Unique Southeast Asian Peat Swamp Forest Habitats Have Relatively Few Distinctive Plant Species

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Unique Southeast Asian peat swamp forest habitats have relatively few distinctive plant species

W. Giesen1, L.S. Wijedasa2 and S.E. Page3

1Euroconsult Mott MacDonald, Arnhem, The Netherlands 2Theoretical Ecology & Modelling Laboratory, National University of Singapore and ConservationLinks, Singapore 3Department of Geography, University of Leicester, UK

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SUMMARY

The peat swamp forests of Southeast Asia are often described as having a unique biodiversity. While these waterlogged and nutrient-poor habitats are indeed unique and include a distinct fauna (especially fish), the peat swamp forest flora is much less distinct and shares a surprisingly large number of species with other habitats. Out of 1,441 species of higher plants found in Southeast Asian swamps (from Thailand to Papua), 1,337 are found in the lowlands (< 300 m a.s.l.). Of these 1,337 species, 216 (16.2 %) occur mainly in lowland swamps, 75 (5.7 %) are shared with freshwater swamps and riparian habitats, 49 (3.7 %) are shared with heath forests, 7 (0.5 %) are shared with montane ecosystems, and 86 (6.5 %) are shared with a range of other lowland habitats. Of the 216 species (16.2 %) that occur in lowland swamps, 120 (9.2 %) are restricted to this habitat (which includes freshwater swamps), and 45 (3.4 %) are restricted to lowland peat swamp forests. Thus, more than 80 % (1,152 species) of the known peat swamp forest flora is common to a wide range of habitats, while 12.4 % (166 species) is composed of opportunistic pioneer or secondary forest species.

KEY WORDS: endangered species, flora, Indonesia, lowland swamp, Malaysia, opportunistic species ______

INTRODUCTION METHODS

The peat swamp forests of Southeast Asia are often Over the past eight years, the first author has described as having unique biodiversity, including compiled a comprehensive spreadsheet database of endemic tree species and rare and endangered Southeast Asian swamp plant species, based on animals (van der Meer et al. 2008, Yule 2010). These species habitat records from key taxonomic waterlogged and nutrient-poor habitats host a references (van Steenis 1950–1984, Backer & distinctly adapted, highly endemic fish fauna (Ng et Bakhuizen van den Brink 1963–1968, Whitmore al. 1994, Kottelat & Widjanarti 2005, Kottelat et al. 1972, Whitmore 1973, Ng 1978, Ng 1989, van 2006) and the largest remaining populations of Steenis & de Wilde 1984–1989, de Wilde 1992– threatened animals such as the false gharial 1996, Kalkman 1997, Stevens 2000–2001, (Tomistoma schlegelii), Storm’s stork (Ciconia Nooteboom 2002, Nooteboom & Kirkup 2005, stormi) and white-winged wood duck (Asarcornis Nooteboom 2007–2010, van Welzen 2011–2016), scutulata), plus a large proportion of the Bornean and scientific papers and ‘grey literature’ reports on peat Sumatran orangutans (Pongo spp.; Meijaard 1997, swamp forests (more than 140 references, available Husson et al. 2009, Yule 2010, Wetlands from the first author upon request). Attention was International 2010) and southern Bornean gibbon paid to accuracy, particularly with regard to (Hylobates albibarbis; Cheyne et al. 2008). While excluding potentially doubtful species records the peat swamp forest (PSF) flora has received less because of uncertain taxonomy or locality attention than the fauna, botanists have observed that information. The plant taxonomy follows The Plant it is less distinct, sharing a relatively large number of List (2010) Version 1.0 (www.theplantlist.org/), species with other habitats (Anderson 1963, accessed between 2012 and 2016. The assessment of Whitmore 1984). In this article we attempt to assess whether a species occurred in lowland PSF was made the degree of floristic similarity amongst Southeast by referring to the habitats listed in the key taxonomic Asian peat swamp forests, and their floristic references mentioned above, and consulting relationships with other habitat types. herbarium records made accessible via the Global

Mires and Peat, Volume 22 (2018), Article 01, 1–13, http://www.mires-and-peat.net/, ISSN 1819-754X © 2018 International Mire Conservation Group and International Peatland Society, DOI: 10.19189/MaP.2017.OMB.287

W. Giesen et al. SOUTHEAST ASIAN PEAT SWAMPS HAVE FEW DISTINCTIVE PLANT SPECIES

Biodiversity Information Facility Version 1.2.6 Palangkaraya University (2012) and Giesen (2013). (http://data.gbif.org/), in which all major herbaria Many species are only infrequently recorded in PSF with Southeast Asia collections collaborate. The and site-specific abundances are often not recorded. tendency for taxonomists and scientists to report Therefore, a cut-off of four or more records in PSF limited habitat information required some level of was used to identify ‘common’ or ‘widespread’ leniency when dealing with the specific habitat species. record 'swamp', which could also mean freshwater swamp. While a ‘swamp’ habitat listing was not used to justify adding a species to the PSF restricted list, a RESULTS species already on the list due to information from a different reference was not removed if a ‘swamp’ The swamp plants database includes 1,441 plant listing was found later. Information on geographical species found in Southeast Asian swamps, from distribution and the occurrence of each species in Thailand and Vietnam to Papua. Of these, 1,337 are habitats other than peat swamps was obtained from lowland swamp species (< 300 m a.s.l.) and 1,313 are the key taxonomic references listed above, and from lowland swamp angiosperms. Considering the 1,313 herbarium collections. The chance occurrence of a lowland swamp angiosperms, 216 (16.5 %) occur species outside of PSF was regarded as too rigorous predominantly in lowland swamps (Table 1). a criterion for excluding it from the restricted PSF Amongst these 216 species, 120 (9.1 %) are found in list, so species were not moved from the restricted to lowland swamps only, with 75 (5.7 %) in freshwater the non-restricted PSF list on the basis of one record swamps, 45 (3.4 %) restricted to lowland peat swamp or a few occurrences outside PSFs. Information about forests (Table 2) and the remaining 96 (7.3 %) mainly whether species were pioneer species or common to in lowland swamps. The majority (> 80 %) of the primary and secondary forest was obtained from 1,313 lowland swamp angiosperms are found in both Kostermans (1958), Kessler et al. (1995), Kessler peat swamps and a variety of other habitats, including (2000), van Eijk et al. (2009), Giesen et al. (2009), many non-waterlogged lowland evergreen rainforest

Table 1. Swamp plant species in Southeast Asia: overlap with other habitats.

Number of % of Habitat plant species total 1. All Southeast Asian swamps 1,451 2. Variety of habitats including lowland swamps 1,337 of which angiosperms 1,313 100 3. Swamps, plus lowlands (>300 m a.s.l.) to hills/ridges (300–800 m a.s.l.) 405 30.8 4. Swamps, plus a variety of lowland habitats 331 25.2 5. Swamps, plus a variety of lowland to montane habitats 305 23.2 6. Lowland swamps and montane habitat 7 0.5 7. Lowland swamps and heath forest (kerangas) 49 3.7 8. Predominantly in lowland swamps 216 16.5 8a. Only in lowland swamps 120 9.1

8a.i Lowland peat swamp forests and riparian/mineral soil swamps 75 (5.7) 8a.ii Restricted to lowland peat swamp forests 45 (3.4) 8b. Mainly in lowland swamps 96 7.3 TOTALS 1,313 216 120

W. Giesen et al. SOUTHEAST ASIAN PEAT SWAMPS HAVE FEW DISTINCTIVE PLANT SPECIES

(LERF) habitats. In total, 405 species are shared with shared with heath forests (kerangas) only and seven LERF lowlands and hill/ridge habitats (300–800 m species (0.5 %) are shared with montane/highland a.s.l.) and 331 species are shared with a variety of ecosystems (i.e. > 800 masl) only (Figure 1). Note lowland habitats, while 305 species are shared with a that many more species are shared by PSF and range of habitats from LERF lowlands to montane kerangas (63 species), and by PSF and montane habitats (> 800m asl). Forty-nine species (3.7 %) are habitats (305 species), but are not exclusive to these

Table 2. Plant species restricted to peat swamp forests in Southeast Asia.

W. Giesen et al. SOUTHEAST ASIAN PEAT SWAMPS HAVE FEW DISTINCTIVE PLANT SPECIES habitats and are shared among a range of habitats. In habitat. This contrasts with the findings of Posa et al. total, 166 lowland swamp species (12.4 %) are (2011), who report 172 plant species (11 % of their pioneer species or are characteristic of secondary total) restricted to peat swamp forests. Given that the habitats. Of the 200+ most common lowland peat total numbers of PSF species are similar (1,337 in our swamp species in Southeast Asia, the largest report versus 1,524 in Posa et al. 2011), this percentage (97 %) occurs on Borneo, followed by difference can probably be explained by a different Peninsular Malaysia (87 %), Sumatra (82 %) and definition of ‘restricted’. In a more general study on Thailand (56 %) (Appendix). tree species distribution across five habitats including The 45 species restricted to PSF (Table 2) are PSF and kerangas, Cannon & Leighton (2004) found found in Borneo (41 species or 91 %), Sumatra that while 67 % of common species were (15 species), Peninsular Malaysia (14 species), significantly associated with one habitat, few species Singapore (5 species) and Thailand (3 species). None were restricted to a single habitat, although the have been recorded in Vietnam or Papua. peatland habitat had the most profound effect on species distribution. They also found that 16 % of their species appeared to be habitat generalists. DISCUSSION In the present study, if habitat records were unclear, herbarium records were consulted as these Floristically, PSF is less diverse than everwet often include habitat descriptions. Often, species that lowland forest on mineral soils in Southeast Asia. have been described as ‘typical for peat swamps’ The count of 30–122 tree species per hectare in PSF (e.g., Campnosperma coriaceum, Combretocarpus is lower than the 70–220 or even 100–280 tree rotundatus, Cratoxylum arborescens, Eleiodoxa species per hectare commonly recorded in Malesian conferta, Gonystylus bancanus, Ilex cymosa, everwet lowland forest on mineral soils (Whitmore Lophopetalum multinervium, Madhuca malayana, 1984, Posa et al. 2011). This is also reflected in the Syzygium zeylanicum, Tetramerista glabra; overall numbers of tree species in specific regions, Whitmore 1984) were also found to occur regularly with only 234 tree species being recorded for in other habitats and are, therefore, not exclusive to Sarawak and Brunei PSF compared to 1800–2300 in PSF. However, a total of about 200+ species (similar lowland forests (Whitmore 1984). to the number of restricted species according to Posa Only a relatively small number (45 or 3.4 %) of et al.) are commonly found in PSF and are less species found in PSF are truly restricted to this common in other habitats; therefore, these can be

Figure 1: Plant species shared between lowland swamps and other habitats in Southeast Asia.

W. Giesen et al. SOUTHEAST ASIAN PEAT SWAMPS HAVE FEW DISTINCTIVE PLANT SPECIES regarded as (non-exclusive) PSF species (listed in the This indicates a possibility of relative advantage and Appendix). The fact that 41 of the 45 PSF restricted specialisation; although these species occasionally species occur on Borneo, and 25 of these are endemic occur outwith PSF, they are apparently considerably to the island, further confirms the importance of better adapted than other species to peatland. Borneo for biodiversity (Raes et al. 2009, de Bruyne PSF is declining rapidly and by 2010 only 2.1 % et al. 2014). The 17 PSF restricted species that are and 4.6 % was left in a pristine condition in common or widespread are either found on Borneo Kalimantan and on Sumatra, respectively (Miettinen only (eight species) or on Borneo, Sumatra and & Liew 2010); while a dramatic 70 % decline in key Peninsular Malaysia (nine species). No known plant PSFs in the Rajang Delta (Sarawak) occurred during genera are restricted to PSF, nor does any ecological 2000–2014 alone (Hooijer et al. 2015). Wetlands characteristic (e.g. physiognomy, growth rate) stand International (2010) conclude that “No example of a out as being a common denominator for the restricted hydrologically intact peat dome remains anywhere in species. Malaysia”. Given the rate and extent of change, it is The (exclusive) sharing of species between PSF to be expected that the eight uncommon and 20 rare and habitats such as kerangas and montane forests is PSF restricted plant species may be under threat. Five puzzling, given the physical demands that these of the rare species are known from 1–2 habitats place on the plants. Kerangas forests are locations/collections and three of these are either characterised by very dry and nutrient deficient sandy endangered (Crudia venenosa, known from type soils, and while nutrient deficiency is a trait shared collection in Sabah only) or possibly extinct (Croton with PSF, the waterlogging typical of PSF and the macrocarpus from the now fully converted Telok everwet conditions of montane habitats are the PSF in Selangor, Peninsular Malaysia; Knema opposite of what a plant faces in kerangas. Low pH mamillata, known only from PSF in South is a feature that these three habitats do share, with pH Kalimantan, where this habitat is fully converted). A averaging 2.9–4.0 in kerangas (Proctor 1999), 2.9– number of rare species known only from Sarawak 4.0 in PSF (Yule 2010) and 3.0–4.0 in montane and Peninsular Malaysia can be considered habitats on Borneo (Ushio et al. 2008). Suzuki (2010) endangered given the current state of the hosting proposes that both kerangas and PSF promote the habitat; these include Garcinia apetala, Ixora growth of species preferring cooler habitats, because pyrantha, Litsea crassifolia, Pandanus vinaceus, of the periodic occurrence of water on the ground Tarenna adpressa, Ternstroemia hosei and surface. That would also explain the overlap with Tristaniopsis beccarii. Lophopetalum sessilifolium, montane habitats. Studies by Nishimura & Suzuki which is known from Sarawak and the now (2001) on growth patterns in kerangas and PSF trees disappeared PSF of Sungai Landak in West shows that plants allocate resources where they are Kalimantan, further extends the list of endangered required (e.g., increased root depth and smaller species. Even common/widespread species (see leaves in kerangas, more lateral root growth in PSF), Appendix) are locally under threat. Combretocarpus and that there is plasticity within species. Nishimura rotundatus has disappeared from Peninsular & Suzuki’s (2001) study on intraspecific Malaysia (where it was never really common), and differentiation in two species (Canarium sp. and five species that are common/widespread overall Shorea teysmanniana) that occurred in both PSF and have disappeared from Singapore in recent decades kerangas shows that phenotypic plasticity enables (Austrobuxus nitidus, Calophyllum calaba, Litsea them to grow in these environmentally contrasting gracilipes, Neesia altissima and Syzygium forests. As a result of their phenotypical adaptations, leucoxylon). Extinctions may also have occurred drought affects PSF trees more than kerangas trees, elsewhere in the region, but gone unnoticed because with higher mortalities occurring in PSF during local recording is less assiduous than in Singapore. prolonged El Niño associated dry spells (Nishimura The coastal/sub-coastal PSFs of Southeast Asia et al. 2007). are generally only 5–15,000 years old (Page et al. Interestingly, while it seems that only a few 2010, Dommain et al. 2011), but peat formations species are exclusively specialised for PSF 75 m below present sea level (dated at 13,000 years environments, some reach levels of dominance not BP) off the east coast of Peninsular Malaysia (Voris seen in other forest types. For example, Shorea 2000) indicate that the habitat itself is older. The albida dominates PSF in Sarawak (Whitmore 1984, PSFs of Borneo, Sumatra and Peninsular Malaysia Bruenig 1990), while Combretocarpus rotundatus may have been (relatively) interconnected until and Dactylocladus stenostachys dominate the central 8,000–10,000 BP, when the Sunda Shelf was still parts of peat domes in Kalimantan (Anderson 1983). exposed (Voris 2000), but due to sea level rise since

W. Giesen et al. SOUTHEAST ASIAN PEAT SWAMPS HAVE FEW DISTINCTIVE PLANT SPECIES the last glacial maximum (10,000–13,000 BP) PSFs endemics to the dynamic geomorphological setting of probably now occur at their highest altitude for these peatlands, although environmental filtering and millennia. According to Hanebuth et al. (2011), the dispersal limitations may also play a role. As more development of PSFs would be especially affected by information becomes available about the vegetation rates of coastal migration, and the rate of sea level of peatlands across the tropical zone, it will be rise would have influenced their degree of interesting to make further comparisons and to development and duration at any specific location. It establish whether any commonalities can be is hypothesised that this relatively rapid ‘landward identified in terms of the processes that determine the retreat’ of the PSF habitat may have contributed to assembly of peatland plant communities. the relative paucity of unique plant species and an abundance of opportunistic species able to adapt. Based on our dataset, 166 lowland peat swamp ACKNOWLEDGEMENTS species (12.4 %) are pioneer species or are characteristic of secondary habitats, compared with The authors thank Mires and Peat editors, an 4 % for lowland rain forest in Peninsular Malaysia anonymous reviewer and Dr Chuck H. Cannon for (Putz & Appanah 1987) and 8.7 % for similar habitat constructive comments which have significantly in eastern Borneo (Slik & Eichhorn 2003). contributed to improvement of this article. Alternatively, the last glacial maximum was also associated with lower rainfall, and according to proponents of the ‘savannah corridor hypothesis’ REFERENCES (e.g. Bird et al. 2005, Raes et al. 2014) the middle part of Sundaland may have been a savannah corridor Anderson, J.A.R. (1963) The flora of the peat swamp and hence too dry for PSF habitats to survive. forests of Sarawak and Brunei, including a Models described by Cannon et al. (2009) and de catalogue of all recorded species of flowering Bruyn et al. (2014) provide an alternative explanation plants, ferns and fern allies. Gardens Bulletin for the relative paucity of PSF restricted species (Singapore), 20, 131–228. (C.H. Cannon, personal communication 2017). At the Anderson, J.A.R. (1983) The tropical peat swamps of last glacial maximum, sea levels were at their lowest Western Malesia. In: Gore, A.J.P. & Goodall, and the entire continental shelf of Sundaland was D.W. (eds.) Mires: Swamp, Bog, Fen and Moor - exposed. Areas with favourable topography and Regional Studies. Ecosystems of the World 4B, drainage for coastal peat formation would have been Elsevier, Amsterdam, 181–199. limited, although scattered peatlands would have Backer, C.A. & Bakhuizen van den Brink, R.C. occurred on more elevated ground inland. This (1963–1968) Flora of Java. Volumes 1–3, N.V.P. setting would present a severe bottleneck to PSF Noordhoff, Leiden, The Netherlands, 648, 641 specialists. When the Sunda Shelf subsequently and 761 pp. flooded during deglaciation, vast areas of peat could Bird, M.I., Taylor, D. & Hunt, C. (2005) have formed (6,000–15,000 BP) because of the flat Palaeoenvironments of insular Southeast Asia topography of the shelf and the large amount of water during the Last Glacial Period: a savanna corridor running onto it from rivers draining Indochina, in Sundaland? Quaternary Science Reviews, 24 Borneo and Sumatra. This might have forced lowland (20–21), 2228–2242. specialist species to become at least tolerant of PSF Bruenig, E.F. (1990) Oligotrophic forested wetlands conditions. In general, the historical instability in in Borneo. In: Luga, A.E., Brinson, M. & Brown, extent and distribution of this unique habitat has S. (eds.) Forested Wetlands. Ecosystems of the probably played a central role in determining how World 15, Elsevier, Amsterdam, 299–334. many tree species became exclusively specialised. Cannon, C.H. & Leighton, M. (2004) Tree species Interestingly, a recent study of the floristic distributions across five habitats in a Bornean rain composition of lowland tropical peatlands in northern forest. Journal of Vegetation Science 15, 257– Peru shows that these comparable ecosystems, 266. although in a very different geographical setting, Cannon, C.H., Morley, R.J. & Bush, A.B.G. (2009) probably have no endemic tree species (Draper The current refugial rainforests of Sundaland are 2016). Instead, the peatlands provide habitat for unrepresentative of their biogeographic past and many generalist tree species, as well as for some highly vulnerable to disturbance. Proceedings of specialists from adjacent white sand and floodplain the National Aacademy of Sciences, 106(27), forests. Draper (2016) attributes the paucity of 11188–11193.

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Author for correspondence: Wim Giesen, Euroconsult Mott MacDonald, Amsterdamseweg 15, 6814 CM Arnhem, The Netherlands. Tel.: +31(0)263577137; E-mail: [email protected]

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Appendix. Plant species that are common or widespread in Southeast Asian peat swamp forests.

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