|Forests, Climate, and Hydrology: Regional Impacts (UNU, 1988, 217 pages)|
|2. The living past: Time state of the tropical rain forest|
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 banyanswhich 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.
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-fernsome of which have led to the angiospermsand 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.