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Southeast Asian Dipterocarp Forest Ecosystems

N. Manokaran

Abstract

The Southeast Asian dipterocarp forests are located in the non-seasonal humid zone stretching from Sumatra in the west, through the Malay archipelago, to New Guinea in the east. They are evergreen, hygrophilous in character, at least thirty metres high, rich in thick-stemmed lianes, and in woody as well as herbaceous epiphytes.

In this paper, the dominance of the family Dipterocarpaceae in the Southeast Asian dipterocarp forests is discussed, stratification in the dipterocarp forest described, and an example given of the woody composition of this stratification.

Some aspects of tree population dynamics in 2-ha plots in Bukit Lagong and Sungei Menyala, the longest running (36, 38 years) plots in primary forests of the tropics, and in Pasoh (13 years), all in Peninsular Malaysia, are discussed in relation to the temporal stability of these rain forests. In discussing floristic composition, changes in species composition with time, common and uncommon species, forest structure, and sapling and adult populations, it is concluded that while there are changes within the natural forest, the composition of the canopy is unlikely to change in the future in the absence of large-scale perturbations, and the forest would remain recognisably of the same type.

Keywords: Dipterocarp forests, Dipterocarpaceae, species richness, tree population dynamics.

Introduction

Dry-land tropical rain forest has been described by Schimper (1903) as evergreen, hygrophilous in character, at least thirty metres high, rich in thick-stemmed lianes, and in woody as well as herbaceous epiphytes. Those rain forests occur in three regions, in Central and South America, in Africa and in the Indo-Malayan region.

In the Indo-Malayan region, the tropical rain forest lies as a belt of evergreen vegetation extending through the Malay archipelago from Sumatra in the west to New Guinea in the east (Whitmore 1984). This is the non-seasonal humid zone of the Southeast Asian dipterocarp forests. The tropical rain forest extends from the shoreline to the tops of all except the few very highest mountains. Outliers are found in Southern Thailand, in Sri Lanka, India, Queensland in Australia and in the Melanesian islands of the Pacific.

With the exception perhaps of New Guinea and the eastern part of the region, the tropical rain forests of the Indo-Malayan region are characterised by family dominance of the Dipterocarpaceae.

Altitudinally dipterocarps are not found in montane oak forests and at higher elevations. They are, however, represented in edaphic climax formations such as peat swamp forests, fresh-water swamp forests and heath forests.

The Family Dipterocarpaceae

The family, revised by Ashton (1982), consists of three subfamilies.

The subfamily Pakaraimoideae consists of a monotypic genus Pakaraimaea, locally found in the south of former British Guyana, in lowlands and hills of the tropics below 1800 m.

Subfamily Monotoideau is represented in Africa and Madagascar, with some 36 species of Monotes and a few species of Marquesia.

Subfamily Dipterocarpoideau, comprising 12 genera and some 470 species ranges from the Seychelles through Sri Lanka to the south of Peninsular India, and then to East India, Bangladesh, Burma, Thailand, Indo-China, to continental South China (Yunnan, Kwangsi, South Kwangtung, Hainan) and through Malesia (natural botanical kingdom comprising Peninsular Malaysia, Sumatra, Java, Lesser Sunda Islands, Borneo, the Philippines, Celebes, the Moluccas, New Guinea and the Solomons).

The Dipterocarpaceae is the main timber family in the forests of Southeast Asia, and usually forms a high proportion of the emergent and main canopy strata of the forest. For example, in surveys covering a total of about 3200 hectares of lowland and hill dipterocarp forests in Peninsular Malaysia, 30% of trees 30 cm diameter and greater at breast height, and 55.5% by volume were dipterocarps (Symington 1943). In another survey in Peninsular Malaysia, Poore (1968) found that in a 21 ha area in a lowland dipterocarp forest in Jengka, 28% of trees of 29 cm dbh and greater were dipterocarps, and these occupied 43% of the basal area in the plot. When woody trees are considered to a dbh as low as 1 cm, dipterocarps are still a significant part of the vegetation. In a 50-ha plot in a primary lowland dipterocarp forest in Pasoh, dipterocarps formed over 9% of the total number of stems, second only to the Euphorbiaceae representing more than 13% of the stems, and formed over 24% of the total basal area with Euphorbiaceae a distant second with over
7% of the basal area (Manokaran et al. 1991).

Structure and Composition of Dipterocarp Forest

At least three-quarters, if not more, of the forests of Southeast Asia are dipterocarp forests. In Malaysia, for example, dipterocarp forests form about 85% of the forested areas.

The classic view is that there is stratification of layering of vegetation into five strata, A-E, in dipterocarp forests (Richards 1952). The A stratum consists of the top layer of the biggest trees, the emergents, whose crowns appear as broad and spreading above the general canopy of the forest. This is a discontinuous layer, dominated by the family Dipterocarpaceae, and with an average height of about 38 m. The B stratum, the main storey, is a continuous layer, and trees here are deep-crowned. The trees are from diverse families, with an average height of about 22 m. Trees in the C or understorey stratum are of an average height of about 9 m. This is a more-or-less continuous layer, and the tree crowns tend to be narrow and deep. This stratum contains many young individuals of the A and B strata. The D or shrub stratum is variable in height up to about 5 m, and is fairly open. The E layer consists of forest floor herbs and small seedlings.

A comparison of tree species in the A, B and C strata of three dipterocarp forests in Peninsular Malaysia by Manokaran and Kochummen (1990) showed that about half are species of the B stratum, and emergent species are about 10% of the total number of species. Secondary forest species were found to be rare, forming less than 3% of the total (Table 1).

The richness of the flora of the tropical rain forest is reflected in results of studies in Indonesia and Peninsular Malaysia (Table 2), and the richest tropical rain forests are considered to have > 200 species of tree greater than 10 cm diameter in one hectare (Whitmore 1989). In a recent study in Pasoh Forest Reserve, a lowland dipterocarp forest in Peninsular Malaysia, 820 species in 294 genera and 78 families were enumerated in a 50-ha plot of all trees of 1 cm dbh and larger (Kochummen et al. 1990). The species number is almost one third of the total number of tree species found in Peninsular Malaysia, indicating that small areas include a surprisingly large percentage of a region's tree and shrub flora.


Table 1. Proportion of species groups (trees of 10 cm dbh and larger) in 1971 in lowland and hill dipterocarp forests in Peninsular Malaysia (adapted from Manokaran and Kochummen 1990).

Species group

Bukit Lagong (Lowland dipterocarp

Sungei Menyala (Lowland dipterocarp forest)

Pasoh (Hill dipterocarp forest) forest)

Emergent species, % of total spp.

8.3

9.5

12.6

Main canopy species, % of total spp.

47.4

50.4

46.2

Understorey species, % of total spp.

41.5

37.5

39.6


Table 2. Species diversity for trees of 10 cm dbh and larger in dipterocarp forests in Indonesia and Peninsular Malaysia.

INDONESIA*

PENINSULAR MALAYSIA+

Wanariset

Lempake

Ketambe

Bukit Lagong

Sungei Menyala

Pasoh

1.6 ha

1.6 ha

1.6 ha

2 ha

2 ha

2 ha

No. of species

239

205

172

253

232

259

*after Kartawinata et al. 1981

+ after Manokaran and Kochummen 1990


Tree Population Dynamics in Dipterocarp Forests

Relatively few studies on tree population dynamics have been carried out in the species-rich tropical rain forests, and these have been summarised in Swaine et al. (1987). Population studies over a long term can provide an insight into the temporal stability of communities in these rain forests. The longest-term studies that have been reported are those of Wyatt-Smith (1966) (12 years), Manokaran and Kochummen (1987) (34 years), Manokaran (1988) (13, 36, 38 years), Primack and Hall (1991)(20 years) and Manokaran and Swaine (1994) (13, 36, 38 years). In this paper some aspects of population dynamics of trees in 2-ha plots in primary forests in Peninsular Malaysia are discussed.

Wyatt-Smith (1966) initiated studies to examine the dynamics of tree populations in a lowland dipterocarp forest at an altitude of 30 m above sea level at Sungei Menyala Forest Reserve in 1947 and in a hill dipterocarp forest at an altitude ranging from 460-550 m above sea level at Bukit Lagong Forest Reserve in 1949. Trees of 10cm diameter and larger at breast height in a 2-ha plot in each of the reserves were enumerated by Wyatt-Smith at two-yearly intervals until 1963 and the data summarised by him. Longer running enumerations in the sample plots until 1985 are summarised by Manokaran (1988) and Manokaran and Swaine (1994). Results of tree population studies over a shorter time frame of 13 years (1971-1984) in four 2-ha plots at an altitude of 90m above sea level in Pasoh Forest Reserve, another lowland dipterocarp forest, summarised by Manokaran (1988) and Manokaran and Swaine (1994), are based on an initial vegetation survey by Ashton (1971a, 1971b) and Soepadmo and Kira (1977).

Floristic Composition

The floristic composition at the three sites was compared using the ordination technique of detrended correspondence analysis (DECORANA) of Hill (1979) and Hill and Gauch (1980). This form of principalcomponent analysismaking use of quantitative data is efficient for heterogenous data (Greig-Smith 1983). Each of the 2-ha plots at the three sites was separated into two 1-ha samples to compare floristic composition in the two halves of the plot for the common enumeration year of 1971.

The results from DECORANA indicate quite clearly that hill forest at Bukit Lagong is different floristically from the lowland forest sites (Figure 1). Based on total species present in each of the six 2-ha plots, about 38% of the species present in Bukit Lagong were not present in any of the five lowland forest plots. Sungei Menyala is different from the Pasoh samples but within Pasoh itself the variation between plots is at least as great. This variation at Pasoh is considered to be due to physiographical differences between the plots.


Fig 1 (31k)

Figure 1. Ordination of floristic composition in 1971 in 1-ha samples in the 2 ha-plots in Bukit Lagong, Sungei Menyala and Pasoh forest reserves. The two 1-ha samples in each of the plots are connected as shown. Ordination was only on species appearing in at least two of the 12 samples.


Changes in Species Composition with Time

Austin and Greig-Smith (1968) evaluated ordinations of heterogenous data from rain forest and concluded that the less abundant species contribute very little information, with less than 25% of the flora in their study being able to provide an efficient ordination. To compare changes in species composition over time for the five lowland forest plots, only species with 10 individuals at any enumeration were used for ordination with DECORANA. Thus, 11.4% of the total species at Sungei Menyala and 11.0%, 9.3%, 10.6% and 11.3% of the species at the four plots at Pasoh were used for ordination. The resulting ordination (Figure 2) shows that variation over time was very slight compared with variation between sites, even within Pasoh Forest Reserve where the 2-ha plots are separated by only a few hundred metres.

In terms of numbers, species richness varied between 248 and 264 in a total of 53 families in the Bukit Lagong plot (Table 3, subspecies treated as separate taxa). In the plot at Sungei Menyala there were between 232 and 248 species in 45 families over the study period. In the four plots at Pasoh, the figures were 255 to 264 species in 45 families, 231 to 240 species in 45 families, 257 to 261 species in 45 families, and 276 to 284 species in 49 families.

Species richness in 2 ha at the three forest sites was variable within narrow limits with no clear difference between the hill forest and the lowland forests. Between the lowland forests, Pasoh had slightly more species and genera than Sungei Menyala.

The number of species present during the first census but absent during the last (local extinction), number of new accessions (local immigration) and number of species present only at sometime during the first and last census, are given in Table 4. In percentage terms local extinction of species was 16% of the total number of species recorded throughout the study period for Bukit Lagong, 17% for Sungei Menyala, and lower figures of 9%, 10%, 8% and 8% for the shorter duration of 13 years for the Pasoh plots. Local immigration was 14% for Bukit Lagong, 17% for Sungei Menyala and 6%, 8%, 7% and 10% for the Pasoh plots. Local extinction and immigration involved rare species, i.e. species with very low densities, and the magnitude appears to be a function of time higher rates were seen at Bukit Lagong and Sungei Menyala which were under observation over periods of about three times those of the Pasoh plots. Local extinction at the level of 10 cm dbh and larger does not in any way mean that the species does not exist as juveniles. Persistent species, those that were present throughout the periods of observation, were
69% at Bukit Lagong, 63% at Sungei Menyala, and 85%, 81%, 84% and 81% for the Pasoh plots respectively.


Fig 2 (16k)

Figure 2. Ordination of changes in species composition over time in the 2-ha plots in the lowland forest sites at Sungei Menyala and Pasoh. Dots mark 15 censuses between 1947-85 for Sungei Menyala and three censuses
between 1971-84 for Pasoh. Ordination was on species with 10 or more individuals at any one census.


Table 3. Changes in the number of families, genera and species in the 2-ha sample plots in (a) Bukit Lagong Forest Reserve between 1949 and 1985 and (b) Sungei Menyala Forest Reserve between 1947 and 1985, and in four 2-ha samples in (c) Pasoh Forest Reserve between 1971 and 1984. Subspecies are treated as separate taxa.

(a) Bukit Lagong
Parameter 1949 1951 1953 1955 1957 1959 1961 1963 1971 1975 1977 1979 1981 1983 1985 Mean
Families 51 51 51 51 51 50 50 50 51 51 52 52 53 53 53 51.3
Genera 141 142 141 143 143 142 141 140 139 140 140 140 139 137 137 140.3
Species 256 258 260 264 264 263 262 258 253 253 255 256 253 248 248 256.7
(b) Sungei Menyala
Parameter 1947 1951 1953 1955 1957 1959 1961 1963 1971 1975 1977 1979 1981 1983 1985 Mean
Families 44 44 44 44 44 44 45 45 45 45 45 45 45 45 45 44.6
Genera 125 122 122 122 119 119 121 120 116 121 120 121 122 122 122 120.9
Species 248 246 246 247 244 240 241 240 232 237 238 241 248 247 246 242.7
(c) Pasoh
Pasoh 3 Pasoh 4 Pasoh 6 Pasoh 7 Mean
Parameter 1971 79 84 71 79 84 71 79 84 71 79 84
Families 45 45 45 45 44 42 45 45 43 48 49 48 45.3
Genera 131 130 126 125 127 125 135 133 129 141 139 136 131.4
Species 264 260 255 235 240 231 261 259 257 276 279 284 258.4


Table 4. Species changes between the first and last census at the 2- ha plots in Bukit Lagong, Sungei Menyala and Pasoh forest reserves.

Site

Number of species

(a) Bukit Lagong

    Present in 1949, absent in 1985

    Present in 1985, absent in 1949

    Present after 1949, disappeared before 1985

49

41

2

b) Sungei Menyala

    Present in 1947, absent in 1985

    Present in 1985, absent in 1947

    Present after 1947, disappeared before 1985

54

52

6

(c) Pasoh

Present in 1971, absent in 1984

Present in 1984, absent in 1971

Present only in 1979

Pasoh 3

25

16

1

Pasoh 4

26

22

1

Pasoh 6

23

19

2

Pasoh 7

24

32

1

Common and Uncommon Species

The ten most abundant species accounted for 28.4% of all trees in 1949 and 31.2% of all trees in 1985 at Bukit Lagong, 23.2% (1947) and 22.9% (1985) at Sungei Menyala, and in four times the area in Pasoh, 17.1% (1971) and 16.8% (1984). Uncommon species, represented by only one individual, consisted of 39.5% of the total trees in 1949 and 44.8% in 1985 at Bukit
Lagong, 36.7% (1947) and 43.5% (1985) at Sungei Menyala, and in Pasoh with the 4-fold increase in plot size, 19.1% (1971) and 22.7% (1984). Thus, while about two-fifths of the
species in 2 ha occurred as a single individual only, the population of uncommon species reduced to about one-fifth with a four-fold increase in plot size. With a plot as large as 50 ha, and woody trees censused to a diameter as low as 1 cm, 273 species (about one-third) of 814 species (revised species number) occurred at densities below one per hectare, and 24 had one individual only in the 50 hectare (Manokaran et al. 1992).



Table 5. Changes in density and basal area of trees of 10 cm dbh and larger in a 2-ha sample plot in (a) Bukit Lagong Forest Reserve between 1949 and 1985 and (b) Sungei Menyala Forest Reserve between 1947 and 1985, and in four 2-ha samples in (c) Pasoh Forest Reserve between 1971 and 1984.

(a) Bukit Lagong
Parameter 1949 1951 1953 1955 1957 1959 1961 1963 1971 1975 1977 1979 1981 1983 1985 Mean
Tree density (ha-1) 544.5 542.5 547.0 549.0 548.5 542.5 535.5 519.5 494.0 492.0 490.5 492.0 458.5 477.5 466.5 515.0
Basal area (m2 ha-1) 42.35 41.09 40.99 41.57 42.32 42.29 42.23 42.03 41.11 40.72 41.18 42.47 41.81 41.52 41.29 41.66
(b) Sungei Menyala
Parameter 1947 1951 1953 1955 1957 1959 1961 1963 1971 1975 1977 1979 1981 1983 1985 Mean
Tree density (ha-1) 537.5 518.5 506.0 510.0 507.0 491.0 487.0 478.0 476.5 478.5 466.5 461.5 484.0 469.0 461.5 489.0
Basal area (m2 ha-1) 33.09 33.30 33.05 33.52 33.18 32.54 32.78 32.25 31.81 31.78 30.43 31.06 32.64 32.76 33.16 32.49
(c) Pasoh
Parameter 1971 1979 1984 Mean
Tree density (ha-1) 545.5 540.0 514.8 533.4
Basal area (m2 ha-1) 29.13 28.54 26.90 28.19


Fig 3 (325k)

Figure 3. Diameter class distribution for trees of 10 cm dbh and larger at first and last censuses at (a) Bukit Lagong (2 ha): histogram, 1949 (N = 1089); 1985 (N = 932); (b) Sungei Menyala (2 ha): histogram, 1947 (N = 1075); 1985 (N = 923); (c) Pasoh (8 ha): histogram, 1971 (N = 4364); 1984 (N = 4118). For each site the two distributions are not significantly different (see text).


Table 6. Comparison of species composition of trees (10cm dbh and larger in 2 ha) and saplings (1.5 m height to < 10 cm dbh in 0.08 ha) of tree species at Sungei Menyala Forest Reserve. Tree species composition are those of the 1951 and 1983 enumerations while those of the saplings are the mean of the 1949, 1951 and 1955 enumerations, and the 1982 enumeration.

All species

Canopy species only

1950s

1980s

1950s

1980s

Total number of tree species

246

247

145

146

Number of tree species present as saplings

106

(43.1%)

98

(39.7%)

58

(40.0%)

55

(37.7%)

Number of tree species absent as saplings

140

149

87

91

Total number of sapling species

188

149

98

80

Number of sapling species absent as adults

82

51

40

25

Forest Structure

The density of trees in the 2-ha plot at Bukit Lagong averaged 515 ha-1 over the period
1949-85, but showed a general decline of 14.4% (Table 5). The corresponding figure for the
2-ha plot at Sungei Menyala was an average of 489 ha-1 for the 38 years from 1947 to 1985. The general decline here was similar to that at
Bukit lagong, being 14.1%. At Pasoh, for the 13-year period from 1971 to 1984, tree density averaged 533.4 ha-1 for the 8 ha, with a general decline of 5.6%.

Average basal area for the corresponding periods at Bukit Lagong and Sungei Menyala were less variable, at 41.66 m2 ha-1 and 32.49 m2 ha-1 respectively. The average basal area at Pasoh was 28.19 m2ha-1, but there was a decline of 7.7% over the period 1971-84.

For each of the study sites, diameter class distributions at the first and last census periods were not significantly different (Figure 3, Bukit Lagong: X2 = 8.01, P>0.25; Sungei Menyala: X2 = 5.80, P>0.5; Pasoh: X2 = 7.41, P>0.25). Despite changes in tree density, basal area and dbh class distributions were approximately constant, implying that net loss of trees is made up by incremental growth in surviving trees, and that these are fully-stocked forests. Stability in the forest structure is an indication of stability in the composition of tree species.

Sapling and Adult Populations Compared

Sapling enumerations had been carried out by Wyatt-Smith (1966) in 1949, 1951 and 1955 in a sample area of 0.08 ha in the 2-ha plot at Sungei Menyala. All woody plants of height of 1.5 m and greater to 10 cm diameter were sampled. The woody plants with the same dimensions were renumerated in 1982 by the author in the identical strip of area. The 1950s and 1982 sapling composition, of species which normally grow to tree size of 10 cm dbh and larger, is compared with tree species composition in 1951 and 1983 respectively in Table 6.

In both the 1950s and 1980s, about 40% of the tree species were also represented as saplings, for all species as well as for canopy species alone, in only a 4% area of the 2-ha plot. There was also a great flux in sapling species composition in the 0.08-ha area over 30 years; 77 (40%) of the 188 species in the 1950s were not present in 1982, and 39 (about 26%) of the 149 species in 1982 had not been there in the 1950s. These results suggest that an enumeration of saplings over a larger area of the 2-ha plot is likely to provide evidence that most trees in the Sungei Menyala Forest Reserve are present as juveniles.

Canopy species that were abundant as adults (10 or more individuals) at either period, were generally well-represented as saplings. If the sapling density had been low in the 1950s, numbers had often increased by 1982. This was in line with early observations in Peninsular Malaysia that lowland dipterocarp forests such as Sungei Menyala and Pasoh had a high stocking of seedlings and saplings of good merchantable species to regenerate the species naturally (Walton 1936, Walton et al. 1952, Barnard 1954, 1956, Wyatt-Smith 1963, 1987). Indeed this was the basis on which the Malayan Uniform System, the silvicultural system for the lowland dipterocarp forests of Peninsular Malaysia, was developed (Wyatt-Smith 1963).

The results from Sungei Menyala are in contrast to the observation by Aubreville (1938) that in the tropical forests of West Africa, many common canopy trees were poorly represented as seedlings and saplings, Richards (1952) considered this as a charateristically African feature and termed Aubreville's account of forest regeneration as the `Mosaic Theory'. The contrast between adult and juvenile species composition has also been observed in the Malesian region in East Kalimantan (Riswan 1982). Many authors have considered such observations as evidence for changes in the future composition of the canopy. However, in noting that Aubreville's conclusions were based on juvenile enumeration of seven canopy species only, Swaine and Hall (1988) have shown that while there were clear differences in composition between adult and juvenile samples from six forest sites in Ghana, over the whole of the forest zone the variation in the floristic composition between size classes at any site was slight.

The study at Sungei Menyala has shown that canopy species that were abundant as adults were generally well-represented as saplings and therefore the composition of the canopy is unlikely to change in the future in the absence of large-scale perturbations. The 50-ha plot study at Pasoh of all woody plants of 1 cm dbh and composition in the 0.08-ha area over 30 years; 77 (40%) of the 188 species in the 1950s were not present in 1982, and 39 (about 26%) of the 149 species in 1982 had not been there in the 1950s. These results suggest that an enumeration of saplings over a larger area of the 2-ha plot is likely to provide evidence that most trees in the Sungei Menyala Forest Reserve are present as juveniles.

Canopy species that were abundant as adults (10 or more individuals) at either period, were generally well-represented as saplings. If the sapling density had been low in the 1950s, numbers had often increased by 1982. This was in line with early observations in Peninsular Malaysia that lowland dipterocarp forests such as Sungei Menyala and Pasoh had a high stocking of seedlings and saplings of good merchantable species to regenerate the species naturally (Walton 1936, Walton et al. 1952, Barnard 1954, 1956, Wyatt-Smith 1963, 1987). Indeed this was the basis on which the Malayan Uniform System, the silvicultural system for the lowland dipterocarp forests of Peninsular Malaysia, was developed (Wyatt-Smith 1963).

The results from Sungei Menyala are in contrast to the observation by Aubreville (1938) that in the tropical forests of West Africa, many common canopy trees were poorly represented as seedlings and saplings, Richards (1952) considered this as a charateristically African feature and termed Aubreville's account of forest regeneration as the `Mosaic Theory'. The contrast between adult and juvenile species composition has also been observed in the Malesian region in East Kalimantan (Riswan 1982). Many authors have considered such observations as evidence for changes in the future composition of the canopy. However, in noting that Aubreville's conclusions were based on juvenile enumeration of seven canopy species only, Swaine and Hall (1988) have shown that while there were clear differences in composition between adult and juvenile samples from six forest sites in Ghana, over the whole of the forest zone the variation in the floristic composition between size classes at any site was slight.

The study at Sungei Menyala has shown that canopy species that were abundant as adults were generally well-represented as saplings and therefore the composition of the canopy is unlikely to change in the future in the absence of large-scale perturbations. The 50-ha plot study at Pasoh of all woody plants of 1 cm dbh and larger has confirmed that most common canopy species are indeed well-represented as juveniles (Manokaran et al. 1992).

Conclusion

The plots at Bukit Lagong and Sungei Menyala are the longest running in primary tropical rain forest. These small 2-ha plots, for which some results have been reported here for periods of 36 and 38 years respectively, are still being enumerated regularly, and data collected now covers periods of 46 and 48 years respectively. Data collected for the Pasoh plots now span a period of 24 years. Information generated by these plots at Bukit Lagong, Sungei Menyala and Pasoh will be of help in understanding the dynamics of tree popupations in these primary dry land forests, and will be a benchmark against which could be measured what happens in forests manipulated by man. This is especially so in Southeast Asia where a large part of dipterocrp forests have been logged and an even larger part in some contries have been converted to other land uses.

In undisturbed natural tropical forest such as those at Bukit Lagong, Sungei Menyala and Pasoh, we expect to find elements of stability in demographic processes and therefore in forest structure and species population size. The expectations are that undisturbed tropical forest will remain recognisably of the same type.

At each of the three sites sampled, the forests were structurally stable in terms of size-class distribution and basal area despite a certain imbalance in mortality and recruitment. This is an indication of stability in the composition of tree species, borne out by ordination that showed that changes in species composition over time were very slight compared with variation between sites.

Although not monitored in detail, changes in the composition of the saplings appeared to be much greater, although without any clear trend. This is to be expected because of higher motality rates in juveniles and the frequent restocking from seed dispersal.

Canopy species that were abundant as adults were generally well-represented as saplings. This is an indication that the composition of the canopy is unlikely to change in the future in the absence of large-scale perturbations.

These results therefore represent a characterisation of the dynamics of undisturbed forest: natural forest is continually changing in all respects, but the forest remains recognisably of the same type.

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OE Nov 21, 1996