With special reference to British Guiana and British West Africa
by P. W. RICHARDS,
Professor of Botany, University College of North Wales, Bangor
Under the auspices of UNESCO a Symposium on the Study of Tropical Vegetation was organized in Ceylon during March 1956. In the organization of this Symposium UNESCO consulted FAO's Forestry and Agriculture Divisions and invited participation of FAO representatives. A number of foresters in Southeast Asia were also invited, at FAO's suggestion, to present papers. A list of the papers presented is given on page 165. This article by Professor P. W. Richards is reproduced with the kind permission of the author and of UNESCO.
Following the presentation of the papers, conclusions and recommendations were formulated by the participants on such major topics as ecological factors, vegetation types and their methods of study, mapping, concept of climax in the tropics, and regimentation as a factor for judging the status of vegetation types. In view of the fruitful results of the meeting, it was recommended that UNESCO should organize future symposia, first on soils, a second on climate, both in relation to vegetation; and a future symposium on co-ordinating aims and methods of study of tropical vegetation, climate and soil.
PROBLEMS of tropical ecology appear somewhat different from those of temperate regions, but fundamentally they are similar. Superficially and practically they may seem different because the flora of the tropics, above all that of the humid forested regions, is immensely richer in species than that of the temperate zone, especially those parts of it which like Europe have been impoverished by glacial periods. This richness of the tropical flora has important consequences. Practically, it means that tropical plant communities generally have very numerous constituents which are difficult to recognize in the field and often poorly known taxonomically. A more important consequence, which bears directly on the chief aim of this symposium, is that in the tropics often, but not invariably, there tend to be innumerable slightly different combinations of species rather than the relatively few fairly well defined plant associations to which we are accustomed in Europe and North America. A further difference between the humid tropics and most temperate countries is that though, in the former, modification of the vegetation by human activities is almost universal and often very profound, there are large areas in which it is less severe or at least much more recent than in the latter. The apparent distinctness of many European and North American plant associations is probably often due to anthropogenic influences to which they often owe their sharp boundaries. One reason for the apparent lack of clearly recognizable plant associations in many parts of the tropics may thus be the comparatively low intensity and recent incidence of disturbance by man.
In dealing with the problems of classifying and naming tropical vegetation, reference is made specifically to two humid tropical regions, British Guiana in the northeast of South America and British West Africa.
These two areas, though showing fairly comparable ranges of climate, are in a number of respects very different and may be regarded as exemplifying two extreme situations which may confront the student of tropical vegetation. The discussion is confined to the more humid types of forest - the tropical rain forest in the broadest sense of the term.
British Guiana is a country in which the great majority of the population, and nearly all the cultivated area, are concentrated in a narrow coastal strip; the interior extends for over 700 kilometers from the coast, but has a very sparse, mainly Amerindian, population or is quite uninhabited. The vegetation apart from the coastal strip is thus comparatively little affected by man and much of it consists of unmodified or only slightly modified climax communities. The area in the neighborhood of Moraballi Creek on the Essequibo river, which was studied by the author and his colleagues on the Oxford University Expedition to British Guiana in 1929, contained no grassland and was wholly forest-covered except for very small patches of shifting cultivation. Apart from this, which owing to the smallness of the population was relatively unimportant, the only human activity affecting the vegetation was timber working, consisting, at that time, of highly selective extraction of greenheart [Ocotea rodiaei (Schomb.) Mez] only. Where the forest showed no traces of former cultivation or of timber working it could be regarded as climax vegetation in equilibrium with the climate and had doubtless existed little changed from a fairly remote geological period.
As the area studied was not more than a few square kilometers in extent and had no striking topographical features, it could be assumed to have a uniform climate apart from very slight microclimatic differences. Differences between plant communities were not due to shifting cultivation or timber felling, but must be due to other factors, presumably factors depending on topography such as drainage and exposure to wind or edaphic factors. This being 80, it was surprising and not in accordance with the then accepted ideas about tropical vegetation, to find that both vegetation and soil were remarkably varied. Distinct and easily recognizable plant communities - referred to in 1929 by the non-commital term "forest types" - were found, and in many cases, though not always, the boundaries between one plant community and the next were sharp. Each "forest type" was associated with a distinct type of soil, or at least with a distinct combination of soil and topography.
Mr. T. A. W. Davis and the author have given a detailed descriptive account of the vegetation of Morabali Creek in a paper published in the Journal of Ecology in 1933-34 and a much briefer account appeared in The Tropical Rain Forest (1952). It will be sufficient to say here that in addition to "Low Bush" (secondary forest on sites of former cultivation or timber felling, i. e. stages in secondary successions) there were five types of primary forest, Mora, Morabukea, Mixed, Greenheart and Wallaba. The Mora forest in which the Leguminous species Mora excelsa Benth, formed 67 percent1 of the larger trees (41 centimeters and more in diameter) occurred on the most low-lying situations in the floodplain of the Moraballi Creek and occasionally ascended to higher levels on slopes where the bed rock was near the surface and the soil stony. Morabukea forest, in which another species of Mora, M. gonggrijpii (Kleinh.) Sandw., formed about 60 percent of the larger trees, occurred on better drained sites where the soil was a heavy and impervious reddish clay. Mixed forest in which the most abundant tree species were Eschweilera sagotiana Miers, Licania venosa Rusby and Pentaclethra macroloba (Willd.) Benth., but where there was no strong tendency to "single-species dominance", occurred on light loamy soils with good drainage which could be regarded as the optimum habitat of the area. The two remaining "forest types," that dominated by greenheart [Ocotea rodiaei (Schomb.) Mez] and the Wallaba forest in which Eperua falcata Aubl. (Soft Wallaba) formed some 67 percent of the larger trees, occurred respectively on brown sand and the peculiar strongly leached "white sand" which the author regards as the tropical lowland equivalent of a temperate podsol.
1 These figurer and other data referring to floristic composition were obtained from single sample plots measuring 400 feet (122 m.) square (c. 1.5 ha.) in which all trees 4 inches (10 cm.) and over were counted.
These five "forest types" form a series and were usually met with in the order given (More to Wallaba) if a transect were made from the river bank to the low plateaux and hill ridges which formed the highest ground in the Moraballi Creek area.
Floristically each "forest type" is distinct, though many species were common to all five of them; the composition of each was fairly constant and characteristic. The three central types of the series, Morabukea, Mixed and Greenheart were very similar in composition and there were very few if any species which occurred in one which were not found in all three, though the proportions in which the species were present were very different. The Mora forest and the Wallaba forest, the communities forming the two ends of the series, were strongly differentiated from the other types and each had a number of subordinate species peculiar to itself. There were some structural differences between the five communities in addition to their differences of composition, but they need not be discussed here.
Where the soil or topographic conditions characteristic of one forest type merged gradually into those characteristic of another, there was also a gradual transition from one community to the other, but where, as was the case particularly where the Greenheart and Wallaba forest adjoined, there was an abrupt change of soil conditions, the boundary between two communities could be very sharp.
There is an obvious similarity in the relationship between the Mixed forest with the Greenheart and Wallaba communities on the one hand, and with the Morabukea and Mora communities on the other, to the state of affairs in the "Mixed Mesophytic forest" of temperate North America where Dr. Lucy Braun has depicted an "undifferentiated" Mixed association flanked by "association segregates" with fewer dominants occupying steep slopes, outlying geographical areas and other situations where the conditions are presumably non-optimal for the majority of the species. In a similar way the Mora, Morabukea and Greenheart forest, and possibly even the Wallaba forest, could be regarded as "association segregates" of the Mixed forest. If a more orthodox terminology than Braun's is preferred, however, we can regard the Mixed forest as an association and the Mora excelsa, Mora gonggrijpii, Ocotea and Eperua communities as consociations, all five "forest types" being regarded as units of the same climax.
Since 1929, Fanshawe has made extensive studies of the vegetation of British Guiana which he has summarized in his valuable but unfortunately very brief Vegetation of British Guiana: a Preliminary Review (1952). This work helps us to see the plant communities of Moraballi Creek in a wider perspective. It also allows the five primary "forest types" to be put in their places in the classification of tropical American climax communities which was originally developed by Beard (1944; 1955) in Trinidad and which Fanshawe has adopted and extended on the basis of his experience in British Guiana. While the author is critical of some features of Beard's classification and would not wish to accept without question all the assumptions of which it is based, it is the most serviceable classification of tropical American vegetation types yet produced and it should be given an extensive trial until ultimately it can be improved or superseded.
According to Beard's scheme, as extended by Fanshawe, the Moraballi Creek forest communities belong to three different "formation series" as shown in the following Table:
|
Davis |
Unit |
Formation |
Formation- |
|
Morabukea consociation |
Mora gonggrijpji |
Rain forest |
Rain forest |
|
Mixed forest |
Mixed faciation |
|
|
|
Greenheart consociation |
Ocotea rodiaei faciation of |
|
|
|
Wallaba |
Eperua-Eperua |
Wallaba forest |
Dry ever-geen forest |
|
Mora |
Mora excelsa |
Mora forest |
Swamp forest |
Thus in the British Guiana rain forest there are a number of clearly differentiated climax plant associations (Fanshawe has described many more than those mentioned here), which are readily recognized and defined. They could be mapped and their characteristics can be precisely stated. In the forests of British West Africa there is found a very different situation.
The five British West African territories, the Gambia, Sierra Leone, the Gold Coast, Nigeria and the British Cameroons (Trusteeship territory), are all strips of country of varying width and length extending inwards from the coast and they therefore consist of varying cross-sections of the vegetation belts which extend across the whole of West Africa from west to east. The conditions found in Nigeria and the Cameroons appear to be common to the Gold Coast and Sierra Leone.
On first acquaintance there seem to be certain great differences from British Guiana, though as has been said already, there is much similarity in range of climate between the two countries.
The humid forest zone of Nigeria (the equivalent of the forêt dense of Aubréville and other French writers) forms a belt parallel to the coast of the Gulf of Guinea. At its western extremity it is less than 100 kilometers wide (further west in Dahomey it disappears altogether), but it widens eastwards; in Nigeria west of the river Niger and in the Cameroons it extends some hundreds of kilometers from the coast. At its inland margin the forest belt is adjoined by the zone of Guinea savannas.
Perhaps the most important differences between this humid forest belt of Nigeria and that of British Guiana are those dependent on density of population. While the rain forest of British Guiana is almost uninhabited, the forest belt of Nigeria is densely populated and includes some of the most thickly populated areas of the whole country. Because this is so, it is not surprising that by far the largest proportion of the forest zone has become either permanent agricultural land (plantations of cocoa, oil palms, etc.), or is farmed under a system of cultivation locally known as the "bush-fallow". Almost all the remaining forest is now enclosed in forest reserves under the protection of local or regional governments. Many of these reserves are being actively exploited for timber and it is only in the hilly country in the extreme east, bordering on the Cameroons, that there are considerable areas of forest which are not at present being exploited and are in a relatively undisturbed condition. In the smaller forest reserves of Western Nigeria, some fine fragments of forest still remain, but these are probably all old secondary communities, not relics of virgin forest. In the Okomu Forest Reserve, for example, Dr. E. W. Jones (1955) has shown that throughout the "high bush" there are fragments of pottery, charcoal, etc. in the soil which prove conclusively that the area was at one time cultivated and inhabited. The present forest dates from perhaps the eighteenth century when, for reasons which are not yet clear, the district became depopulated.
The dense population and consequent high degree of human interference in the Nigerian forest means that undisturbed climax forest is never met with, or very rarely; all stands of forest are a patchwork consisting of secondary communities of many different ages, i.e., a mosaic of seral stages. This state of affairs makes a formidable difficulty for the study of vegetation. in addition to the variation due to climate, soil and topography there is superimposed considerable variation depending on a time factor (which cannot easily be measured) and on the kind and intensity of human interference.
If allowance is made as far as possible for the effects of human interference, can distinct associations or other units of vegetation be recognized in the humid forest belt of Nigeria? It can be said at once that "forest types" as distinct as those met with in one area in British Guiana do not seem to exist in Nigeria, partly perhaps because there is less variety of soil, but some variation in the vegetation exists which can be correlated with differences of climate, and some (but of rather small importance) which seems to depend on soil; in addition there are differences between the composition of the forests in Eastern and Western Nigeria which may be due to historical factors and the geographical barrier of the river Niger.
The first distinction which was recognized in the Nigerian forest apart from that between the upland and lowland forests was that between the Evergreen forest and what earlier writers called Mixed Deciduous forest.
In the most humid parts of the forest belt, i.e., in those with the shortest and least severe dry season (in Nigeria, there is nowhere less than two consecutive months with under 10 centimeters rainfall), the forest is evergreen; though "tropical rain forest" in the broad sense of the term, it approximates physiognomically to what Beard calls evergreen seasonal forest in tropical America. Towards its inland boundary this forest becomes more or less gradually different in floristic composition though not fundamentally different in structure or physiognomy except for the greater proportion of trees deciduous for a considerable period. This forest is the Mixed Deciduous forest (forêts tropophiles or semi-cadurifoliées of French and Belgian authors). The change from Evergreen to Mixed Deciduous forest coincides with a slight change of climate and also with a change in the soil-forming material from sedimentary to crystalline rock. Some recent writers have argued that Mixed Deciduous forest is heterogeneous, that it is not climatically determined but dependent on the substratum and that the name is misleading or inappropriate. More recent studies in Nigeria, in 1955, caused one to revert to the older opinion that there is a distinctive type, determined mainly by climate, for which the name Mixed Deciduous forest is at least as good as any other which has been suggested.
When we try to make further distinctions, e.g., to recognize different associations, faciations or other units within the Evergreen or the Mixed Deciduous forest, inadequate data and the fragmented state of the remaining forest. present formidable difficulties. Rosevear (1953) has suggested very tentatively that there are four types of lowland rain forest in Nigeria, characteristic of Ondo Province, Ijebu Province and Benin Province in Western Nigeria, and of the Eastern Provinces respectively. Rosevear's Ondo type largely corresponds with what is here referred to as Mixed Deciduous forest. The other three types are variants of Evergreen forest; to what extent the differences are due to climatic factors, edaphic factors or to geographical barriers remains to be determined. It seems probable that the marked differences between the Western and Eastern forests are due largely to historical factors and the geographical barrier formed by the Niger and Cross rivers, but whether climatic differences are also important is uncertain. The differences between the Ijebu and Benin forests are perhaps dependent on differences of soil. To what extent these two types represent stages in a single continuum is difficult to determine when so much of the forest of the intervening region has been destroyed.
The study of tropical vegetation raises problems of classification and nomenclature. The aim of ecology (in its gynecological or phytosociological aspect) in the tropics as in other parts of the world, is to describe, name and classify plant communities, not simply for classification's sake, but in order to bring to light causal relations between the vegetation on the one hand, the environment on the other. This should lead to the clarification of what Emberger terms homoecies (perhaps best rendered in English as "equivalent environments"); this, apart from its great scientific importance, is of great potential value for agriculture, forestry and land usage generally. The comparison of British Guiana with British West Africa may at least indicate how much remains to be done and some of the difficulties which will have to be surmounted before such aims can be realised in the humid tropics.
As is well known, there is no general agreement as to how temperate plant communities should be classified and named. It is not to be expected that agreement should be reached any more easily for the highly complex plant communities of the tropics. The first constructive suggestion to make is that even more important than an agreed scheme of classification and nomenclature is some measure of standardization in methods of describing and sampling vegetation units. A natural classification of any set of natural phenomena presupposes a fairly complete knowledge of the facts on which the classification can be based. With tropical vegetation such knowledge is far from complete. In 1939, the late Professor Tansley, Dr. A. S. Watt and the author published a scheme for The Recording of Structure, Life-form and Flora of Tropical Forest Communities as a Basis for their Classification. This scheme was intended to give foresters and others working in the tropics some guidance in collecting descriptive data about tropical vegetation. It seems to have proved useful and a new version, brought up to date and perhaps extended to include communities other than forest communities might be even more useful and would perhaps help towards the objects in view.
Secondly, there are several fairly complete schemes of classification of tropical vegetation in existence; all of these have their shortcomings and all are intended by their authors to apply to one region of the tropics alone. There are, for example, Beard's scheme for the vegetation of tropical America which has already been mentioned and Professor van Steenis' very valuable classification of the vegetation types of Malaysia. For India and Ceylon there is Professor Champion's elaborate classification of the forest types of India and Burma. Though premature attempts to equate plant communities in one part of the world with those in other very distant parts are to be avoided, it might be well worth while for those with the right experience to compare these schemes. It might then be worth examining how far Beard's scheme could be applied to Africa and Asia or Champion's to Africa and tropical America. But in doing this the question should not be that of pure classification and nomenclature but rather the tracing of causal relations between the vegetation and the factors which determine it.
BEARD J. S. (1944) - Climax vegetation in tropical America. Ecology, Vol. 25: 127-168.
BEARD J. S. (1955) - The classification of tropical American vegetation-types. Ecology, Vol. 36: 89-100.
CHAMPION, H. G. (1936) - A preliminary survey of the forest types of India and Burma. Indian. Rec. (New series), Vol. 1, No. 1.
DAVIS, T. A. W. and RICHARDS, P. W. (1933-34) - The vegetation of Moraballi Creek, British Guiana: an ecological study of a. limited area of tropical rain forest. Parts I and II. J. Ecol. Vol. 21: 350-84, Vol. 22: 106-66.
FANSHAWE, D. B. (1962) - The vegetation of British Guiana: a preliminary review. Imperial Forestry Institute, Oxford, Institute Paper No. 29.
RICHARDS, P. W. (1962) - The tropical rain forest, an ecological study, Cambridge.
RICHARDS, P. W., TANSLEY, A. G. and WATT, A. S. (1939). The recording of structure, life-form and flora of tropical forest communities as a basis for their classification. Imperial Forestry Institute, Oxford, Institute Paper No. 19 (also printed in J. Ecol. Vol. 28: 224-239, 1940).
ROSEVEAR, D. R. (1953) - Chapter XV, Vegetation, in the Nigeria Handbook, London.
STEENIS, C. G. G. J. VAN (1935) - Maleische vegetatie schetsen. (AN OUTLINE IN MALAYSIAN VEGETATION) Tijdschr. ned. aardrijksk. Genoot. (PERIODICAL OF THE NETHERLANDS GEOGRAPHICAL SOCIETY) Reeks (SERIES) 2, 52: 26-87, 171-203, 363-98
LIST OF SCIENTIFIC COMMUNICATIONS IN ORDER OF PRESENTATION
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UNESCO |
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NS/HT/46 |
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G.S. PURI |
Problems in the Ecology of the Humid Tropics |
Annexe 1 |
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A. DILMY and A.J. G.H. KOSTERMANS (Indonesia) |
Research on the Vegetation of Indonesia |
Annexe 2 |
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R.A. de ROSAYRO |
Tropical Ecological Studies in Ceylon |
Annexe 3 |
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J. WYATT-SMITH |
A Note on the Study of Tropical Vegetation |
Annexe 4 |
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P.W. RICHARDS |
The Study of Tropical Vegetation with special reference to British Guiana and British West Africa |
Annexe 5 |
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P.W. BEDARD |
Reconnaissance Classification and Mapping of Philippine Forests |
Annexe 6 |
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F.R. FOSBERG |
Vegetation of the Islands of Oceania |
Annexe 7 |
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D. CHATTERJEE |
Tropical Vegetation of Eastern India |
Annexe 8 |
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C.G.G.J. VAN STEENIS |
Concise Outline of Malaysian Vegetation Mapping and Literature |
Annexe 9 |
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R. MISRA |
The Study of Tropical Vegetation in Madhya Pradesh and the Gangetic Valley |
Annexe 10 |
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J.W. PURSEGLOVE |
The Vegetation of Humid Tropics with special reference to Singapore |
Annexe 11 |
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F.R. BHARUCHA |
Methods for the Study of Tropical Vegetation Annexe 12 |
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C.H. MACFADDEN |
Humid Tropical Vegetation Research by Aero-Field Techniques and Photography |
Annexe 13 |
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C.H. HOMES |
The Broad Pattern of Climate and Vegetational Distribution in Ceylon |
Annexe 14 |
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G. MANGENOT |
Les recherches sur la végétation dans les régions tropicales humides de l'Afrique occidentale |
Annexe 16 |
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B.W. TAYLOR and G.A. STEWART |
Vegetation Mapping in the Territories of Papua and New Guinea conducted by CSIRO Annexe 16 |
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E.K. JANAKI AMMAL |
An Introduction to the Genetical Analysis of the Humid Forest Flora of South Asia |
Annexe 17 |
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S. HEDAYETULLAH |
The Study of Tropical Vegetation with particular reference to Pakistan |
Annexe 18 |
Copies of all the papers can be obtained from UNESCO, South Asia Science Co-operation Office, New Delhi, India.
