Running title: Plant endemism patterns in Borneo and Malay Peninsula

Patterns of Plant Endemism and Rarity in Borneo and the Malay Peninsula

K.M. Wong

Forest Research Centre, P.O. Box 1407, 90715 Sandakan, Sabah, Malaysia

Fax No. Malaysia 089-531068


Abstract

Plant endemism patterns in Borneo and the Malay Peninsula, as they relate to phytogeographical subprovinces and special units of plant diversity, are described. Special phytogeographical subprovinces in Borneo include parts of the Riau Pocket in northwest Borneo, an east coast Sabah area, and a core area of influence from seasonal Asiatic and east MalesianAustralian elements in north Borneo; while those in the Malay Peninsula include the Riau Pocket along its eastern and southeastern flank, a special area of high diversity and endemism in the Perak region, and a core area of seasonal Asiatic influence from the north. In both Borneo and the Malay Peninsula, special units of diversity are pointed out, including mountain peaks (notably Kinabalu) and ranges, areas of ultramafic and volcanic substrate, limestone outcrops, quartzite and sandstone ridges and plateaux, swamp-heath forest systems, and river systems, most of these characteristically habitat islands. Endemic species from the lowland forests, where land conversion and timber extraction activities are most significant, are potentially the most difficult to census and locate. Forest fragmentation has severe implications for the survival of these endemics and other rare plants, and in highly disturbed isolates even the common canopy species have poor chances of regenerating. Management in timber production areas where forest modification is greatest, although these are not specially protected for conservation, ought to consider leaving a network of large and smaller, intact, forest fragments as local refugia for rare plant species and seed source areas.

 

Keywords: Plant endemism, Rarity; Historical phytogeography; Ecological phytogeography; Borneo; Malay Peninsula; West Malesia; Lowland forest diversity; Effects of forest fragmentation.


Patterns of Plant Endemism and Rarity in Borneo and the Malay Peninsula

In conservation terms, narrow endemics and rare species are potentially at the highest risk of endangerment when not adequately represented in protected areas. Endemic species may exist because of extinction throughout much of their range (the so-called relict species or paleoendemics) or because an actively evolving group has given rise to localized distinct species (neoendemics). Rare species, whether endemic or not to a given region, include those with small, localized populations or whose individuals occur at very low frequencies.

Conservation effort also depends on understanding the patterns of rarity in plant distribution on both local and broader scale, as well as the ability to justify particular sites for special protection. Clearly, in a floristically rich region such as Malesia (van Steenis 1950, Whitmore 1984), many groups of plants show remarkable extents of endemism to the various territories (van Welzen, Kort & Ratanopas in Johns 1995). In the lowlands, the intense species richness of forests is significantly due to the predominance of species rare on a local scale (Ashton 1984; Kochummen et al. 1992; Manokaran et al. 1992). Gentry (1986) has already underscored the importance of habitat specialization and explosive, recent speciation in neotropical plant communities, which also applies to various situations elsewhere. Ashton (1989) pointed out that the occurrence of rare species on a local scale in the species-rich forests of Malesia is related to local site heterogeneity and influenced by both disturbance and edaphic factors, and is largely unpredictable. This modifies our conservation perspective for preserving only particular sites of inferred rich biodiversity, to include the question of conserving as much of the evolutionary functions in tropical forests that give rise to randomly generated rare and endemic species (Gentry 1986), a hallmark of these species-rich communities.

As conservation biology consolidates as a discipline (Primack 1993) and the need for adequate habitat protection becomes more emphasized as an issue (e.g., Kiew 1991a), a practical understanding of plant rarity and endemism based on actual examples from a region becomes relevant. The present paper considers the extent of endemism and reviews the forms of rarity among the plants of Borneo and the Malay Peninsula, two of the richest territories in Sundaland (which includes the large islands of Sumatra and Java).


Plant Diversity and Endemism Among the Main Sundaland Islands

In a general context, Borneo and New Guinea are the richest islands in Malesia, both with extremely high extents of endemism at the generic and specific levels (Johns 1995). New Guinea (east Malesia) has 86 endemic genera while Borneo, with 63 endemic genera (Table 1) (and also an endemic family, Scyphostegiaceae), is by far the richest island in west Malesia.

Many families have not been recently revised for the entire region, and therefore general statistics of endemism in a comparative sense for all the islands are not immediately available. Moreover, it will be expected that some groups of plants will be richer in one island than in others, and the different sizes, geographical position and physiography of the islands will, to a certain degree, portend varying overall richness as well as diversity in particular families and genera.

Nevertheless in west Malesia, Borneo is a particularly rich island, approached in overall species diversity only by the Malay Peninsula: many taxa are significantly more speciose in Borneo than the Malay Peninsula, irrespective of their size, life form representation, whether they are predominantly lowland or montane, and even if they are overall more speciose in other parts of Malesia (Tables 2 and 3). Other studies show similar trends: the Myrsinaceae are represented by 157 species in Borneo and 110 in the Malay Peninsula (Stone 1992); Monophyllaea (Gesneriaceae) has 17 species (16 endemic) in Borneo and only 4 (3 endemic) in the Malay Peninsula (Burtt 1978); palms are represented in Sarawak by 213 species (out of 290 species in Borneo, with a 40% endemism) and in the Malay Peninsula by 197 species (43% endemic)) (J. Dransfield, pers. comm.). The Euphorbiaceae (340 species in Borneo, 371 in the Malay Peninsula) (Airy Shaw 1975, Whitmore 1972b, respectively) are one of the plant groups that are apparently more speciose in the Malay Peninsula, as are the Labiatae and other groups with a greater diversity in the Indo-Chinese area.

This could be due in part to Borneo having a larger land area and a median position within the Malay Archipelago where it is more isolated from the Asian mainland. Additional factors that probably also contribute to Borneos high plant diversity include the greater physiographic complexity (Mt Kinabalu, 4101 m, is the highest mountain in west Malesia, and the generally sharp and high ridges of the mountains and abundance of streams give a highly dissected landscape and a large altitudinal spectrum) and larger edaphic range (extensive swamp systems, tropical heath or kerangas forest, sizeable basaltic and ultramafic areas, and substantial limestone outcrops in addition to the generally poor soils developing on sandstone and shale). These habitats may also be present in other territories, but in Borneo they are represented by large extents and several areas.


Phytogeographical Provinces and Special Units of Diversity

A phytogeographical unit can be taken in various senses, at a wider or more local scope, and can indicate a special relationship of the flora to that in a different region or an especially high diversity or significant concentration of endemics unique to it. Whereas the student of historical plant geography may be interested in identifying distinct, or related, provinces and subprovinces that reflect relationships between the floras of different land masses and regions, others with conservation as a priority emphasize more the documentation and protection of areas and localities of special richness or diversity, which are frequently linked to special habitats or communities (ecological biogeography). Both areas are relevant to conservation, as both point out special aspects of the flora. Studies of species richness (Ashton, 1984, 1989, 1992; Primack, Hall & Ashton, 1992) suggest that while such special units of plant diversity should be given special attention in conservation, a generally high diversity with its own suite of rare or endemic species can nevertheless be expected in many lowland forest areas in Borneo and the Malay Peninsula, which are, as seen earlier, core areas of the west Malesian floristic diversity.


Richness and Endemism in Borneo

The size of the flora of Borneo is still very much a matter of conjecture (Wong 1995) but is believed to range between 9,000 (Merrills original estimate in 1921) and 15,000 (the upper limit of Merrills revised estimate in 1950) species of vascular plants.

In general terms, the tree flora of the northwest and northern parts of Borneo appears richer than the remaining portion that is mostly occupied by Kalimantan; this notion was earlier explained by a lower collecting density and paucity of documentation in Kalimantan (Vink, 1981) compared to Sabah, Sarawak and Brunei, but further work on Kalimantan material appears to confirm the comparatively lower diversity of the flora (van Balgooy, pers. comm.). Of the 311 species of tree in Borneo belonging to 31 families included in the first volume of the Tree Flora of Sabah and Sarawak (Soepadmo and Wong, 1995), which included the whole of Borneo in its perspective, 108 species (just over a third) are endemic to Borneo. Of these, roughly a third (30 species) is endemic to Sabah only, another third (31 species) endemic to Sarawak (and Brunei) alone, and a third (34 species) endemic to and occurring in both Sabah and Sarawak (Wong, in press). Only two species are endemic to Kalimantan and not recorded for Sabah and Sarawak.

Even higher levels of endemism among many non-tree groups in Borneo, such as Diplycosia, Vaccinium and the Araceae, have been recorded (Tables 2 and 3). In addition, of the 48 species of Medinilla (Melastomataceae) recorded for Borneo, 39 are endemic (Regalado 1990), 22 of the 28 species of Argostemma (Rubiaceae) in Borneo are not known elsewhere (Bremer 1989), and some 40% of the 290 species of palms in Borneo are also endemic (Dransfield, pers. comm.).


The Riau Pocket in Borneo

Besides the generalized inland flora of the lowlands, Ashton (1992, 1995) has pointed out two other notable lowland floristic subprovinces in Borneo. One is what he has identified as part of the Riau Pocket of Corner (1960), an area defined by the preponderance of species that are characteristic of two or all three main areas on the Sunda Shelf: Central Sumatra and the Riau islands south of Singapore, the eastern and southeastern part of the Malay Peninsula, and northwest Borneo. This area had substantial land connection during the Pleistocene (Umbgrove, 1949) and in a way constitutes a core Sundaland area for the flora. This is represented in Borneo by the area bounded by the Kapuas River from its mouth to the lakes region and the Lupar valley on the western part of the island, and an area that stretches roughly from the mouth of the Rejang River, across the Sibu, Bintulu and Miri districts of Sarawak, Brunei, the Sarawak Limbang area around Brunei, and into the BeaufortSipitang area in southwest Sabah (Figure1).

These two patches of the so-called Riau Pocket lie roughly north of a line between Pontianak and Sipitang, called the PontianakSipitang Line (Ashton, 1992). About 50 of the 281 species of Dipterocarpaceae in Borneo appear not to occur south of the Line (Ashton, 1992), and a host of characteristic Riau Pocket taxa that are rare or unknown elsewhere occur in this special area, such as Dillenia excelsa var. tomentella (Dilleniaceae), Cotylelobium lanceolatum, Dipterocarpus globosus, D. rigidus, Shorea materialis, S. parvifolia ssp. velutinata (Dipterocarpaceae), Sebastiana borneensis (Euphorbiaceae), Didymocarpus sect. Boeopsis (Gesneriaceae; Kiew, pers. comm.), Chionanthus lucens (Oleaceae), Daemonorops scapigera (Palmae), Pandanus epiphyticus (Pandanaceae), Aidia lancifolia, Gardenia pterocalyx, and Rothmannia kuchingensis (Rubiaceae). Trees endemic to this region in Borneo north of the Pontianak-Sipitang Line include the dipterocarps Cotylelobium burckii, Dryobalanops fusca, D. rappa, Hopea enicosanthoides, H. treubii, Parashorea macrophylla, Shorea albida, S. bakoensis, S. flemmichii, S. inaequilateralis, S. pilosa, S. quadrinervis, S. rubella, S. slootenii, Vatica compressa, V. congesta and V. pedicellata, and the oaks Lithocarpus ferugineus, L. hallieri and L. meijeri.

The Riau Pocket flora occurs on freely drained yellow or red soils (humult ultisols), deep podsols and in peat swamps, and it has also been suggested (Ashton, 1972, 1992) that the area of north and northwest Borneo has possibly been a Pleistocene refugium. The occurrence in the Bintulu area, in northwest Borneo, of an endemic species of Nephthytis (Araceae), a genus otherwise known only in tropical Africa (Bogner and Boyce, 1995), may reflect the persistence of a genus present since earlier times and whose representatives became extinct elsewhere in the region. This flora is generally distinct from that occurring on the deep clay soils (udult ultisols) characteristic of most of the other forested lowland parts of Borneo, the Malay Peninsula and Sumatra, and the two distinct types in Borneo are noted as having less than 50% of the major tree species in common (Ashton, 1992). The clay flora in the northern part of Borneo is also exceptionally rich.


The East Coast Sabah Floristic Subprovince

The other notable lowland floristic subprovince is the east coast Sabah subprovince (Figure 1). As circumscribed by Ashton (1992), this includes the area around and on both sides of the Sandakan peninsula, and in the TawauTarakan area (marked A and B, respectively, in Figure 1). Here the endemics include Guioa pterorhachis (Sapindaceae) and the dipterocarps Dipterocarpus fusiformis, Dryobalanops keithii, Hopea ovoidea, Shorea symingtonii and S. waltonii, and in total at least 65 of the 87 species of Shorea in Sabah are found in east Sabah, with 54 dipterocarp species occurring in the now isolated Sepilok Forest Reserve in Sandakan (Fox, 1973). The east Sabah area is also home to some 35% of the rattan species in Borneo, although diversity is more notable in local pockets (Dransfield, 1984, and pers. comm. 1996).

There appears to be a good basis for delimiting a larger area as an east coast Sabah floristic subprovince. This coincides, at least in large part, with the area east of a rough arc that stretches from near Mt. Kinabalu along the eastern flank of the Crocker Range (through Tambunan and Keningau), veering southeast (through the Sook, Pendawan and Maliau basins) into the Tanjungredeb area just north of the Sangkulirang peninsula in Kalimantan. Much of this path is interpretable in tectonic terms as the Kinabalu Suture Zone, marking the margin of cohesion between an East Sabah Terrane (that had rifted from east Asia during the late Cretaceous to early Tertiary through sea-floor spreading of the South China Sea Basin) and the main portion of Borneo (Tjia, 1988, Tjia et al., 1990). This cohesion of the two terranes was completed in the early-middle Miocene, during which a rift corridor (the Sandakan Rift) developed from the Maliau Basin northeast to the Sandakan area, probably related to the opening of the Straits of Makassar east of Borneo (Hutchison, 1988; Tjia et al., 1990).

It is within this area that the endemic dipterocarps Parashorea tomentella and Shorea macroptera ssp. sandakanensis are most common. Although they have different soil preferences and almost never occur in strict mixture (Ashton, pers. comm. 1996), they are usually found in the lowlands along and east of the Sandakan Rift corridor; westwards these taxa become scarce or are absent altogether. The area is markedly different from the west Sabah region, which is dominated by the Crocker Range that has Kinabalu at its northern extreme. At the same time, some species that are more widely distributed in Borneo are conspicuous by their absence in this east coast Sabah region, such as Upuna borneensis (Dipterocarpaceae).

A zone of ultramafic rocks (mainly serpentinite and peridotite), interpreted as part of the oceanic crustal material that has intruded overlying Tertiary sedimentary rocks in relation to nearby tectonic activity, occurs within this region and separates the two areas marked by Ashton (1992) as representing the east coast Sabah floristic subprovince. This ultramafic zone bears a distinctive subset of the flora discussed later.


The Seasonal Asiatic-Australasian Intrusion in North Borneo

Not as well defined is the occurrence in Borneo of taxa that are most well developed in the more seasonal areas of Asia, usually from India, Indo-China, and South China and from the region east of Borneo, from the Philippines to the Australasian area. This is related to the more marked seasonality in the extreme north of Borneo, with its core from around Kudat to the lower Kinabatangan area in the northeast, although such elements occur as far south as Kota Kinabalu, Ranau (Kinabalu area) and along the east coast islands (Figure 1).

Examples include Antidesma ghaesembilla (Euphorbiaceae), distributed from the Himalayas and south China, to north Borneo (Sabah) and northern Australia, but absent in Sumatra, the Malay Peninsula, and in Borneo almost completely absent from Sarawak and Kalimantan. This species is common in the Kudat peninsula in north Sabah. Likewise, Severinia (Rutaceae; represented in Borneo by 2 species known from north and northeast Sabah only) is a genus most well developed in south China, Indo-China, and the more seasonal parts of Malesia, in the Moluccas, Java and parts of New Guinea. The same is true of Pleiospermium longisepalum (Rutaceae), endemic to Sabah but with the genus most well developed in the IndiaVietnam area and Sumatra and Java. Recently two unnamed species of the largely IndianIndo-Chinese genus Ceriscoides (Rubiaceae) were discovered in Sabah, in the Ranau area and the lower Kinabatangan area in the northeast. Schrebera (Oleaceae), a pantropical genus which in Asia is best developed in the seasonal areas, is represented in Malesia only by S. kusnotoi, documented from the Ranau area in Sabah and the Sangkulirang peninsula in Kalimantan (Kiew, pers. comm.). The bamboo genus Yushania is represented in Taiwan and Luzon (Philippines) by Y. niitakayamensis, which is vicariant with Y. tessellata occurring in Sabah at Kinabalu and the Crocker and other nearby ranges; the affinities of the genus are with the Asian mainland and indeed, some botanists have attempted to include Yushania in Sinarundinaria or Fargesia from there.

A genus with its centre of diversity in the East Asian-Australasian region and including the Philippines, Sulawesi and Lesser Sunda Islands, but which appears to avoid the everwet cores of West Malesia is Aphananthe (Ulmaceae): this is represented in north Borneo by A. cuspidata (which ranges from India and Indo-China, through the Philippines to Sulawesi and the Lesser Sunda Islands), in the Ranau (Kinabalu) area. Likewise, Exocarpos (Santalaceae) ranges from Indo-China, through the Philippines, down to the rest of central and eastern Malesia and Australia and the Pacific, and avoids the everwet Malayan and Bornean core, but occurs on Gaya Island near Kota Kinabalu and parts of the Sabah east coast.


Other Areas of Special Plant Diversity

her areas of high or special plant diversity in Borneo include the major mountain ranges (of which Kinabalu and the Crocker Range in the north must be the most well studied but still incompletely documented), and areas with unusual substrate (Figure 2). The latter includes the chain of ultramafic areas from Palawan southwards through Kinabalu-Tambuyukon and veering eastward through the Labuk valley and Telupid to the upper Segama and Darvel Bay area; the limestone outcrops (principally Sarawak and Sabah but also notable at Mt. Suwaran and the Mangkalihat peninsula of East Kalimantan); the tropical heath or kerangas forests developed on coastal podsols, raised beach terraces and sandstone ridges (widespread in northwest Borneo and Kalimantan); and the peatswamp complexes (best developed in Kalimantan, Sarawak and the northern part of Sabah).


The Mountain flora

Mt. Kinabalu, with some 4500 vascular plant species in its area alone (Beaman and Beaman,1990), perhaps some 10% of the vascular flora of the entire Malesian region, is the richest botanical site in Borneo and in Asia west of New Guinea. Rising from the lowlands to 4101 m, this spectacular mountain has 73 genera represented by 10 or more species, of which 30 genera have 20 or more species. The largest are Ficus (Moraceae) (98 species), Bulbophyllum (Orchidaceae) (85 species), Syzygium (Myrtaceae) (66 species), Dendrobium (Orchidaceae) (61 species), and Lithocarpus (Fagaceae) (46 species). Among ferns, 50 species of the 609 recorded for Kinabalu (8%) are endemic there (Parris, Beaman and Beaman, 1992). In Medinilla (Melastomataceae), 17 of the 48 species in Borneo occur on Kinabalu, and 7 are endemic to Kinabalu and the Crocker Range (Regalado, 1990). Some 25 species of Rhododendron (Ericaceae) are known on Kinabalu, including R. lowii (shared with Mt. Trus Madi) and R. abietifolium, R. buxifolium, R. ericoides and R. maxwellii (endemic to the mountain alone). About half of some 1500 species of orchids known in Borneo are found on Kinabalu (Wood and Cribb, 1994).

Not only is endemism significant among many plant groups on Kinabalu (and the Crocker Range, with which it is contiguous but which is lower, with major hills 1200–1800 m high), but also many taxa of montane or temperate affinity are present on account of its high elevation. Examples are Agathis (a genus of Australasian affinity, 3 species on Kinabalu) (Araucariaceae), Dacrydium (also an Australasian genus, 5 species on Kinabalu) (Podocarpaceae), Drapetes ericoides (known otherwise from New Guinea mountains) (Thymelaeaceae), Euphrasia borneensis (endemic representative of a largely temperate genus) (Scrophulariaceae), Photinia davidiana and P. prunifolia (elsewhere known only from China, Vietnam and Sumatra) (Rosaceae), Potentilla parvula (known otherwise from some high Philippine, Celebes and New Guinea mountains), P. borneensis (known otherwise from north Sumatra mountains) (Rosaceae), Ranunculus lowii (endemic representative of a largely temperate genus) (Ranunculaceae), and Trachymene saniculaefolia (elsewhere known only in Mindoro and east New Guinea) (Umbelliferae).

Although the mountain chains running south of the Crocker Range in Sabah, into Brunei and Sarawak/Kalimantan (Figure 2), are not as high or as rich as Kinabalu, they nevertheless are important appendages of the unique and rich mountain flora of Borneo. An increasing number of species first known from Kinabalu are now found on mountains to its south: the tree Prunus oocarpa (Rosaceae) (now also known in the Pagon and other ranges in Sarawak), the bamboo Kinabaluchloa nebulosa (occurring also on the Pagon Range in Brunei and Sarawaks Kelabit Highlands), the rattan Calamus gibbsianus (also on the Crocker Range and Kelabit Highlands) and the palms Pinanga keahii (also on Mt Mulu in Sarawak and Mt Halau-Halau in the Meratus Mountains in Kalimantan) and Areca kinabaluensis (widespread on mountains usually at about 1000–1500 m throughout Borneo), the orchid Entomophobia kinabaluensis (on mountains in Sabah, Sarawak, Brunei and Kalimantan), and the herb Sonerila kinabaluensis (Melastomataceae) (known also on Mt Mulu in Sarawak) are examples that join a more general Bornean montane alliance comprising a suite of such taxa as Rhododendron borneense, R. crassifolium, R. fallacinum, R. lanceolatum, R. suaveolens, R. orbiculatum, Vaccinium spp. (Ericaceae), Litsea spp. (Lauraceae), Ficus spp. (Moraceae), Embelia spp. and Rapanea spp. (Myrsinaceae), Syzygium spp. (Myrtaceae), Nepenthes tentaculata and N. veitchii (Nepenthaceae), Chelonistele (Orchidaceae), Dacrydium gracilis, D. xanthandrum and Phyllocladus hypophyllus (Podocarpaceae), and others.

Moreover, different mountains and ranges often have their own specialities, for instance, Dacrydium gibbsiae (Mt. Kinabalu) and D. ericoides (Mt. Dulit and the Merurong Plateau in Sarawak) (Podocarpaceae); Rhododendron buxifolium (Kinabalu), R. baconii (Mt. Tambuyukon, Sabah) and R. burttii (Mt. Lotung, Sabah and Mt. Murud, Sarawak) (Ericaceae); and Nepenthes boschiana (Mt. Sakumbang, S. Kalimantan), N. campanulata (Mt. Ilas Bungaan, E. Kalimantan), N. mollis (Mt. Kemul, E. Kalimantan), N. clipeata (Mt. Kelam, W. Kalimantan) and N. muluensis (Mt. Mulu and Mt. Murud, Sarawak) (Nepenthaceae). No doubt more localized endemicity in montane representatives of such genera as Begonia (Begoniaceae), Elatostema (Urticaceae), Impatiens (Balsaminaceae) and Sonerila (Melastomataceae) will be demonstrated when work progresses towards their revision.

Altitudinal segregation of taxa probably occurs on many mountains and is often difficult to confirm with a limited knowledge of distribution, but is well displayed on Kinabalu by the closely related bamboos Racemobambos hepburnii (well developed at 1100–2200 m) and R. gibbsiae (2000–3000 m), which are reproductively segregated with no coincident flowering (Wong, Chan and Phillipps, 1988), and also R. hirsuta (on ultramafic soils, 800–1500 m) and R. rigidifolia (ultramafics above 1600 m). On a few other (lower) Sabah mountains R. hirsuta is the higher-elevation species with R. pairinii lower down, again with no coincident flowering. Such altitudinal segregation is also reflected by several Machaerina species (Cyperaceae) on Kinabalu: while the more widespread M. disticha and M. glomerata are common from the lowlands to about 1500 m and M. falcata is found at 1900–3300 m, the endemic M. aspericaulis is well developed at around 1500–1650 m. Among the species of the herbaceous genus Pentaphragma (Pentaphragmataceae), it is known that the montane P. longisepalum and P. aurantiacum show such segregation to some extent; on Mt. Kinabalu and Mt. Murud, respectively, the former occurs at 830–2170 m and 1740–2130 m, while the latter is only known at higher elevations at 2000–3000 m and 2400–2650 m (Kiew 1990). In addition to the localized endemicity of different mountains, and examples of altitudinally or edaphically isolated congeneric endemics on the same mountain, the commonness of variant forms and natural hybrids observed among montane species of genera such as Nepenthes (Nepenthaceae), Prunus, Rubus (Rosaceae) and Rhododendron (Ericaceae) suggest these mountains and ranges as islands of rather intense speciation.

The commonness of a number of congeners co-occurring at particular sites or as altitudinally differentiated species has been attributed by Kitayama (in press) to two different processes which can occur on mountains. In the first, newly evolved alleles or species may coexist in stability with existing ones when they are adapted to different parts of the resource coordinate system (i.e., there is niche differentiation), or because they are maintained by stochastic (site-disturbance) processes which create habitat heterogeneity. This increases alpha diversity on site. In the second, the emergence of a new allele or species may impose competition on existing ones when they are very similar in resource demands. Here, selection for one is at the expense of another, and fitness dictates which is maintained. This can diminish the distribution range of an ancestral or pre-existing entity, and both can then be spatially segregated, e.g., in altitude. This contributes to an increase in beta diversity between sites.

 


The Flora on Ultramafics and Basic Volcanic Rock

The flora on ultramafic soils (Figure 3) is distinctive by its endemics, including the trees Atuna cordata (Chrysobalanaceae), Borneodendron aenigmaticum (Euphorbiaceae), Quercus kinabaluensis (Fagaceae), Dipterocarpus ochraceus, Shorea micans and S. tenuiramulosa (Dipterocarpaceae), and Scaevola micrantha (Goodeniaceae, shared with Philippines and Palawan), the epiphytic orchids Paraphalaenopsis labukensis and Renanthera bella, the bamboos Dinochloa darvelana, D. obclavata, D. prunifera and D. robusta (the last shared with Palawan), and the rattan Daemonorops serpentina, one of 11 species of rattans confined to lowland ultramafic areas. Where the influence of ultramafics combines with that of elevation, narrow endemics are typical: examples are Rhododendron meijeri, R. baconii (Ericaceae), Scaevola verticillata (Goodeniaceae) (known only on Mt. Tambuyukon); Dacrydium gibbsiae (Podocarpaceae), Dendrobium beamanii, D. maraiparense, D. piranha, Paphiopedilum rothschildianum (Orchidaceae), Leptospermum recurvum (Myrtaceae), Nepenthes villosa (Nepenthaceae), Pittosporum linearifolium (Pittosporaceae), Scaevola chanii (Goodeniaceae) (on Mt. Kinabalu only); Lithocarpus rigidus (Fagaceae), Nepenthes burbidgeae, N. edwardsiana, N. rajah (on both Kinabalu and Tambuyukon); Corybas serpentinus (Orchidaceae), Pittosporum silamense (known from Mt. Silam only); and Nepenthes macrovulgaris (known on both Kinabalu and Silam). As yet unnamed species of Antirhea, Porterandia and Timonius (Rubiaceae) (Wong, in prep.) that are endemic to ultramafic areas have also been noted, but in general our census of plants from ultramafics is still far from complete.

Besides specialist species, the ultramafic areas in east Sabah also sustain outlying populations of species typical of the northwest Borneo area (Ashton 1992), such as Shorea andulensis and S. venulosa (a possibly distinct form). In Borneo, the widespread Malesian fern Sphenomeris retusa is known only on ultramafic areas on Kinabalu (Parris, Beaman & Beaman 1992) and in east Sabah (Shim 1992).

Volcanic rock in Borneo (Figure 3) ranges from basalt to basic andesites, acid dacites and rhyolites (Leong 1976, Yin 1987, Shu et al. 1994). These occur extensively in east Sabah (Tawau, Kunak and Semporna areas: where there are significant areas of dacite, basalt and andesite), and in central Sarawak in the Ulu Anap / Bukit Mersing area (basalt), Sungai Arip area (rhyolite), Bukit KajangHose Mountains area and Usun Apau plateau (dacite), and the Batu Laga and LinauBalui plateaux (basalt). The occurrence of Bornean endemics common on or restricted to such volcanic rock, such as Hopea andersonii ssp. basalticola (central and northeast Sarawak, east Sabah, southeast Borneo), Shorea mujongensis (central Sarawak, east Sabah) (Dipterocarpaceae), and Dendrochilum hosei (central Sarawak) (Orchidaceae), relate to these areas as having a special floristic aspect. Nevertheless, information on the flora of these areas is still scanty.


Limestone Flora

Exposed limestone in Borneo is principally distributed in the western and northeastern part of Sarawak; the northern islands of Balambangan and Banggi, the KinabatanganMadai area of northeastern Sabah; and in the Mt. BuntungMt. Suwaran area of the Mangkalihat Peninsula (Sangkulirang district), the area west of Mt. Seribu (Krayan County) near the Sabah/Sarawak border, the Meratus Mountains (where there are extensive parallel outcrops), and at Tiang Laju in the upper Kapuas area in Kalimantan (Figure 3). Of these, the flora of the limestone areas in Sarawak have been better collected (Anderson, 1965), the ones in Sabah and Kalimantan virtually unknown, and a special study of these key limestone sites is long overdue, as studies in the Malay Peninsula (Chin, 1977, 1979, 1983a, 1983b; Kiew and Yong, 1985; Kiew, 1991b) have demonstrated the abundance of endemics among herbs, even though the woody flora is generally poor. The inselberg” (island mountain) nature of the majority of the outcrops appears to have promoted speciation among such groups as the gesneriads, balsams and begonias. In Sarawak, the high elevation of the Mulu Park limestone on Mt. Benarat and Mt. Api, the latter reaching 1700 m, sustains montane vegetation on these moist peaks, and there it is possible the potential isolation factor that underlies speciation is further increased.

Nearly all of the 15 species of Paraboea in Borneo are locally endemic to limestone (Burtt, 1984; Xu and Burtt, 1991), and all 17 species of Monophyllaea have been recorded on limestone, 16 being confined to it (Burtt, 1978). Plants confined to the Melinau limestone in Sarawaks Mulu National Park include, as a very incomplete mention, 6 species of Monophyllaea, 3 species of Paraboea, 3 species of Cyrtandra (Gesneriaceae), the spectacular slipper orchid Paphiopedilum sanderianum, the screwpines Pandanus calcinatus and P. leptophilus (Pandanaceae), the palms Calamus nielsenii and Salacca rupicola, and the tree Scaevola muluensis (Goodeniaceae). The Bau-Padawan limestones farther south in Sarawak have, as endemics, 6 species of Monophyllaea, Paraboea havilandii, 2 species of Cyrtandra, Nepenthes northiana (Nepenthaceae), Daemonorops unijuga (Palmae) and Paphiopedilum stonei, among others. Turpinia calciphila (Staphyleaceae) is one of the species common to both the Mulu and Bau limestone in Sarawak. It is also known that the Gomantong outcrop in northeastern Sabah has its own endemics, such as Begonia malachosticta (Begoniaceae), as well as species shared with other outcrops in the vicinity, such as Antirhea inaequalis (Rubiaceae). The Batu Ponggol outcrop in south Sabah is known to have the orchid Ania ponggolensis endemic there. The extensive limestone complex in the Mangkalihat Peninsula also bears its own endemics, such as Osmoxylon kostermansii (Araliaceae) and Monophyllaea stellata (Gesneriaceae).

The little that is known from very limited collections from the islands of Balambangan and Banggi, including their limestone outcrops, shows that they share specialities with Palawan and Balabac (politically part of Philippines), and this may appear to lend support to the idea (Merrill, 1926, Stone, 1980) that these northernmost islands of the Sunda Shelf perhaps form a special biogeographical subprovince. While limestone species such as Pandanus occultus (Pandanaceae) and Antirhea caudata (Rubiaceae) are common to Balambangan and Palawan, and there are many endemics in this cluster of islands (e.g., Amorphophallus hottae, Araceae, from limestone, and Fagerlindia emanuelsoniana, Rubiaceae, from ultramafics, both known only from Palawan), this is not quite sufficient evidence. Palawan has a substantial number of taxa in common with either Borneo or the main Philippines islands, Balambangans flora has more in common with north Sabah, and there are unique distribution patterns among some plants there. For instance, the bamboo Dinochloa robusta occurs only in Kudat (north Sabah), Balambangan, Banggi and Palawan, and Agathis celebica (Araucariaceae) is known only in Celebes (Sulawesi), southern Philippines and Palawan whereas A. philippinensis, widespread in the Philippines and Celebes, appears absent from Palawan. The occurrence of endemic taxa is not unexpected in a cluster of islands which share such special edaphic variants as limestone and ultramafics, and which are to some extent isolated from other such sites in either the Philippines or Borneo, and speculation at this time will have to contend with the fact that these islands are very much in the transition between the Philippines and Borneo.


The kerangas, kerapah and Peatswamp Flora

The flora of kerangas forest (and its waterlogged equivalent, kerapah) and peatswamp forest (Figure 4) is overall poorer than that of Mixed Dipterocarp Forest but still includes many species, and shows a distinctive array of specialized and endemic species, probably on account of the extent of this edaphically controlled vegetation type in Borneo. Examples of specialist species of kerangas include the dipterocarps Cotylelobium burckii, Dryobalanops fusca, Shorea retusa, S. revoluta and Vatica compressa, the oak Quercus kerangasensis (Fagaceae), the rattans Calamus ashtonii, C. nanodendron, C. sabalensis, Daemonorops maculata and D. oblata, and orchids such as Coelogyne bruneiensis and Dendrochilum gravenhorstii. Some of these are more widespread in many kerangas areas in Borneo, others are more confined in distribution to one or very few localities.

Anderson (1963) has listed 393 species of flowering plants for the peatswamp forests of Sarawak and Brunei. Kerangas, kerapah and peatswamp forest, often in close proximity and intergrading or occurring as a complex of mosaics in the coastal areas of northwest Borneo and Kalimantan, share a number of species (Anderson, 1983; Bruenig, 1990), and these complexes have their own specialist species endemic to Borneo, including the trees Dryobalanops rappa, Shorea albida, S. inaequilateralis, Shorea pachyphylla (Dipterocarpaceae) and Dactylocladus stenostachys (Crypteroniaceae). The rattan Plectocomiopsis triquetra and the sedge Mapania maschalina are found in both peatswamp and kerapah, but the pitcher-plant Nepenthes bicalcarata and the screwpine Pandanus andersonii (Pandanaceae) might be true peatswamp endemics. Large areas of highland kerangas and kerapah occur in central Sarawak, such as on the Merurong plateau, in Sabahs Maliau Basin, and in Kalimantan, but the flora of these remain little known, although expected to have their own rare or endemic species.


Rheophytes and Species of Seasonally Flooded Riversides and Freshwater Swamps

The special group of plants adapted to the flood zone of fast-flowing, rocky streams, called rheophytes, has been given special attention by van Steenis (1981) on account of their diversity. Rheophytes are not distributed in discrete areas or habitat islands, so that their conservation is taken together with that of areas of the various forest or vegetation types, in particular at places of scenic value or pristine condition. Nevertheless, van Steenis points out that the diversity of rheophytes is particularly notable in Sarawak and Brunei. Widespread west Malesian rheophytes include the fern Dipteris lobbiana, the shrubs Homonoia riparia and Phyllanthus chamaepeuce (Euphorbiaceae), and the tree Dipterocarpus oblongifolius (Dipterocarpaceae). Rheophytes endemic to but common in Borneo include the palms Pinanga rivularis and P. tenella, the shrubs Myrmeconauclea strigosa (Rubiaceae) and Osmoxylon borneense (Araliaceae), and the trees Antidesma linearifolium (Euphorbiaceae), Fagraea stenophylla (Loganiaceae), Sandoricum borneense (Meliaceae) and Syzygium tetragonocladum (Myrtaceae). There are also endemic rheophytic species of restricted distribution, such as Aglaia rivularis (Meliaceae; east Sabah only), Antidesma stenophyllum (Euphorbiaceae; Sarawak), Phyllanthus kinabaluicus (Euphorbiaceae; Kinabalu in Sabah) and Schismatoglottis gillianae (Araceae; Bruneis Temburong district). Areca rheophytica (Palmae) is a rheophyte known only from streams through ultramafic substrates in Sabah.

Although not defined as rheophytes in the sense of van Steenis (1981), a number of plants are restricted to riverbank forest that is seasonally (less often) flooded over, typically in the lowermost stretches of some rivers. Some of these also occur in patches of freshwater swamp near such stretches of rivers. The bamboo Schizostachyum terminale, confined to these habitats in the lower Kinabatangan and lower Belait rivers in Sabah and Brunei, and elsewhere in Kalimantan, Sumatra and the Malay Peninsula, and an unnamed species of Ceriscoides (Rubiaceae) in the lower Kinabatangan, are examples. The earth-fig, Ficus uncinata (Moraceae), which is common along some fast-flowing stretches of streams, also grows along the banks of more slowly flowing and clearly alluvial stretches of rivers, and may be more able than the other plants named here to disperse, so that any riverbank flat or terrace that is liable to be flooded over is a potential habitat for establishment: this wider adaptation is perhaps commensurate in some way with the species also occurring in Sumatra and the Malay Peninsula, although this alone does not explain why other rheophytes are not similar and not more widely distributed. Perhaps other factors are involved, such as more abundant light and a more humid surrounding along rivers and streams.


Richness and Endemism in the Malay Peninsula

Keng (1970) has estimated the Malayan flora to comprise just over 8000 species, in about 1500 genera, of seed plants, of which a third of the genera are centred in Malesia, another third in Asia, and roughly a third also with a centre of development spanning the Asian-Malesian-Australian region or which is pantropical. A small proportion (less than 4%) of the genera were attributed by Keng to an Australian affinity, and less than 1% to a Pacific affinity. In addition, Keng (1970) also listed an overwhelming number of genera shared with Borneo than with Sumatra, so that on the whole the Malayan flora seemed more related to the Bornean flora than to that of Sumatra. This, however, could be a superficial view due to the intense diversity in Borneo and the relative paucity of Sumatra (Table 1), although Sumatra remains less explored botanically.

Of the 2830 species of tree enumerated in a revision of the tree flora, 746 (or 26%) are endemic to the Malay Peninsula (Ng, Low and Mat Asri, 1990). The largest families of tree, Euphorbiaceae (286 species), Rubiaceae (222) and Myrtaceae (204), had 86 (30%), 74 (33%), and 73 (36%) endemic species. The largest tree genera, Eugenia (=Syzygium) (Myrtaceae, 191 species), Diospyros (Ebenaceae, 70 species) and Shorea (Dipterocarpaceae, 57 species), had 73 (38%), 28 (40%) and 7 (12%) endemic species.

There are also significant levels of endemism among non-tree groups, especially among a number of dicot herb genera, in which the proportion of endemic species is remarkably higher than that found among trees: Argostemma (Rubiaceae) (88%), Begonia (Begoniaceae) (96%), Didissandra (Gesneriaceae) (100%), Didymocarpus (Gesneriaceae) (94%), and Sonerila (Melastomataceae) (87%) (Kiew, 1991b). About 43% of the palm flora (83 of some 195 species) is also endemic (Kiew, 1989).


Phytogeographic Provinces

Three special floristic areas in the Malay Peninsula were recognized by Corner (1960):

(a) The Riau Pocket, a part of which according to him was localized in the Southeast PahangEast Johore area, but which, with current additional information might be extended to include the entire coastal area along the eastern part of the Malay Peninsula (Ashton, 1992);

(b) The west coast region in which there was much similarity to the Sumatran flora: this coincides, in part, with the Perak subprovince of Ashton (1992); and

(c) An enclave of southward invasion by Burmese-Thai floristic elements, corresponding to a V-shaped area with its base at Kuala Lumpur: this might be understood as a core area of seasonal Asiatic intrusion in the Malay Peninsula.

 

These are shown, as currently interpreted, in Figure 1. Of the three subprovinces, the first has been elaborated in the discussion concerning the Riau Pocket in northwest Borneo, and here it is mentioned only that there are also endemics to this area, such as: Alseodaphne corneri (Lauraceae, east Johore), Cleistanthus major (Euphorbiaceae, Terengganu east coast), Dillenia albiflos (Dilleniaceae, southeast Johore), Hopea polyalthioides (Dipterocarpaceae, southeast Johore), Madhuca tomentosa (Sapotaceae, Pahang, east Johore), M. tubulosa (Terengganu, Pahang, east Johore), Shorea collina (Dipterocarpaceae, along the eastern fringe of the peninsula), and Trigonostemon rufescens (Euphorbiaceae, southeast Johore). Herb species endemic to this area in the peninsula include Begonia corneri, B. rajah (Begoniaceae), Didymocarpus craspedodromus (Gesneriaceae) and Musa gracilis (Musaceae). Further comment is here offered only on the other two areas.

The Perak subprovince includes the state of Perak (the core area), the adjacent parts of Kedah and Selangor, and the island of Penang, and harbours an extraordinary number of endemics not found elsewhere, such as the dipterocarps Dipterocarpus perakensis (known only from the PenangDindingsPangkor area) and Shorea lumutensis (only in the Dindings area), Chisocheton perakensis (Meliaceae, only in the Larut Hills), Hieris curtisii (Bignoniaceae, Penang only) and Maingaya malayana (Hamamelidaceae, Penang and Perak only), the last two representing monotypic genera. Among the 11 states of Peninsular Malaysia (equivalent to the Malay Peninsula proper), the tally of hyper-endemic tree species (defined as species endemic to one state only) was highest for Perak (103), and distinctly lower for the states around it: Kedah (13), Kelantan (9), Terengganu (18), Pahang (76) and Selangor (21) (Ng, Low & Mat Asri 1990).

A number of species that are shared with Sumatra occur in the Perak area, such as Parashorea globosa and Vatica perakensis (Dipterocarpaceae), but such species can be generally distributed along the entire west coast of the peninsula (e.g., Brachylophon curtisii, Malpighiaceae; and Lithocarpus eichleri, Fagaceae), restricted to the northern half of the peninsula (e.g., Diospyros daemona, Ebenaceae; and Dipterocarpus concavus, Dipterocarpaceae), restricted to the southern half of the peninsula (e.g., Shorea blumutensis, Dipterocarpaceae), or even scattered throughout the peninsula (e.g., Durio malaccensis, Bombacaceae; Anisoptera curtisii, Shorea lepidota and S. ochrophloia, Dipterocarpaceae; Carallia eugenioidea, Rhizophoraceae; Pandanus stelliger, Pandanaceae), so that in effect there is no marked occurrence of the so-called Sumatran elements in one area of the peninsula. Elements of the Riau Pocket flora are found also in the Perak subprovince, including Dipterocarpus lowii and Shorea macrantha (Dipterocarpaceae).

To the north of Peninsular Malaysia, the KangarPattani Line marks the northwestern limit of the Malesian flora, respected by at least 200 genera from the Indo-Chinese area and 375 genera from the Malesian area (van Steenis, 1950).

The core area of seasonal Asiatic intrusion (Figure 1) is marked by the presence of a number of taxa, some of which are lone representatives of genera that are well developed only farther north, such as Ceriscoides perakensis (Rubiaceae; Perak), Lonicera malayana (Caprifoliaceae; on the Main Range mountains northeast of Kuala Lumpur), and the bamboo Bambusa farinacea (Gramineae; occurring in the northern states of Kelantan, Kedah and Pahang and petering out just north of Kuala Lumpur) (Wong, 1993). Some species with clearly more northerly core ranges reach the Malay Peninsula, e.g., Amesiodendron chinense (Sapindaceae) (China, Indo-China, reaching north Sumatra and extending in the peninsula south to Gombak near Kuala Lumpur); Phoenix paludosa (the mangrove date-palm, Palmae) (India to north Sumatra and north Malaya); Viola inconspicua (Violaceae) (Himalayas, Burma, China, Sumatra, Java, Philippines and in the peninsula found on Penang Island in the northwest); Shorea siamensis (Dipterocarpaceae) (Burma, Indo-China, Thailand, to Langkawi off northwest Malaya); and Munronia pinnata (Meliaceae) (Ceylon, India, Java, Lesser Sunda islands, in the peninsula only in the extreme northwest).

A number of species from more seasonal climates farther north persist in the Malay Peninsula on the drier limestone habitat, and these maximally reach the Batu CavesTakun Hill complex, the most southerly limestone outcrops on mainland Asia, just outside Kuala Lumpur (discounting the slight outcrop of some 20 m high farther south on the eastern slope of Mt. Sumalayang in east Johore: Chin, 1977). These include: Colona merguensis (Tiliaceae) and Cycas pectinata (Cycadaceae) (Lower Burma, Thailand, extending to the northwest Malay Peninsula); Hopea ferrea (Dipterocarpaceae) (Indo-China, Thailand, reaching limestone in the northwest Malay Peninsula); Euphorbia antiquorum (Euphorbiaceae) (south India, Burma, Indo-China, in the Malay Peninsula wild only in the extreme northwest); Actephila excelsa (Euphorbiaceae) (Indo-China, Thailand, extending in the peninsula to limestone in Kelantan, Perak and the Batu Caves just outside of Kuala Lumpur); Sapium insigne (Euphorbiaceae) (Himalayas to Burma, and in the Malay Peninsula known only from the Batu Caves); Ligustrum confusum (Oleaceae) (S India, Indo-China, S China, Thailand, reaching Takun Hill just north of Kuala Lumpur).

This accumulation of northern elements, significantly Thai-Burmese, on the Perlis and Langkawi limestones in the extreme northwest accounts for their strong floristic difference from the limestone vegetation farther south (Chin, 1977).


The Mountain flora

The Peninsular Malaysian mountain flora has been estimated to include about 3000 species of vascular plants, including some 2125 species restricted to mountains (Kiew, 1995). Individual mountains, depending on their height and area covered, have different tallies: Stone (1981) recorded 460 species at the summit of Mt. Ulu Kali on the Main Range, while an earlier survey on the Taiping Hills on the western side of the Main Range (Burkill and Henderson, 1925) listed 1939 species.

Woody taxa that are notably montane, such as the Araliaceae, Ardisia (Myrsinaceae) and Theaceae, have significant proportions (25-40%) of endemic montane species (Table 4). Among predominantly montane herbaceous dicots, endemism can be higher, such as in Argostemma (Rubiaceae) (88% of 41 species), Didymocarpus (Gesneriaceae) (94% of 87 species), and Begonia (Begoniaceae) (96% of 47 species) (Kiew, 1991c). Among the orchids, which are richly represented in montane areas, some 50% of 850 species in Peninsular Malaysia are endemic (Seidenfaden and Wood, 1992). In addition, 9 of the 22 genera endemic to the Malay Peninsula belong completely to the montane provenance on the Main Range: Exorhopala (Balanophoraceae), Pernettyopsis (Ericaceae), Micraeschynanthus (Gesneriaceae), Orchadocarpa (Gesneriaceae), Holttumochloa and Maclurochloa (Gramineae), Tricalistra (Liliaceae), and Gentingia and Klossia (Rubiaceae).

The major point about the distribution of montane endemics is that a large number are known from just one or two peaks (Kiew, 1995); for instance, 59% of Argostemma species, 73% of Didymocarpus, and 80% of Sonerila are known in the Malay Peninsula from one peak alone (Kiew, 1991c).

The phytogeographical affinities of the Malayan montane flora are principally Malesian (e.g., Agalmyla, Gesneriaceae; Arthrophyllum, Araliaceae; Rhododendron sect. Vireya, Ericaceae); Asiatic (e.g., Acer, Aceraceae; Lonicera, Caprifoliaceae; Clethra, Clethraceae; Magnolia, Magnoliaceae) and pantropical (e.g., Rapanea, Myrsinaceae). The distribution of Australian elements on mountains (e.g., Weinmannia, Cunoniaceae; Gahnia, Cyperaceae; Leptospermum, Myrtaceae) appears to be edaphically controlled, as they also occur on leached, poor sandy soils at lower altitudes (Wong, Saw and Kochummen, 1987), so that they do not truly form a montane alliance. However, on account of the peninsula lacking truly high mountains (Mt. Tahan, the highest, is just 2190 m), a number of alpine elements, such as found on Sumatran, Javan or Bornean mountains, are conspicuously absent, as already noticed by Ridley (1915).


The Flora of Ultramafics and Basic Volcanic Rock

Much less extensive in the Malay Peninsula than its equivalent in Borneo, ultramafics and basic volcanic rocks are only significant in parts of east Pahang and the northeast Johore coast (the KuantanMersing area), where there is not any noticeable correlation with the vegetation. Nevertheless Shorea kuantanensis (Dipterocarpaceae) has been documented as endemic to basaltic substrate on the Kuantan coast.


Limestone Flora

In Peninsular Malaysia, the limestone flora has been studied by Henderson (1939) and Chin (1977, 1979, 1983a, 1983b) and comprises at least 97 species of ferns and 1119 species of seed plants, some 13–14% of the total seed-plant flora of the peninsula. Among the four largest families, Orchidaceae, Rubiaceae, Euphorbiaceae and Leguminosae, there is some 12–22% representation on limestone (Table 5).

In the Malay Peninsula, limestone outcrops are typically low (reaching just over 700 m at Gua Peningat, Pahang), not quite reaching montane elevations as they do in Sarawaks Mulu area, and are mostly of the tower karst type. They are distributed mainly in the north of the peninsula (the ones along the isthmus and in the extreme northwest at Langkawi distinctly coastal, often forming islands in the sea); the Gua MusangTaman Negara area in Kelantan, Terengganu and Pahang; the Ipoh area in Perak; and in the TakunBatu Caves area just north of Kuala Lumpur (Figure 3).

The phytogeographical affinities of the Malayan limestone flora are predominantly (c. 51%) Malesian or Indo-Malesian (Chin, 1977) and, as noted, a strong Asiatic (or northern) element (c. 19% of the total limestone flora) exists, particularly in the north of the peninsula. Overall the number of species (including non-endemic ones) restricted to limestone number c. 257 (or 21% of the limestone flora). At least 251, or some 21% of the species (including 245 species of flowering plants) are endemic to the Malay Peninsula, and about half of these, some 125 species, are confined to the limestone habitat (Chin, 1977, and corrections in Chin, 1979).

High degrees of endemism occur among specialized saxicolous genera such as Boea, Monophyllaea and Paraboea (Gesneriaceae) and genera that include mostly herbaceous or shrubby forms, e.g., Begonia (Begoniaceae), Chirita (Gesneriaceae), Impatiens (Balsaminaceae), Justicia (Acanthaceae), and Ophiorrhiza (Rubiaceae), and their endemic species restricted to limestone are also generally rare, known from only one general locality or a single outcrop (Table 6). The endemic Malayan genus, Stenothyrsus (Acanthaceae) has only one known species, S. ridleyi, a herb, recorded from only the Ipoh limestone in Perak.


Kerangas, Sandstone Plateaux and Ridges, and Quartzite

In the Malay Peninsula, coastal kerangas-type forest occurs on the raised sandy strips of the east coast and at Tanjung Hantu in Perak (Wyatt-Smith, 1963), on the sandstone plateaux at 400-600 m elevation along the southeastern flank in the EndauRompin area of PahangJohore (Wong, Saw and Kochummen, 1987), and in small patches on some dry quartzite ridges. On the plateaus, a waterlogged facies equivalent to the kerapah forests of Sarawak exists.

A number of genera with affinity to the Australian area occur in these plant communities, their intrusion into the Malesian and other regions, into mountain, coastal and kerangas vegetation, explained by van Steenis (1979, 1985) as due to their basic adaptation to thin and nutrient-deficient soils on which many other taxa cannot compete. These are represented by Dacrydium beccarii (Podocarpaceae), Weinmannia blumei (Cunoniaceae), Gahnia tristis, Lepidosperma chinense, Machaerina maingayi, Schoenus calostachyus, Tricostularia undulata (Cyperaceae), Styphelia malayana (Epacridaceae), Austrobuxus nitidus (Euphorbiaceae), Leptospermum flavescens, Rhodamnia cinerea and Tristaniopsis merguensis (Myrtaceae) in the plateau heath forests (Wong, Saw and Kochummen, 1987).

A number of endemic species have been documented from these sandstone plateaus and ridges, most notably the fan palm Livistona endauensis (Dransfield and Wong, 1987), known only from the EndauRompin area, where it can form dense stands, and in Terengganu (Dransfield, pers. comm.), and which appears unable to establish well in other habitats where shade-tolerant, taller and possibly faster-growing trees predominate (Weiner and Corlett, 1987). Others include Morinda hispida (Rubiaceae) and Phyllagathis stolonifera (Melastomataceae), the latter apparently restricted to rocky banks of streams draining the kerapah forest on the Temambong sandstone plateau and moist rockfaces and boulders in hill forest in Endau-Rompin (Kiew, 1987).

The rare Loxocarpus tunkui (Gesneriaceae), found only at some localities in the northern part of the Endau-Rompin area, occurs on stream boulders and near-vertical rockfaces kept moist by spray and drip from waterfalls and runoff down the edge of almost horizontal plateaux (Kiew, 1987). Elsewhere, the endemic Didymocarpus flavobrunneus (Gesneriaceae) also appears to be restricted to exposed sandstone boulders and outcrops in the Tahan valley in Pahang (Kiew, 1987).

Although noticeably poor in species, the vegetation at a number of quartzite (= metamorphosed quartz or sandstone) ridges is also of interest on account of the occurrence of several narrowly endemic species. The quartzite ridges at Klang Gates and Kanching in the lowlands and hills just north of Kuala Lumpur are of particular significance (Kiew, 1982) as they harbour at least five endemic species in a relatively small area. At Klang Gates, the endemics are Aleisanthia rupestris (Rubiaceae, sole species of a genus endemic to the Malay Peninsula), a low shrub that grows on the crest of the ridge and in crevices on the rock face; Didymocarpus primulina (Gesneriaceae), a herb that is known from only one locality at the south side of the ridge; the tree Ilex praetermissa (Aquifoliaceae), growing on the north and south sides (Kiew, 1983); and the fern Tectaria rumicifolia, growing on the crest and rock face (Allen, 1963).

At the Kanching end of the Klang Gates quartzite ridge, a solitary population of Acrymia ajugifolia (Labiatae), also the sole representative of an endemic Malayan genus, grows at the base of a rock face (Chin and Kiew, 1985). In that locality, too, is the only known population of the dipterocarp Hopea subalata, which is a triploid and apomictic species (Ashton, pers. comm. 1996). Elsewhere, Dendropanax maingayi (Araliaceae) is also documented as a shrub growing on quartzite in some Perak hills.

Such large exposed quartzite dykes, like limestone or sandstone outcrops, or mountain peaks, are in effect island habitats where soil development is poor and are surrounded by forest developed on the surrounding soil from granitic or sedimentary parent material. It is likely that the restriction of a number of species to quartzite in highland areas, as it may conceivably exist, will be somewhat obscured by the generally lower stature of the vegetation and the lush growth of other plants indifferent to this substrate. Quartzite areas exist on the Main Range, such as around Fraser Hill, on the isolated Mt. Jerai in Kedah, and on the Mt. Tahan massif in Terengganu-Pahang, places where a notable number of endemics are known to exist, probably as a result of various factors that contribute to such sites being isolated habitats.


The Flora of Peat Swamps and Freshwater Swamps

Peat swamps in the Malay Peninsula are not developed to the same extent as in Borneo (Figure 4), and are not generally associated (or co-occurrent) with coastal kerangas as exists there. Significant areas of peat swamp are found in Perak, Selangor and Johore in the Malay Peninsula, and contain characteristic west Malesian peatswamp elements, shared with Sumatra and Borneo, such as Cyrtostachys renda (Palmae) and Gonystylus bancanus (Gonystylaceae). There are a number of species known only from peatswamp forest and endemic to the Malay Peninsula, e.g., Vatica flavida (Dipterocarpaceae, south Perak only), Croton macrocarpus (Euphorbiaceae, the Telok peat swamp in Selangor only), Tristaniopsis pontianensis (Myrtaceae, Johore only), Ardisia praetermissa (Myrsinaceae, Selangor and Malacca only) and Tarenna adpressa (Rubiaceae, known from Pahang, Selangor and Johore).

However, most species occurring in peatswamp forest are not inflexibly faithful to this habitat. Some, like Ilex maingayi (Aquifoliaceae) also occur on limestone, and others, such as Endiandra maingayi (Lauraceae), Tetractomia tetrandra (Rutaceae, also in Borneo) and Timonius flavescens (Rubiaceae, also Borneo) are known to occur in montane forests as well. Some others, e.g., Elaeocarpus acmosepalus (Elaeocarpaceae), Scaphium longiflorum (Sterculiaceae), Neesia malayana (Bombacaceae), Ganua motleyana (Sapotaceae), Campnosperma coriaceum (Anacardiaceae) and Tetramerista glabra (Tetrameristaceae), occur in both peat swamps and freshwater swamps, and perhaps these are truly adapted to swampiness.

As in Borneo, there seem also to be species specially adapted to riverbank forest or alluvial flats that are seasonally flooded over, and where peat is not developed, exemplified by the endemic Enicosanthum praestigiosum (Annonaceae), Mesua rosea (Guttiferae), Millettia galliflagrans (Leguminosae), Gaertnera obesa (Rubiaceae), and Madhuca sessiliflora (Sapotaceae), all from the Johore area in the south. The screwpine Pandanus yvanii (Pandanaceae, Malaya and Sumatra) is found in such habitats and not infrequently stands in the course of small seasonal streams through lowlying forest. Calophyllum subsessile (Guttiferae, Perak only) has been documented from both such habitats as well as freshwater swamps, and Dillenia albiflos (Dilleniaceae), Ardisia mawaiensis (Myrsinaceae) and Ternstroemia corneri (Ternstroemiaceae) are Malayan endemics documented only from freshwater swamps. In contrast, Pandanus immersus (Pandanaceae), known only from the Tasik Bera area in Pahang, forms thickets in open freshwater swamps.


Rheophytes

Joining a host of rheophytes common to the Sunda area are similar species, with various habits and adapted to different levels along fast-flowing streams, that are endemic to the Malay Peninsula. These include the streambank trees Antidesma salicinum (Euphorbiaceae) and Dysoxylum angustifolium (Meliaceae, eastern flank of the peninsula and Johore); the shrubs Ardisia tahanica (Myrsinaceae, east and south Malaya), Eugenia (= Syzygium) graeme-andersoniae (Myrtaceae, often lining large stretches of stream banks), Garcinia cataractalis (Guttiferae, known only in Terengganu) and Phyllanthus watsonii (Euphorbiaceae, northeast Johore only); the herb Piptospatha ridleyi (Araceae, on rocky stream beds in Johore); and the tiny tufted fern Grammitis universa (Grammitidaceae, in crevices on boulders in and beside torrential streams).

Corner (1940) has classified Saraca streams, common in the headwaters of rivers all over the peninsula, by the predominance of Saraca spp. (Leguminosae) along them (mostly S. declinata and S. thaipingensis, which grow beside and in narrow rocky streams, forming tangles of root along stream banks and beds, as well as the moist valley sides), and neram rivers, which are lined by neram trees (Dipterocarpus oblongifolius, Dipterocarpaceae), common on the east coast of Terengganu and Pahang.


Some Biological Correlates of Rarity

 The notion of rarity here involves plant species that are known only from a single site; known from several localities representing a narrow distribution range; known over a wider range but occurring at very low densities or locally within only one or few sites or populations; and restricted in distribution due to population depletion through human activity (including the so-called anthropogenic endemics).

The reasons for such restriction in distributional range are various (Primack 1993) and include low rates of population increase, ineffective dispersal, specialized niche requirements and intolerance of changes in the environment, biological dependence on other species (as with Rafflesia parasites that depend on particular Tetrastigma host species), and (unnaturally) intensive or extensive destruction of habitats.

 

All this is made extremely academic if and when plant biologists’ requests for systematic land-use planning are not well heeded, viz., to protect samples, large (up to tens of thousands of hectares) and small (up to several thousand hectares), of a range of vegetation and sites, with surrounding buffer areas where applicable, conscientiously repeated here at great risk of appearing naive to cynics on the matter—

1. Mixed dipterocarp (lowland) forests: in the different climatic and biogeographical subprovinces, and on different soil zones;

2. Natural vegetation on specialized” substrates, in wetlands or podsolic areas, especially those important in coastal protection, hydrological stability or fisheries and wildlife conservation: such as mangroves, estuaries, peatswamp forests, coastal and upland kerangas and kerapah, lakes and freshwater swamps, beaches;

3. Natural vegetation on well-drained specialized” substrates: ultramafic sites on islands, in the lowlands and on mountains; quartzite and sandstone ridges of scenic or endemism value, or of special geological formations; areas on basic volcanic rock: in different districts;

4. All or large ecologically important parts of isolated islands, or of some islands in clusters;

5. Whole large limestone outcrops in different districts, at least several of every cluster of smaller outcrops and whole isolated outcrops even when small;

6. Large contiguous tracts of montane and hill forests on different ranges, including their ridges and drainage systems: especially in scenic places, areas of special biological richness or endemism, and all catchment zones, with selected sites of low-impact development, such as small resorts that are mindful of special natural features; all or large parts of isolated peaks, especially including their summit regions;

7. Complete drainage systems from highland to lowland, in different districts and on different coasts, and when contained within special geological formations.

 

If the list sounds ideal, all is not well, and the most important of all biological correlates of rarityhabitat destruction through human actionespecially when not enough resource scientists are performing the positive tasks of documenting species richness and habitat specialities where it mattersmust have taken its toll.


Viability Versus Vulnerability: The Lowland Forests 

Still one matter remains, which concerns, in tropical rain forest regions, conservation outside of specially protected areas, particularly in the lowlands and increasingly so in the hills because, obviously, not all vegetation should disappear outside of protected areas. At least two important underlying notions dictate special wisdoms in the way such areas, including especially timber production areas, are managedone from the point of view of dynamics and the maintenance of viable populations of the rarer species, and the other from the point of view of sustainability, related also to the first point.

Mixed dipterocarp forests anywhere typically have a high number of species represented at very low densities, including series of co-occurring congeners, such as in Shorea (Dipterocarpaceae), Syzygium (Myrtaceae), Diospyros (Ebenaceae), Knema (Myristicaceae), and so on (Ashton, 1984, 1992). Not only is diversity high in the lowland forests. The level of endemism in the various plant families that are largely lowland” is mostly between 15% and 33% in the Malay Peninsula, and 27% overall (Table 7).

Manokaran and Swaine (1994) report that in 2-ha plots of rain forest in Bukit Lagong and Sungei Menyala, Peninsular Malaysia, uncommon species (represented by only one individual) accounted for between 37% and 45% of all trees (diameter 10 cm and above) censused, but over a larger area in Pasoh forest about four times this size this proportion was between 19% and 23%. At Pasoh, the inventory data of Manokaran et al. (1992) for a census of trees and shrubs 1 cm diameter and above in a 50-ha plot give 159 species (19.5% of the 814 enumerated) represented by 20 individuals or less, 118 species (14.5%) by 10 individuals or less, and 79 species (9.7%) by 5 individuals or less. Also, Kochummen et al. (1990) make it quite clear that some taxa are not well represented in certain regions, so that it cannot be assumed that a single large site at one locality serves an adequate conservation function.

Species have a critical minimum population size and range for optimal existence and very small residual populations, such as in small forest fragments, are unlikely to persist, because, for instance, of breeding failure (frequencies of mature individuals too low, pollination mechanisms interrupted, etc.) or genetic deterioration (reduced genetic exchange within the residual population) (Gilpin, 1987; Simberloff, 1992).

Smaller plot studies of rain forests have revealed that the canopy species, which represent the principal targets of a timber-based exploitation of a forest area, were either well represented or poorly represented by the regeneration on site, whereas over a larger area the regeneration of canopy species will have better chances of being included (Swaine and Hall 1988; Manokaran and Swaine, 1994). Moreover, the importance of source areas outside of a particular site in influencing on-site species richness cannot be underestimated (Pickett and Thompson, 1978; Janzen, 1983), and the edge effect, which includes invasion and modification of the species composition by a few more common species abundant outside the fragments, is greater the smaller the fragment. Fragments, large or small, where the canopy and forest floor have been unduly disturbed, and if sufficiently isolated from source areas of the so-called desirable species” in timber-tree management, stand little chance of regrowth at optimum rates and of subsequent inclusion of the favoured species taken in the first place (Ingram, 1990). Large areas overly disturbed by logging are comparable to such fragments in being non-sustainable from the foresters viewpoint.

Thus, not only must foresters continue to aspire to mark trees for retention on site by loggers moving in, and perform their other functions in attempting to obtain minimum disturbance from the logging, forest managers should not take lightly the retention of forested islands, large and small, where no intrusion is permitted, throughout a production area.


Acknowledgements. I thank Dr Cheng-I Peng (National Museum of Natural Science, Taiwan) for encouragement to prepare this paper and Prof. Peter Ashton (Harvard University), Dr John Dransfield (Royal Botanic Gardens, Kew), Prof. Ruth Kiew (Universiti Pertanian Malaysia, Serdang), S.P. Lim (Forest Research Centre, Sandakan), Prof Christian Puff (Institute of Botany, University of Vienna), and Dr. L.G. Saw (Forest Research Institute, Malaysia) for comments on earlier drafts. Jon Noad of the Department of Geological Sciences, Birkbeck College, University of London, generously shared information and provided interesting discussions on the geology of Sabah, and S.P. Lim assisted with the preparation of the maps used here.


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Captions to figures

Figure 1. Special phytogeographical areas in the Malay Peninsula and Borneo.

Note that the East Coast Sabah subprovince has earlier been recognized as two discrete pockets, marked A and B here. 

Figure 2. The major mountain ranges of the Malay Peninsula and Borneo.

Areas above 1000 m shown in black; that between 500 and 1000 m stippled. 

Figure 3. Distribution of major limestone outcrops, sites of ultramafic geology and areas of volcanic rock in the Malay Peninsula and Borneo.

Major limestone outcrops marked by crosses; sites of ultramafic geology shown in black; areas of volcanic rock stippled. 

Figure 4. Peatswamp areas in the Sunda shelf territories.

Areas with sizeable tracts shown in black; areas with scattered occurences marked by crosses. After Bruenig (1990).