levees or other dry areas receive more sunlight and are hotter than those nearer wet, shaded swamp, the sex ratios of hatchlings will be different between these habitats (Ferguson and Joanen 1982). It is likely that the sexes of Sumatran crocodiles are also determined by incubation temperature, and this is important for management of the species. Felling of swamp trees, particularly those near riverbanks, will reduce the amount of shading and may increase the average ambient temperature of any area. This in turn may increase the number of male crocodiles. However, the efficiency of nest temperature regulation by the female is unknown as are the social constraints to any alterations in sex-ratio.