666 Becker, Ong, and Goh

BlOTROPlCA 30(4): 666-671 1998

Selective Drought Mortality of Dipterocarp Trees: No Correlation with Timber Group Distributions in Borneo'

Key words: biogeography; Borneo; Dipterocarpaceae; drought vulnerability; mortality; mycorrbime.

VUI NFRARII I T Y T O DROUGHT IS THOIJGHT TO PREVENT the Red Meranti group Of Shorea (Dipterocarpaceae) from invading the seasonal evergreen forests north of the Kra ecotone near the Thailand-Peninsular Malaysia border (Whitmore 1984, Turner & Whitmore 1991). North of this ecotone, where rainfall becomes increasingly unreliable, the forests are characterized by abundant White Meranti Sborea and White Seraya Parashorea. In general, the presence of even a short (2 1 mo) but regular dry period (<lo0 mm rain/mo) profoundly affects the number and kind of dipterocarp species present (Ashton 1982).

In Borneo, droughts are more severe in eastern Sabah and parts of eastern Kalimantan than in Sarawak and Brunei (Walsh 1996). Drought in this region can greatly affect forest dynamics-the one in 1877- 78 caused up to one third of trees to die in stands along the Mahakan River in the Kutai region of East Kalimantan (Allen et al. 1989). Like Ashton (1989), Davies and Becker (1996) suspected that irregular, strong drought might account for reduced species richness of lowland rain forests in eastern Borneo compared to those in northwestern Borneo. Newbery et al. (1996) hypothesized that drought has inter- acted with topography to control the understory species composition of the dipterocarp forest at Danum Valley in eastern Sabah. Taking a broader geographical perspective, eastern Sabah appears to comprise a zone of taxa that are best developed in the more seasonal areas of Asia (Wong 1997).

The severe and widespread drought of 1982-83 in northeastern and eastern Borneo was among the most extreme in this century (Walsh 1996). Exacerbated in some forests by logging and fire, it caused reduced growth and extensive mortality of large and small trees (Leighton & Wirawan 1986, Woods 1989, Ong & Kleine 1995). Marsh (1996), citing an unpublished report (Anonymous 1985), noted that mortality was greater in the Light Red Meranti Sborea than among other dipterocarp timber groups in iesponse to this drought in eastern Sabah.

Large scale studies in tropical forests of differential mortality following drought are rare (eg., Leighton & Wirawan 1986, Condit et al. 1995). Here we test the null hypothesis that dipterocarp timber groups are equally vulnerable to drought by comparing their adult mortality at a site in Sabah just after the 1982-83 drought. Next we test the hypothesis that the relative abundance of individuals and species of drought-vulnerable Red Meranti does not differ between northwestern Borneo and more severely drought- affected eastern Borneo. This is done using all available data for plots whose composition (censused in different years) reflects exposure to drought and other selective forces during at least the past century. Finally, we briefly examine whether species richness in eastern Borneo is reduced compared to that of northwestern Borneo.

Normally, standing dead trees represent much less than 0.5 percent of stems in the size-class surveyed in timber inventories (C. L. Ong, pers. obs.), but high mortality after the 1982-83 drought led to a claim for compensation by a concessionaire. Therefore, a ten-percent systematic inventory was made in 1985 of 4038 ha representing the unburnt 1986 and 1987 coupe areas of Silam Forest Products in the Ulu Segama Forest Reserve, located 52 km northwest of the Silam Forest Products Mill, Sabah (4"58'22"N, 118'12'57"E). All standing dead and living trees 2 60 cm DBH (diameter at breast height) were censused in 20-m wide strips at 200-m intervals. The great majority of dead trees had died since the 1982-83 drought. Trees were classified by bark slash and fallen leaves into timber groups, which corresponded to supraspecific dipterocarp taxa.

For consistency, timber groups in the mortality inventory were converted to the Malaysian standard nomenclature (Wong 1982). Thus, Light and Dark Red Meranti (LRM and DRM, respectively) corre- spond to Shorea sections Rubella, Brachypterae, PachycaTae, Ovalis, and Mutica (Ashton 1982), with the Dark Red Meranti producing darker, heavier timber. Yellow and White Meranti (YM and WM, respec- tively) are Shorea sections Richetioides and Anthosborea, respectively; White Serayas (WS) are lighter

' Received 24 August 1996; second revision accepted 2 June 1997.

Notes 667

TABLE 1. Numbers o f dead and live trees (260 cm DBH) in a ten-percent inventory of the 1986 and 1987 coupe areas (4038 ha) of Silam Forest Products in Ulu Segama Forest Reserve, Sabah.

Percent Timber group Dead Alive dead

Light Red Meranti Merawan White Seraya Kapur Dark Red Meranti Yellow Meranti Keruing White Meranti Balau Other

238 1538 13.4 8 66 10.8

51 528 8.8 55 63 I 8.0 12 146 7.6 28 368 7.1 26 615 4.1

1 25 3.8 9 28 1 3.1 0 33 0.0

Parashorea, especially I? malaanonan in Sabah; and Balau (BL) are from Shorea section Shorpa. Keruing (KR) are Dt;oterocalpus, Kapur (KP) are Dvyobalanops, and Merawan (MR) includes the lighter Hopea, especially H. sangal in Sabah. Anisoptera, Cotylelobium, heavy Hopea, Upuna, and k t ica were classified as Other (OT). For the biogeographical study of timber groups, species were classified by reference to Meijer and Wood (1964) and Ashton (1964a, 1982).

Mortality was substantial (9.2% of 4659 trees) for the large trees at Ulu Segama, and differed significantly among dipterocarp timber groups (Table 1 with White Meranti and Other combined; G = 85.66, P < 0.001). An unplanned test of all subsets of rows in Table 1 (White Meranti and Other combined) by BIOM-PC (Sokal & Rohlf 1995; p. 741) revealed 14 homogeneous subsets, indicating however, that there was no simple statistical subdivision of timber groups into mortality classes. Light Red Meranti had significantly higher mortality (13.4%) when tested against all other dipterocarp timber groups combined (7.1%; G = 59.17, P < 0.001). Merawan also had fairly high mortality, while Keruing, White Meranti, Balau, and Other had much lower mortality. Although not directly comparable in that the standing dead trees censused represent mortality accumulated over several years, some context for the above results is provided by observing the annual mortality rates of ca 2 percent for trees (>60 cm DBH) in Malaysian dipterocarp forest (Manokaran & Kochummen 1987) and a median of 1.6% for trees (> 10 cm DBH) in a variety of tropical forests (Lug0 & Scatena 1996).

There was no consistent broad geographical variation in the representation of timber groups, most of which showed substantial differences in relative abundance among sites (Table 2). Light Red Meranti were notably abundant at Danum in Sabah (Table 2). Based on fluctuations in the frequency of large trees and a high density of lianas there, Newbery et al. (1992) speculated that drought may have killed numerous emergents and opened the canopy on several occasions during the past century Many of the Light Red Meranti are almost pioneer species. They respond to increased light by rapid growth and are capable of competing with smothering climbers that invade large gaps (Whitmore 1984). Their dispro- portionately high abundance at Danum would be consistent with recent disturbance there, although vulnerability to herbivory can offset the response of some fast growing species to high light regimes (Whitmore & Brown 1996).

The percentage representation of Light Red Meranti individuals and species in northwestern (Sarawak and Brunei) and eastern Borneo (eastern Sabah and E. Kalimantan) did not differ significantly (Table 2 with Anderson’s Andulau, Lempake, and Kutai excluded; Mann-Whitney U = 9.50 and 9.00, P = 0.90 and 0.81, respectively). On a broader scale, the Light Red Meranti constitute 14 percent of dipterocarp species in Brunei (Ashton 1964a) and a statistically indistinguishable 16 percent (G = 0.075, P = 0.78) of the eastern Sabah dipterocarps (Meijer & Wood 1964).

White Meranti are uncommon everywhere in northwestern and eastern Borneo (Table 2), suggesting that these areas have less seasonal climates than the dipterocarp forests north of the Kra ecotone. On the other hand, White Seraya are relatively common in two eastern Sabah plots (Table 2), and the abundance

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Notes 669

of this group distinguishes the forests of Sabah from those of Sarawak and Brunei in northwestern Borneo (Whitmore 1984). Fox (1973) observed, however, that White Seraya are absent from the presumably drought-prone sandstone hills and ridges of Sabah, and they experienced relatively high drought mortality at Ulu Segama (Table 1). The occurrence of White Seraya forest in Perlis and Langkawi (Peninsular Malaysia) is usually attributed to the presence of a relatively arid, seasonal climate, but this interpretation does not accord with the available, although possibly inaccurate, climatic map (Turner & Whitmore 1991). Thus, a climatic basis for the Kra ecotone is not well established. Nepstad et al. (1994) have shown how deep roots extracting soil ground water can complicate the relationship between climate and forest type distribution.

It is likely that timber group distributions depend on factors in addition to climate. Most of Ashton’s (196413) generalizations about the occurrence of timber groups on different soil types concerned soil texture, which was generally not reported. So this question cannot be examined with the data in hand. Because of differences in plot size and tree numbers among sites it is difficult to assess geographic gradients in species richness. For plots <2 ha, dipterocarp species richness was on average 50 percent greater in northwestern compared to eastern Borneo, even though the latter plots tended to be larger (Table 2). A similar difference in species richness was shown more convincingly for the entire tree flora from the same sites (Davies & Becker 1996). Sist (1996), however, has noted that the remarkably high dipterocarp species richness of the plots at Berau (Table 2) contradicted the generalization that western Borneo is richer than eastern Borneo.

We found no evidence that the vulnerability of large Light Red Meranti to drought is reflected in their present Bornean distribution. This could be because the period from 1916-1967 in eastern Sabah was nearly drought free with more frequent, severe droughts having occurred during prior and subsequent periods (Walsh 1996). There is evidence that drought-causing El Nifio Southern Oscillation events of increasing strength and duration have developed during the past two decades as a result of global warming (Trenberth & Hoar 1996). Thus, the drought vulnerability of contemporary Light Red Meranti adults would not have been fully expressed until recently and might not yet be reflected in species distributions and relative abundances. A similar, nonequilibrium argument was advanced to account for the distribution of drought-intolerant shrub species in a Panamanian forest (Condit et al. 1995).

Perhaps vulnerability to drought at the seedling stage is most crucial to dipterocarp distribution, as alluded to by Ashton (1989). The irregularity of droughts in eastern Borneo may afford sufficient wet years for seedlings to escape and produce substantial juvenile and adult populations. Note that the lower size limit for our distributional data (Table 2) was 10 cm DBH compared to 60 cm for the mortality data (Table 1). The suggestion is that the greater mortality of large Light Red Meranti compared to other dipterocarp timber groups has not reduced regeneration capacity enough to affect relative species abundance inclusive of small size classes. Furthermore, the effect of catastrophic mortality on tree turnover can be low when compared with background mortality on long time scales (Lug0 & Scatena 1996).

Why might large Light Red Meranti be especially vulnerable to drought? Ectomycorrhizae, which are characteristic of all dipterocarps, confer drought resistance (Bowen 1973). Ashton (1 989) has speculated that some mycorrhizae may be host specific at the sectional level in Shared, which is a plausible proposition ( c - Molina et al. 1992, Smits 1994). If this is true and if the different mycorrhizal associations vary in their drought resistance, this could be the basis for differences in drought mortaliry among sections (timber groups). Also, differences in the water stress (xylem tension) required to induce embolism have recently been demonstrated for important species from dipterocarp and tropical heath forest in Brunei (Tyree et al. 1998). We suspect that a similar comparative study of Red and White Meranti would demonstrate a lower vulnerability to embolism among the latter.

l? S. Ashton, C. Marsh, D. M. Newbery, T. C. Whitmore, and K. M. Wong provided useful com- ments on the manuscript. Thanks to D. M. Newbery and J. Proctor for sharing unpublished data, and to D. M. Newbery for his generous efforts in processing the data. ALLEN, B., H. BROOKPIELI~, AND Y. BYRON. 1989. Forest and drought through time and space, part 11: the written,

ANLXRSON, J. A. R. 1957. The enumeration of 235 acres of dipterocarp forest in Brunei. Malay. For. 20: 144-150. ANONYMOUS. 1985. Report on the tree mortality inventory of Silam Forest Products 1986 and 1987 coupes. Timber

ASITON, P. S. 1964a. Manual of the dipterocarp trees of Brunei State. Oxford University Press, London, England.

oral, and proxy records and their meaning. Mtn. Res. Devel. 9: 279-305.

Production Department, Yayasan Sabah, Kota Kinabalu, Malaysia. Unpublished report.

670 Becker, Ong, and Goh

. 196411 Ecological studies in the mixed dipterocarp forests of Brunei State. Oxford For. Mem. 25. Clarendon, Oxford, England.

. 1989. 1)ipterocarp reproductive biology. In H. Lieth and M. J. A. Werger (Eds.). 'Tropical rain forest eco- systems: biogeographical and ecological studies, pp. 2 19-240. Elsevier, Amsterdam, The Netherlands.

BOWKN, G. L). 1973. Mineral nutrition of ectomycorrhizae. In G. C. Marks and T. T. Kozlowski (Eds.). Ectomycor- rhizae: their ecology and physiology, pp. 15 1-205. Academic Press, New York, New York.

< : o N I ) I I , R., S. I? HLJBBRLI., AND R. B. FosreK. 1995. Mortality rates of 205 Neotropical tree and shrub species and the impact of a severe drought. Ecol. Monogr. 65: 419-439.

I>Avirls, S. J., ANI) P. BECKER. 1996. Floristic composition and stand structure of mixed dipterocarp and heath forests in Brunei Darussalam. J. Trop. For. Sci. 8: 542-569.

FAO-UNESCO-ISRIC. 1988. FAO-UNESCO soil map of the world. Revised legend. World Soil Res. Rep. 60. FAO, Rome, Italy.

Fox, J . E. D. 1973. A handbook to Kabili-Sepilok Forest Reserve. Sabah For. Rec. No. 9. Borneo Literature Bureau, Kuching, Malaysia.

KAKIAWINATA, K., R. AH[)[Jl.HAL>l, ANI) T. ~ ' A K . I ~ ~ M I I I A I ~ I ~ J ~ . 1981. Composition and structure of a lowland dipterocarp forest at Wanariset, East Kalimantan. Malay. For. 44: 397406.

L I ~ K X I I ~ O N , M., AND N. WIKAWAN. 1986. Catastrophic drought and fire in Borneo tropical rain forest associated with the 1982-1983 El Nifio southern oscillation event. In G. T. Prance (Ed.). Tropical rain forests and the world atmosphere, AAAS selected symposium, 101, pp. 75-102. Westview Press, Boulder, Colorado.

I,UGO, A. E., ANI) F. N. SCATFNA. 1996. Background and catastrophic tree mortality in tropical moist, wet, and rain forests. Biotropica 28: 585-599.

MANOKAKAN, N., AND K. M. KOC:HIJMMI:N. 1987. Recruitment, growth and mortality of tree species in a lowland dipterocarp forest in Peninsular Malaysia. J. Trop. Ecol. 3: 315-330.

MAKSII, C. W. 1996. A brief history of the forests of the upper Segama region of eastern Sabah. Sabah SOC. J. 13: 33-56.

MI..IITR, W., ANI) G. H. S . Wooi>. 1964. Dipterocarps of Sabah (North Borneo). Sabah For. Rec. No. 5. Forest Department, Sandakan, Malaysia.

MOI INA, R., H. MASSICOTI A, ANI? J. M. TRAITI:,. 1992. Specificity phenomena in mycorrhizal symbioses: Community- ecological consequences and practical implications. In M. F. Allen (Ed.) Mycorrhizal functioning: An integra- tive plant-fungal process, pp. 357-427. Chapman & Hall, New York, New York.

I. A. S~ONI:, S. E. 'TwMaoRe, AND S. V ~ I K A . 1994. The role of deep roots in the hydrological and carbon cycles of Amazonian forests and pastures. Nature 372: 666-669.

Ni~wiw:u, 13. M., E. J. F. CAMPBELL, Y. F. LEE, C. E. RISDALE, AND M. J. SI,II,I.. 1992. Primary lowland dipterocarp forest a t Danum Valley, Sabah, Malaysia: structure, relative abundance and family composition. Phil. Trans. R. Soc. 1,ond. B 335: 341-356.

-- ~ , J. PKOC.I.OK, AND M. J. Srir .~. . 1996. Primary lowland dipterocarp forest at Danum Valley, Sabah, Malaysia. Species composition and patterns in the understorey. Vegetatio 122: 193-220.

ONL, R., ANI) M. Ki.r; iNa. 1995. DIPSIM: A dipterocarp forest growth simulation model for Sabah. Forest Research Centre Research Papers No. 2. Forestry Department, Sabah, Malaysia.

PCXJISI;N, A. D., I. C . Nwi~cN, S. TAN, ANI) H. BAI.SI.EV. 1996. A quantitative inventory of trees in one hectare of mixed dipterocarp forest in Temburong, Brunei Darussalam. In D. S. Edwards, W. E. Booth, and S. C. Choy (Eds.). Tropical rain forest research-current issues, pp. 139-1 50. Kluwer Academic Publishers, Dordrecht, The Netherlands.

RISWAN, S. 1987. Structure and floristic composition of mixed dipterocarp forest at Lenipake, East Kalimantan. In A. Kostermans (Ed.). Proceedings of the third round table conference on dipterocarps, pp. 435458. UNESCO, Jakarta, Indonesia.

Sisr, 1). 1996. Structure and diversity of dipterocarps in a lowland dipterocarp forest in East Kalimantan. In S. Appanah and K. C;. Khoo (Eds.). Proceedings fifth round-table conference on dipterocarps, pp. 60-86. Forest Research Institute of Malaysia, Kuala Lumpur, Malaysia.

-~ . 1982. Dipterocarpaceae. Flora Males., Ser. I, 9: 237-552.

NIWI .A~) , 1). c . , (:. R. DE CARVAI k l ~ > , E. A. DAVIIISON, 1? H. Jlpp, A. LEFEBVKI:,, G. H. NI't iRIilROb, E. 1). D A sll.VA, > .

S M ~ , W. T. M. 1994. Dipterocarpaceae: mycorrhizae and regeneration. Tropenbos Series 9. SOKAI, R. R., A N D F. J. ROIIIJ. 1995. Biometry. 3rd edition. W. H. Freeman and Company, New York, New York. SUSI~I.O, '1.. B., A N D S. RISWAN. 1987. Compositional and structural pattern of lowland mixed dipterocarp forest in the

Kutai National Park, East Kalimantan. In A. Kostermans (Ed.). Proceedings of the third round table conference on dipterocarps, pp. 459470. UNESCO, Jakarta, Indonesia.

TI:L:.NBIXIH, K. E., AN13 1: J. HCMR. 1996. The 1990-1995 El Niiio-Southern Oscillation event: longest on record. Geophys. Res. Lett. 23: 57-60.

Toiwi:.it, I. M., AND 'I-. C. WHITMORE. 1991. The northern limit of red meranti in the Malay Peninsula. In 1. Socrianegara, S. S. Tjitrosomo, R. C. Umaly, and 1. Umboh (Eds.). Proceedings of the fourth round-table conference on dipterocarps, pp. 295-305. BIOTROP Special Publication No. 41.

I'YKEE, M. T., S. PAN~INo, A N D 1'. BECKEK. Vulnerability to drought-induced embolism of Bornean heath and dipter- ocarp forest trees. Tree Physiol. 18: 583-588.

WAist5, R. I? D. 1996. Drought frequency changes in Sabah and adjacent parts of northern Borneo since the late nineteenth century and possible implications for tropical rain forest dynamics. J. Trop. Ecol. 12: 385-407.

WIIIIMORV., 'I: C. 1984. Tropical rain forests of the Far East. 2nd edition. Clarendon, Oxford, England.

Notes 671

AND N. D. BROWN. 1996. Dipterocarp seedling growth in rain forest canopy gaps during six and a halfyears. Phil. Trans. R. Soc. Lond. B 351: 1135-1203.

WONC, K. M. 1998. In press. Patterns of plant endemism and rarity in Borneo and the Malay I’eninsula. In (;.-I. Peng (Ed.). Rare, threatened and endangered floras of Asia and the Pacific Rim. Institute of Botany, Academia Sinica, Monograph Series No. 16, Taipei, Taiwan.

WON<;, T. M. 1982. A dictionary of Malaysian timbers. Malay. For. Rec. No. 30, Appendix 11. WOODS, I? 1989. Effects of logging, drought, and fire on structure and composition of tropical forests in Sabah,

Malaysia. Biotropica 21: 290-298.

Peter Becker

Biology Department Universiti Brunei Darussalam Gadong BE1410 Brunei (SE Asia).

Ong Chye Lye

#12, 582134 47300 Petaling Jaya Malaysia.

and

Francis Goh

lnoprise Corporation Sdn. Bhd. P. 0. Box 11 623 8881 7 Kota Kinabalu Sabah, East Malaysia

BlOTROPlCA 30(4): 671-676 1998

Litterfall in Rain Forest on Christmas Island, Indian Ocean: Quantity, Seasonality, and Composition’

Key words: Christmas Island Indian Ocean; leaffall; litter; litte+all; rain forest.

SMALL LITTERFALL IS A MAJOR PATHWAY by which organic matter and mineral nutrients are returned to the forest floor in tropical rain forests (Edwards 1982, Vitousek & Sanford 1986), and many studies have documented litterfall in a variety of mainland tropical forests (Songwe et al. 1988, Veneklaas 199 1, Lips & Duivenvoorden 1996; see Proctor 1984 for a review of over 200 studies until 1983). However, there have been no studies reported for lowland rain forest sites on oceanic islands. In this study I describe the quantity, seasonality, and composition of small litterfall on Christmas Island, Indian Ocean.

Christmas Island (105”40’E, lO”30’S) is a small (135 km2) oceanic island lying 360 km south of Java in the northeastern Indian Ocean. The island rises in a series of cliffs and terraces to 361 m elev. Almost 70 percent of the island is covered by rain forest. The island is situated at the southern extremity of the Asian summer monsoon, and by Mohr’s classification (1933 in Whitmore 1984), the climate is seasonal with a mean Q of 0.68 (N = 53 yr). Average annual rainfall is ca 2000 mm, most of which falls during the November-May wet season (Stoddart 1971). There are two major forest types (Mitchell 1975, Du Puy 1993); “plateau” forest occurs over relatively deep soils at higher elevations, where 12 common species form a canopy up to 40 m tall and “terrace” forest occurs over shallower soils on the terraces and plateau shoulder, where up to 25 species form a canopy 20-30 m tall.

Litterfall was measured at two sites in plateau forest over 31 months from August 1988-March 1991. These sites (US and LS) have been described in detail elsewhere (Green 1997). Twenty litter traps were erected at each site. Traps consisted of a circular steel hoop enclosing an area of 0.5 m2, held 1.4 m

’ Received 6 May 1996; revision accepted 21 July 1997.