Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
2. The living past: Time state of the tropical rain forest
Abstract
Historical
background
The modern
forest
Man and
the forest
The
forest strikes back
References
D. J. Mobberley
Early European ideas on tropical vegetation were based on temperate preconceptions and on initial contact with secondary and pantropical vegetation types. Modern angiosperm forests in the context of fossil tropical forest are a recent phenomenon. They are maintained as a mosaic of regenerating gaps affected by environmental factors but also historical factors such as continental drift, vulcanism, and animal extinction and by slow rates of colonization by trees as well as the long lifespans of the latter, which may thus outlive their ecological associations and exist as "living fossils." Man's early effect was to modify the forest framework in terms of gaps and by consciously and unconsciously selecting certain trees. Modern destruction has increased this modification and also the number of anachronisms. In response, many trees have been found to be far more flexible than was previously supposed and to survive under the new regime, though the "new" forests differ markedly from those less affected by man.
Exploration
The first that the Western world knew of tropical vegetation was when Alexander the Great, having defeated Darius in 331 B.C., pushed on over the Khyber Pass to the Punjab: the Indus became the eastern boundary of his extended Asiatic empire (Steam 1977). His army saw mangrove swamps (which upset conventional views on trees), jakfruit, mangoes, bananas, cotton, and banyans—which upset everybody's views on what roots are supposed to do. These findings were incorporated in Theophrastus's Inquiry into Plants, which was translated into Latin by Pliny at the beginning of the Christian era: through this, it was to become part of the corpus of plant knowledge, to be copied, misconstrued, bowdlerized, and generally misrepresented until the Renaissance. Firsthand knowledge was not available again until the great voyages of the Portuguese and Spanish, the Dutch and, later, the British and French.
At first only the plants and animals of settlements (the plants largely pantropical weeds) and the seashore, a remarkably uniform vegetation throughout the tropics, were collected. Indeed, by the eighteenth century, Linnaeus, that great cataloguer of the living world, felt that the tropics had few new things to add to the 6,000 or so species of plants he had recorded in his Species Plantarum of 1753. Writing this great work, a superb synthesis of what came before, led to his having a nervous breakdown. It is difficult to gauge what would have happened had he realized that he had recorded only some 2 or 3 per cent of the species now known.
Of course, the early explorers rarely penetrated the forest and, when they did, they applied their temperate knowledge to what they saw: cauliflory was clearly parasitism to them and is embedded in nomenclature, in that the plant Linnaeus's pupil, Osbeck, called Melia parastica is the common cauliflorous Dysoxylum parasiticum (Meliaceae) of the Malay Archipelago (Mabberley 1983, 2). Around the settlements were the tangles of secondary forest known in India as jangal, to become the jungle of the colonists and to give the bad reputation of tropical rain forest to the layman, for whom it will always be jungle. The earliest specimens of tropical plants are preserved in the Sloane herbarium in the Department of Botany, British Museum (Natural History) and in the Sherard herbarium in the Department of Botany in the University of Oxford: the oldest snippets are some 300 years old. It is well to remember that when these scraps were being gathered some of the mature trees now being felled in Borneo and the Malay Peninsula were already well over half-way through their lives, while some living brazil-nut trees are thought to have germinated in South America within a few decades of the death of Pliny or, at least, the abandoning of Britain by the Roman legions.
Geological History
Let us go back further. The first tropical rain forests were dominated by woody ferns and club-mosses: those of the Carboniferous (c. 300 million years ago), growing in the tropical belt, are preserved in the coal measures of England. The plants were of varying constructions, some resembling modern angiosperm trees (Mabberley 1983, 15). Some had holes in their leaves, suggesting attack by herbivorous animals; they were wind dispersed spore-trees and their survivors are the tree-ferns. Such forests were followed by the first seed-forests, various kinds of seed-fern—some of which have led to the angiosperms—and the earliest gymnosperms, of which the tropical survivors include the cycads and the majestic araucarias of the west Pacific: these are still the tallest trees in the Old World tropics. We know rather little about the early seed-forests or how they worked, about the undergrowth or, indeed, the hydrological cycle. However, they were in place for a long time— from the Permian, which extinguished the spore-forests in its desiccation, until the rise of the angiosperms in the Cretaceous about 150 million years ago.
Only in the last 30 to 50 million years, a tiny postscript to geological time, has there been a flora at all characteristic of what we think of as typical tropical rain forest. However, these forests have a feature quite new. One of the great things the early angiosperms are thought to have been able to do (Ashton 1977) was to exploit the gapphase, that is, the opportunity for colonization offered by the collapse of some aged gymnosperm and the consequent puncturing of the forest canopy. No known gymnosperm can do this so well. Indeed, the whole of modern tropical rain forest ecology hinges on gaps: their frequency in space and time and their filling.
Dynamics and Diversity
Largely through the work of Aubreville, in francophone Africa, and of E. W. Jones at Oxford and A. S. Watt at Cambridge, both writing on temperate vegetation, built on by T. C. Whitmore and G. S. Hartshorn, working in Asia and the Neotropics respectively, we now have some idea of the dynamics of tropical rain forest. This, in turn, gives us some idea of how to interpret its structure. It is simplest to visualize the forest as a patchwork quilt, each patch representing a gap caused by the collapse of a tree or other local disaster, at a particular point in a secondary succession of refilling. The "climax" forest is thus an amalgam of the careers of individual trees. And so some gaps will be a chaos of fallen limbs with high insolation and great diurnal ftuctuations in temperature; others will have rapid-growing, colonist or pioneer trees with stings, barbs and thorns, and living-in ants; yet later phases will have a consolidated canopy, possibly of just one enormous tree. Not that that tree will be simple at all, for as E. J. H. Corner (1964, 116) has written:
On its canopy birds and butterflies sip nectar. On its branches orchids, aroids and parasitic mistletoes offer flowers to other birds and insects. Among them ferns creep, lichens encrust, and centipedes and scorpions lurk. In the rubble that falls among the epiphytic roots and stems, ants build nests and even earthworms and snails find homes. There is a minute munching of caterpillars and the silent sucking of plant bugs. On any of these things, plant or animal, a fungus may be growing. Through the branches spread spiders" webs. Frogs wait for insects, and a snake glides. There are nests of birds, bees and wasps. Along a limb pass wary monkeys, a halting squirrel, or a bear in search of honey; the shadow of an eagle startles them. Through dead snags fungus and beetle have attacked the wood. There are fungus brackets nibbled round the edge and bored by other beetles. A wood pecker taps. In a hole a hornbill broods. Where the main branches diverge, a strangling fig finds grip, a bushy epiphyte has temporary root, and hidden sleeps a leopard. In deeper shade black termites have built earthy turrets and smothered the tips of a young creeper. Hanging from the limbs are cables of lianes which have hoisted themselves through the undergrowth and are suspended by their grapnels. On their swinging stems grows an epiphytic ginger whose red seeds a bird is pecking. Where rain trickles down the trunk filmy ferns, mosses and slender green algae maintain their delicate lives. Round the base are fragments of bark and coils of old lianes, on which other ferns are growing. Between the buttress-roots a tortoise is eating toadstools. An elephant has rubbed the bark and, in its deepened footmarks tadpoles, mosquito larvae and threadworms swim. Pigs squeal and drum in search of fallen fruit, seeds and truffles. In the humus and undersoil, insects, fungi and bacteria and all sorts of animalculae participate with the tree roots in decomposing everything that dies.
This tree is not alone: in a single hectare there may be 200 different species of tree, represented by individuals with boles greater than 10 centimetres in diameter. They are of all sizes and shapes, from the unbranched palms in the lower reaches to the tall legumes with feathery foliage, open and dense crowns, upward-thrusting or weeping branches, simple leaves or pinnate ones, branches behaving like leaves, leaves behaving like branches, flowers on the twigs, branches, or bole, pollinated by bees, beetles, birds, or bats, even marsupials, and dispersed by wind, bats or birds and mammals, including man. With such diversity and the nature of forest dynamics, it is largely unpredictable when or where a particular species will be found.
Historical Factors
There are, of course, well-known preferences for sites, but even the most extreme represent ecological rather than physiological optima: mangrove trees associated with high levels of salinity will grow perfectly well inland in the absence of competition from other trees. Nonetheless, a major influence on why a particular species grows where it does is its history, not only the local turbulent dynamics but major geological and meteorological changes. Schuster (1972) has attributed the species richness of the Malesian region to the collision of the Australasian and Asiatic floras, evolved in isolation, as a result of continential drift. Only four million years ago did the isthmus of Panama become consolidated such that the equids, mastodons, tapirs, and llamas could move south, the sloths and armadillos move north. Plants are somewhat less restrained by water barriers, such that Wallace's Line through the Malay Archipelago, with placental mammals to the west and marsupials to the east, is less significant for plants, in that the lowland rain forest of New Guinea has much in common with that further west, for example. This raises an interesting question, for the plants are now in contact with a different fauna—they must be rather flexible in their interactions with animals.
The Far Eastern tropics seem to have had a history less marked by desiccation than have the tropics of Africa and America. The poverty of Africa in certain groups, notably palms, has been attributed to extinctions during dry periods. The femurs that lived there are gone, as are the iguanas: both survive in Madagascar, where the great grazing ungulates and pachyderms have not penetrated. The effect on the vegetation of Africa of what have been called the bulldozer herbivores (Kortlandt 1984), in terms of their foraging, debarking, uprooting, digging, and trampling, has been, on the other hand, to promote the mosaic nature of the forest. When the bulldozer herbivores are gone, so is much of the diversity. Similar could be said of the old coppice-with-standards system of woodland management in northern Europe eastwards to central Asia, where a deliberate cyclical disturbance form of management has been practiced for centuries. The cessation of this practice leads to rather boring secondary woodlands, which trend conservationists are trying to reverse, by reintroducing the old management regime, to promote species diversity of all kinds of animals as well as plants. Even that speck of tropical rain forest—well known to all American tropical ecologists — Barro Colorado Island in the Panama Canal, has become less diverse since the small-scale farming formerly practiced there has been discontinued.
What evidence we do have of the Far Eastern tropics does suggest, however, that in Borneo at least, the climate was formerly more seasonal (Mabberley 1983, 19): it is argued that in general there was 30% less precipitation per annum than there is now. Moreover, it is alleged that we are now experiencing an exceptionally high rainfall regime. Furthermore, the incidence of cyclones in the tropical belt has been steadily increasing in recent years. Nevertheless, it is the absence of marked seasonality, rather than overall annual precipitation, that governs the distribution of tropical rain forest. An increase in the seasonality is marked by an increased gregariousness of trees, a lower canopy, an increase in numbers of deciduous trees, fewer large lianes and, indeed, a greater susceptibility to destruction by fire. In the most unseasonal climates, though, there are seasons of flushing of tree foliage, of flowering, and of fruiting. Pollinators and disperal agents respond accordingly. In the mast years of dipterocarps in the Far Eastern tropics, other unrelated species flower and fruit in unison. We have a few ideas on how this is synchronized and know something of the rather general pollination mechanisms involved, but where do the dispersal agents come from and where do they go to?
Major geological upsets, such as vulcanism, push a succession back to the start. Perhaps the most celebrated is Krakatoa, lying between Sumatra and Java. It erupted just a century ago: within 50 years it had 271 plant species. Some of these have now disappeared again, but the overall total is increasing, though the forests there have reached a kind of truncated stage, because the 40 km from the neighbouring islands is too much for some of the later successional trees to cross. Even on dry land the colonization by such later successional tree species is painfully slow: dipterocarps invade al a rate of 2 m per annum. This has an important lesson for those who study extremely isolated island groups being colonized afresh, Hawaii for example, where only groups of plants with small or long lived propagules can colonize and diversify.
What I have tried to stress so far is that a good deal of the ecology of tropical rain forest is rooted in history, from the vicissitudes of continental drift and palaeoclimate to the history of particular gaps. But trees remain in place for a very long time. What local fluctuations in climate have there been in the career of a brazilnut growing up since the time of St. Augustine? And these trees have a considerable effect themselves, not only in shading and providing, as it were, niches for other organisms but in affecting the edaphic conditions: the brazil-nut itself may produce toxic litter while other plants produce a very acid humus, supporting a very particular set of understorey plants, and around the roots of dipterocarps, the soil becomes bleached sand.
Living Fossils
One of the dipterocarps, Dipterocarpus crinilus, is a common tree of the rain forests of the Malay Peninsula (Ng 1983). It produces fruits regularly and copiously, yet seedlings have not been found. It seems that the seeds are sterile and that the tree is dying out in Malaya. What has happened? We cannot even guess, but must conclude that this widespread tree was happier in former times, that it is in fact a living fossil: a monument to times past.
To turn to the New World, around 10,000 years ago over 15 genera of Central American herbivores became extinct: these included the gomphotheres (mastodon-like proboscidians), ground sloths, and equids (Janzen and Martin 1982). Like the dipterocarp, the trees which such animals dispersed and still survive may be seen as vegetable anachronisms. Indeed, there may be other features of plants that reflect past adaptations. On the islands of Hawaii are extraordinary tree-lobelioids with thorns and toxins, apparently noxious to the types of herbivores not found on those islands but possibly to those encountered by their ancestors on the mainland (Mabberley 1975).
A dispersal anachronism, which is surely the most spectacular, is the double coconut, Lodoicea maldivica, long known only from the fruits washed up in the Indian Ocean. This "coco-de-mer," rumoured to have been cast into the sea by the Devil to mislead the amorous intentions of mariners, was, with the discovery of the Seychelles, found to be the fruit of a palm now restricted to a few valleys on one island in that group. Savage and Ashton (1983) have shown that the trees, of which there are males and females, live to be about 350 years old. The seed is the largest of any plant, living or fossil: it may weigh 20 kg and a female tree may carry as much as 500 kg of fruits in the crown. The tree has one of the largest leaves known. It is a giant in botany but is very restricted in its distribution, for, remarkably, and quite unlike the coconut, it cannot be dispersed by sea, as seawater kills the seeds. How did this plant reach the Seychelles? Perhaps it is easier to argue that it has been there all the time, for, unlike most oceanic islands, the Seychelles include ancient granites, thought to have been part of the old Gondwanaland. If this is so, then this tree may have been well adapted to the conditions in Gondwanaland but, now, it is adapted, if the conceit be allowed, to becoming extinct. Biologists, while accepting the fossil record and mechanisms of speciation, seem remarkably unwilling to accept that extinction must take place, even without the intervention of man, though this must be a logical consequence of a belief in evolution.
Origins of Agriculture
But what of man? We are used to the idea that modern man got going outside the forest and, indeed, the grazing typical of animal husbandry as well as the plants and animals typically domesticated are not of the forest. Pre-agricultural man, if we could define him as man, has left little trace, though Corner (1962) has argued that traces of those ancient days are embedded in modern life: a digging stick of the forest became a spade, still with a wooden handle, lianes led to string and rope, palm fibre and bark to textiles, bamboo to piping and, perhaps most outrageously, the range of war-clubs in the Pacific shows the gradation from the wrenched-up sapling with the root-bases as spikes, to the mace, carried in honour in our Parliament.
Of recent forest-people, the Australian aborigines are thought to have removed much of the megafauna of tropical Australia and destroyed much of the Araucaria forest, leading to the advance of the tougher Eucalyptus there. Drainage ditches some 9,000 years old have been claimed for the highlands of New Guinea, a region under intensive clearing for some 5,000 years. Modern shifting cultivation, partially mimicking the gapformation in the natural mosaic of tropical rain forest, has been claimed as the best form of agriculture in terms of soil conservation: it is important to seek out some of the differences between the "natural" and man-made gaps in our consideration of the state of tropical rain forest. In Malaya the Temiar (Carey 1976), one group of the orang ask, the forest peoples thought to have been in the peninsula for some 25,000 years, have no personal land tenure, though certain fruit-trees, notably durians, may belong to particular people and can be inherited. When a new site is chosen for cultivation, the undergrowth is cleared as are some of the trees, though stumps are left as are, for superstitious or practical reasons (often these coincide), certain big trees, like the tualang (Koompassia excelsa, Leguminosae), which has useless timber but harbours bees' nests. Useful fruit-trees are left and are therefore under selection pressure. In the New World, the cultural practices of Maya times are thought to be "fossilized" in the forests of Mexico and Guatemala, for the present-day abundance of mahogany, chicle, and ramon may be explained by their being selected and encouraged to regenerate. When cultivators move on, they leave a tangle of exotic and native plants, including fruit-trees, which they have deliberately planted or encouraged. They may scatter seeds of useful plants, like the peoples of New Guinea, where Artocarpus is sown and pandans are planted in gullies and bogs. Furthermore, the seeds may be dispersed in the faeces of humans. Ridley (1930, 341) records that rambutan seeds are dispersed thus and notes that the allied fruit, mata-kuching, which has thin flesh, is always swallowed by Malays, who consider this to be the only healthy way of eating this type of fruit. The distribution of such fruit-trees may thus represent, at least in part, old cultivation sites, camps, or once-only latrines. Indeed, Lieberman and Lieberman (1980) have argued that during man's early evolution, he had "proto-gardens" of such trees and other plants around occupation sites and that he was initially unaware how edible-fruited plants came to be there, something which might well be called the Eden Syndrome. It is then argued that the evolution of seedgardening involved fleshy fruits and not grasses, legumes, or tubers, for which specialized techniques and/or open country is required. If this is so, then the origins of "agriculture" are much earlier than suspected and, more germane to our discussion now, the association between man and the forest is much longer than is generally suspected.
Modern Destruction
Much better documented, of course, is the twentieth-century rape of the world's tropical rain forests (Myer 1980). The figures of the rates of telling are so unreal to most people that we are in danger of overexposing the tragedy, with consequently smaller and smaller effect on the comfortable citizens of the developed world. I will add only a few remarks. That which survives in many places is in blocks too small to regenerate and is affected by being surrounded by destroyed vegetation (Janzen 1983): such include the blobs of forest in the middle of Kuala Lumpur and Singapore, for example. Doomed to become impoverished and increasingly secondary in nature, these islands are, in effect, reversing the process by which islands are usually populated in the first place. Elsewhere, the removal of animals by shooting has had the effect of removing the dispersers (Mabberley 1983, 106). Only elephants can swallow the pyrene of an African tree called Panda oleosa: the seed will germinate only after passing through the elephantine gut and being deposited in the droppings. If the elephant goes, then the Panda becomes an anachronism. A notorious and much-publicized example was of the species of Calvaria (Sapotaceae), confined to Mauritius. Only geriatric trees were found in recent years and it was guessed that perhaps it had been dispersed via the alimentary canal of the dodo, extinct since 1681. So turkeys were force-fed and the extruded seeds were said to have germinated and thus brought back the tree from the brink of extinction. Francis Ng at Kepong (1983) has made a list of a number of trees of the Malay Peninsula that will only germinate after the fruit has been opened by monkeys, or the arils removed by animals: with the loss of these animals, trees laden with untouched fruit are an increasingly common sight.
Opportunism
On the other hand, some trees are flexible enough to make a comeback. The trees once dispersed by the gomphotheres of Central America have spread again with the introduction of horses and cattle which eat the fruit and spread the seeds. The papaya, originating in tropical America, is readily pollinated by midges when it is introduced to Africa. An outstanding example of this flexibility is from Hawaii. The climbing relation of the pandans, Freycinetia arborea (Cox 1983), has long been considered to be pollinated by rats, but this is now denied. Indeed, it has been shown that it is pollinated by a bird, a white-eye introduced from Japan in 1929. What pollinated it before? There was some circumstantial evidence from published records that some drepanids, now largely extinct through shooting, collected pollen, so Cox examined the hundred-year-old skins of these birds in a museum at Harvard: in the head-feathers were pollen grains of this species and no other. Fortunately for the Freycinetia, it is more flexible than the Panda, and it is attractive to the exotic white-eye. It survives.
The introduction of birds in tropical rain forests can be very misleading and result in apparently widely disjunct distributions of them. One such is the great-tailed grackle in Veracruz and also in the Valley of Mexico: this is now explained (Haemig 1978) by the bird-fancying attributes of the Aztec emperor Auitzotl, who introduced it at the end of the fifteenth century. Even more subtle perhaps are the tiger teeth in caves in Borneo: there is no other evidence that the animal lived in that island and it is concluded that the teeth were put there by man.
The Status of Tropical Rain Forest
To return to tropical Amercia, there is a good deal of evidence of European archaeological remains, not to speak of earlier aboriginal settlements throughout areas now re-covered with forest. It is estimated (Myers 1980) that since 1825 the forests of part of Venezuela increased from 21% of their original range to 45% in 1950. As Myers points out, it is difficult to decide whether these are "restored" forests or of a new type. Certainly in Asia and also, incidentally, in Europe, however, "new forests" are considerably lower in stature than the original ones: those in Malaya are compressed into three-quarters or even half of their original height (Ng 1983). There is a long history of settlement in West Africa and Central America and it could be argued that rain forests there represent old secondary forests. But, even the "primary forests" typical of Borneo, for example, have been long influenced by man, as the naturalized fruit-trees and caches of mediaeval Chinese pottery indicate. In short, most tropical rain forests are "disturbed," but some are more disturbed than others.
Many ecologists and, I suspect, hydrologists have a wish to work in primaeval, original, virgin, untouched, or primary forests. I hope that with eucalyptus, sloths, femurs, bulldozer herbivores, Krakatoa, brazil-nuts, gomphotheres, coco-de-mer, and Emperor Auitzotl with his great-tailed grackle it is clear that when we are dealing with tropical rain forest, where it survives, we are dealing with the living past.
Ashton, P. S. 1977. "Ecology and the durian theory." Gdns.' Bull., Sing., 29: 19-23.
Carey, I. 1976. Orang ask. Oxford University Press, Kuala Lumpur.
Corner E. J. H. 1962. "Botany and prehistory." Symposium on the Impact of Man on the Humid
Tropics Vegetation, Goroka, 1960, pp. 38-41. Unesco.
———. 1964. The life of plants. Weidenfeld and Nicolson, London.
Cox, P. A. 1983. "Extinction of the Hawaiian avifauna resulted in a change of pollinators for the ieie, Freytinetia arborea." Oikos, 41: 195-199.
Haemig, P. D. 1978. "Aztec Emperor Auitzotl and the great-tailed grackle." Biotropica, 10: 11 - 17.
Janzen, D. H. 1983. "No park is an island: Increase in interference from outside as park size decreases." Oikos. 41: 402-410.
Janzen. D. H.. and P. S. Martin. 1982. "Neotropical anachronisms: The fruits the gomphotheres ate." Science, 215: 19-97.
Kortlandt, A. 1984. Vegetation research and the bulldozer herbivores' in tropical Africa." In A. C. Chadwick and S. L. Sutton, eds., Tropical rain forest. Ecology and management, Suppl. Vol. Blackwell Scientific Publications, Oxford.
Lieberman, M., and D. Lieberman. 1980. "The origin of gardening as an extension of infra-human seed dispersal." Biotropica, 12: 316-317.
Mabberley, D. J. 1975. "The giant lobelias: Toxicity, inflorescence and tree-building in the Campanulaceae." New Phytol., 75: 289-295.
———. 1983. Topical rain forest ecology, Blackie and Son, Glasgow.
Myers, N. 1980. Conversion of tropical moist forests. National Academy of Sciences, Washington, D. C.
Ng. F. S. P. 1983. "Ecological principles of tropical lowland rain forest conservation." In S.L. Sutton et al.. eds., Tropical rain forest Ecology and management pp. 359-375. Blackwell Scientific Publications, Oxford.
Ridley, H. N. 1930. Dispersal of plants throughout the world. L. Reeve and Co., UK.
Savage, A. P., and P. S. Ashton. 1983. "The population structure of the double coconut and some other Seychelles palms." Biotropica, 15: 15-25.
Schuster, R.M. 1972. "Continental movements, 'Wallace's Line' and Indomalayan-Australasian dispersal of land plants: Some eclectic concepts." Bot. Rev, 38: 3-86.
Steam, W. T. 1977. "The earliest European acquaintance with tropical vegetation." Gdns.'Bull., sing., 29: 13 - 18.