Problems of the quaternary on mountains of the Sunda-Sahul region

Problems of the quaternary on mountains of the Sunda-Sahul region

Quaterna~ Science Reviews, Vol. 15, pp. 549-555, 1996. Pergamon PII: S0277-3791(96)00017--0 Copyright © 1996 Elsevier Science Ltd. Printed in Great ...

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Quaterna~ Science Reviews, Vol. 15, pp. 549-555, 1996.

Pergamon PII: S0277-3791(96)00017--0

Copyright © 1996 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. 0277-3791/96 $32.00

PROBLEMS OF THE QUATERNARY ON MOUNTAINS OF THE SUNDA-SAHUL REGION J.R. FLENLEY

Geography Department, Massey University, Palmerston North, New Zealand Abstract - - Two specific problems are identified in understanding the data on the Quaternary of Montane areas in the Sunda-Sahul region. These are: (1) The Upper Montane rainforest, according to palynological evidence, appears to have been absent in the Late Pleistocene, although it is well represented today• (2) The estimates of temperature lowering in the Late Pleistocene are strikingly greater in the mountains than in the lowlands. A single possible explanation for both problems is proposed. It is argued that altitudinal zonation of vegetation is partly controlled by ultraviolet-B light which is strikingly increased at high altitudes in the tropics. This fact could account for both the above problems, not only in the Sunda-Sahul region, but also in other tropical regions. Copyright © 1996 Elsevier Science Ltd

QSR

Whitmore, 1975), but a useful one for our purpose is as follows (Fig. 2). The Lowland Rain Forest (LRF) occurs up to about 1000 m (exceptionally 1400 m). Above this is a Lower Mountain Rain Forest (LMRF) up to about 2900 m (exceptionally 3100 m). In Sumatra, and perhaps elsewhere, it is possible to divide this zone into LMRF I (below ca. 1800 m) and LMRF II (above ca. 1800 m) (Newsome and Henley, 1988). The LMRF gives way at about 2900 m to the Upper Mountain Rain Forest (UMRF) which continues up to the limit of woody vegetation at ca. 3800 m. The UMRF has been subdivided by the definition of various other vegetation types such as Sub-alpine forest and Alpine Shrubbery (Wade and McVean, 1969) and these are perfectly valid subdivisions, but they will not be needed in the present paper. The physiognomy of the UMRF is peculiar (Grubb, 1977). The trees are stunted, especially at the higher altitudes. Most species have exceptionally small leaves with a thick cuticle and, frequently, a hypodermis. Many species have high concentrations of pigments, especially of anthocyanins, in the leaves. The above zones are defined physiognomically, but nevertheless have some floristic validity. The zonation obtained by Walker and Guppy (1976) based on computer analysis of floristics at the generic level in New Guinea included a major break between 2800 m and 3000 m. Their upper mountain forest, above this level, was 'an altitudinal attenuation of the lower mountain forest, but many of its species ..... do not occur at the lower level' (Walker and Flenley, 1979). Above the forest limit lies the tropic-alpine grassland. In some areas this is sufficiently rich in tree ferns (Cyathea atrox) to form a distinctive Tree Fern Grassland

INTRODUCTION: THE MOUNTAINS OF T H E SUNDA-SAHUL REGION The region extending from the Malay peninsula through the Sunda and Philippine islands to New Guinea has, as a whole, a certain botanical (floristic) unity, and was defined by Good (1947) as a major botanical region of the world, which he named Malaysia. This term, and even its re-spelling as Malesia (Henley, 1979) are however politically unacceptable to Indonesians. The term Sunda-Sahul region is therefore used here instead• It refers to the Sunda platform, which bears the Malay peninsula and the islands of Sumatra, Java and Bomeo, as well as the shallow South China and Java Seas. The Sahul shelf is the continental shelf which connects New Guinea to Australia and continues out to the west. Between the two lies the Island of Sulawesi, surrounded by deep water, but included within the concept of the Sunda-Sahul region. The region (Fig. 1) bears high mountains especially on New Guinea (up to 5040 m), Borneo (up to 4101 m), Sumatra (up to 3381 m) and Java (up to 3676 m). There is permanent snow on Mt Jaya and other mountains in New Guinea, and evidence of former glaciation on several New Guinean mountains, on Mt Kinabalu in Borneo (Koopmans and Stauffer, 1968) and on Mt Leuser in Sumatra (van Beek, 1982). The present firn line in New Guinea is at about 4500 m altitude, and the geomorphological evidence from cirque floors is that this was about 1000 m lower during the Late Pleistocene (Loftier, 1984). The vegetation of the mountains changes with altitude, and has been divided into zones based primarily on physiognomy. Various schemes for this have been proposed (e.g. van Steenis, 1934--36; Grubb, 1974; 549

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FIG. 1. The Sunda-Sahul Region, showing some places mentioned in the text. The -200 m contour has been used to show the possible maximum extent of dry land during the Pleistocene.

PRESENT DAY

LATE PLEISTOCENE : 18K B.P.

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FIG. 2. The altitudinal zonation of vegetation in the Sunda-Sahul Region, now and in the Late Pleistocene. Effects of recent human activity are omitted.

(TFG). T F G occurs mainly around the upper limit of U M R F , but also down to 3000 m where U M R F is absent for any reason, such as human disturbance. The t r o p i c alpine zone continues up to the permanent snow at about 4500 m. Some authors distinguish a tundra zone within the upper part o f the tropic-alpine zone. It must be clearly understood that this zonation is highly generalised. There are many local differences of physiognomy and floristics.

The generalisation will, however, suffice for the present purpose. There has been a c o m m o n assumption, but little evidence, that the above zonation is causally related to temperature. This seems reasonable, for there is a general decline in mean annual temperature with altitude (the lapse rate) at the rate of about 0.61°C per 100 m in the region. Mean annual sea surface temperatures in the

J.R. Flenley: Problems of the Quaternary of the Sunda-Sahul Region vicinity are about 28.5°C (U.S. Hydrographic Office, 1969) with a steady decline until the firn line is reached at a mean annual temperature of ca. I°C at ca. 4500 m (Hope and Peterson, 1975; Allison and Bennett, 1976). This places the limit of woody vegetation (top of the UMRF) at ca. 5°C, the UMRF/LMRF boundary at ca. 11.5°C, and the LMRFI/LMRFII boundary at ca. 17.5°C. Of course, these figures are approximate for several reasons. There is a limited number of empirical measurements. Lapse rates are normally curvilinear, not linear. Also, local topography can affect the lapse rate (Hastenrath, 1968). It must also be pointed out that mountain climates are characterised by pronounced diurnal temperature variation, and this increases with altitude. The above mean annual approximations are, however, adequate for the present purpose.

THE QUATERNARY IN T H E R E G I O N

Climap Project Members (1976) concluded that sea surface temperatures were lowered by ca. 2°C at 18 ka BP, relative to the present day in the region. Sea level was lowered at the time, certainly to -50 m (Geyh et al., 1979) and possibly to -75 m (Biswas, 1976). A value o f - 1 0 0 m will be used in this paper, in line with other parts of the world. The lowland climate at 18 ka BP is still uncertain, but there is a suggestion that it might have been somewhat drier. The evidence for this includes the palynological evidence from the Misedor borehole in the Mahakam delta, Borneo (Caratini and Tissot, 1985) in which phases with high values for Gramineae, perhaps indicating savannas, coincided with indications of low sea level. Offshore cores from near the island of Halmahera (in eastern Indonesia) show increases of Casuarina and Lithocarpus pollen in the Late Pleistocene, suggestive of drier and cooler conditions (Barmawidjaja et al., 1989: van tier Kaars, 1990). In the mountains of the region, palynological evidence strongly indicates Late Pleistocene lowering of the forest limit and other vegetational zone boundaries (Fig. 2). In New Guinea the forest limit apparently fell to ca. 2200 m, a depression of 1600 m relative to the present day (Walker and Flenley, 1979; Hope, 1976). The LMRF I/ LMRF II zone boundary in Sumatra was lowered by a lesser amount, perhaps 700 m (Morley, 1982; Newsome and Flenley, 1988). The situation at 18 ka BP and subsequent events are summarised in Fig. 3. This diagram does not include many other pollen records, especially from New Guinea, and does not adequately demonstrate the variability in the timing of the rise of the forest limit to present levels during the terminal Pleistocene and early Holocene. These matters have, however, been discussed by Roberts et al. (1981) and Walker and Sun (1988), and will not be elaborated here. The purpose of this paper is to draw attention to two problems of the Quaternary in the Sunda-Sahul mountains, and to suggest a single solution to both. The problems are as follows.

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~Pr6bleM l: The Disappearance of the U M R F As will he clear from Fig. 3, the LMRF occurred at lower altitudes in the Late Pleistocene. The UMRF, however, was not usually present at lower altitudes, but more or less disappeared. Presumably it survived somewhere, perhaps as isolated fragments near the forest limit, or its elements may have survived at the upper edge of the LMRF, but it was not usually indicated in the pollen records as a distinct formation. A similar circumstance has now been reported from another tropical rain forest region, the Andes (Salomons, 1986), so it cannot easily be written off as a local peculiarity.

Problem 2: The Temperatures in the Late Pleistocene The temperature at 18 ka BP appears to have been everywhere lower than at present. But the amount of lowering differs strikingly. At sea level it was ca. 2°C (Climap Project Members, 1976). The lowering of the LMRF I/ LMRF II boundary in Sumatra is ca. 700 m which, at the present lapse rate of ca. 0.61°C/100 m, translates into ca. 4°C. The movement of the forest limit in New Guinea (ca. 1600 m) similarly translates into ca. 10°C lowering. The movement of the snow line (ca. 1000 m from geomorphology) implies a cooling of ca. 6°C. It is extremely difficult to reconcile these differing estimates. An attempt to do so was outlined by Walker and Flenley (1979) and amplified by Flenley (1984). This attempt was made by proposing that the 18 ka BP lapse rate was much steeper: 0.8°C/100 m instead of 0.61°C/ 100 m. This explained everything except the snow line, which appeared to be too high in the Pleistocene. The dry conditions implied by the steep lapse rate were thought to indicate a snow line kept high by low precipitation. The dry conditions also appeared to fit evidence of lowland desiccation. Unfortunately, this explanation is unacceptable to climatologists. Webster and Streten (1978) argue that the vertical temperature profile should be close to the moist adiabatic even if the atmosphere were drier. The point has been reiterated more recently by Kutzbach and Guetter (1986). We are therefore left with two problems, which have appeared intractable for over ten years.

A SOLUTION That such intractability should persist for so long suggests that there could be something wrong with our initial assumptions. I here investigate the possibility that the assumption of a causal relationship between vegetational zonation and temperature is partially erroneous. As Whitmore (1975) and Flenley (1979) point out, there are factors other than temperature which vary with altitude: atmospheric pressure and ultra-violet insolation, for example. It was already suggested by the late Francis Merton (pers. commun., 1973) that ultraviolet insolation

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FIG. 3. Changes in vegetation during the Late Quaternary in the Sunda-Sahul Region, as evidenced by palynology. Each horizontal bar represents a pollen diagram (modified after Flenley, 1984).

could be involved in vegetational zonation on tropical mountains. Caldwell (1971) calculated that significantly more of the biologically damaging UV-B radiation (280315 nm wavelength) should be present above ca. 3000 m in the Rocky Mountains of North America (Fig. 4). Brettner (1969) had already measured UV-B on tropical African mountains as double that in the lowlands, and Robertson (1972) found that in the New Guinea mountains the UV component responsible for human skin cancer was exceedingly high. The Merton hypothesis was independently restated by Lee and Lowry (1980), with special reference to Mt Kinabalu in Borneo. They argued that light intensities generally, and especially UV-B, must have been high on tropical mountains both now and in the Pleistocene. "Since the light climate at a given altitude is mainly determined by the atmospheric thickness, which is independent of climatic change, plants adapted to intense light conditions would probably not descend the slopes (during a glaciation). Thus, the zone where these intenselight-adapted plants could survive was either reduced to thin bands next to the ice fronts, reduced to non-glaciated rocks, or it disappeared altogether" (Lee and Lowry, 1980). These authors, therefore, correctly predicted the disappearance of the UMRF in the Late Pleistocene (Problem 1). The Merton hypothesis has received further support in

recent experimental work. Teramura (1983) and Murali and Teramura (1986) subjected various crop plants to high UV-B. The plants became stunted, with smaller leaves possessing a hypodermis: all characteristics of the UMRF. They also produced extra pigments, but flavonoids rather than anthocyanins. Yellow pigments are, however, often common in vegetation at high altitude, e.g. in the subalpine scrub at ca. 1300 m on Mt Ruapehu, New Zealand (pers. obser.). Flenley (1992) argued that the cloud regime on tropical mountains could be particularly damaging to vegetation. This is because of photo-reactivation, a process by which plants repair the damage done by UV-B. Photo-reactivation requires visible-light insolation. The upper zones of many mountains are sunny in the early morning, with light (presumably including UV-B) intensities increased by reflection from clouds in the valleys below. Dense clouds later envelop the summits, reducing them to near darkness in the afternoons. Thus damage done in the mornings might have no chance of being repaired. Having provided a possible solution to problem 1, let us turn to problem 2. Following the argument above, let us suppose that: UV-B Insolation is significantly higher above 3000 m; LMRF is adapted to lower UV-B and temperatures above I 1.5°C; UMRF is adapted to high UV-B and temperatures 11.5°C-5°C; TFG is adapted to high or lower UV-B and temperatures 11.5°C-5°C but is

J.R. Flenley: Problems of the Quaternary of the Sunda-Sahul Region [

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unable to compete with U M R F because of the greater stature of the latter. This would lead to the following scenarios: Present:

LMRF occurs up to ca. 2900 m (ca. 11.5°C). Above that altitude both temperature and UV-B are limiting. UMRF occurs 2900 m (ca. 11.5°C)-3800 m (ca. 5°C). Below 2,900 m temperature is too high and UVB too low. Also the trees are unable to compete with the taller-growing LMRF. Above 3800 m the temperature is too low.

18 ka BP:

II boundary from Sumatra), and of ca. I°C at ca. 3500 m (the lowered firn line), we obtain Fig. 5. This shows an 18 ka BP lapse rate of ca. 0.71°C per 100 m, compared with the present lapse rate of ca. 0.61°C per 100 m. Such a rate will presumably be much more acceptable to climatologists and also removes the discrepancy between the shifts of vegetational zones and the snow line.

~) PresentValues , ~ Values18K B.P. 30

TFG occurs mainly around 3800 m (ca. 5°C), but down to 3000 m where UMRF is absent for any reason, e.g. human disturbance. Below 3000 m it is usually absent because it is unable to compete with the UMRF. Above 3800 m the temperature is usually too low, LMRF occurs up to 2200 m (ca. 11.5°C). Above that altitude temperature is limiting.

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as there is nowhere available with high UV-B and temperature in the range 11.5°C-5°C. All the places with high UV-B are too cold. TFG occurs ca. 2200 m (ca. 11.5°C) to ca. 3000 m (5°C). Below 2200 m it is too warm, and anyway cannot compete with the LMRF which is taller. Above 3000 m it is too cold.

These scenarios correctly reconstruct the vegetation at 18 ka BP (as evidenced by palynology) and require temperatures at 18 ka BP of 11.5°C at 2200 m and of 5°C at 3000 m. If we plot these on a temperature/altitude graph, along with the figures of 26.5°C (28.5 minus 2°C) for the 18 ka BP sea level (-100 m cf. present sea level), of ca. 17.5°C at ca. 1100 m (the lowered L M R F I/LMRF

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FIG. 5. Temperature-altitude graph for the SundaSahul Region, showing the lapse rates now and as reconstructed for the Late Pleistocene.

CONCLUSION This paper has i n d i c a t e d two p r o b l e m s of the Quaternary in the S u n d a - S a h u l region, and made a single

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proposal which would solve both of them. Although, by O c c a m ' s Razor, such a proposal is attractive, it can be accepted only after further investigation. At least the following investigations appear to be needed: 1. Further measurements of UV-B on tropical mountains. 2. Controlled laboratory investigations of the effects of UV-B (or its absence) on significant species. Since it is difficult to simulate the sun's UV-B spectrum, it might prove easier to use the naturally high UV-B on tropical mountains and to control the temperature environment there. Such experiments should be done in any case to prepare us for an ozone-depleted world in the future. 3. Investigations o f UV-B in the Pleistocene. The assumption in this paper was that UV-B was constant in the past. To check on this, we should measure the thickness of the hypodermis in fossil and modern leaves of selected species. This research is already under way.

ACKNOWLEDGEMENTS I am grateful to Dr A Agnew and Dr M Bush for helpful discussion. REFERENCES Allison, I. and Bennett, J. (1976). Climate and microclimate. In: Hope, G.S., Peterson, J.A., Radok, U. and Allison, I. (eds), The Equatorial Glaciers of New Guinea, pp. 61-80. Rotterdam, Balkema. Barmawidjaja, D.M., de Jong, A.F.M., van der Borg, K., van der Kaars, W.A. and Zachariasse, W.J. (1989). Kau Bay, Halmahera, a Late Quaternary palaeoenvironmental record of a poorly ventilated basin. Netherlands Journal of Sea Research, 24, 591-605. Biswas, B. (1976). Bathymetry of Holocene foraminifera and Quaternary sea-level changes on the Sunda shelf. Journal of Foraminiferal Research, 6, 107-133. Brettner, K.J.K. (1969). The effects of natural sunlight on human skin. In: Urbach, F. (ed.), The Biologic Effects of Ultraviolet Radiation, pp. 237-249. Pergamon, London. Caldwell, M.M. (1971). Solar UV irradiation and the growth and development of higher plants. In: Giese, A.C. (ed.), Photophysiology, pp. 131-177, Vol. VI. Academic Press, New York. Caratini, C. and Tissot, C. (1985). Le Sondage Misedor. ~tude palynologique. Etudes de geographie tropicale, No. 3, pp. 49+25 plates. Centre d'etudes de geographie tropicale, Centre national do la recherche scientifique, Domaine Universitaire de Bordeaux. Climap Project Members (1976). The surface of the Ice-Age Earth. Science, NY, 191, 1131-I 137. Flenley, J.R. (1979). The Equatorial Rain Forest: A Geological History. Butterworths, London, 162 pp. Flenley, J.R. (1984). Late Quaternary changes of vegetation and climate in the Malesian mountains. Erdwissenschaftliche Forschung, 18, 261-267. Flenley, J.R. (1992). UV-B insolation and the altitudinal forest limit. In: Furley, P.A., Proctor, J. and Ratter, J.A. (eds) Nature and Dynamics of Forest-Savanna Boundaries, pp. 273-282, Chapman & Hall, London. 616 pp. Geyh, M.A., Kudran. H.-R., and Streif, H. (1979). Sea-level changes during the late Pleistocene and Holocene in the Strait of Malacca. Nature. 287, 324-326.

Good, R. (1947). The Geography of the Flowering Plants. 1st edn. Longmans, Green and Co., London, 403 pp. Grubb, P.J. (1974). Factors controlling the distribution of foresttypes on tropical mountains: new facts and a new perspective. In: Flenley, J.R. (ed.), Altitudinal Zonation in Malesia, pp. 13-46. Transactions of the Third Aberdeen-Hull Symposium on Malesian Ecology, Hull 1973, University of Hull, Department of Geography, Miscellaneous Series No. 16. Grubb, P.J. (1977). Control of forest growth and distribution on wet tropical mountains. Annual Review of Ecology and Systematics, 8, 83-107. Hastenrath, S. (1968). Certain aspects of the three-dimensional distribution of climate and vegetation belts in the mountains of central America and southern Mexico. Colloquium Geographicum. 9, 122-130. Hope, G.S. (1976). The vegetational history of Mt Wilhelm Papua New Guinea. Journal of Ecology, 64, 627-663. Hope, G.S. and Peterson, J.A. (1975). Glaciation and vegetation in the high New Guinea Mountains. Royal Society of New Zealand Bulletin, 13, 155-162. Koopmans, B.N. and Stauffer, P.H. (1968). Glacial phenomena on Mount Kinabalu, Sabah. Borneo Region, Malaysia Geological Survey Bulletin, 8, 25-35. Kutzbach, J.E. and Guetter, P.J. (1986). The influence of changing orbital parameters and surface boundary conditions on climate simulators for the past 18,000 years. Journal of Atmospheric Science, 43, 1726-1759. Lee, D.W. and Lowry, J.B. (1980). Solar ultraviolet on tropical mountains: can it affect plant speciation?. American Naturalist, 115, 880-883. Loftier, E. (1984). Pleistocene and present day glaciations in the high mountains of New Guinea. Erdwissenschaffliche Forschung, 18, 249-259. Morley, R.J. (1982). A palaeoecological interpretation of a 10,000 year pollen record from Danau Padang Central Sumatra, Indonesia. Journal of Biogeography, 9, 151-190. Murali, N.S. and Teramura, A.H. (1986). Intraspecific differences in Cucumus sativus sensitivity to ultraviolet-B radiation. Physiologia Plantarum, 68, 673-677. Newsome, J. and Flenley, J.R. (1988). Late Quaternary vegetational history of the Central Highlands of Sumatra II. Palaeopalynology and vegetational history. Journal of Biogeography, 15, 555-578. Roberts, N., Street-Perrott, A. and Perrott, A. (1981). The "Late-glacial" in the tropics. Quaternary Newsletter 35, 15. Robertson, D.F. (1972). The prophylaxis of ultraviolet radiation damage: a physicist's approach. In: Melanoma and Cancer, pp. 273-294. Government Printer, Sydney. Salomons, J.B. (1986). Paleoecology of volcanic soils in the Colombian Central Cordillera (Parque Nacionai Natural de los Nevados). The Quaternary of Colombia. 13, 1-212. Teramura H. (1983). Effects of ultraviolet-B radiation on the growth and yield of crop plants. Physiologia Plantarum, 58, 415-427. van Beek, C.G.G. (1982). A Geomorphological and Pedological Study of the Gunong Leuser National Park, North Sumatra, Indonesia. Wageningen Agricultural University, Wageningen. van Steenis, C.G.G.J. (1934-36). On the Origin of the Malaysian mountain flora. Bulletin of the Botanic Gardens, Buitenzorg Series III Part 1. 13, 135-262; Part II, 13, 289--417; Part III 14, 56-72. van der Kaars, W.A. (1990). Late Quaternary Vegetation and Climate of Australasia as Reflected by Palynology of Eastern Indonesian Deepsea Piston Cores. Academisch Proefschrift, Universteit van Amsterdam, 71 pp. + Figs. U.S. Hydrographic Office (1969). World Atlas of Sea Surface Temperature, 2nd edn. U.S. Hydrographic Office Publications No. 225, Washington.

J.R. Flenley: Problems of the Quaternary of the Sunda-Sahul Region Wade, L.K. and McVean, D.N, (1969). Mt Wilhelm Studies I. The alpine and subalpine vegetation, 225 pp. Australian National University. Research School of Pacific Studies. Department of Biogeography and Geomorphology. Publication BG/I. Walker, D. and Flenley, J.R. (1979). Late Quaternary vegetational history of the Enga District of upland Papua New Guinea. Philosophical Transactions of the Royal Society, B286, 265-344. Walker, D. and Guppy, J.C. (1976). Generic plant assemblages in the highland forests of Papua New Guinea. Australian Journal of Ecology, 1, 203-212.

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Walker, D . and Sun, X. (1988). Vegetational and climatic changes at the Pleistocene-Holocene transition across the eastern tropics. In: Whyte, P. et al. (eds), The Palaeoenvironment of East Asia from the Mid-Tertiary, pp. 579-591. Centre of Asian Studies, Hong Kong University. Hong Kong. Webster, P.J. and Streten, N.A. (1978). Late Quaternary Ice Age climates of tropical Australasia: interpretations and reconstructions. Quaterna~' Research, 10, 279-309. Whitmore, T.C. (1975). Tropical Rain Forests of the Far East. Clarendon Press, Oxford, 282 pp.