Biological Problems of Tropical Vegetation * Richard P

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Biological Problems of Tropical Vegetation * Richard P. Aulie, Department of History of Science and Medicine, Yale University, New Haven, Connecticut

Rain forest once covered much of West Africa. In recent years this has been replaced in many areas by savanna and secondary forest. All three vegetation types represent activities of great interest to the biologist. The author is a former high school biology teacher who spent a year in Liberia making these studies. The article demonstrates an excellent ecological study with clear implications for teachers wishing to conduct similar projects. It was wriften in Monrovia, Liberia during 1963-64. Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Richness of Vegetation must learn the conditionsof their genesis and If the travelernotices a particularspecies growth, together with the continual changes and wishes to find more like it, he may often occurringamong them. As a British ecologist turn his eyes in vain in every direction.Trees has written, ". . . the understanding of what of varied forms, dimensionsand colours are happensis fundamentaland the study of what around him, but he rarely sees any one of is preliminaryto it." (5). them repeated (1). The purpose of this article is to present a broad discussionof West African vegetation With these words Alfred Wallace in 1878 types, and to point out some unsolvedbiolog- provideda glimpseof the richnessof tropical ical problems that are involved. The study, vegetation. Although the celebrated English largely ecological and genetic, is based on explorerdid his epoch-makingsurveys in the observationsmade in several forest regions MalayArchipelago and Amazonia, his descrip- of Liberia during the period 1962-64 while tions to a great extent fit what may be found filling a UNESCO teaching appointmentat in Liberia,and in much of West Africa today. the Universityof Liberia (6). This article is One need not have the practiced eye of a aimed at the kind of broad understandingto Wallace, though, to be impressedby the ex- which Waddingtonrecently has applied the uberantvegetation of the tropics. useful term "syntheticbiology," which seeks There is the richness of componentparts; to present a more "coherentpicture of the around600 species of trees have been identi- whole realm of living matter" (7). fied in the Cote d'Ivoirealone (2, 3) and over The need for this approachis now becom- 235 in Liberia (4). There is the richness of ing clear, for it is only in recent years that structure,in which dozens of organismsare West Africa has receivedthe sustainedatten- superimposedone on the other, masking by tion of biologists(8). The famous expeditions seeming disarrayits clear conformitiesof or- of the 19th century,such as those of Charles ganization.To be sure, nature has a horror Darwin,Alexander von Humboldt,and Alfred vacui and rapidlyfills every niche with some- Wallace, turned rather to the forests of the thing green. There is the richnessof dynamic equatorialAmericas and Malaya.As a result, organization.Any changein one part, such as by the beginning of the 20th century there soil, microclimate,or vegetation, results in was an almosttotal ignoranceof West African compensatingadjustments in the others. plantlife. Indeed,so isolatedwas West Africa The tropicallandscape is the richestbiolog- from serious biologicalwork that the phrase, ical environmentof any land mass in the "ceux violent la foret," was often applied to world. It is composedof integratedgroups of those who first traversedthe area early in the communitiesconstantly interactingin space 1900's (9). and time. Indeed, this richness of vegetation There was a good reasonfor this isolation. is an obstacleto its understanding,for a plant Due to the extremelyunhealthy condition of communityis not "understood"if we are con- the climate, as comparedwith other tropical tent only to list the species composing it. regions of the world, West Africa properly After determiningthese constituentunits, we earned the title during the late 19th century

331 332 THEAMERICAN BIOLOGY TEACHER Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Fig. 1. Tropical forest profile in Liberia. Area in foreground has been cleared for farming. Note the 'umbrella trees" (Parasolieres). They are Musango cecropoides, and rapidly invade clearings in response to increased light during early stages of succession on Bong Mountain Range. of the "white man's grave." Malaria, dysen- He was followed by M. A. Aubreville, who teries, and dengue fever raised the death rate continues to bring out important publications among soldiers, officials, missionaries, and in which he describes new species and dis- traders to 10% for people between the ages cusses forest populations (14). The Kew gar- of 18 and 50. However, with the widespread dens in London have extensive collections, use of drugs in recent years and access to good and today most of the important species have medical care, disease no longer holds the same been described in print (15). In more recent threat to physical and mental well-being. Las years there has been an increasing interest in Palmas, Paris, and New York are only hours the management of Liberian forests as a val- away by jet. The enervating climate has be- uable national asset, in large measure due to come tolerable and the death rate is around the efforts of the Bureau of Forests of the 10 per 1000, which is considered fairly nor- Liberian Ministry of Agriculture and its mal for the same age group (10, 11). USAID advisers, the German Forestry mis- The great pioneer of scientific exploration sion, one that is engaged in a forest survey, was Auguste Chevalier of the French Acad- and the educational program of the College emy. He conducted extensive surveys of the of Forestry of the University of Liberia, C6te d'Ivoire in 1906-7, and also explored the largely staffed by FAO (16). forests of Liberia in the region of Mount The early explorers distinguished 3 different Nimba. His work marked out the forests as vegetation types. (a) The most prominent, of objects of serious biological inquiry (12, 13). course, is the rain forest, which the French BIOLOGICALPROBLEMS OF TROPICALVEGETATION 333 call "la foret dense," mile on mile of green wilderness blanketing the earth to the horizon. Since time immemorial its canopy of high trees has enclosed an almost complete solitude (Fig. 1). (b) Then, too, the early travelers noticed that abandoned land, once cleared for farming, would, under certain conditions, re- build itself into rain forest. A stage in this rebuilding process, and manifestly quite different in character, is secondary forest, often appearing as a labyrinth of tangled under-

growth, vines, creepers, and trees. This is Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 really the "jungle" of travel books and films (Fig. 3). (c) The 3rd area is the park-like savanna (man-made), consisting of isolated trees, often the oil-palm, surrounded by thriving grasses and low-lying shrubs. This particular savanna is man-made, since it occurs wherever there is regular firing of the grasses (Fig. 4). Of course, one cannot always expect to find clear geographical boundaries between these 3 "communities." One may grow accustomed to the sight of palm trees in savanna land- scapes only to come upon them thriving on the edge of secondary forests, wherever con-

Fig. 3. Secondary vegetation. Lianas climbing a high tree toward the light. From the "Para Bush" near the ELWA radio station, Monrovia. ditions of light are optimal,and even in small clearings.Then, too, the later, more mature stages of secondaryforest are often indistin- guishablefrom primaryrain forest, which is to be expectedif the formeris a precursorof W.I~ the latter.And one may find remnantsof all 3 in areas that have been cleared for farming. p~~~ As a matter of fact, almost all of the "rain forest" has been affected in one way or another by human activities. Even in the finest examplesof maturerain forest one can often find fragmentsof pottery,palm nuts, mounds, and subsurfacelayers of charcoal,all evidence that the forest at one time had been cleared and inhabited(17). A biologist approachingthe study of tropical vegetationmay start by collectingplants, gathering into an herbarium collection the greatestnumber of species in the shortestpos- sible time. However, such a collection rarely Fig. 2. Aerial photograph of tropical forest bordering representsa random sampling of the entire Atlantic ocean shown at bottom, near Monrovia, Liberia. Mottled, circular depressions are clearings in the forest geographicrange of a species. Since a biol- used for farming. Note logoon behind sand bar. Large ogist usually collects where he can, he can areas of coastal savanna in lower half. River cuts never be sure he has through forest at top of picture. (Courtesy Hansa-Luftbild sampledall the variation Munster) patterns that exist within a single species. 334 THEAMERICAN BIOLOGY TEACHER

caps on the biologist who is interested in as-

t F- ' 0 _ 00 ;ji 0 > 0 it N z- ;l i ;: s : certaining broad relationships therein, never- ~~~~~.'.gi.... ' . . s ;..... theless this same abundance confers important advantages. Much that is new to biology will doubtless emerge from studies of tropical plant life simply because so little is at present

'. ...:' .. ...T ; f - 0 known. Whereas most ecological studies have

.Et-E itV,; i: ~~~~~~~~~~...... '. !i .;".'.-i been done in temperate climates, the floris- 0:; ; ;;, '.-::0:..E is 'liS4',-~~~~~~~~~~~~~~.-...... -..;. ;: ,0:00::;',;'. :::V tically rich communities of the tropics hold : ....',...... '.';,, :_ unrivalled opportunities for the 0 . . . .;....ls:...B.. x i ...... |: : biologist, particularly as regards studies of evolution, adap-

tations, and succession. Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Evolution The floristic richness of the tropics is no Fig. 4. Savanna landscape. Found on Liberian coast, near ELWA radio station. Typical savanna trees include Para- doubt due to the fact that evolution has pro- narium macroph,yllumand Eleis guineensis. ceeded largely without interruption since earliest geological times. Furthermore, during Indeed, examination of an herbarium might the Oligocene period, vegetation closely re- lead one to think there are few variations at sembling modem tropical rain forest covered all, as suggested by the older, more static con- large parts of the world, even as far north as cept of the species. the American Great Lakes. This may be one A species is a complex mosaic of freely reason for thinking that the more specialized interbreeding populations, affected as they are and modified temperate communities have by ecological and genetic factors extending been derived from the relatively unspecialized, through space and time. These variations re- rain forest communities now found largely in flect underlying genetic diversities. We there- the humid tropics. fore need to go beyond the herbarium collec- More detailed studies of the changes in tion. Many different disciplines are needed for plant populations would undoubtedly elucidate studying tropical vegetation, for even with the how this evolution might have occurred, and most extensive herbaria, representing the best this is one of the tasks of modem biology. of classical botanical systematics, the problem Stated in genetic terms, such observed changes remains of visualizing physiological and would represent modifications in the distribu- genetic processes. tion of zygotic frequencies. One would then Today, very little is known about the need to clarify in particular the natural selec- physiology and genetics of tropical plants. tive agencies that continually modify gene Using established techniques, though, perhaps frequencies in space and time. These agencies one might make a beginning. For example, have eliminated, by definition, all but the most peculiarities of geographic distribution, if favorable gene mutations and recombinations, good herbaria are available, would tell some- which then become expressed on the popula- thing about the rate and direction of gene flow tion level as adaptive or inadaptive traits. in time. Changes in proportions of plants in a Evolution is considered here to be, not so given area over a number of years would much the origin of species per se, but the reveal clues about fluctuations in gene fre- modifications that occur in the hereditary quencies. But difficulties of technique would characteristics of the population. Therefore, be immense, and a final union of physiology, changes that a careful biologist might note in genetics, and classical systematics probably the proportions of various trees composing a awaits the biology of the future. It will thus forest population under natural conditions be seen how formidable are the problems of would constitute evolution, a process that tropical plant biology. presumably has resulted in the creation of the rain forest as we find it today. This view of What we may learn evolution does not deny the existence and While it is certainly true that the abundance operation of intelligent design, or divine prov- of tropical vegetation imposes serious handi- idence, as many biologists in Darwin's time BIOLOGICALPROBLEMS OF TROPICALVEGETATION 335 thought it did. Rather, it simply implies that could not survive. What determines, then, the species were not created ex nihilo. It im- whether a juvenile communityis to become plies also that the designing agency, which savanna or secondaryforest? It is generally does not denote "vitalism,"has been contin- agreed that secondaryforests are progressive ually acting, or creating,for millions of years stages toward the establishmentof the more throughprocesses that may be unravelledby dynamicallystable rain forest. This processis biologicalinquiry. Indeed, on this view, evolu- "succession."Under other conditions,such as tion is the method of divine creation. repeated grazing and firing of farmland, It might be reasonableto view the broad- savannacommunities result, although some of leaved tropical vegetation as the generalized these, too, can become intermediatestages. type from which the temperatecommunities There have been few studies made on the are derived. Indeed, under ideal conditions, successions of which secondary forests and Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 perhaps a biologist ought to study tropical savannaseem to be either actual or potential vegetationfirst, and then go on to the deriva- stages. This is unfortunate,since perhapsno tive temperatevarieties, but, of course, this area of ecology is of greater theoreticalim- is usually impossible. portance in understandingwhat is occurring in large areasof the tropicallandscape. With- Adaptations out doubt, successions are complex. They A newcomer to a tropical forest, partic- vary in length of time and composition of ularly if he is familiarwith the landscapeof species from place to place, depending on North America, likely will be impressedby many local factors. Such studiesrequire years the extraordinaryprevalence of certain com- of painstaking,quantitative observations. mon structuralfeatures of the vegetation.As compared with the temperate communities, tropical communities, so rich in unrelated West African Vegetation Types species, often display similarindividual struc- General features of Liberia tures. For example, there are the wing-like The coastal plain is just a few feet above extensionscalled "buttressroots" found at the sea-level and extends inland over gently roll- bases of many large trees. And again, leaves ing hills for approximately80 kilometers. of many unrelatedspecies are similar in ap- Wide, sandy beaches fringe the coast. Behind pearance, often to the point of monotony. them lie brackish lagoons and mangrove These physiognomicfeatures are not typical marsheswhich reflect tidal fluctuations.Begin- of particular families, but rather they are ning at about 80 kilometers, the elevation characteristicof widely scattered systematic graduallyrises to 180 or 220 meters, with groups.In other words, among the many dif- low hills rangingsomewhat higher. Mt. Nimba, ferent species of trees one may find adaptive that was first explored by Chevalier,has an traits that look alike. Similarityof form con- altitudeof 1752 meters. Much of this area is trasts markedly with taxonomic diversity. now being exploited for its rich iron ore de- Understandingthe causal processes that lead posits, said to be near 69% pure,by LAMCO, to evolutionaryadaptations must necessarily the Liberian-American-Swedish Minerals come from careful analysis of plant popula- Company.The native rock is pre-Cambrian, tions, using many tools that disentanglethe and so there are no fossils. It pierces the sur- effects of environmentalfactors from their face in the coastal region, as evidenced by genetic and developmentalcomponents. scatteredrocks and bouldersand again inter- ruptsthe coast by occasionalpromontories of Succession varying height, such as Mamba point at Tropical communities are examples par Monrovia. excellence of "ecosystems," to use Tansley's At one time Liberia was almost entirely word (18), where complex interactionsoccur coveredby rain forest. In the last half century, between soil, fauna, flora, climate and the however,large sections have been clearedfor interventionof man. Like a living organism, temporaryuse in shiftingcultivation, including they exist in a dynamicstate, and theirparts- much of the coastal plain. Each year 50,000 trees, shrubs, and vines-are renewedby the acres are cleared in Liberia by this method ceaseless adaptiveactivity without which they alone. Shiftingcultivation is perhapsthe most 336 THE AMERICAN BIOLOGY TEACHER prominent of man's activities in the tropics, probably a greater influence on the existing rain forest than any other single factor. I have found striking evidence of such interference. One day I was walking through an area of forest with my "tree-finder" in search of a IC. suitable tract for study. We found an area whose trees he clearly identified as "secondary" species. Proceeding further, we came upon a huge specimen of Piptadeniastrum africanum, which is a common rain forest

species. I asked how this could be, that this Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 rain forest specimen should be growing in the middle of a secondary forest. He explained 5o 2 that long ago the rain forest had been cleared for farming, except for this particular tree which was left standing because of its large size. In due time, the land was abandoned, to lz7-- be overtaken by secondary forest. In a second

tract that we established as rain forest we J P M A M J J A S 0 N D passed low mounds, which marked the spot Fig. 5. Monthly rainfall in Liberia during 1963. Measure- where a village once stood, perhaps 2 or 3 ments were taken at the Firestone Research Station in centuries ago. This second tract was obviously Harbel. Average annual rainfall for 1936-63 was 129.15 inches. The total for 1963 was 136.93 inches, or 3478.1 much older than the first. It may be that all mm. (Courtesy, Firestone). the tropical forest has been subject to such human interference at various times in the far richer than can be found anywhere else on past (19). earth (Figs. 5, 6). As such, rain forest is re- The rubber plantations have also replaced ferred to as "climax," since it is the final and considerable rain forest. Along the 66-kilom- Fo C? eter drive to Monrovia from Robertsfield, the international airport, a newcomer might easily 9d, _ _ 32e mistake the mass of green on every side for rain forest. Actually, none is visible. Farm - land abandoned by tribal people, and this W0 _, __27 e usually occurs after 1 or 2 years' cultivation, generally is replaced by savanna and second-

ary forest, both of which are now prominent 70'- 2i? features of the Liberian landscape. One may also note an occasional high tree, such as M~1 J J A S O N D J F ? Piptadeniastrum africanum, mentioned above, 90% thrusting its flattened crown above a dense canopy of green (Fig. 7). It has survived destruction, and provides a clue to the past. 80%

Rain Forest The rain forest is the characteristic vegeta- 70% tion of the humid tropics. Wherever there is a combination of heavy rains and sufficiently 60% high temperatures-and this is the case for many equatorial areas-one can expect to find 4 J J3 A S 0 N D J F M A this rich organization of plant life. A yearly Fig. 6. Temperature and relative humidity in the tropics at rainfall around 3500 mm, most of it falling approximately sea-level. (Courtesy, Firestone) from to October, and temperatures con- (a) Monthly march of temperature for 1963. May (b) Relative humidity fluctuations for 1963. Two readings tinually from 250 to 35?C, insures that it is were taken daily. BIOLOGICALPROBLEMS OF TROPICALVEGETATION 337 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 A

Fig. 7. Savanna and tropical forest. The savanna tree in the foreground could not survive in a mature forest. Two "emergent" trees, Ceiba and Piptadeniastrum, rise above the "canopy." Note the sharp, perpendicular boundary between the 2 communities. Palm trees partly compose the interface. Poro Bush. most stable vegetation that may be evoked by stage, according to their nature. this combination (20). The great von Humboldt referred to South Despite extensive depletion around the American rain forests as "Hylaea" in his eco- world, particularly in the last 50 years, these logical writings of the last century (23). luxuriant evergreen formations still thrive in "Rain forest" was coined by Schimper in his West and Central Africa, the far east, and in classical studies on plant geography in 1898, South America. Smaller stands still survive and this is the generally recognized term in as relicts in such widely scattered areas as English used today (24). The most authorita- certain regions of Australia, India, China, and tive source in English is by P. W. Richards islands of the sea (21). (25). The term "la foret dense" is often used Perhaps the earliest description of rain in the extensive French literature, and serves forest is found in the account of Christopher as the title of the very fine survey by Schnell Columbus' visit to the New World. He wrote (26). The excellent study with an experimen- in 1492 that the island of Espaniola was (22) tal approach done by Schulz in Surinam filled with trees of a thousand kinds and should also be noted, and also the works of tall, and they seem to touch the sky. And Mangenot (27, 28, 29). A number of differ- I am told that they never lose their foliage, ent types of tropical rain forest such as moist as I can understand for I saw them as green forest, dry deciduous, moist montane, and high and as lovely as they are in Spain and in forest are usually recognized, each with par- May and some of them were flowering, ticular characteristics. "Rain forest" is used in some bearing fruit and some in another this article in a general sense to include any of 338 THEAMERICAN BIOLOGY TEACHER these types that may be regardedas mature. will show differentdimensions, depending on It is essential,of course, to have a uniform local conditions, and in some cases, it may nomenclaturein order to avoid the confusing not even be possible to detect stratification terminologythat so often appearsin popular (32). In Fig. 10 the canopyis not completely and even in scientificliterature. For example, closed. one comes upon the non-technical"scrub," Figs. 9 and 11 show the heightsof the trees "jungle,"and "bush,"the latter of Australian in the profilediagrams arranged in increasing origin. These are not useful in biology, since order, making possible the delineation of 3 they are often appliedindiscriminately to vari- separate height classes. Notice that in Fig. ous kinds of tropical, and even to temperate 9 there are four emergent trees, rising above vegetation(30). 40 meters,8 that are 25 to 35 meters,forming A few years ago all of the vegetationtypes the canopy, and 12 scattered through the in West Africa were classifiedand described understory.In Fig. 11, the height classes are Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 by an internationalgroup of plant ecologists not as clearly defined since the canopy was meeting at the town of Yangambi in the found to be fairly open. Similarstratification Congo. They employedthe suggestionsof the promptedvon Humboldtto describe such an French biologist, A. Aubreville,as the basis organizationof trees as a kind of forest above of their report. The Yangambireport, which a forest (33). is very importantfor biologistswho are inter- In general, rain forest is characterizedby ested in the ecology of the area, containsthe (a) the large numberof tree species present, following accurate definition of rain forest and (b) their unique spatial arrangement. (31). Thus, they differfundamentally from temperate forests. Whereas a deciduous forest of Closed stand with several strata including the AmericanMiddle West may have only 2 an upperstratum of large trees; understory dominants,such as a maple-beech,or an oak- of shrubsevergreen; grass stratum generally hickoryassociation, a rainforest has hundreds absent or, if present,of species with broad of species. The other main feature is shown leaves. by its stratification.The trees at maturity reach differentheights. Their branchesstick out near the As do Figs. 8 and 10 are "profilediagrams," illus- tops. they so, they meet branches extending from their trating developing stratificationin a tropical neighbors, thus a of forest. As may be noted in the diagrams,it is producing layering height classes. However, this feature is not convenientto refer to 3 "layers"or "strata": obvious because of the visual impact (1) the "A", emergentlayer, composedof the caused by the mass of highest trees, reachingin these examples 60 greenery. Since the forest is are meters; (2) the "B", canopy layer, formed alive, there many trees when the crowns of trees coalesce about 30 immature between one stratumand the either meters, to form the closed, upper canopy of next, that have not yet grown to the or are the forest; and (3) the "C", understory, upper level, too precocious for the lower. The visual whose trees, many of them juveniles,reach 10 consequenceis an apparent to 20 meters. vertical continuity,as shown in Fig. 1. One must not draw the impressionfrom necessarilysimplified diagrams like these that Secondary Forest these "strata" are easy to recognize, like Secondaryforest is the name applied to shelves on a wall. They are not. You do not those communities,composed principallyof see "strata"in the picture shown in Fig. 1. trees and lianas, that grow up quickly on What is meant by "layers"or "strata"is that abandonedland. Since farm land is generally when height measurementsare made of tree abandonedafter 2 or 3 years' use, there are populations,as in these examples, groups of now vast areas in the tropics covered with trees seem to vary in height about a mean. this growth, particularlynear the population In the mature forest community, there are centers. It is called "secondary"because it several such height classes, reflectingthe inti- is the second growth after "primary"rain mate interactionof trees with vertical varia- forest has been removed.It growsup by itself, tions in environment.Different strips of forest as an adaptiveresponse to humaninterference. BIOLOGICALPROBLEMS OF TROPICALVEGETATION 339 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Fig. 8. Profile diagram of late secondary forest, Bomi Hills, Liberia, showing developing stratification with regenerating Tetraberlinia. Diagram represents a strip of forest 60 meters long and 10 meters wide. Trunks less than 25 centimeters in circumference are not shown. All open spaces are actually filled with branches whose trunks are sometimes outside the strip. These trees are analyzed further in Fig. 9. Measurements made in May, 1963. A, Anthonotha fragrans K, Klainedoxa gabonensis B, Berlinia confusa 0, Ongokgea gore Br, Brachystegia spp. Pa, Paranarium excelsis C, Ceiba pentandra Pb, Parkia bicolor Co, Coula edulis S, Strombosia pustulata T, Tetraberlinia tubmania

Like savanna, therefore, it is essentially diagramsprepared for this study, the second- "man-made,"in the sense that it develops ary forest happensto be higher than the rain after man removes the natural growth of a forest specimen. region. Unlike savanna, on the other hand, Arnold Toynbee ponders at length "this it is transitory;its very structurechanges with challengeof the tropicalforest" in the genesis the years, as it gradually, albeit ineluctably, of civilizations, and assesses the various re- becomes transformedinto mature rain forest. sponseshuman societies have made in the past There are fundamentaldifferences between to the rigorsit has imposed (34). More accu- secondaryforest and rain forest. Where the rately, it is undoubtedlythe fast-growingsec- formeris, indeed, difficultof access, owing to ondarygrowth that has constituteda challenge the extraordinaryprevalence of woody plants, to human achievement. Often an "island"of with the tangle of lianas, herbs, and shrubs, secondaryforest entirely surroundedby rain one can walk throughrain forest with compar- forest is evidence of former habitation. It is ativeease. Then, too, the formeroften does not this heavy growth that some less accurate grow as tall as the latter, and secondaryforest writers of travel books and adventurestories has no clear stratification,except, of course, in often describe as "jungle,"with the implica- the more advanced stages. In the 2 profile tion that it is the original growth of the 340 THEAMERICAN BIOLOGY TEACHER

10 20 30 40 30 60 sal, long dormancyperiods, and quickgrowth: Brachystega ---- Tetraberlinia these are the characteristicsthat have enabled Ongokgea ----- "A", emergent, 45-58 m (4) secondaryforest to spread so fast in the last Tetraberlinia ------"B", canopy, 25-33 m (ll) ..- _ understorey, 12-19 m (JO) few decades. We thereforeare left with visu- _____"C", ... alizing relativelysmall societies of secondary flora formingthe edge of the rain forest, and Anthonotha ______their seeds lying dormanton the edge of the Tetraberlinia ______Coula ------forest floor, often for long periods of time, Strombosia ------springingup quicklyand colonizingnew habi- Tetraberlinia ------Coula ------tats when any chanceoccurrence brings newly favorableconditions of illumination. Berlinia ______--______

has fewer Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 Strombosia ______-______The secondaryforest population Coula ------species and they requirerelatively more light Klainedoxa ______--______-__ their are se- Tetraberlinia ?______--______-__ for propagation. These species lected for growth when the conditionsof increased light are since also Paranari ------met, and, they Parkia _- are capable of wide dispersal, they quickly Ceiba -_ _ _ - become establishedwherever there is an aban- lo0 0 0 4 o 6) Height, meters doned clearing,in gaps made by fallen trees, or as the outer curtainof the forest. Fig. 9. Heights of secondary forest trees of the Fig. 8 specimen. This chart shows 3 height classes correspond- ing to the "A", "B", and "C" levels. As the forest grows Savanna older, presumably Tetraberlinia will become one of the A dominants in the upper levels, as succession proceeds typical savannais dominatedby grasses toward climax. Note that the larger size trees are rep- and scattered trees. There are, of course, resented by few species and individuals among the various kinds of savanna throughout the juveniles. world, includingthose with almost no trees, and othersthat have them organizedin groves tropics, which, of course, it is not. and clumps. Althoughin West Africa special- Secondary growth in the tropics, as any- ists usually find it difficultto distinguishlate where else, requires more light. Any area secondaryforest from rain forest, even a new- that is open to the sun, such as an abandoned comer can easily recognizethe distinctivefea- field, a gash slashed in the forest cover by a ture of the coastal savanna landscape. This fallen giant, or a newly-cut road, soon exhibits feature is the continuousgrass cover that is a vigorous outburst of growth. New masses punctuatedby isolated trees (Fig. 4). of leaves, vines, creepers, saplings quickly The proposedYangambli classification lists coalesce in a dense, chaotic tangle. One can 4 kinds of savannafound in the sub-Sahara: often observe this growth covering a fallen woodland, tree, shrub, and grass. The tree log. Secondary growth also forms a curtain of savanna, which is found in Liberia, particu- green on either side of a road cut through rain larlyin the coastalregions, is given this defini- forest, as well as its outer boundary. Any open tion (37): space, therefore, acts as a selective agency; it confers an advantage on those organisms Formationof grasses at least 80 cm high, that are adapted to increased illumination. forming a continuous layer dominatinga lower stratum. Usually burnt annually. The question may then be asked, where Leaves of grasses flat, basal, and cauline. did secondary vegetation come from? For Woody plants usually present. Trees and example, how can Elaeis guineensis quickly shrubs scattered. appear in the space made by a fallen tree, when the nearest kindred palm stands miles The Liberian savannawith its tall grasses away (35)? In general, secondary vegetation and scatteredtrees, forms a unique park-like has always been present as the outer bounda- appearancethat is altogetherpleasing to the ries of rain forest proper, thereby forming the eye. In addition,a rich mixtureof smallherb- green curtain one sees from the roadside (36). aceous plants clings to the surface of the The answer lies also in the other character- ground. There is abundantflowering during istics of secondary growth. Wide seed disper- the rains,usually evidence of firing,and often BIOLOGICALPROBLEMS OF TROPICALVEGETATION 341 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Fig. 10. Profile diagram of rain forest, Tchien, Liberia, to illustrate variety and density of species. Heights and crown shapes are approximate. Notice canopy toward right is fairly open. This particular strip represents a forest that is either untouched ("virgin" forest), or extremely old secondary. It may therefore be regarded as a climax specimen. Measurements made in April, 1964. C, Carapa procera M, Monopetalanthus compactus Co, Coula edulis P, Paranarium excelsis Di, Didelotia unifoliata Pe, Pentadesma spp. D, Diospyros spp. S, Strombosia glaucescens G, Gilbertiodendron preussii Ta, Tarietia utilis K, Klainedoxa gabonensis T, Trichilia heudelotia Ma, Mammea africanum sabine U, Uapaca guineensis X, Xylopia spp. a sharp boundary with the nearby forest, due bution of rainfall, which annually varies be- to the quite different ecological equipment tween 500 and 1000 mm (38). Together, possessed by these respective communities the climatic savanna zones form part of the (Fig. 12). north-south ecotone as they merge with the This savanna is man-made and is generally steppe on the north and the tropical rain forest found in the humid coastal regions. There is on the south to form the series, desert- a pronounced climatic gradient from the hu- steppe-savanna-rain forest. mid coast, which has only a short dry season, Liberian coastal savanna vegetation is quite to the relatively arid interior, some hundreds complex. The isolated trees include the oil- of miles inland, where the rain forest gives palm, Elaes guineensis, and also Anthosterna way to "climatic savanna," which is not man- senegalensis, Paranarium macrophyllum, and made. Climatic savanna is composed of the Sacoglottis spp. The imported mango, Magni- Sudan and Guinean zones, that arc across fera indica, also has a wide distribution. It is Africa to the north of Liberia. The vegeta- not a native African plant, nor is it particu- tion of these 2 zones (savanes boise'es) is larly characteristic of savannas, except where dependent largely on a more seasonal distri- it has been planted. All of these trees are; 342 THEAMERICAN BIOLOGY TEACHER

10 20 30 40 50 Coastal savannas, where rain forest is the Strombosia ------climax vegetation,rarely take part in succes- Uapaca _____ ------sion, and one would thereforethink they also Carapa ------Didelotia ------"A", emergents, 40-48 m (6) representa kind of climax. But they depend Coula ------"3", canopy, 28-35 arr(14) ]Conopeta3anthus ______"C", understorey, 10-25 ra (20) for their varyingdynamic stabilities on fire or Yylopia ------

St-ombos5ia ------local soil conditionsthat renderthem inhospi- table to the establishmentof high tree sap- Lonopetalanthus ------lings. It may thereforebe questionedwhether Diospyros ------there is any such thing as a "tropicalsavanna M-mmea ------__ ------Monopetalantlhus ------climatic climax," in the sense that rain forest

Strombosia ------is a climaticclimax (39). On the other hand, Diospyros ------in typically savanna regions, as in the Sudan Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 Coula ______.--______-____ Pentade sma ______--______-_-______and Guinea zones, the climate is not suitable StromboBia ------Klainedoxa ------for rain forest. There, a savanna woodland Ca rapa ------Monopetalantlhus ------climax could theoreticallyappear if firing did Tarrietia ____ ------Trichilia ------?------not occur, but this is rarelythe case. Xylopia ------Monopetalanthus _------Paranari ------Aspects of Ecology Uapaca ------Monopetalanthus ------.----?------A. Evolution of Rain Forest Trees ... a pro- Gilbertiodendron - ?---- __.- ----?------posed genetic model. The Mosaic Theory

onopetalanthus -----?---- ______I------Tarrietia ------? --- ______------In 1938 the Frenchbiologist Aubreville ad- G ilbertiodendron ------?------vancedthe mosaic theory of rain forest regen- eration (40). He found that both young and Height, meters 20 30 40 50 old tree species do not grow in equal propor- Fig. 11. Heights of rain forest trees of the Fig. 10 specimen tions, as would be the case if the climax forest arranged in order of their increasing height. The height classes are not as distinct in this specimen as the one were static. Often, indeed, young seedlings shown in Fig. 9, and the emergents are not as high. are completely different from the dominant Notice that there are 40 trees here, considerably more trees characterizingthe upper levels of the than in the secondary specimen. forest. His views are most likely true for the tropics, and, as such, they may suggest modi- equippedwith efficientdispersing mechanisms. fications of our own views of the temperate They migraterapidly into any open areas that climax forest, which is generally regardedas appear. Assorted grasses form the ground of unvaryingfloristic composition (41). Fur- cover. This rich vegetation, together with its thermore,if the mosaic theory were coupled discontinuousdistribution, suggest the great with modernviews of populationgenetics, we antiquityof savannaflora. mightbetter understand how hundredsof tree From the Ducor Hotel in Monroviaone can species could have evolved in the tropics, and often see duringthe dry season bluish smoke thus advance our notions of the evolutionary flowing low across the distant landscape as process. We could perhaps make useful pre- tell-taleevidence of burninggrass (les feux de dictions as well about the future composition brousse). As a result of such firing, grasses of forest areas. grow right up againstthe perpendicularforest The changesin species that characterizethe wall that is composed of fire-resistanttrees secondaryforest, it will be remembered,are and vines. The fact that savanna can thus accountedfor in terms of succession. That is occur in close proximityto forest in a region to say, the highest trees in a secondaryforest where the rainfall may exceed 3500 mm per are usuallynot representedby equal numbers year raisesthe questionof what relationit has of juveniles in the lower levels, and I have to naturalclimaxes. It arises chiefly through found this to be the case, as shown in the excessive firing with consequent degradation profilediagram. This is not so much an evolu- of soil that follows shifting cultivation. Sa- tionaryprocess as it is rather a movementto vannas thriving in predominantlyrain forest a new ecologicalequilibrium. In a rain forest, areas are thus almost all producedby human by contrast, the changes in the species are activities. explained by the mosaic theory. As a result BIOLOGICALPROBLEMS OF TROPICALVEGETATION 343 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Fig. 12. Typical boundary configuration between savanna and secondary forest. The perpendicular interface formed by the trees is caused by firing the adjacent savanna. Forest trees are fire-resistant. The path in the foreground connects 2 towns in the forest. On the highway to Robertsfield. of his studies of the foret dense in the C6te size may be occurring.On the other hand, if d'Ivoire, Aubreville came to the conclusion a particularspecies has a long life, a small that dominantsdiffer from place to place, and number of saplings will insure its survival. that, in a given locality,they also changefrom So small may be this number that in a re- time to time. This is true even though the strictedarea the species may not be growing maturerain forest itself, consideredas a cli- at all, or, in the words of Aubreville,ne fond max, is undergoingno succession. The com- jamais des petits (40). Abundant saplings binationof many trees comprisingits organi- of another species may all be eliminatedby zation are not fixed in space and time, he said, natural selection, and will not be found in but rather,the combinationschange and suc- the adult stage. The saplings one finds in a ceed one another cyclically from century to particulararea of original, undisturbedrain century. Accordingto this view the forest is forest are not necessarilythose that will char- a kind of restless patchwork, composed of acterize the future rain forest. As the adult constantlyshifting groups of species. Such a trees of a particularcommunity die off, they processis undoubtedlyconducive to evolution. may be replacedby entirely differentgroups Trees reaching to the upper levels of the of species. Fig. 10 may be interpretedin this forest are often representedby few corre- way. spondingjuveniles. There are various reasons Since Aubreville'sforest is a living, inte- why this should be so. The most obvious ex- gratedwhole, in dynamicequilibrium with its planationmay be that juvenilesare found just environment,it is reasonableto supposethat, a few feet outside the area under considera- like a living organism, its parts-trees-are tion. Or, temporaryfluctuations in population replaced,even though the communityretains 344 THEAMERICAN BIOLOGY TEACHER its organizationalintegrity. The floristiccom- process. The most ideal population size, ac- position of a particulararea changes and is cordingto Wright,would be large enough to succeededby a differentcombination of spe- promotevariability, and small enough so that cies, all selected from the larger store of new mutations would not be drowned out. genetic variants. As such, the forest thrives The most effective size for reproductionis as an inner, organic unity, not as a static alwaysless than the apparentsize of this pop- aggregationof discrete elements. ulationin question.An estimateof its theoretical outer limits sets them at 250 and 250,000 Population Genetics individuals (48). It would be interestingto know the effective population size for rain The mosaic theory is to be expected from forest species. the modem views of genetics that have to do with applying the statistics of Mendelian Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 heredity to the processes of evolution. Such Isolation workers as Fisher, Dobzhansky, and more particularly,Wright, have worked out the Effective size is determinedby the degree mathematicaldescriptions of populationsthat of isolationto which the populationis subject. are undergoingspeciation (43, 44, 45, 46, 47). In the forest this partial sub-division into However, even though their work was begun functioningunits may be achievedin a variety more than 20 years ago and has continued of ways that serves to restrict the ease of since, no one has made studies, as far as I reproduction,or the degree to which mem- know, of forest populationswith these math- bers can efficientlyexchange genes. One sim- ematical models in mind. Current literature ple isolatingmechanism may be local hills and reveals little on the genetics of tropical streamsthat impede gene flow. Or again, the vegetation. dispersalrange of pollen mightisolate individ- Implicit in Aubreville'smosaic theory of uals as surely as a large geographicbarrier forest regenerationis the concept of popula- such as a mountainor desert.Partial isolation tion size. Since a forest is composedof many may thus occur even among species whose tree species, we must envisage the mosaic as geographic range is essentially continuous. composedof populationsof sympatricspecies, But isolation is not absolute. Migration of living in the same general area, each with a genic materialinevitably occurs and this also high degree of heterozygosity.These larger promotes the instabilityof gene frequencies, units, or communities,are composed of pop- which, in turn, initiates evolutionarydiver- ulations that are intermediatein size between gencies (49). the very large and the very small. And these It is likely that forests are partiallyisolated populations,while integratedinto larger,more into units whose populationsare intermediate complex units, the largestof which is the rain in size. These units have been optimalin pro- forest per se, are themselvespartially isolated motingthe large amountof speciationthat has from each other in space and time. resultedin tropical vegetation as it is today. Wrighthas pointedout that populationsize Given the intermediatesize, however,we still is criticalin evolution. He has developedthe need to know a great deal about the factors idea that a populationintermediate in size is that modify gene frequenciesand thus bring most efficientfor rapid speciation.In a very on speciation.We need to know the rates of largepopulation, which probably cannot occur mutation in a forest population, and how in nature,new mutationscannot spread easily, selection acts on the genetic variabilitythus gene frequencies therefore remain generally obtained, difficult problems, indeed. Then, constant, and evolution is not optimal. Very too, does the effective size of a species con- small populations,on the other hand, would form actually to the predicted values? And have fairly low variability;they would tend again, how fast do genes migrate across a towardhomozygosity, and any selectionwould geographicrange? It would be of great theo- rapidlylead to extinctionrather than to evolu- retical importanceto see how these genetic tionary divergences.But in a population of concepts that have been developingfor more intermediate size, selection would rapidly than 20 years may be comparedwith actual changethe frequencyof a gene, and this kind conditions in the tropical rain forest. Here of change is the essence of the evolutionary is anotheruntouched field for research. BIOLOGICALPROBLEMS OF TROPICALVEGETATION 345 Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

Found nsecodary frest. .Ciapnada obca.Goswdl seegn re omnnm s"otn

Fig. 13 Drptis Ths ar juenle fro 3: different failes shwnF:ga: simla shapemarp:lm a. Rutaceae. Fudin scnay foet.b Ceibo;00;petnda Bobcee Grow00tXswidely as emrgn tre Como nam is "ctton-

tree." c Entandophragmaangolene, Melioeae.ran.forestspecies AnAfrican ahogany An emerent,

tree."c3. Driptips.opheseagme juvngolensefrme3iaceaent Afrianiis mahogany.A emeilrgsaent rainfogres spcies.lm R

B. Problems of Adaptations . . . a genetic an advantage where there is very little move- approach. Convergent evolution ment of air which would increase water loss. The extraordinary similarity of leaf size There is scarcely any wind at any time, and and shape in the lower levels of the forest has even convection currents are kept at a min- often been regarded with deep interest by imum by the overhanging canopy of the "B" ecologists. In particular, the wide prevalence layer. Drip-tips are a good example of evolu- of "drip-tips" are striking features which even tionary convergence in plants, an "invention" the casual observer may note while walking that has aided many diverse species to spread through the forest. Leaves of widely separated ubiquitously through the tropics (50). taxonomic affinities display this unique fea- This similarity of leaf shape seems to be ture (Fig. 13). It is as though the forest related to the peculiar environmental condi- "molded" the foliage of all members coming tions found near the ground. There is a pro- under its influence into one particular form. nounced vertical climatic gradient in the This trait is seen most clearly when leaves are compared with temperate varieties, which "A"l Ci" show much more variations in size and shape. Number species' 70 25 measured The question then arises, in what way are Average length 2.1 9.1 drip-tips an adaptation to microclimatic con- acumen (mm.), "drip-tips" ditions that prevail near the ground? Water Number species runs off quickly, and thus eggs, larvae, spores without acumen. 37 3 and soluble materials that otherwise might be detrimental to the health of the plant are Fig. 14. Relation of leaf apex and tree height, showing that drip-tips occur more frequently in the lower levels rather quickly washed off. This is obviously of the forest. (Harley Herbarium, Monrovia) 346 THE AMERICANBIOLOGY TEACHER

the product of natural selection operating over millions of years. To fully comprehend how this occurred would require a detailed knowl- edge of the genetics of the converging species, a near impossibility because of the crossing difficulties. We must rather visualize the accu- mulation of mutations in populations of these different species. From the store of variability provided by these mutations, appropriate similarities may have been selected by environmental agencies. Only a modest mutation rate would be necessary to provide an enor- Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 mous amount of genetic variability. Sets of mutations producing elongation of the acu- mena in leaves would have a selective advantage; that is to say, they would live to reproduce more offspring, and thus over many genera- Fig. 15. Juvenile and adult leaves of Entandrophragma (African mahogany). Note that the pronounced drip-tip tions this adaptive trait would increase in fre- of the juvenile on the left, from a sapling about 2 quency until it would characterize a large meters high, is much diminished in the adult form, proportion of the populations involved. shown on the right, taken from about 50 meters above the forest floor. Natural Selection at Work forest. The lower levels are characterized by One quickly notices in the forests the but- relative constancy of temperatures. At 40 tress and stilt roots that characterize a large meters above the ground the climate will be proportion of the trees. These are peculiar much different from what it is on the surface. features of the lower levels of the trunks Canopy and emergent foliage are exposed to (empattiments) and constitute one of the wide fluctuations in illumination, temperature, most remarkable features of the forest archi- wind velocity, and relative humidity. Trees and shrubs that in the adult stage occupy the lower half of the forest are adapted -~- generally to one set of conditions, namely, to a relatively uniform, equable microclimate near the surface. But the taller members of the community, including secondary individuals, the emergent "A", and the "B" canopy trees must, during their lifetime, adapt first to the poorly-lit, unvarying surroundings near the forest floor, and, as they reach adulthood, they must become accustomed to much more variable conditions (Fig. 14). Juveniles of these trees have drip-tips, and, as they push to the higher levels, "discard" them for leaves of more variable shapes and sizes (51). Such differences are thus quite clearly correlated with the vertical climatic gradient that charac- terizes the internal climate of the forest. How could this similar adaptation appear, or "converge," as it were, in so many unrelated forms? This extraordinary similarity in the lower levels cannot be due to chance. It would be useless to speculate on the random fixation of homologous genes in all these dif- Fig. 16. Supporting root systems. Stilt roots (rocines- a6riennes) of Uapaca guine6nsis. Apex of roots is about ferent species. The adaptation is, of course, 4 meters high. BIOLOGICALPROBLEMS OF TROPICALVEGETATION 347

leaves the domain of botany, we should say that the winged roots are like the massive buttresses of our roman cathedrals, and the aerial roots play the role of the slender buttresses of our gothic naves.

It is true that buttresses and stilts give stability to the tree, as Schnell affirms. They are thus an adaptation for anchorage, since sub-surface roots by themselves do not always confer adequate stability to a tree that may

reach well over 30 meters in height. Tropical Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 root systems proceed along the surface of the soil and do not as a rule penetrate more than a few dozen centimeters. The large roots are all superficial roots, and they ramify abun- dantly near or on the surface. Tropical soils tend to be poorly aerated, and competition for oxygen and organic materials precludes deep #71 penetration. On the other hand, there are more favorable supplies of water and minerals at deeper levels. The balance between these 2 conditions determines the depth of penetration, which, for tropical trees, is not very great It would Fig. 17. A road-cut reveals the roots of Ceiba ramifying (Fig. 17). be easy to conclude, near the surface. Near the town of Gbanga. therefore, that buttressing is an adaptation for support, serving in an analogous role to that tecture. On first sight it is easy to conclude of the deep root systems of temperate trees. they help support the tall trees, and that, like But it may be doubted whether this adapta- the features of leaf shape, they are another tion has considerable survival value. If but- example of convergent evolution (Fig. 16). tressing does confer greater survival value for Buttress roots (racines palettes) are large, anchorage, then the extra mechanical support thick plates, often as high as a man, that ex- would represent a selective advantage over a tend out from the trunk like upturned tables. species in which they are absent. But the They originate in the vigorous cell division, adaptation is not found in temperate trees called epinastic growth, that occurs along the that would clearly profit by them. Sequoia upper surfaces of the lateral roots growing out gigantea in the Sierras, for example, which is along the surface of the ground. As a result, much taller, has no buttresses, yet it is quite the buttress roots actually grow up to assume able to withstand violent windstorms. On the their peculiar shape. They are found in trees other hand, tall tropical trees are known to that extend as high as the "A" and "B" layers. topple over in the wind, and so more efficient Stilt roots (racines echasses), on the other means of anchorage surely would be an ad- hand, spring from the trunk about 1 to 3 vantage. Then, too, buttresses and stilts are meters above the ground and arch down into sometimes absent from trees that have large the superficial layers of the soil. Often they crowns to support. For example, Gilbertio- anastomose in the air. The trunk appears to dendron rises with a straight bole to 50 meters be propped upright by these stilts, like so with no buttressing. many legs of an immense spider. Most trees Various theories of buttress formation have with stilts belong to the "C" story. One also been put forward, but there is at present no finds them in other habitats, such as in adequate explanation for their development marshes and lagoons. In the words of Schnell (53). No doubt growth hormones play an (52): important part in this phenomenon, and more information on their secretion and transloca- If we wish to make a comparison that tion, perhaps in the epinastic regions, would 348 THEAMERICAN BIOLOGY TEACHER help solve this biological problem. Of course, i 8 u4 * ~ Bg 3 8 a. there is a genetic basis as well. Nuclear mate- rial is expressed epigenetically in the rather close correlation that exists between buttressing habit, root system, and the type of soil involved. In any case, buttressing has arisen by the process of natural selection acting on groups of random mutations. These mutations act on 7.- particular processes of development and me- tabolism in such a way that phenotypical dif- I@ 1 ferences appear between a parent generation 6 6. t i M2 ?4 . it. and its offspring. Accordingly, modifications Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 Fig. 18. Hourly variations of temperature and relative of the lower trunk become more pronounced humidity on a forest floor during a 3-day period, Feb- from generation to generation, and spread ruary 13-15, 1963. Poro Bush. through the populations until the adaptation known as buttressing has been established. most prominentof these communitiesin tran- Natural selection, of course, does not cause sition are the secondary forests. Sometimes this continuing contrast, like some mysterious savannaalso may develop in a similarfashion deus ex machina acting within living things. (Fig. 19). When a communityhas changed Rather, it is the contrast, and, as such, is a sufficientlythrough the years so that it may powerful explanation for the variations we have an entirelydifferent character, then suc- observe in nature. cession has taken place (55, 56). However, the problem still remains of de- Tropical succession means the gradualre- termining how particular mutations, them- placement of secondary growth and, under selves with little or no survival value, are certainconditions, the replacementof savanna, selected and spread through the population. by a process that results inevitably in the It would seem that natural selection acts in- dynamicallystable rain forest climax, that is, directly on these mutations. A theoretical in relative equilibriumwith its environment. explanation might involve the pleiotropic This process occurs in communitiesof every effects of a single group of genes. That is, possibledescription, such as those along road- 2 or more character differences may appear sides, under power lines, or on open fields. together in successive generations, and are said Many secondarycommunities have been de- to be correlated with each other, if they are scribedin the tropics,but so far there are only produced by a single set of genes. In other a few systematicstudies of the particularsuc- words, a single group of genes may be respon- cessionsof whichthey are stages.One of these sible for one particular trait that is adaptive, is a study done in Nigeria (57). and also might cause the appearance of but- That secondarygrowth does in generalrep- tressing that does not have as high a survival resent stages in the reconstitutionof mature value (54). The trait with the higher survival rain forest is generallyaccepted by those who value would produce more offspring and are familiar with tropical vegetation, begin- would drag the buttressing habit with it ning as far back as Chevalier,almost 50 years through the population. Of course, there is no ago (58). Most studentswould agreewith the experimental evidence for this point-of-view. eminent British authority, P. W. Richards, Nevertheless, pleiotropy probably occurs in who has written(59): plants in a rather high frequency, and likely is responsible for the establishment by selection Secondary communities derived from Trop- of character traits which may not themselves ical Rain Forest, whether they have the aspect have high survival value. of forest, scrub, savanna or (as is often the case) of a chaotic wilderness of trees, shrubs, C. Patterns of Secondary Succession herbs, and climbers, are always more or less Many ecological communities in the tropics unstable and are thus stages in secondary suc- are only temporary stages that in time are re- cession. Left to themselves and protected from placed by other kinds of communities. The felling, burning, and grazing, they are grad- BIOLOGICALPROBLEMS OF TROPICALVEGETATION 349

1st centuxry RAIN FOREST, climax or more., Parkia, Terminalia, / Uapa%a, Khaya, Anopyxis, Gilbertiodendron, etc.

Micro-climate Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021

beyond SECONDARYFOREST, transition 25 years Musanga, Ceiba, Fagara, Macaranga, etc.

protection ;7 K from ' S,' fire \N. SAVANNA, sub-climax Eleis, Paranari, / Saccoglottis, Anthostema, etc.

\ / z F - -~~fire

3rd to 6th Pioneer stage year Cissus, Ipomea, Lippia, Chance factore Arvundinella, etc.

Cleared Land, Abandoned

Suggested paths of ecological succession in Liberian coastal areas-.

Fig. 19. Suggested paths of ecological succession in Liberian coastal areas. 350 THEAMERICAN BIOLOGY TEACHER ually invaded by primaryforest species and resulting community. For example, one often there is little doubt that the climatic climax finds patches of grasses and vines thriving side would ultimately re-establish itself, though by side. A competition inevitably occurs be- probablyafter a very long time. tween these invaders, the outcome of which determines whether the young community will To be sure, one does not see succession become savanna or secondary forest. Some of taking place, for the re-establishmentof a the factors which determine the outcome in- as much as a cen- climax forest may require clude the nutritive quality of the soil, the one tury or even more. Nevertheless, may developing microclimate, the proximity of visit forest. Rain forest easily secondary ju- already established savanna, and the degree veniles may be growingin its shade. Or, when of firing. If left to itself, as Richards points one comes upon Musanga cecropioides grow- out, the secondary species will eventually ing along the edge of a clearedfield, or by a overcome any savanna vegetation. Vines, if Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 roadside, where there is plenty of light, we the field is untouched and protected from fir- know again that succession is in operation. ing, may enclose the grasses and gradually This tree is never found inside a shady forest, stifle them, thus making way for invasion by for it cannot regeneratein shade. It is gen- secondary juveniles. On the other hand, firing erally a one-generationtree, and may be con- ( le feu de brousse) quickly clears the surface sidered a pioneer species, thus aiding the and halts the growth of ligneous shrubs and of development an unstable into a stable saplings. Fire acts as a selective agency by ecosystem. favoring the continued installation of grasses Chance Factors whose extensive root systems send forth new Althoughsuccession in generalmay be ini- growth in a matter of days (61). tiated by diverse agencies, in the tropics it is the abandonmentof cleared land that gen- Microclimate erally sets in motion this complex process. In the second stage, which lasts from 3 or Various interlockingcauses bring about the 4 years to perhaps a quarter century, woody reorganizationof new biotic equilibria,until plants gradually gain dominance and tend to une formation en equilibre is established. suppress the herbaceous plants and grasses. Schnell speaks of the "progressiveevolution" Chance factors gradually cease to operate as of secondaryforest, and, on the other hand, the competition for light increases. The flor- of "regressiveevolution," whose ultimate ex- istic composition of the emerging forest is pression is savanna (60). Research is badly determined more and more by the character neededto elucidatethis process. of the microclimate that is being established There seem to be 3 stages in the reconsti- (Fig. 19). Younger generations of seedlings tutionof the rain forest. The first stage is very cannot thrive in the ecological milieu-shade short, probablylasting no more than 3 or 4 -their elders have created. For example, years. It is characterizedby colonization of some trees, such as Musanga, cannot regen- the open field. The initial colonizers may erate in their own shade, and therefore tend either come from the seeds alreadypresent in to last only a single generation (62). Other the soil at the time of abandonment,or they trees in this stage may include Fagara macro- may be herbaceous invaders from nearby phylla, Macaranga barteri, and Ceiba pen- fields. The colonizers may also be derived tandra. The closed carpet of herbs gradually from a combinationof these 2 sources, de- disappears as the amount of shade increases. pending on local conditionsprevailing at the As the third and final stage commences, time of abandonment.If the soil has survived more and more rain forest seedlings invade burningwithout extensive damage then con- the congenial surroundings of such a micro- ditions for germinationwill favor seeds al- climate, and, because they can regenerate in ready present just below the surface. If, on the shade of secondary trees gradually gain a the otherhand, severesoil degradationaccom- foothold. These new invaders may include Ter- panies burning,then grasses and shrubsmay minalia superba, Paranarium glabra, Parkia invade from neighboringareas. bicolor and Tetraberlinia tubmania, the latter This short first stage seems to be critical found only in Liberia. As these increase in and probablydetermines the characterof the number they in turn suppress the secondary BIOLOGICALPROBLEMS OF TROPICALVEGETATION 351 species, a process that may requirethe better Genetic Future part of a century, or even more (63). The Rain forest once covered large areas of result is that the structureof the secondary West Africa. In recent years, however,under forest changesin time. Figs. 8 and 9 show the the influence of rapid economic progress, analysis of a plot of trees in a developing much of this has been replaced by savanna secondary forest. The varying height classes and secondaryforest. Saturatedas they are are distinct characteristicsof the coming rain by the end productsof evolution,all 3 vegeta- forest per se. tion types representactivities of great interest Each stage tends to preparethe way for the to modem biology. Together they constitute next. It providesthe necessaryenvironmental a vast outdoor laboratorywhere many prob- conditionsin which succeedingjuveniles may lems involving the operation of living things

get a start(64). As the microclimatedevelops, await solution. However, undoubtedlymany Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 climax species graduallybecome more dom- years will pass before the biology of the trop- inant (Fig. 18). The first 2 stages are rapid- ics catchesup with modem experimentalbiol- growing and light-demanding,typical of sec- ogy. In the meanwhile, les ge'ants are cut ondaryforest. The last stage requiresless light down. and grows much more slowly, more charac- The encroachmentsof secondaryforest will teristicallyof the emergingrain forest. In the continueto bring on furtherdepletion of rain words of Schnell (65), forest until the numbers of particulartrees decrease to a level too small to insure sur- * . . Ia foret secondaire evolue progressive- vival. We can therefore expect that in the ment, et tend peu a peu a acquerirune com- future many species of rain forest trees will position voisine de celle de la foret primaire. become extinct-small genetic populations again-thus decreasingthe store of variability availablefor adaptationsand migrationelse- where. For many individualspecies no doubt this process is already irreversible.But it can be slowed, and probably stopped altogetherfor many others by setting aside large areas of mature rain forest as natural preserves or parks,the largerthe better, under the type of AlI ~ ~ ~ ~ A managementthat will insure their remaining untouchedas they are, and carefullypreserved for furtherstudy. At the same time, increasing attentionshould be directedalso to the unique secondaryforests, rich in biological organization and possible foci of future evolution, :4 *.i' * /, .* where study may well yield rich dividendsin the understandingof living systems. Citations 1. Wallace, A. R. 1878. Tropical nature and other essays. London, Macmillan and Co. (365 pp., preface) p. 65. 2. Aubreville, A. La flore foresticre de la Cote d'Ivoire. Centre Technique Forestier Tropical, Nogent-sur-Marne. Paris, La Rose, 1936 (trois tomes), Tome I, p. 8. 3. Norman, Didier. Atlas des bois de la Cote d'Ivoire. Centre Technique Forestier Tropical. 45, bis, avenue de la Belle-Gabrielle.Nogent-sur- Maine (Seine)-France, 1950. I, II. 4. The Harley Herbarium at the University of Fig. 20. Ficus. The central core is a dead trunk, having Liberia contains 230 forest species among 44 been surrounded by the "strangler fig," which forms the families. It is named for Dr. George W. Harley, living portion of the structure. who spent 20 years with the Methodist Mission in 352 THEAMERICAN BIOLOGY TEACHER

Ganta, and contains his collection. 21. Cain, Stanley A. Foundations of plant geog- 5. Watt, A. W., 1961. "Ecology," pp. 115-131, in: raphy. New York, 1944. (556 pp.) Contemporary botanical thought. Macleod, A. 22. Jane, Cecil (Tr. and ed.), 1930. Select docu- M., and L. S. Cobley, eds. Chicago, Quadrangle ments illustratingthe four voyages of Columbus. Books. p. 130. Vol. I, The first and second voyages. London, 6. Much of the field work in preparation for this The Hakluyt Society. pp. 4 and 6. paper was done in secondary forest and savanna 23. Humboldt, Auguste von. 1852. Personal narra- surrounding Monrovia; in a forest relict, called tives of travels to the equinoctial regions of the "Poro Bush," near the ELWA radio station; the new continent, 1799-1804, Williams, H. M., the Bomi Hills; a visit to Mt. Nimba; rain forest Tr. London, Longmans Green, 1824 (7 vols.) on the Bong Range and near the town of Tchien; 24. Schimper,A. F. M., 1903. Plant geography upon the use of the "ecological tower" in a stand of a physiological basis. Fisher, W. R., English tr., forest on the grounds of L'Institut d'enseigne- Oxford. pp. 839, 260. ment et de recherches tropicales; and at the 25. Richards, P. W., 1957. The Tropical Rain Foret de Banco, Abidjan, Cote d'Ivoire in April, Forest. Cambridge, at the University Press. Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 1963. (450 pp.) 7. Waddington, C. H. "Evolutionary adaptations." 26. Schnell, Op. cit. Perspectives in biology and medicine. University 27. Schulz, J. P., Ecological Studies on Rain Forest of Chicago Press, Vol. II, pp. 379-401. Summer, in Northern Surinam. Amsterdam, North- 1959. p. 379. Holland Publishing Company, 1960. (267 pp.) 8. For accounts of early scientific explorations of Extensive bibliography. West Africa see Aubreville, Op. cit., pp. 13-15, 28. Mangenot, G. 1950a. Les forets de la C6te and Hutchinson, J., and J. M. Dalziel, Flora of d'Ivoire. Bull. Soc. Bot. France, 97.156-7. West Tropical Africa. London, The Crown 29. 1950b. Essai les forets denses de la Agents for the Colonies, Kew. Ed. 2, 1954. Pre- C6te d'Ivoire. Bull. Soc. Bot. France. 97.159-62. face, Vol. I, Part 1. See also Graham Greene's 30. Tansley, A. G., 1935. "The use and abuse of novel, Journey Without Maps, an account of his vegetational concepts and terms." Ecology, trip across Liberia in 1942. 16.284-307. 9. Schnell, R., La fore?tdense. Introduction a l'e- 31. Boughey, A. S., "The physiognomic delimination tude botanique de la region forestiere d'Afrique of West African vegetation types." pp. 148-165. occidentale, 1950. Paul Lechevalier, Editeur. Journal of the West African Science Association. 12, rue de Tournon, Paris VIe. p. vi. A well- August, 1957. p. 151. written survey of tropical vegetation. 32. Richards, Op. cit., pp. 24-39. 10. Bates, M., Where Winter Never Comes. New 33. Humboldt, Op. cit., et passim. York, Charles Scribners Sons, paper, 1963. (316 34. Toynbee, A. S., 1934. A Study of History. Vol. pp.) p. 127. I, The genesis of civilizations, p. 321. New York, 11. Personal communication from WHO, Brazza- Galaxy, Oxford, 1962. (484 pp.) p. 321. ville, Congo. 35. For a full discussion of this problem see 12. Chevalier, A., Rapport sur une mission scienti- Richards, Op. cit., pp. 386-387. fique en Afrique occidentale. Recherches de 36. Schnell, Op. cit., 1950, p. 19. 1906-07 'a la Cote d'Ivoire. Nouvelles Archives 37. Boughey, Op. cit., p. 153. des Missions Scientifique et Litteraires, xviii, 38. Schnell, Op. cit., 1950, pp. 7-13. fascicule 3, Paris, 1909. 39. Richards, Op. cit., pp. 340-343. 13. Lechevallier, Paul, ed. Exploration botanique de 40. Aubreville, A., 1938. La foret coloniale: les l'Afrique occidentale francaise. Tome I. Enume- forets de l'Afrique occidentale francaise. Aca- ration des plant recoltees. Paris, 12, rue de demie des sciences coloniales, annales 10. Paris, Tournon, 1920. 795 pp. 9, 1-245. p. 140, et passim. 14. Aubreville, A., Climats, forets et desertification 41. Richards, Op. cit., pp. 49-53. de l'Afrique Tropicale. Paris, 1949. p. 15. 42. Ibid., p. 43. 15. Hutchinson, and Dalziel, Op. cit., Vols. I, II, et 43. Dobzhansky, Th., "Mendelian populations and passim. their evolution." In: Dunn, L. C., Ed., Genetics 16. Kunkel, G., "The Pteridophytes of the Gbi Na- in the 20th century, pp. 573-589. New York, tional Forest, Liberia." Journal of the West Macmillan Co., 1951. (634 pp.) Africa Science Association, August, 1963. pp. 44. Evolution, genetics, and man. New 121-126. York, John Wiley, 1955. (398 pp.) Chs. 6, 7. 17. Schnell, R., Plantes alimentaires et vie agricole 45. Fisher, Ronald A. The genetical theory of nat- de l'Afrique noire, 1957. (222 pp.) Editions ural selection. New York, Dover, 1958 (1929). Larose, ii, rue Victor-Cousin, ii, Paris Ve. p. 79. (291 pp.) Chs. 2, 4, 5. 18. A. G. 1935. Introduction to plant ecol- Tansley, 46. Wright Sewell, "Evolution in Mendelian popula- George Allen, 1946. (260 pp.) ogy. London, tions." Genetics, March, 1931. pp. 97-159. p. 207. 19. Corner, E. J. H., "Impact of man on the vegeta- 47. 1940. "The statistical consequences tion of the humid tropics." Nature, 189.4758. of Mendelian heredity in relation to speciation." January 7, 1961. pp. 24, 25. In: Huxley, Julian, The new systematics. Oxford 20. Clements, F. E., "Nature and structure of the University Press, pp. 161-183. climax." Journal of ecology, 1936. 24. 252-284. 48. Simpson, George Gaylord. Tempo and mode in BIOLOGICALPROBLEMS OF TROPICALVEGETATION 353

evolution. New York, Columbia University Press, the sprouting of some savanna species." Journal 1950. (237 pp.) p. 68. of the West African Science Association. Feb- 49. Stebbins, G. Ledyard. Variation and evolution ruary, 1963. Vol. 7, No. 2. pp. 154-162. in plants. New York, Columbia University Press, 62. Richards, Op. cit., p. 387. 1950. (643 pp.) Ch. 6. 63. Taylor, C. J., Synecology and silviculture in 50. Richards, Op. cit., pp. 80-89. Ghana, 1960. New York, Thos. Nelson and Son. 51. Jones, E. W., 1955-56. "Ecological studies on Chapter 2. the rain forest of southern Nigeria." IV. The plateau forest of the Okomu Forest Reserve. 64. Schulz, Op. cit., Part 1. Journal of ecology, 43, 564-594; 44.83-117. 65. Schnell, 1950, Op. cit., p. 60. (translation from the French). 52. Schnell, 1950, Op. cit., p. 21. Acknowledgements 53. Cain, Stanley A., and G. M. de Oliveura Castro. 1959. Manual of vegetation analysis. New York, I am grateful to many persons for generous Harper. (325 pp.) pp. 78-82. assistance in the preparation of this paper, Downloaded from http://online.ucpress.edu/abt/article-pdf/27/5/331/21457/4440974.pdf by guest on 29 September 2021 54. Stebbins, Op. cit., pp. 121-123, discusses indirect particularly the members of the staff of the action of natural selection. See also Cain, Ibid., pp. 377, 388. College of Forestry of the University of Libe- 55. Watt, A. S., 1947. "Pattern and process in the ria; Mr. David Queway, Liberian "tree finder," plant community."Journal of ecology, 35. 1-22. who gave on sight the Latin names of all the 56. Clements, Frederic E., 1916. Plant succession: trees in the 2 profile studies; Mr. Hans Otto an analysis of the development of vegetation. New York, Hafner Publishing Co. (512 pp., 61 Schaefer of the German Forestry Mission in pls., 51 figs.) Liberia for numerous suggestions and help in 57. Ross, R. 1954. "Ecological studies on the rain identifying species; the staff of l'Institut d'en- forest of southern Nigeria." III. Secondary suc- seignement et de recherches tropicales, Abid- cession in the Shasha forest preserve. Journal of jan, Cote d'Ivoire, who gave me complete ecology, 42. 259-282. 58. Chevalier, A. 1917. La foret et les bois du access to their library and herbarium during Gabon. Les vegetaux utiles d'Afrique tropicale a visit in April, 1963; and to Professor P. W. francaise, Fasc. 9, Paris. Richards of the University College of North 59. Richards, Op. cit., p. 277. Wales (UK), who kindly read the entire man- 60. Schnell, 1950, Op. cit., p. 91; and, Lebrun, J. 1936. Repartition de la fore^tequatoriale et des uscript and offered valuable comments. All formations vegetales limitrophes. Bruxelles. photographs were taken with a Rolleiflex T 61. Hopkins, Brian. "The role of fire in promoting camera on verichrome film.

Deer Spy Smoking Film Their deer friends might consider it a bit The color film Is Smoking Worth It?, is sneaky, but several deer trained by specialists available through the local unit of the Ameri- of the Arizona Game and Fish Department can Cancer Society at no cost for school or are being used to gather detailed information group showings. Teachers are urged to show on their wild brethren. The captive deer are junior and senior high school students this first taught to load in and out of a truck film and help them understand the facts about voluntarily. When taken into the field, the smoking, the strong social and psychological animals are released and allowed to feed at pressure on teenagers to smoke, and the grave will. Their trainer, a research biologist, uses diseases that are risked by those who do a tape recorder to make notes on what vege- smoke. This is an important, authoritative tation is available, what the deer eat, how film designed to help students decide for much they consume, and what they pass up. themselves that the answer is, "No." When the deer have satisfied their appetites, they return to the truck and climb in. Since the tame deer apparently feel insecure in the Cover Picture woods by themselves or with wild deer, they A termitarium made of mud, found only in stay close to the observer and so far have not forests and serving as a ventilator for its sub- run off. For obvious reasons, however, the terranean termite inhabitants. A liana is feeding observations will not be conducted shown in the foreground. The picture was during hunting season. taken by Richard Aulie in West Africa.