Quaternary Science Reviews, Vol. 6, pp. 77-92, 1987. Printed in Great Britain. All rights reserved.
0277-3791/87 S0.fr0+ .50 Copyright (~) 1987Pergamon Journals Ltd.
LIGHT ON TROPICAL
D. Walker and Y. Chen*
Research School of Pacific Studies, Australian National University, G.P.O. Box 4, Canberra A.C.T. 2601, Australia Although there are more than a hundred Quaternary pollen diagrams from the non-arid tropics, very few are from strictl~ lowland ever-wet rainforest. Nevertheless, those from more upland or seasonal sites imply major changes of size and location of all tropical forests which must have had repercussions on the composition of and niche realization in the lowland ever-wet forest. Among the ecologically most important implications are that these features of the forest have been constantly changing at varying rates for at least a million years and still are changing. While it is no longer possible necessarily to associate diversity with changelessness, the alternative extreme view that diversity is the outcome of Quaternary environmental change has yet to be demonstrated palynologically. The courses of expansions of some trees in the early stages of establishment of an Australian rainforest in the Holocene are fitted to standard population growth curves and the significance of fine resolution pollen analysis for the study of rainforest ecology is demonstrated.
INTRODUCTION Thirty years ago tropical rainforest was thought of as the archetypal biome in which biological diversity and ecosystem stability were the joint products of long and slow evolutionary history operating more-or-less in the regions where the rainforest grows today. As recently as 1975, Whittaker (1975) could write of 'the tropical rainforests with their rich life, relatively stable populations, wealth of strikingly adapted forms and occurrence of survivors of some ancient and primitive groups'. Palaeoecological data which implied criticism of this view was beginning to appear in the early 1960s but when Flenley (1979) drew this together in 1979 his book evoked the response from one reviewer: 'Permit me to think that this can only be a problem to him who is disinclined to recognise that the lowland rainforest is the product of a long evolution under stable conditions' (M.J. 1980). The significance of major changes in the location of forest boundaries in the tropics during the last 20 ka was not lost on the palaeoclimatologists who were also drawing evidence from geomorphology for previously unthinkable associations of climatic components such as coldness'with drought. The apparently widespread occurrence of dry conditions in many parts of the world at about the time of the last glacial maximum was siezed upon by another group of scientists challenging the notion that rainforest diversity is the product of age-long evolution in a constant environment. These were largely taxonomists confronted with the high degree of endemism in many animal groups of Amazonia and the congruence of their distributional patterns as well as the locations of hybrids (e.g. Prance, 1982). Despite all this and a number of relevant reviews (e.g. Livingstone, 1975; Livingstone and van der Hammen, 1978), much discussion about tropical ecology and its significance for ecological theory in
general seems to continue virtually uninformed of the evidence for recent and striking changes in both the locations and compositions of all major rainforest regions of the world. Most of this evidence derives from pollen analysis which is itself now developing the capacity to trace the courses of individual species population changes hitherto unknowable by any means. The purposes of this paper are to review the pollen analytical evidence for change in the location and composition of humid tropical vegetation, to point to its ecological significance, and to indicate how it might be used still more potently. It will dwell on the facts of change, rather than its causes. THE NATURE OF THE DATA
Geographical Distribution Pollen analytical sites are few in the tropics, compared with their numbers in many other parts of the world. Although some generalizations are already possible from them, it is important to be quite clear about the nature of the subject-matter, particularly the distribution of sites amongst the various tropical vegetation types. Very species-rich tropical rainforest in which architectural dominance of one or a very few species is rare and local, is a lowland phenomenon. It is found below ca. 800 m in the Amazon basin, below 1300 m in the Congo and below 700-1200 m in Southeast Asia and on parts of the far northeast coast of Australia. The most important environmental correlates seem to be high temperatures with little diurnal or seasonal variation, high rainfall throughout the year and, at least initially, moderately fertile soils. Within the lowland rainforest area, species diversity is often quite low, and dominance may develop, in environmentally extreme localities, such as perennially water-logged swamps (e.g. of Sarawak and Brunei: Anderson, 1963) or mineral deficient soils (e.g. also of Sarawak and Brunei:
* Present address: Institute of Botany, Academia Sinica, Beijing, China.
l). Walker and Y. Chcn
Brunig, 1974). Secondary forest, following removal of the original biota and its mineral contents, is very slow to approach the diversity of primary forest. Many rainforest canopy trees are thought to be long-lived. As yet there are very few late Quaternary pollen diagrams from anywhere within the primary lowland rainforest. Although the transition from lowland tropical rainforest to upland forest takes place across a wide altitudinal band, there is little difficulty in relegating any particular stand to one or the other type. Although there may be some species in common, some characteristically upland additions are very evident, sometimes gregarious and dominating large areas. Taxonomic diversity is significantly less than that of the lowland rainforest, average bole size is smaller and unbuttressed and the canopy, except for emergent individuals, is not so high. This lowermost band of upland forest is often termed 'lower montane' (e.g. Richards, 1952; Whitmore, 1984; Johns, 1982; Paijmans, 1976; Hamilton, 1982) (but Subandean in South America: van der Hammen, 1974) to distinguish it from an 'upper montane forest'. There are several major late Quaternary pollen diagrams from this zone (Table 1). There is often a gradient, or at least pronounced patchiness, in floristic diversity, biomass, gregariousness and other attributes across the lower montane forest, but there comes an altitude above which the major features of the lowland rainforest are totally absent. This elevation (ca. 1500 m in Malaysia (Whitmore, 1984), 3000 m in New Guinea, 3200-3500 m in the Colombian Andes (van der Hammen, 1974), ca. 3000 m on Mt. Elgon, Uganda-Kenya (Hamilton, 1982)) marks the transition to upper montane forest which often contains genera which are not commonly represented below. The upper montane forests are floristically poor compared with those at lower altitudes, lower (usually <20 m), often microphyllous and tangled. All these characteristics are accentuated upward into the subalpine zone but the forest composition differs greatly from place to place within a region. Several pollen diagrams are available from within this forest type (Table 1). The altitudinal forest limit is reached at about 3700-4000 m in the Colombian Andes (Grabandt, 1980), 3000-3450 m on Mt. Elgon (Hamilton, 1982), 2400 m (but with bamboo and tree heathers to 3900 m) on Ruwenzori, (Livingstone, 1967) and 3800 m in New Guinea. Above it grow a variety of shrubby and herbaceous subalpine and alpine vegetation types which penetrate to lower elevations in swampy, very dry or cold valleys. Numerous pollen diagrams derive from such supra-forest locations (Table 1). Just as there are vegetational gradients and discontinuities altitudinally within the forests of the tropics, so there are at all elevations in response to effective rainfall and its seasonality. In northeast Australia, for example, rainforest occurs on those parts of the coastal mountains and tablelands which receive more than about 1.3 m of rain a year, even though 73% of it falls in 4 summer months. In southwest China semi-ever-
green 'monsoon' forest grows beyond the rainforested river valleys where the soil is inferior (over limestone) or the rainfall strongly seasonal (Li and Walker, 1986). In upland East Africa, Hamilton (1982) and others distinguish 'moist' and 'dry' lower montane forests with distinct floristic and environmental differences. Seasonal rainforests of these kinds usually contain many families and genera, but few species, in common with their ever-wet regional counterparts. They are also less diverse floristically, sometimes of smaller stature and lower biomass and may contain some species with strongly seasonal habits e.g. dry-season deciduousness. The sites of several major Quaternary pollen diagrams lie in these kinds of forest (Table 1). Beyond the point where rainfall, often as modified by soil conditions, can maintain continuous, mesophyllous rainforest, there is more-or-less rapid transition to floristically quite distinct forest, much less diverse and with much lower biomass, such as the various eucalpyt-dominated forests of Australia, the evergreen-broadleaved forests of China, the 'chaco' of Amazonia, and the Tetrameles forests of Java. Where rainfall is even more meagre, these forests give way to savanna, grassland, shrubland and, in extreme cases. desert. Major Quaternary pollen sites derived from such regions are also listed in Table 1. A great deal of the vegetation of the tropics has been affected by human activities, never more than today, ranging from selective food collection through swidden farming to 'industrial' clear-felling. Except for very specialized large swamp habitats, unrepresentative of the general vegetation, we can only say a little about the ecological history of lowland, everwet, tropical rainforest from direct pollen analytical evidence; we simply lack sufficient data. The reasons for this are mainly that appropriate sites are rare; lakes are either large and receiving pollen from enormous areas often of varied vegetation, or have been subject to erosion of their sediments or have been small, filled and overgrown. As will be shown, however, what can be gleaned from the few sites investigated is ecologically revealing. Sites are more abundant at higher altitudes in the ever-wet tropics and in seasonally less wet places, both of which sometimes now lie in forest which shares some of the characteristics of ever-wet lowland rainforest. Phenomena recorded from these sites might be extended at greater or lesser intensity, to the 'typical' rainforest itself. Also, and very importantly, plant geographical changes and shifts in the distributions of the major vegetation units, recorded at any place in the tropics, often have geographical and ecological implications for the lowland rainforest itself.
Pollen Analysis Even under the most favourable conditions, the suite of pollen taxa reaching a site of sediment accumulation is but a selection of the species growing in its vicinity. This applies everywhere but particularly in rainforest, many of the component species of which are not wind-
PalynologicalLighton Tropical RainforestDynamics
pollinated and where many of them are represented by proportionately represented and the patchiness of the few individuals. The identification of a fossil pollen representation may change through time, as when assemblage with a living vegetation type at best changes in the discharge of streams flowing in to a lake depends on indicator species present in both. In alter the quantity of stream-bank plant pollen reaching comparatively simple vegetation the matching process it. Vegetational events which do not affect the whole can be assisted by statistical comparisons between the pollen catchment can sometimes be followed, and even living plant composition of vegetation units and the located, by dating and analyzing pollen from several identities and quantities of pollen reaching sites of cores from selected places in the accumulation site, but accumulation from them at the present day. Transfer the amount of work involved is very large. The functions relating pollen spectrum to vegetation can relationship between the size of a sediment accumuthen be applied, with greater or lesser success, to fossil lation site and the area of its catchment is also assemblages. But in tropical circumstances, for the important and to some degree definable (Prentice, reasons given above, comparisons of this kind are only 1985). Ideally, the pollen analysis site should be chosen useful in detecting which species are likely to be such that its catchment is small enough for its vegrepresented by statistically useful quantities of pollen, etation to have behaved as a single unit in the in distinguishing between grossly different vegetation ecological process to be studied. Put in another way, it units, and in discovering which pollen travels far or is difficult to study processes of vegetation change hardly at all. which are at different stages at the same time within a Spotting the pre-existence of, or changes between, pollen catchment. For example, the course of gap subtly different tropical forest types is presently beyond regeneration in forest cannot be clearly followed in a the powers of pollen analysis. The kind of community catchment many times bigger than average gap size change it can detect very well is one in which pollen unless all the gaps occur at once. indicator taxa for one community are. replaced by those Even in a small pollen catchment it is often difficult of another, even if the quantities of the relevant pollen to separate the pollen-analytical effects of changes in in each fossil sample are small. All this means that, total quantity of a plant from changes in location of the whilst plant geographical changes are more-or-less same-sized population. Replication of sampling can readily discerned in tropical pollen diagrams, details of certainly assist this but, because the analytical process community composition at the ecological level are not. is so labour-intensive, it is rarely carried out. But the first may well imply some important things Having reviewed the sources of our palynological about the second as we have already noted and to data and their limitations and strengths, we now turn to which we shall return (see also Colinvaux, this some examples of their significance for tropical volume). ecology. It can be reasonably asserted (Walker, 1982) that hardly anywhere in the world, and least of all in the IMPLICIT EVIDENCE OF ECOLOGICAL CHANGE tropics, have we made the most of what Quaternary pollen analysis relates to most directly, i.e. measures of The kind of information which orthodox pollen the populations of those taxa quantitatively well- analysis best records explicitly, namely changes in the represented in fossil-pollen counts. There is probably location of vegetation formations, can often have much to be gained from the further development of implications at the ecological scale. It can almost Davis' (1963) pioneering work on the relationships always provide important information about the conbetween populations of species and the amounts of text in which experimental, or observational, ecology their pollen reaching a sedimentation site, including the on living systems is carried out. ways in which these are affected by transport and accumulation (e.g. Davis, 1973; Bonny, 1976; Penning- Late Quaternary Environmental Change and Vegetaton, 1979; Bonny and Allen, 1984; Davis et al., 1984). tional Instability Yet, even with only little information of these kinds, it There is now abundant evidence from every part of is certainly possible to detect significant changes in the tropical world, that forests were differently located pollen quantities of individual taxa through past times before about 10 ka BP than they are today. The date is which must reflect changes in quantity or position of somewhat arbitrarily selected because, where we have the parent plant population within the pollen catch- the necessary data, major change seems often to have ment. Together with good chronological control, been going on as much as 16 ka BP and in some places sampling at short time intervals, replication of obser- to have continued until 7 ka BP. For example, on Mt vations, and the proper statistical treatment of results, Kenya in East Africa the altitudinal forest limit was 700 such fine-resolution pollen analysis is already becoming m lower before 11 ka BP than it is today (Coetzee, a useful method in palaeo-population ecology. But this 1967; Hamilton, 1982). In the central highlands of New is only possible under circumstances in which the pollen Guinea the corresponding difference was about 1500 m catchment is definable and, preferably, small. The at 18 ka BP (Walker and Flenley, 1979; Walker and pollen incorporated in a sediment is the integral of that Hope, 1982) and in the Colombian Andes about produced in the pollen catchment, by definition, but 1200-1500 m before 13 ka BP (van der Hammen, 1974; not all parts of the potential catchment may be Grabandt, 1985). In West Africa, the latitudinal shift
D. W a l k e r a n d Y. C h e n
T A B L E 1. Q u a t e r n a r y p o l l e n a n a l y t i c a l sites in the n o n - a r i d t r o p i c s , o r d e r e d a c c o r d i n g to (a) v e g e t a t i o n t y p c in w h i c h t h e y n o w lic, (b) r e g i o n , (c) l a t i t u d e , (d) a l t i t u d e . S o m e f r a g m e n t a r y sites h a v e b e e n o m i t t e d . L a t i t u d e s a n d l o n g i t u d e s a r e c o r r e c t e d to n e a r e s t d c g r c c , a l t i t u d e s to n e a r e s t 10 m a n d a g e s to n e a r e s t t h o u s a n d y e a r s . T h e list is p r o b a b l y i n c o m p l e t e a n d s o m e sites m a y h a v e b e e n e r r o n e o u s l y a t t r i b u t e d to p a r t i c u l a r v e g e t a t i o n t y p e s
Lowland Ever-Wet Rainforest
14°N 16°N 17°N
90°W 89°W 90°W
Rondonia (3 diags)
2% 6°S 7°S 3°N
140°E 146°E 147°E 103°E
Nigeria Senegal Brazil
Panama Guyana Kalimantan Malaysia Sarawak
Lowland Seasonal Rainforest
Other Low- to Mid-Altitude Forest and Woodland
SE Asia: India: Savanna, Shrubland Africa: and Grassland, Americas: some Secondary
Lower Montanc Rain forest
Hordorli Yanamugi L. Wanum Tasek Bera Ofuabo Crcck Tanma L. Cumina L. do Caju Terra Nova L. Surara Katira Creek Gatung L. 7 diagrams Padang Karaya Pekan Nanas Marudi Ganges delta
Bosumtwi L. Barrinc L. Euramoo L. Eacham Bromfield Swamp Lynch's Crater Ouincan CratEr
5°N 6°E 15°N 17°W l°S 56°W 3°S 60°W 3°S 60°W 4°S 60°W 9°S 63°W 90N 80°W 5-7°N 57-58°W 2°S II4°E 2°N III3°E 4°N 1140E 22°N 89°E
Alt m a.s.I, 650 ca. 0 ca. 150 --
560 170 35 25 -------ca. 150 1) ca. 0 ca. 30 ca. 1l ca. 0 ca. 11
Age range, ka BP
Tsukada and Dccvcy, 1967. Tsukada and Decvey, 1967. Tsukada, 1966; Tsukada and Dccvey, 1967. Quaternary Absy and van der Hammen, 1976; van dcr Hammen, 1972. >33-0 2-(1 10-0 4-0 ->35-0 10-1 4-0 2-0 3-0 5-0 Plcistocenc 11-0 mostly <45 Holoccnc IIoloccnc 4-0 7-0
t7°S 17% 17°S 17°S 17°S 17°S
146°E 146°E 146°E 146°E 146°E 146°E
7211 730 750 750 770 800
12-5 10-1 5-0 11-1 ca. 200-5 7-ca. 0
14-0 41l-6 22-1) 13-11
L. Victoria L. Abiyata
lshiba Ngandu L. Valencia
L. Amatitlan Menghai Kakathopc
14°N 22°N 1I°N
91°W 100*E 77°E
1190 1200 2320
ca. 3 - 0 36-<20 35-0
Oyo El Junco
L. dc Agua Sucia
Sacred L. L. Bunyonyi Katenga Swamp Butongo Swamp Muchoya Swamp Kamiranzovu Swamp Kuwasenkoko Swamp Kilimanjaro Mera L. de Pedro Palo Fuquenc
0 l°S 1°S l°S l°S 2% 2°S 3°S l°S 5°N 5°N
37°E 311°E 30°E 31FE 30°E 29°E 29°E 37°E 78°W 74°W 74°W
24(10 1950 1980 2020 2260 1950 2340 26511 1100 211t10 25811
Viecntc Lachncr Birip Inim Sirunki Tclefomin Manton Kindeng Ambra Draepi-Minjigina Kosipe Tari-Koroba
9°N 5°S 5°S 5°S 6°S 6°S 6°S 6°S 6°S 9°S 6°8
Tanzania Ecuador Colombia
Costa Rica Australasia: Papua-N.G.
84°W 144°E 144°E 144°E 142°E 144°E 144°E 144°E 144°E 147°E 143°E ca,
2400 1900 2500 25~) 1500 15911 1600 16211 1890 21)00 15011
ca. 1-0 ca. 2 - 0 ca. 4 - 0
L. Cuscachapa L. Isabal L. de Petenxil
El Salvador Guatemala
Australasia: Indonesia (lrian Jay°) Papua-N.G.
Lowland Ever-Wet Swamps and Riverinc Lakes
tfope, pcrs. comm. Garrctt-Joncs, 1979. Garrett-Jones, 1979. Morley, 1981a, Sowtmmi, 1981. Medus, 1984. Absy, 1979. Ahsy, 1979. Absy, 1979. Absy, 1979, van dcr H a m m c n , 1972. Bartlett and Barghoorn, 1973. van dcr H a m m c n , 1963. Morley, 1981b. ttascldonckx, 1977. Anderson and Muller, 1975. Vishnu Mittrc and Oupta, 1971l; Gupta, 1981. Maley and Livingstone, 1983; Talbot et al., 1984. Chen, I986. Kershaw, 1970. Grindrod, 1979; Goodficld, 1983. Kershaw, 1975. Kcrshaw, 1976, 1978, 1983. Kershaw, 1971. Kendall. 1969. Lczine, 1982; Lczinc and Bonncfillc, 1982. Livingstone, 1971. Bradbury et al., 1981; Salgado-Labouriau, 1982. Tsukada and Deevey, 1967. Liu et al., 1986. Gupta, 1971.
Ritchic, Eylcs and Haynes. 1985. Colinvaux, 1972; Colinvaux and Schofield, 1976. Latc Absy, 1979. 1982; Holocene Wijmstra and van der Hammcn, 1966. Wijmstra and van der tlammen, 1966; >10-<6 van der Hammcn, 1974. Wijmstra and van der Hammcn, 1966: >4-<2 van dcr Hammen, 1974. Tsukada and Decvcy, t967. < 10-0 33-(I ca. 1-0 ca. 8 - 0 ca. 8-1 >13-<6 >43-<13 >14->2 5-0 33-26 >12-<11 ca. 30-11
Coctzcc, 1967. Morrison and tlamilton, 1974. Morrison and Hamilton, 1974. Morrison and Hamilton, 1974. Morrison, 1968; Hamilton, 1972. ttamilton, 1982. Hamilton, 1982. Coctzec, 1967. Liu and Colinvaux, 1985. van der Hammen, 1974. van Gccl and van der tlammen, 1973; Grabandt, 1985. Martin, 1964. >>36-0 Walker and Flcnlcy, 1979. 2-1) Walker and Flenley, 1979. 10-0 Walker and Flenlcy, 1979. 30-11 ca. 20-0 Hope, 1983. 5-1) Powcll, 1982. 2-1) Powell, 1982. 32-0 Powell, 1982. Powcll, 1982. 36-0 Hope. pets. comm. 36-11 ca. 40-32 Williams, McDougall (fragments) and Powell. 1972.
Palynological Light on Tropical Rainforest Dynamics T A B L E 1. (continued)
Country Fiji Indonesia (Sumatra)
Danau Padang Danau di Atas Tau Sipinggan Pea Sim-sim
2°S 2°S 20N 20N
102°E 102°E 990E 99°E
950 1530 1440 1450
10-0 3112-0 18-0
Morley, 1982. Henley, 1984. Maloney, 1981. Maloney, 1980.
Mahoma L. Kitandra L. Laboot Swamp Cberangani Swamp
0 0 I°N 1°N
30~E 30"E 35°E 35°E
2960 3990 2880 2900
15-0 7-ca. 0 24-ca. 5 28-0
Alsacia S. de Bogota Funza
El Abra Paramo de Palacio
El Billar Siete Cabezas Totare le Quindio L. de Ocubi
50N 5"N 5*N 6°N
75°W 75°W 750W 72°W
3600 3700 3760 2800
ljomba Mire Lr. Pindaunde Imbuka hog
4°S 6°S 6°S
137°E 145°E 145°E
3630 3510 3550
ca. 14-0 4-0 13-0
L. Ratundu L. Bujuku Koitobos Bog L. Kimilili Danka Bog • Badda Mire La Guitarra La Primavera Andabobos Gobernador La Rabona Qnebrda Africa V. San Carlos Buenos Aires C. del Visitador
0 0 I°N ION 7~N 8*N 4ON 4°N 4°N 4"N 4°N 5ON 5*N 5°N 6°N
37"E 30~E 35OE 35°E 40"E 40OE 74°W 74°W 740W 740W 74°W 75°W 75°W 75°W 73°W
3140 3920 3940 4150 3830 4040 3450 3520 3750 3810 4000 3770 3850 3850 3300
>11-0 3-?0 7-? 11-0 8-?0 12-0 15-0 12-0 14-0 10-1 mid-Holocene 11-0 13-<11 3-1 13-0
L. de los Bobos Rio Corralitos S. Nevada del Cocuy (7 diags) Nevado de Sumapaz
6°N 60N 6°N
73°W 72°W 730W
3800 3860 3800 -4000 4100
Brass Tarn Summit Bog
4100 -4500 3300 -3800 3910 4420
Indonesia (lrian Jaya)
Kenya Uganda Kenya
Peru Australasia: Papua-N.G.
Age range, ka BP
Alt m a.s.L
Australasia: Indonesia (lrian Jaya) Papua-N.G. Sub-Alpine and Alpine Non Forest
Upper Montane and Subalpine Forest
L. Junin (3 diags)
van Zeist, Polhaupessy and Stuijts, 1979. Holocene van Zeist, Polhaupessy and Stuijts, 1979. Livingstone, 1967; Hamilton, 1972. Livingstone, 1967; Hamilton, 1972. Hamilton, 1982. van Zinderen Bakker, 1964; Coetzee, 1967, Hamilton, 1982. Melief, 1985.
3.5 My- Hooghiemstra, 1984. Holocene 5-0 van der Hammen and Gonzalez, 1965a. Penult. glacial van der Hammen and to Holocene Gonzalez, 1960a. >50-0 Shreve-Brinkman, 1978. ca. I I-0 van der Hammen and Gonzalez, 1960b. 12-0 Melief, 1985. >4-<3 Melief, 1985. ?6-?I Melief, 1985. Holocene van der Hammen et al., 1980. Hope and Peterson, 1976. Hope, 1976. Hope, 1976.
Coetzee, 1967. Livingstone, 1967. Hamilton, 1982. Hamilton, 1982. Hamilton, 1982. Hamilton, 1982. Melief, 1985. Melief, 1985. Melief, 1985. Melief, 1985. Melief, 1985. Grabant, 1985. Melief, 1985. Melief, 1985. van der Hammen and Gonzalez, 1965b. 25-1ate van der Hammen et al., 1980. Holocene >3-0 van der Hammen, 1962. Holocene van der Hammen et al., 1980. 13-0 Gonzalez, van der Hammen and Flint, 1965. Late Melief, 1985. Holocene Holocene Salgado-Labouriau and Schubert, 1977. 13-< 12 Salgado-Labouriau, Schubert and Valastro, 1977. 7-2 Saigado-Labouriau and Schubert, 1976. >43-0 Hansen, Wright and Bradbury, 1984. 1.2-0
Hope, 1976. Hope, 1976.
3-1 Last interglacial -0 22-0
Hope and Peterson, 1976. Agwu and Beug, 1982, 1984. RossignoI-Strick and Duzer, 1979.
D. Walkerand Y. Chen
between forest and savanna was probably through about 5° latitude as judged from off-shore sediments (Agwu and Beug, 1982, 1984). On the Atherton Tableland of northeastern Australia, several sites record a change from something like savanna woodland to seasonal rainforest beginning before 10 ka BP and completed by about 5 ka BP (Kershaw, 1970, 1971, 1975, 1983; Chen, 1986). And at Lake Valencia in Venezuela, a process of replacement of arid grassy vegetation by semi-evergreen forest began about 10.5 ka BP and culminated about 2000 years later (Bradbury et al., 1981; Salgado-Labouriau, 1982). It is sometimes difficult to be sure that tropical forests were less extensive before 10 ka BP rather than much the same size as today but somewhere else. Thus, when the forest was absent from the Atherton Tableland there was a hypothetically larger area available to it on the then emerged continental shelf. In general, however, entirely independent evidence of gross climatic and associated environmental change very strongly implies much smaller habitats for tropical forest before 10 ka BP and not just a shift in their location. Such evidence, combined with topographic considerations, makes it quite certain that the forests of central New Guinea occupied only 75% of their present area and 60% of their altitudinal range at about 18 ka BP. Hamilton's (1976) tentative reconstructions of changes in the area of African lowland rainforest suggest much greater alterations, namely about a quarter and three times the present area at 20 ka BP and 8 ka BP respectively. In East Africa, a hypothesized refuge for forest during the last glacial in Rwanda seems indeed to have been confirmed from the pollen analysis of Kamiranzovu Swamp spanning the period from about 40 ka BP to about 12 ka BP (Hamilton, 1982). The next question, of course, is whether these changes in location and size were proportionately distributed among all tropical forest types or whether some escaped them or were, at least, relatively little affected. We cannot gain any hints about this from geological or palaeo-climatological data either because they are insufficiently precise or create major, and as yet irreconcilable, difficulties. For example, CLIMAP reconstructions show tropical sea surface temperatures around Southeast Asia only 2°C colder than present at last glacial maximum about 18 ka years ago, on the basis of which it might be supposed that lowlands which were wet enough might even then have harboured lowland rainforest (CLIMAP, 1976; Prell et al., 1980; but see also Aharon, 1983; Rind and Peteet, 1985). On the other hand, the movement of the altitudinal tree line in central New Guinea implies temperatures there about 10°C below those of today which, simply extrapolated downhill parallel with today's temperature lapse rate would surely have excluded some lowland forest types at sea level. Meteorologists are reluctant to accept a change of lapse rate sufficient to accommodate this anomaly (Webster and Streten, 1978), so it is probably more profitable to seek fossil evidence of vegetation distribution patterns at that time
than to argue from environmental potentials. In New Guinea itself there is a suggestion that vegetation changes were less drastic around 10 ka BP at 1500 m a.s.1. (Telefomin) and at 2000 m a.s.1. (Kosipe) than they were at higher altitudes, but there were changes which, by analogy with today's vegetation, reflect altitudinal shifts in the same direction. At Hordorli, only 560 m a.s.l., on ultrabasic rocks, a rich mixture of gymnosperms (e.g. Dacrydium, Araucaria, Phyllocladus) gave way to a mixed, mainly angiospermous forest at about the same time (Hope~ 1983, pets. comm.). For Sumatra, Flenley (1984) has tentatively concluded that critical vegetation boundaries were more removed from today's levels the higher their altitudes today. Thus, the alpine-subalpine boundary, now at 2400 m, was at 1400 m around 17 ka BP; the main division between montane forests, now at 1800 m, was at 1400 m before 1.2 ka BP; and the montanesubmontane boundary now at 1400 m, was at 1050 m before 8.6 ka BP. Unfortunately, the age ranges of sites are such that these differences may underestimate apparent movements at the lower altitudes. In Africa (Ghana) the record from Lake Bosumtwi, only 100 m a.s.l., indicates the presence there before 8 ka BP of a 'wooded grassland' of a type now growing ~300-400 m higher and 750 km to the west of the site (Maley and Livingstone, 1983; Talbot et al., 1984) before 8 ka BP. Although implying a temperature difference from the present day of only 2-3°C, these data unequivocally illustrate that, at least in some places, lowland tropical vegetation patterns did change substantially at about the time that the high mountain forest limit was displaced. During the last two decades, there has accumulated some irrefutable evidence of more extensive aridity near the time of the last glacial maximum than occurs today in tropical latitudes. From this has stemmed the notion that all the equatorial lowland forests of today have arisen from comparatively small refugia of that time which can be specified today either from hypotheses about the locations of least climatic exigency or observations on the distributions of endemicity and hybridity in particular taxa, mostly short-lived animals (see, for example, papers in Prance, 1982). It seems very likely that insufficient water and differences in the seasonal availability of the year's total did restrict tropical rainforest to smaller and less continuous areas than it occupies today in some regions. But~ equally probably, this was not universally the case (Walker, 1982), while in some regions of undoubted forest contraction and expansion evolution seems not to have been especially fast (Hamilton, 1976). Certainly there is as yet no direct fossil evidence, one way or the other, from the heartlands of present lowland rainforests. Lack of low- and middle-altitude pollen analytical sites makes it difficult to assess the degree to which the big changes at the tropical forest borders were directly reflected throughout it. It is, however~ striking that at almost every site studied some changes in forest composition occurred across the 10 ka BP marker zone.
Palynological Light on Tropical Rainforest Dynamics
It seems safe to suppose, therefore, that the spatial relationships of plant species in the low altitude tropics were different before about 10 ka BP, from those we see today. Because of the gross restrictions, both altitudinally and areally, the variety of landscapes available to the forest must have been smaller than it is today so that, in order to accommodate much the same flora, niche differentiation must have been greater then than now. Whether this was expressed through smaller patches of forest types with much the same species compositions as we recognize today, or whether the species associations were entirely different is a matter that calls for further data but, such as it is, our existing evidence favours the latter interpretation over the former. With the maximum of the last glaciation dating to about 18 ka BP, environmental conditions before 10 ka BP can be loosely called 'cryomeric'. Such conditions, in the most general terms, occupied the greater part of the last million, perhaps even two million, years, punctuated by a rather small number of 'thermomeres' climatically comparable with today. This suggests that the arrangement of tropical plants immediately prior to 10 ka BP was much more representative of the last million years than are today's vegetation patterns and species associations. In experimental studies of tropical rainforest today we study a system which is temporarily 'relaxed', one in which realized niches are closer to potential niches than to minimal niches for most species. It is, of course, quite wrong to suppose that environmental 'syndromes' switch quickly from one extreme to another; if it were generally so, plant and animal extinctions would be more common than observed. Whatever surrogate for climatic change is taken (e.g. oxygen isotope variations in deep-sea cores, sea level change), its maximum amplitude can be traversed in as little as 20 ka years, particularly at the beginnings of major thermomeres. In the long interval from the beginning of a cryomere to its maximum, which can take about 100 ka years, there are many superimposed oscillations through as much as 40% of maximum amplitude (Chappell, 1983). At all stages there are smaller, nested, oscillations, down to those which are evident from places in which reliable instrumental records became available in the nineteenth century (Lamb, 1977). The period straddling 10 ka BP, the beginning of the current thermomere, therefore, may have represented the most extreme of environmental events in the tropical Quaternary but, by analogy, events which were big enough to cause major vegetational change have been happening continuously for at least a million years. Nor is it necessarily the case that the components of the physical environment important for plant growth and affecting their competitive performances (e.g. temperature, effective rainfall) always altered in the same sense at different times in the past. So we can no longer substantiate any observations about species numbers or species relationships in tropical rainforest by appeal to stability of
either environment, geographical distributions of vegetation types, or biotic mixes. It is not now, and for long has not been, a setting in which speciation and ecological relationships are dominated by purely biotic forces operating in an unchanging physical environment. Long-lived trees, of which the lowland tropics have a large share, are buffered against the effects of short-term environmental fluctuations, even those of large amplitude. But when climatic change progresses in the same general direction for about 8 ka before reversing, survival must depend on a breadth of environmental tolerance, an ability to re-locate the population, or both.
Holocene Plant Species Migrations and Population Expansions How quickly can such migration happen? Once again, we have no direct information from within the lowland rainforest itself. But in the central highlands of New Guinea, the altitudinal tree line rose from about 2750 m to 3900 m in about 2000 years (i.e. 6 m/century) (Hope, 1976). Differences in the local availability of species able to take advantage of this opportunity led to differences in the composition of the upper montane forests of individual mountains which are still evident. It is very likely that some species are still arriving there from below, perhaps because they lack pioneering aptitudes or because they had further to travel. In some places it seems that the initial colonists of a previously unforested area may be hard to replace because they combine the capacities for rapid establishment with those of gregariousness and in situ regeneration; such is the case of some Nothofagus stands in New Guinea (Walker and Hope, 1982). On the whole, however, on the basis of pollen analytical data it can be argued that, in the tropics as in temperate lands, the actual migrations of many tree species onto 'vacant' land can be much faster than is indicated by, say, modal seed dispersal distances of the same species determined experimentally. It must be that the chance long dispersals and the creation of outlying nucleii of pioneers by them was important in those circumstances. Changing physical conditions probably resulted in much slower change within the forests themselves, damped by competition from existing, even though by then less well-fitted, individuals. Another kind of forest edge brings us insight into the complexity of the migration process. On the Atherton Tableland of northeastern Australia six similar palynological sites, all within a circle of 13 km radius, show clear records of the replacement of grassy sclerophyllous (Eucalyptus) woodland by rainforest shortly after 10 ka BP. At five of these sites we can measure the time from the beginning of the replacement of the woodland through a sequence of changes culminating in the filling of the site's pollen catchment with rainforest. Even though the rainforest was evidently coming close to the limits of its expansion at that time, the differences between the times of its initiation, 10 ka BP and 7 ka BP, are surprising. Once under way,
I). Walker :,ml Y. (?hen
however, the transition was completed at all sites in 1000 to 1500 years. At one of these sites (Lake Barrine) fine-resolution pollen analysis together with the counting of fine charcoal particles from the same samples has provided insight as to one source of these differences (Chen, 1986). Before the advent of rainforest in the region the grassy woodland seems to have sustained fires with a mean return period of a decade or so as judged from time series analysis of both charcoal particles and apparent palynological responses. At about 9.3 ka BP, the first traces of rainforest pollen attest that it was in the region though not in the Lake Barrine catchment. Along with other evidence, we take this to mean that the regional climate had changed to favour rainforest by that date. But it was not until about 2500 years later that pioneer rainforest trees actually established themselves at Lake Barrine, during which time the fire occurred in the catchment at 220year to 240-year average intervals. The sensitivity of rainforest seedlings to fire is well known and we suppose that this was the factor inhibiting the forest's development. Pollen representation of the major plants (e.g. Casuarina, Eucalyptus, Gramineae), is strongly correlated with that of charcoal with appropriate lags. What caused a change in fire regime is unknown. There might not have been any change in mean frequency of fire, only a particularly long and statistically rare interval which could have happened at any time since 9.3 ka BP. Given the regional context, however, the switching mechanism is likely to have been site-specific and to have been at least one factor affecting the variation of date of rainforest establishment around the pollen analytical sites and elsewhere on the Tableland. Short of experimentally manipulating the fire regime over hundreds of years, contemplation of the vegetation pattern would probably not have led us to such a clear understanding of its environmental controls and of its patchiness. It reinforces the fact that sometimes the most potent determinants of plant patterns operate covertly and are only exposed by observations lasting very long times; this is no less true of rainforests than of other kinds of vegetation. In Africa, the only sites within lowland rainforest investigated palynologically are Lake Bosumtwi (Ghana) and the northern end of Lake Victoria. Bosumtwi is strictly in moist semi-deciduous forest about 140 km and 70 km from the wet evergreen forest and savanna respectively (Maley and Livingstone, 1983; Talbot et al., 1984). As yet, pollen counts have been made at only about 1000-year intervals but it is evident from available data that continuous forest cover was not established until about 8.5 ka BP. From Pilkington Bay, on Lake Victoria, at about 1134 m and only 30 km north of the Equator, Kendall (1969) has published a detailed pollen influx diagram covering the last 12 ka. The site receives its tree pollen from terrestrial vegetation which is now a patchwork of disturbed moist semi-deciduous forest, thicket, treesavanna and grassland. Walker and Pittelkow (1981), treating each of the 26 major tree or shrub taxa separately, showed them to demonstrate continuous
100 0 J l
5x10 a ,I
o. 6.1 m u) tr ),-
0 u) a Z ,< $2
:D 0 1I-
FIG. 1. Percentages of sclerophyll woodland and rainforest pollen and the numbers of charcoal particles (cm 2 yr-i) in samples averaging 34 years apart in the early Holocene at Lake Barrinc. northeast Queensland, Australia.
change in forest composition resolved at 500-year intervals in which most of the plants behaved independently in the sense that the changes in their populations occurred at different times. Of 24 possible segments in which changes could have occurred, only 7 lacked at least one of the first order. Yet there were periods (e.g. 10.5-7.5 ka BP, 2.5-1.5 ka BP) during which more changes occurred than at others. In the same paper, an attempt was made to fit curves to the influx values for Macaranga pollen which might hint at the main trends in the populations of the species represented through different stages of the last 12 ka. The best description was of a population following a cubic function about a low mean from 12-9.5 ka BP, followed by a four-fold increase in about 500 years, then a quadratic diminution to about earlier levels by 5 ka BP, interrupted by temporary maintenance of higher than predicted values along the way. This approach was further developed by Bennett (1982) in a study of the early Hoiocene of southeast Britain and by Chen (1986) in describing the synthesis of the rainforest of Lake Barrine in northeast Australia. Each of the pollen diagrams from the Atherton Tableland of northeast Australia (Kershaw 1970, 1971, 1975, 1986) can be divided into 3 or 4 zones since the establishment of rainforest there, indicating substantial changes in forest composition during that time. That this was nearly continuous has been demonstrated by Goodfield (1983) at Lake Eacham where closer dating and better statistical methods have also distinguished two major half-millennial periods of greatest change during the past 5 ka. Lake Barrine is another crater lake on the Atherton Tableland and is still surrounded by rainforest although close to its western climatic limit. The lake has no
Palynological Light on Tropical Rainforest Dynamics
80 t 70
4- 4- 4++~
~ 20 ¸
X v X D I::
yrs after 10,000 BP
y ~ after 10,000 BP
=50 ~ 2o0.
yrs after 10,000BP
'42bo ' doo'
yrs after 10,000BP
FIG. 2. Exponential curves fitted to the pollen influx values (grains cm -2 yr-t) for periods during the early Holocene at Lake Barrine, northeast Queensland, Australia (for comments see text).
major inflows but is maintained at a more-or-less constant depth by an overflow which is only active in wet summers. This depth is 67 m at the centre of the lake which has a surface area of about 1 km 2 and a pollen catchment roughly twice that size. The stratigraphy is well known, some 6 m of organic clay-mud overlying a mixed deposit of clay, sand and gravel, the transition dating to about 10 ka BP. The regional shift from sclerophyll (Eucalyptus) woodland to rainforest begins above this stratigraphic break. Chen (1986) has constructed influx diagrams for the major sclerophyll and rainforest pollen taxa and for charcoal particles through the period 10-5 ka BP. The chronology, and therefore pollen influx calculation, is based on 21 radiocarbon dates. Each counted sample must represent some 17 years and the gaps between samples 34 years. It is argued that the main rainforest trees expanded at differing rates, temporary stability of composition not being achieved until 5 ka BP. In order to establish the best description of the behaviour of each pollen taxon, the actual values have been fitted against two standard population expansion equations, namely the exponential
Nct) = Noe", and the logistic N(O = KI(1 + eC-'), where Nt is the number of individuals in the population at time t, No is the number of individuals at the start of expansion, r is the intrinsic rate of population growth, K is the carrying capacity of the catchment for that taxon, and c is a constant acquired during integration. Although all the taxa tested can be fitted at reasonable levels of significance (p < 0.9) to either of these equations, some fit more closely to both than do others whilst some are clearly more significantly described by one model than the other (Fig. 2). Agathis values, for example, are relatively small and, although better fitting an exponential than a logistic curve, are not convincingly related to either. Podocarpus, another exponential preference, might better be described by a straight line with a pollen increase gradient of 5000 grains/cm2/year over 200 years. Eugenia and Rapanea, however, show convincing fits to the exponential
D. Walker and Y. Chen
8 x 30.
yr= after 10,000
4~0o 50bo 10,000 BP
3400 r yrs atter 10,000 BP I
FIG. 3. Exponential and logistic curves fitted to the pollen influx values (grains cm 2 yr- ~) for periods in the early Holocene at Lake Barrine, northeast Queensland, Australia (for comments see text).
equation. Some other taxa are best treated in separate periods, the first of which shows exponential increase and the second either exponential with different r (e.g. Trema, Moraceae-Urticaceae) or logistic (e.g. Elaeocarpus, Macaranga-Mallotus). Although it is important to be able to show that at least some trees in pioneer circumstances follow the Verhulst-Lotka equations equally well as do some more easily manipulated experimental organisms, at least as much ecological interest attaches to the courses followed by the less orthodox taxa and to such derivatives as population doubling times which can be deduced from these statistics (Fig. 4). Data such as these are only ever likely to be obtainable from unusual sites at which the extent of the pollen catchment and the regularity of the sedimentation processes can be well defined but the results already obtained in this way from Lake Barrine clearly promise substantial and explicit contributions to rainforest ecology through their wider application.
Prehistoric Anthropogenic Impacts Some types of change in plant environments which can be investigated in part by palaeoecological means
are cumulative in their impact. Amongst these, in the tropics, must be human victualling activities. Although these doubtless began and continued for millennia as a kind of broad based sophistication of basically animal techniques, even hunter-gathering systems develop preferences for some natural products which must have led to ecologically selective pressures. Today, for example, the Malaysian Negritos are said to 'methodically search the jungle for all that is edible' (Carey, 1976) and the Mbuti pygmies of the Itur Forest of the Republic of the Congo obtain all their necessary food in this way (Turnbull, 1978). The development of horticulture brought with it the even more purposive modification of the forest, which is shown amply in pollen diagrams. True swidden horticulture in which a rather small patch of forest is cleared, cropped and then deserted for a very long time, much as the Temiar of Malaysia (Carey, 1976) operate even today, allows the re-establishment of the rainforest during the fallow. But it introduces into the dynamics of the forest a phenomenon not there previously. At simplest assessment it must bring increased opportunity for pioneer species and those that successively follow
Palynological Light on Tropical Rainforest Dynamics Agathis (5200-4300 BP) Podocarpus (6600-6000 BP)
Podocarpus (4500-4000 BP)
Eugenia (6000-5300 BP) C
Elaeocarpus (6800-6500 BP) Elaeocarpus (6000- 5500 BP)
Cunoniaceae (6700-6200 BP) C
Repartee (6500-6000 BP)
Balanops (6750-6200 BP) Blepharocarya (4600-3800 BP) -41-
Mecaranga/Mallot us'(6800-6485 BP)
Bars denote 95% confidence intervals
Macaranga/Mallotus (5100-4600 BP) Moraceae/Urticaceae (6800-6500 BP)
Moraceee/Urticaceae (5100-4600 BP) Trema (6800-6500 BP) Trema (52004950 BP)
FIG. 4. Doubling times of the populations of some pollen taxa from the early Holocene at Lake Barrine, northeast Queensland, Australia, derived from fitted curves.
them. Although the area evidently 'disturbed' at any one time may be small, the regional repercussions as the p,erturbation flows through a territory over a thousand years or so, eventually impinging again on previously affected areas, must result in a changed balance of species. If interactions of this kind have been widespread over long periods in the past, it may be mistaken to underestimate their persisting effects on the dynamics of rainforests today. In a pollen diagram from the lower montane forest of the central New Guinea highlands (Walker and Flenley, 1979) the first unequivocal evidence of clearing in the nearby forest dates from 4.3 ka BP, only about 200 years after the major forest compositional adjustments to the earlier climatic changes were complete. Partial regeneration took place about 300 years later but high forest was not totally re-established until about 3 ka BP. That forest never became the same as it had been before the initial disturbance; it continued to contain a greater proportion of 'secondary' trees. A thousand metres lower, at 1500 m, the same change had evidently taken place by 5 ka BP, and was repeatedly reinforced thereafter (Poweli, 1982). Human habitation of lowland New Guinea is archaeologically documented as far back as 40 ka BP, (Chappell et al., 1986). By 6 ka BP, and perhaps even 3 ka earlier, major swamp drainage was being undertaken to create repeatedly used gardens in major highland valleys (Golson, 1985;
Powell et al., 1975). Valley sides, and at lower altitudes ridge tops, were also permanently cleared of forest as the human population grew to levels locally among the most concentrated in the non-urban world (e.g. 120200 persons/km2; Hope et al., 1983). In the lowlands, a pollen diagram from 35 m a.s.l. (Garrett-Jones, 1979) shows a fluctuating but increasing component of nonforest taxa accompanied by charcoal particles since about 8.5 ka BP. Elsewhere, at 560 m a.s.l., another site recently studied has a similar record, originating from vegetation changes in the nearby valley (Hope, pets. comm.). In Africa, the long history of human evolution seems to have taken place largely outside the tropical forest. Hamilton (1982), notes only a few pollen analytical indications of early agriculture disturbing forest, all of them above 2000 m. Traditional tropical gardeners also make free use of forest products, sometimes on a large scale although their populations are sustained by the horticultural staple and sometimes to a degree by husbanded animals (e.g. the Yanomamo of Venezuela-Brazil [Chagnon, 1983] the Kwele of the Congo [Turnbull, 1978] and many New Guinea groups [Powell, 1976]); today they may also be sold for cash (Carey, 1976). The activities of similar peoples must have had significant effects on the balance of tropical rainforests for a long time although Bellwood (1985) doubts whether lowland
D. Walker and Y. Chen
rainforest settlement in Malaysia is older than the Holocene (i.e. the last 10 ka) and Meggers (1984) can report only one date older than about 7 ka BP to suggest ancient occupation of Amazonia. Yet close to the present day population densities of 0.2 to 14.6 persons/km 2, depending on the ecosystem occupied, suggest that human activity has been ecologically important in both major Amazonian lowland rainforest types. In experimental studies, it is often assumed that such effects are negligible, and have always been so, unless there is direct and obvious local evidence to the contrary. It might be better to begin by assuming the forest to have been subject to human selection pressures for a very long time unless positive contrary evidence is available. Of course, there are some tropical rainforest regions in which such extreme caution might not be necessary by reason of unusual histories of human cultural development, e.g. the lowland rainforest of northeast Australia, but even there it has been argued that human demands have at least modified the forest edges over a very long period. There are certainly other places where the rainforest is very recently established, perhaps in association with active human occupation, including farming, as seems to have been the case in parts of lowland Guatemala where it happened in Postclassic Mayan times, more recently than about 1 ka BP, (Tsukada, 1966; Tsukada and Deevey, 1967; see also Binford et al,, this volume).
environmental controls. The most interesting event recorded, however, is a wave-like invasion by Calophyllum retusum across much of the area studied roughly co-eval with the reduction or disappearance of other established trees (e.g. Lithocarpus spp., llex spp.). Other strictly lowland sites also often record more about local plant responses to drainage and flooding changes than about the composition of the nearby dryland forest. This is Absy's (1979) preferred interpretation of five pollen diagrams together spanning the last 6 ka in the central Amazon basin. Yet they clearly show that the periodically inundated forests, which form a substantial component of the Amazon forest in general, were subject to repeated change. Farther south, however, upstream along the Rio Madeira, there is better representation of dry-land rainforest in the pollen diagram and indications that it has succeeded savanna vegetation in the recent, but undated, past (Absy and van der Hammen 1976; v e n d e r Hammen 1972, 1974). In peninsular Malaysia, a pollen diagram from a large riverine swamp-lake complex, covering the last 4.5 ka, shows that mixed dipterocarp forest, similar to that found in 'undisturbed' parts of the region at the present day, dominated the low hills nearby but has been subject to clearance of varied intensity with partial regrowth during the last 700 years (Morley, 1981a).
EXPLICIT EVIDENCE OF ECOLOGICAL CHANGE
Table 1 demonstrates the paucity of pollen diagrams from lowland tropical rainforest itself and even from fioristically closely related but seasonal rainforests. Of those which have been published none shows the forest composition in its pollen catchment to have been changeless through the last few thousand years. Some of the diagrams derive from sites such as peat swamp forests where changes in forest composition accompany peat accumulation through time. Coastal peat swamps that originated in mangrove swamps (e.g. Bartlett and Barghoorn, 1973; Medus, 1984) were common on the shelves, exposed during the greater part of Quaternary time (now covered by shallow seas), particularly in Southeast Asia. There, Anderson and Muller (1975) have shown that the succession of vegetation types leading from mangroves to the present Combretocarpus rotundadtus- Dactylocladus- Stenostachys (Sphagnum junghuhnianum), through Shorea albida and other forests, on the thickest peat accumulations, is mirrored by the more-or-less concentric zonation on the bog surface today. This demonstrates that, even in the lowland tropics, extreme habitats select a limited range of plants with quite specific relationships with their physical environments. Furthermore, in Southeast Asia, Morley (1981a, b) has shown that many of the plants involved in the synthesis of another ombrogenous peat forest, formed over a freshwater swampy lake, behaved in strongly individualistic ways, presumably in response to local
The most arresting conclusion from all these data is that, although there is still very little pollen-analytical data from the lowland rainforests of the world, results from nearby vegetation clearly imply that their extent is now substantially greater than it was before 10 ka BP. Moreover, the area of rainforest just prior to 10 ka BP was probably closer to that which it has occupied for the greater part of the last million years. Yet, seeing how major locational changes have roughly paralleled big climatic changes in the last 20 ka it is reasonable to suppose that, just as the climate has not been stable for more than a million years, so has the extent of tropical forest continuously altered at a variety of speeds through that time. This must mean that the patterns of species distributions within the forests have also been changing at rates and in ways further complicated by characteristics of their life histories. The major processes of this kind liberated by the climatic change at the end of the last glacial maximum still continue, modified by other pressures imposed by both the physical environment and human activities. Data from the ever-wet lowlands confirm these implications although their expression is more subtle than at higher altitudes or in drier climates. Ecological and biogeographical theories which relate the complexity of rainforest to its supposed long-term stability are no longer tenable. However, the other extreme view that species diversity derives from the former dissection ot lowland rainforest has yet to be supported palyno-
Palynological Light on Tropical Rainforest Dynamics
logically and, in some regions, seems highly unlikely to be true. Another important conclusion is that any measures we care to make experimentally of a relationship between tropical forest species now is very unlikely to expose its full potential range. The same is true of determinations of the correlations with aspects of the physical environment. The degree to which observed balances between the components of a biota and its environment are optimal or limiting needs to be assessed in the light of the knowledge of change. This does not mean that correlations based on field observations and experiments are wrong; only that their predictive value can be improved by relating them to their historical context. There is a tendency in some ecological theorizing to stress the results of purely biotic phenomena, particularly as applied to rainforest. It now seems that no ecological model is acceptable if it does not incorporate a changing physical environment. In all probability, models of tropical vegetation dynamics will need the kinds of information about plant population cycles and trends which are now beginning to be accessible through refined pollen analysis. The implications of all these findings for rainforest conservation need careful consideration lest they be misunderstood. It seems undeniable that rainforest genetic pools can persist entirely naturally in a total rainforest area substantially smaller than that which they occupied before the impact of industrial forestry, perhaps smaller than that they occupy still. But we still cannot identify the places in which this has happened in the past and might be possible in the future. More importantly, however, reduction of area without genetic loss requires that the rate of such reduction must be slow, comparable with that imposed by natural fluctuations in the physical environment over the last million years. The rate at which tropical rainforest is being destroyed today is orders of magnitude greater than this. Similarly, although a degree of perturbation is clearly part and parcel of a rainforest's existence, the amplitudes and frequencies of such 'disturbances' and the gradients of trends about which they are scattered, are all very much smaller than those imposed by the most conservative industrial forestry today. ACKNOWLEDGEMENTS We are grateful to E.S. Deevey, J.R. Flenley, G.S. Hope, D.A. Livingstone and T. van der Hammen for help in the compilation of Table 1.
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