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Plant Ecology & Diversity

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Structure and composition of deciduousdipterocarp forest in Central Vietnam: patterns ofspecies dominance and regeneration failure

Thuy T. Nguyen & Patrick J. Baker

To cite this article: Thuy T. Nguyen & Patrick J. Baker (2016): Structure and compositionof deciduous dipterocarp forest in Central Vietnam: patterns of species dominance andregeneration failure, Plant Ecology & Diversity, DOI: 10.1080/17550874.2016.1210261

To link to this article: http://dx.doi.org/10.1080/17550874.2016.1210261

Published online: 26 Sep 2016.

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Structure and composition of deciduous dipterocarp forest in Central Vietnam: patterns ofspecies dominance and regeneration failure

Thuy T. Nguyen a,b* and Patrick J. Bakera

aSchool of Ecosystem and Forest Sciences, The University of Melbourne, Richmond, Australia; bVietnamese Academy of Forest Science,Ha Noi, Vietnam

(Received 16 November 2015; accepted 4 July 2016)

Background: Deciduous dipterocarp forest (DDF) is the most widespread forest type of continental Southeast Asia. Fourdominant canopy species of the DDF are often found in near-monodominant stands, but quantitative structure, speciescomposition and regeneration status of these stands are little understood.Aims: To quantify structural, compositional and regeneration variability of the dominant stands in the DDF at YokDonNational Park in Central Vietnam.Methods: We established seventy 0.04 ha plots across the Park to quantify the structure, species composition andregeneration patterns.Results: We found distinct patterns of one or two of the four dipterocarp species dominated basal area in any given stand.Patterns of seedling dominance were not as distinct as in the canopy, nor were there strong associations between thedominant seedling and canopy species, particularly for Shorea siamensis. The most striking feature of the forest was theabsence of saplings, implying a significant bottleneck in the structure.Conclusions: Our results suggest a potential shift in the dominant canopy species in the DDF. The apparent lack ofrecruitment into the larger size classes and the decoupling of dominant species in the canopy and seedling layer raisequestions about the future dynamics of the DDF.

Keywords: Dipterocarpus tuberculatus; regeneration bottleneck; savannah; Shorea siamensis; YokDon National Park

Introduction

Members of the Dipterocarpaceae family dominate thecanopy of tropical forests from ever wet regions of theMalay Archipelago to strongly seasonal areas of northernIndia and southern China (Ashton 2014). The highly diversemixed forests of theMalay Peninsula and Borneo harbour themost floristically rich dipterocarp communities, are of sig-nificant economic importance for their timber, and have beenthe subject of extensive botanical, ecological and silviculturalresearch (Symington 1974; Wyatt-Smith et al. 1995; Ashton2014). In contrast, the low diversity deciduous dipterocarpforests (DDFs) of continental South and Southeast Asia,which are similarly extensive and economically valuable,have received only limited silvicultural study (primarily forShorea robusta; Troup 1921) and even less ecological andbotanical research (Smitinand et al. 1980). The vast forests ofcontinental Southeast Asia, dominated by deciduous dipter-ocarp species, that stretch from Myanmar in the west toVietnam in the east and parts of India (Bunyavejchewin1983) provide important ecosystem services, such as habitatfor flora and fauna, carbon sink, watershed protection, as wellas being a significant source of timber and non-timber forestproducts (Ho 2008; Vu and Vo 2014).

Deciduous dipterocarp forests are limited regionally toareas with a strongly seasonal climate and locally to areasof nutrient-poor soils (Bunyavejchewin 1983; Dinh 1993).

In areas of more fertile soils, they are replaced by the morespecies-rich seasonal evergreen or mixed deciduous for-ests, where the deciduous dipterocarp species are absent orless abundant. As such, DDF are structurally and floristi-cally impoverished, with relatively few species and rela-tively short-statured stands with one or two canopy strata(Bunyavejchewin 1983). The defining features of the DDFare a ground layer dominated by grass and seedlings of thecanopy trees and a tree layer in which the canopy is almostcompletely dominated by a small group of deciduousspecies of Dipterocarpaceae (Brandis 1921; Troup 1921;Champion and Seth 1968; Kutintara 1975; Smitinand et al.1980; Bunyavejchewin 1983; Santisuk 1988; Tran 1991;Dinh 1993; Ashton 2014). Unlike the DDF of northernIndia, which is dominated by a single dipterocarp species,Shorea robusta (sal; Troup 1921), Southeast Asian DDFsare dominated by four deciduous dipterocarp species:Dipterocarpus obtusifolius, D. tuberculatus, Shoreaobtusa and S. siamensis. However, the patterns of canopydominance and species association in DDFs are complexand poorly understood. Pioneering studies by Kutintara(1975) and Bunyavejchewin (1983) in Thailand and Tran(1991) and Dinh (1993) in Vietnam showed that these fourdipterocarps typically occur in discrete stands in whichonly one or two are dominant in the canopy and that thedominant species are loosely associated with certain

*Corresponding author. Email: thuyn2@student.unimelb.edu.au

Plant Ecology & Diversity, 2016http://dx.doi.org/10.1080/17550874.2016.1210261

© 2016 Botanical Society of Scotland and Taylor & Francis

environmental features of the site such as soil water-hold-ing capacity and elevation. However, the generality ofthese patterns is limited by the small number of studiesand fundamental questions about patterns of relative dom-inance and the relationship between the canopy dominanceand new recruits remain unexplored.

The regenerative capacity of the DDF is of particularimportance because, like other seasonally dry tropical forests(Janzen 1988; Miles et al. 2006), the DDF faces a range ofnatural and human-induced threats (Bunyavejchewin et al.2011). First, regeneration of DDF might be influenced byshifts in the timing of flowering and fruit set which is directlyand indirectly affected by climate variation (Corlett andLafrankie 1998; Cleland et al. 2007). Mass flowering orfruiting of dipterocarps is associated with El-Niño-induceddrought (Ashton 1988; Ashton et al. 1988) and is impactedby satiation of the pollinators during the flowering season(Ghazoul et al. 1998; Sakai 2002). Second, the grass layer ofthe DDF makes it an attractive habitat for large browsingmammals (elephant, banteng, gaur) and their predators(tigers, dhole) (Duckworth et al. 2005; Gray and Phan2011). The large browsers directly impact on the regenera-tion of the DDF through browsing of seedlings, uprooting ofsaplings and physical damage to trees (Fernando andLeimgruber 2011). These large animals may also have indir-ect impacts on DDFs through modifying fire intensity as anindirect effect of browsing the grass and seedling layer(Fernando and Leimgruber 2011). Third, like other tropicaldry forests, DDFs are subject to ever-increasing humanimpacts (Janzen 1988; Stott et al. 1990). The straight stemsand high-quality wood of the dipterocarps (Gautam andDevoe 2006) make them a desirable timber species andsubject to often intense, and sometimes illegal, harvestingpressures (Stott et al. 1990). At the broader landscape scale,the DDF also serves, in many parts of the region, as a bufferbetween agrarian human populations and the remaining sea-sonal evergreen and mixed deciduous forests that harbourspecies-rich plant and animal communities (Bunyavejchewinet al. 2011). Each of these threats alone has the potential tolimit the ability of many tree species in DDF to regeneratesuccessfully and therefore affects the structures and speciescomposition of the DDF (Bunyavejchewin et al. 2011). As aconsequence, threats to the long-term viability of the DDFmay also present potential threats to broader patterns ofdiversity within and across the forested landscapes of con-tinental Southeast Asia.

Perhaps the greatest threat affecting the structure, spe-cies composition and regeneration of seasonally dry tropi-cal forests of continental Southeast Asia is a change in thefrequency and intensity of fires that occur during theannual dry season (Bunyavejchewin et al. 2011). Fireshave a long history in the dipterocarp forests of Southand Southeast Asia (Goldammer and Seibert 1989;Maxwell 2004). Based on microscopic charcoal depositionin lake sediments from north-eastern Cambodia, Maxwell(2004) has shown evidence of an increase in fire activity inDDFs beginning ca. 5500 years ago associated with anincrease in summer monsoon intensity, while an increase

in human-induced fires appeared around 3000 years ago.Historical records of natural or human-induced fires inDDF are scarce; however, most recent fires in these forestsare caused by humans, either intentionally or accidentally,in association with land clearance for cultivation, huntingwildlife, and pest and disease control (Stott et al. 1990;Maxwell 2004). The impacts of fire on deciduous dipter-ocarp regeneration have been widely observed throughoutthe region (e.g. Troup 1921; Stott 1986). Most seedlingsare top-killed by fires because their bark is not thickenough to protect the cambium from the heat of the fires(Stott et al. 1990; Dinh 1993). However, due to theirvigorous resprouting ability, a large proportion of theseedlings and saplings grow back at the onset of therainy season, although they may be subjected to fires inthe following dry season (Stott et al. 1990; Dinh 1993).This pattern of shoot extension and dieback may keepindividuals in seedling or sapling form for decades beforethey can reach adult size (Dinh 1993).

Because the previous studies of Southeast Asian DDFhave focused exclusively on adult trees (Bunyavejchewinet al. 2011), basic questions about regeneration of the DDFthat have direct relevance to future forest structure andspecies composition remain unanswered. For instance, dothe dominant canopy species regenerate in situ, or seed-lings are less likely to establish under conspecific trees?Do non-dipterocarp species replace dipterocarp species asa result? Are there consistent patterns of regenerationacross stands dominated by each of the four dipterocarpspecies? And if not, what are the implications for the long-term floristic composition of the DDF at landscape scales?

Our study had two broad aims. First, we wanted tocharacterise the variability in structure and composition ofthe tree component of the dominant stands in the DDF.Second, we wanted to assess the regeneration status ofthese dominant stands and determine whether overstoreycomposition influenced the abundance or composition ofthe regeneration. To address these issues, we established anetwork of study plots across a landscape in CentralVietnam almost exclusively dominated by DDF. We quan-tified the structure and composition of each plot by mea-suring and identifying all trees and then assessed theregeneration status by sub-sampling seedlings withineach plot.

Material and methods

Study area and site description

Our study was conducted at YokDon National Park, DakLak Province, in the Central Highlands of Vietnam. Thepark was established in 1986 as a protected forest andlaunched as a National Park in 1992 (Ngo 2003).YokDon National Park is one of Vietnam’s largest nationalparks and the only place in the country where large land-scapes dominated by DDF are protected. The Park issituated 40 km north-west of Buon Ma Thuot City, andcovers an area of 115,545 ha (Ho 2008) (Figure 1). The

2 T.T. Nguyen and P.J. Baker

topography of the Park is relatively flat with an averageelevation of 200 m a.s.l. and 97% of the area <300 m a.s.l.(Ho 2008). The highest point is YokDon Mountain (482 ma.s.l.) (Ho 2008). The Park is roughly bisected bySerepork River, a tributary of the Mekong.

The Central Highlands of Vietnam are situated in theseasonal tropics dominated by the Asian Monsoon. Theregion experiences 4–6 months with <100 mm rainfallfollowed by the onset of the monsoons in May/June(Nguyen et al. 1981; Dinh 1993; Ho 2008). Minimumand maximum mean monthly rainfalls occur in January(1.4 mm) and June (248 mm), respectively (Dinh 1993).The majority (87%) of the 1610 mm of annual rainfalloccurs from May to October (Dinh 1993), causing water-logging in some areas of the DDF (Tran 1991). The meanmonthly temperature ranges from 19.8°C in January to27.7°C in April (Dinh 1993).

Deciduous dipterocarp forests cover 80% of the Park(Ho 2008) and are characterised by a tree layer and aground layer including grass, seedlings and sometimevarious Cycas species (Dinh 1993). The tree layer (indivi-dual trees ≥1 cm DBH) of the DDF features an opencanopy that, at the landscape scale, is dominated by thefour deciduous dipterocarp species (Tran 1991; Dinh1993). Other common non-dipterocarp canopy and sub-canopy tree species that occur in the DDF are Aporosa

spp., Buchanania spp., Pterocarpus macrocarpus,Strychnos nux-blanda, Terminalia spp. and Xylia xylo-carpa (Tran 1991; Dinh 1993). The grass layer variesfrom scattered and short to dense and ca. 2 m tall andconsists of mainly Imperata cylindrica and Vietnamosasapusilla (Dinh 1993). Lianas and shrubs are not a commoncomponent of the DDF. All four of the deciduous dipter-ocarp species that dominate the DDF canopy are decid-uous for at least six weeks during the dry season, havethick bark, and are capable of vigorous resprouting (Stott1986; Dinh 1993).

Deciduous dipterocarp forest at YokDon National Parkforms distinct stands that appear to be dominated by one ortwo dipterocarp species and vary in size from several hundredsquare metres to a few hectares. Based on our preliminarysurveys, stands dominated by D. tuberculatus are more com-mon than stands dominated by any of the other three dipter-ocarp species. Soils in the forest are mainly poor in nutrientsand often eroded and thin (Nguyen et al. 1981; Dinh 1993).During the dry season when the DDF is subjected to waterstress, most tree species completely or partially shed theirleaves, while grasses, leaves, twigs and often seedlings onthe ground dry out and become combustible. Annually, alarge proportion of the forest is incidentally or accidentlyburnt in the dry season, primarily as a result of human ignitions(Ho 2008).

Figure 1. Map of the 70 plots across YokDon National Park in Central Vietnam (inset) and the map of Vietnam (small) with Daklakprovince (dark shading) and the Central Highlands (light shading).

Structure and composition of deciduous dipterocarp forest 3

Data collection

In October 2013, at the end of the wet season, we estab-lished 70 plots of 20 m × 20 m (0.04 ha) across YokDonNational Park (Figure 1). Each plot was established withinthe core area of a stand that was, based on a preliminaryvisual assessment, dominated by the one of four diptero-carp species and which was at least 0.1 ha in size. Weavoided areas that had been obviously disturbed by illegallogging. The plots were selected to represent the full rangeof variation in stand development (immature to mature),grass density (low and scattered to tall and dense) and soiltypes (sandy vs. non-sandy).

Within each plot, all trees ≥1 cm DBH (diameter atbreast height, 1.3 m) were identified to species, measuredfor DBH with a diameter tape and total height with a 5-mmeasuring pole or a clinometer. Within each plot, twoseedling plots of 5 m × 5 m were established in thesouth-western and north-eastern corners. In each seedlingplot, all seedlings (classified as individuals <1 cm DBH)were identified to species, the number of stems tallied andthe height and basal diameter of the largest stem measured.Most study sites were quite flat, but where the slope wasnoticeable, it was measured with a clinometer.

Data analysis

In this study, the dominant species in the tree layer of eachplot was defined as the species with the highest importancevalue (IV) and the dominant species in the seedling layerwas defined as the most abundant species. IVs for eachspecies in each plot were calculated as:

IVi ¼ %Ni þ%Gi

2

where %Ni is the proportion of stems of species i relativeto the total number of stems for all species and Gi is theproportion of basal area of species i relative to the totalbasal area of all species. To explore patterns of speciescomposition across the plots, we used non-metric multi-dimensional scaling (NMDS) (Kenkel and Orlóci 1986)with the “vegan” package (Oksanen et al. 2013) in R (RCore Team 2015). The Bray and Curtis method (Beals1984) was used to calculate the dissimilarity in speciescomposition among the plots. NMDS was used to identifythe grouping pattern of the plots for both the tree andseedling layers based on the dominant species of theplots. For the seedling analyses, we pooled the seedlingdata from the two sub-plots within each 0.04 ha plot andtreated them as a single seedling plot for all analyses.

To quantify structural variability of the canopy layeramong the plots, we fitted Weibull distributions to the dia-meter and height distributions for each 0.04 ha plot (Baileyand Dell 1973). The Weibull distribution is a flexible dis-tribution capable of capturing various distributional shapes.The shape parameter (c) of the Weibull function represents acertain type of distribution. When c < 1, the curve has a

reversed-J shape; when c = 1, it is a negative exponentialdistribution; when 1 < c < 3.6, the curve has a positiveskewed distribution; and as c > 3.6, the curve has a negativeskewed distribution (Bailey and Dell 1973).

We used one-way ANOVA to test for differences intree density, basal area, species richness and the Weibullshape parameters for DBH and height among plots domi-nated by the four dipterocarp species. When the ANOVAwas significant, we then used Tukey’s honestly significantdifferences (HSD) test to identify which group(s) of plotsdiffered from the others.

To study the overall height structure of the DDF, wedivided all individual seedlings and trees of the plots intofive height (H) classes: small seedlings (H < 50 cm), largeseedlings (50 cm ≤ H < 100 cm), small saplings (100 cm ≤H < 200 cm), large saplings (200 cm ≤ H < 600 cm) andadults (H ≥ 600 cm). We then calculated the frequency ofeach size class on a per hectare basis for the seedling andtree data separately and combined them into one datasetfor plotting the height frequency of all four species.

Results

Plot characteristics

Across the 70 plots surveyed, the four dipterocarp speciesaccounted for 79% of all trees (DBH ≥ 1 cm) and 83% ofthe total basal area. The number of dipterocarp species inthe tree layer varied. Of the 70 plots, 84% had two or threedipterocarp species in the tree layer, 10% had one speciesand 6% had all four species. In the seedling layer, the fourdipterocarp species accounted for 59% of all seedlings. Ofthe 70 seedling plots, 81% had two or three dipterocarpspecies, 9% had one species and 9% had all four species.One seedling plot did not contain any seedlings of thedipterocarp species.

Although 90% of the plots had more than one diptero-carp species in the tree layer, typically only one or two ofthem dominated in terms of basal area or density.Dipterocarpus tuberculatus was the dominant tree speciesin 37 plots, followed by Shorea siamensis in 16 plots, S.obtusa in 9 plots and D. obtusifolius in 8 plots. Among the70 seedling plots, D. tuberculatus was the dominant speciesin 24 plots, followed by non-dipterocarp species in 23 plots,S. obtusa in 14 plots, D. obtusifolius in 7 plots and S.siamensis in 2 plots. Characteristics of 70 plots collectivelyand the plots dominated by each of the four species werepresented in Tables 1 and 2, respectively. Appendix lists allspecies encountered in the tree layer of the 70 plots.

Dominant species in the tree and seedling layers

The NMDS analysis, which compares tree-layer speciescomposition at the plot level, showed distinct patternsamong the plots dominated by each of the four species(Figure 2(a)). Plots dominated by D. tuberculatus and S.siamensis were tightly clustered, whereas the plots domi-nated by D. obtusifolius and S. obtusa were more diffusely

4 T.T. Nguyen and P.J. Baker

arranged. This suggests that species composition withinthe plots dominated by D. tuberculatus and S. siamensiswas more consistently similar than the species composi-tion within the D. obtusifolius- and S. obtusa-dominatedplots.

In contrast to the composition of the tree layer, theNMDS of the seedling data showed little evidence for adistinct grouping pattern among the plots (Figure 2(b)).Only the plots dominated by seedlings of S. obtusa andD. tuberculatus formed identifiable groups, but neitherwas tightly clustered in the NMDS space. Plots domi-nated by non-dipterocarp seedlings were evenly spreadacross the NMDS space. Dipterocarpus obtusifolius-dominated plots did not show any grouping pattern.Shorea siamensis-dominated plots also showed noobvious pattern, but this was primarily because therewere only two plots in which S. siamensis was thedominant seedling species.

Comparison of dominant species in the tree and seedlinglayers

A comparison of the dominant species in the tree andseedling layers showed little evidence for a strong linkbetween tree abundance and regeneration success, parti-cularly for S. siamensis (Figure 3). Only 12.5% of theplots with S. siamensis as the dominant species in thetree layer had S. siamensis as the dominant seedlingspecies (Figure 3). For plots dominated by the otherthree species, about half the seedling plots were domi-nated by a species other than the dominant canopyspecies (Figure 3). Notably, seedlings of non-dipterocarpspecies were dominant in the seedling layer of 33% ofthe plots.

Variation in species richness and stand structure

The ANOVA showed that species richness differed sig-nificantly in both the tree layer (F = 4.37, n = 70, df = 3,P < 0.01) and the seedling layer (F = 5.24, n = 70, df = 3,P < 0.01) among the plots dominated by each of the fourspecies (Table 2). A pair-wise comparison with Tukey’sHSD test showed higher species richness in plots domi-nated by D. obtusifolius than in plots dominated by thethree other species. This was true for species richness inboth tree and seedling layers.

The density of trees ≥1 cm was not different among theplots dominated by each of the four species (F = 0.68,n = 70, df = 3, P > 0.1), but total basal area of trees ≥1 cmDBH was (F = 3.15, n = 70, df = 3, P < 0.05). The D.obtusifolius-dominated plots had higher total basal area oftrees ≥ 1 cm DBH than the D. tuberculatus- and S. sia-mensis-dominated plots (Table 2). Seedling density wasalso significantly different among the plots dominated byeach of the four species (F = 4.05, n = 70, df = 3,P < 0.05) (Table 2). Tukey’s HSD test indicated that D.obtusifolius-dominated plots had significantly higher seed-ling densities than the plots dominated by the two Shoreaspecies.

The distribution of the Weibull shape parameter forboth DBH and height of the plots provides a quantita-tive comparison of structural variability among the plotsdominated by the four species (Figure 4). The results ofthe comparison of the Weibull shape parameters indi-cated differences in both DBH (F = 7.45, n = 70, df = 3,P < 0.001) and height (F = 8.39, n = 70, df = 3,P < 0.001) distributions among the plots dominated byeach of the four species (Figure 4). In particular, S.siamensis-dominated plots had significantly higherWeibull shape parameter values for both the DBH andheight distributions than the two Dipterocarpus-domi-nated plots, suggesting a preponderance of large, tallstems in the plots (Figure 4).

Vertical structure and regeneration status of the deciduousdipterocarp forest

The most distinctive feature of the vertical structure of theDDF was a lack of small saplings (H = 100–200 cm) forall four dipterocarp species (Figure 5). The number ofseedlings of all species, except S. siamensis, showed asharp decrease from the small size (H < 50 cm) to largesize class (H = 50–100 cm). Similarly, the number ofindividuals of all four species declined sharply from thelarge seedling to the small sapling stage. In terms of theabundance of trees relative to conspecific seedlings, theratio of tree to seedling (on a per hectare basis) of S.siamensis (1:2.4) was much lower than for the threeother dipterocarp species (D. obtusifolius, 1:14; D. tuber-culatus, 1:6.2; S. obtusa, 1:18) (Table 1, Figure 5). Thissuggests that S. siamensis had the poorest regeneration ofthe four dipterocarp species, while S. obtusa had the best.

Table 1. General characteristics of the tree and seedling layersof 70 plots collectively at YokDon National Park in the CentralHighlands of Vietnam.

Tree layerSeedlinglayer

Summed total for all plotsTotal no. of trees/seedlings 3442 4684No. of species 38 51No. of species with less than 10individuals

20 35

No. of Dipterocarpus obtusifolius 235 426No. of Dipterocarpus tuberculatus 1425 1102No. of Shorea obtusa 475 1054No. of Shorea siamensis 584 176Plot meansBasal area (m2 ha−1) 19.12 ± 5.89 n/aTree density (ha−1)– Trees ≥ 1 cm dbh 1229 ± 523 n/a– Trees ≥ 10 cm dbh 590 ± 178 n/a

Seedling density (ha−1) n/a 13,383

Total area surveyed for the trees and seedlings was 2.8 ha and 0.35 ha,respectively.

Structure and composition of deciduous dipterocarp forest 5

Table

2.Characteristicsof

structureandspeciesdiversity

(mean±SD)of

plotsdo

minated

bythefour

decidu

ousdipterocarpspeciesat

Yok

Don

NationalParkin

theCentral

Highlands

ofVietnam

.

Dom

inantspecies

Treelayer

Seedlinglayer

#plots

Density

≥1cm

ha−1

Density

≥10

cmha

−1

Basal

area

≥1cm

ha−1

Basal

area

≥10

cmha

−1

#Species

≥1cm

plot

−1

Weibu

llshapeof

DBH

Weibu

llshapeof

height

Density

ha−1

#Species

plot

−1

Dipterocarpus

obtusifoliu

s8

1322

±81

255

9±20

324

.55±9.44

22.39±9.41

10±5

1.75

±0.78

2.07

±0.61

20,950

±11,669

11±3

Dipterocarpus

tuberculatus

3712

89±52

654

3±14

818

.38±5.16

16.22±5.90

7±2

1.72

±0.43

2.33

±0.67

13,767

±61

368±3

Shorea

obtusa

910

88±25

065

8±18

220

.01±5.52

18.61±6.10

6±2

2.00

±0.54

2.71

±0.70

8911

±71

757±3

Shorea

siam

ensis

161129

±46

567

8±19

917

.62±4.22

15.83±4.10

6±2

2.44

±0.58

3.29

±0.80

11,225

±88

627±3

The

firstcolumnshow

sthenameof

thedominantcanopy

speciesin

theplots.The

identifi

catio

nof

thedominantcanopy

specieswas

basedon

theim

portance

valueof

alltrees≥1

cmDBH

(see

text

fordetails).

6 T.T. Nguyen and P.J. Baker

Discussion

Dominant patterns of the deciduous dipterocarp species inthe DDF

Our data show that the dipterocarp species form distinctstands in which one of the four species dominates in termsof basal area and density. The observed pattern of speciesdominance is similar to those described in previous studies ofDDFs in Thailand (e.g. Kutintara 1975; Bunyavejchewin1983), Vietnam (e.g. Tran 1991; Dinh 1993) and Myanmar(Troup 1921). In all of these studies, the defining feature ofthe DDF is the overwhelming dominance at the stand level ofone or two deciduous dipterocarp species. We also found, asothers (Kutintara 1975; Bunyavejchewin 1983; Tran 1991;Dinh 1993), that there are often sharp boundaries betweenstands with different dominant dipterocarp species. In thefield, we observed individual stands sharing distinct bound-aries with other adjacent deciduous dipterocarp stands or

with other vegetation types, such as riparian forest, evergreenforest or bamboo patches.

One of the obvious distinctions between the DDF atYokDon National Park and the DDF from Thailand is theabsence of pine species in the Vietnamese forests that wesurveyed. At high elevation sites in Thailand, the decid-uous dipterocarp species often occur with Pinus kesiya orPinus merkusii (Kutintara 1975; Bunyavejchewin 1983).Both of these species are found in Vietnam but only atelevations well above the known range of DDF in Vietnam(150–700 m; Tran 1991). The only other DDF – the sal(Shorea robusta) forest of India and Nepal – also formsdistinct stands where S. robusta is the dominant species interms of both abundance and basal area (Troup 1921;Timilsina et al. 2007; Kushwaha and Nandy 2012).However, while none of the deciduous dipterocarp speciesfrom continental Southeast Asia occur in the sal forests,many of the non-dipterocarp species, such as Buchananialatifolia, Cleistocalyx operculatus and Terminalia tomen-tosa do (Timilsina et al. 2007).

Although the patterns of species dominance in theDDF of continental Southeast Asia are well known, themechanisms driving these patterns remain poorly under-stood. Environmental factors are potentially important indriving stand-level patterns of species dominance acrossthese landscapes. Kutintara (1975) and Bunyavejchewin(1983), working on DDF in Thailand, found associationsbetween the dominant stands and both soil properties andtopographic features, primarily those related to soil wateravailability (Bunyavejchewin et al. 2011). However,Timilsina et al. (2007) found no evidence of environmen-tal controls on patterns of species dominance in sal forests.

−0.75

−0.50

−0.25

0.00

0.25

0.50

−0.75

−0.50

−0.25

0.00

0.25

0.50

Adult

Seedling

−0.4 0.0 0.4

NMDS Axis 1

NM

DS

Axi

s 2

Percentage of dominance20406080

Dominant speciesD. obtusifoliusD. tuberculatusS. obtusaS. siamensisNon−Dipterocarp

(a)

(b)

Figure 2. Non-metric multidimensional scaling (NMDS) basedon the species composition in the tree layer (a, stress value = 14)and seedling layer (b, stress value = 24) of the 70 plots atYokDon National Park in the Central Highlands of Vietnam.The size of the shape is proportional to the calculated importancevalue of dominant species and relative abundance.

62.5 2.7 0 6.2

0 59.5 22.2 0

12.5 32.4 22.2 50

25 5.4 55.6 31.2

0 0 0 12.5

D. obtusifolius

D. tuberculatus

S. obtusa

S. siamensis

Non−Dipterocarp

D. obtusifolius D. tuberculatus S. obtusa S. siamensis

Dominant species of the tree layer

Dom

inan

t spe

cies

of t

he s

eedl

ing

laye

rFigure 3. Comparison of the dominant species in the seedlinglayer (y-axis) against the dominant species in the canopy (x-axis)of the 70 plots at YokDon National Park in the Central Highlandsof Vietnam. Circles represent the proportion of the dominantseedlings in plots dominated by each of the four species.

Structure and composition of deciduous dipterocarp forest 7

While there are consistent patterns of species dominancein DDF across the region, these patterns may be broadlylinked to other biotic and abiotic factors, such as seedproduction, seed dispersal and seedling recruitment.Indeed, the Dipterocarpaceae are well known for masting,in which synchronous production of large seed cropsacross whole landscapes or regions occurs at infrequentintervals (Ashton et al. 1988). Dinh (1993) monitored thefruit production of the DDF at several sites near YokDonNational Park from 1987 to 1989 and observed that fruitproduction of dipterocarps in all sites in 1989 was ca. 10times higher than in the previous two years. Mast fruitingprobably allows the species to recruit more in mastingyears due to predator satiation (Janzen 1970; Ashtonet al. 1988). Apart from mast fruiting, limited seed dis-persal due to heavy seeds might also contribute to thediscrete spatial pattern of DDF (Ashton and Kettle2012). Dinh (1993) reported that D. obtusifolius and D.tuberculatus fruits can disperse up to 30–60 m from their

parent tree, while S. obtusa and S. siamensis may dispersefurther, up to 70–80 m in open area and windy condition.Similarly, Smith et al. (2016) used a dispersal model todemonstrate that under normal conditions dipterocarpsspecies typically disperse within a distance of 17–77 m.However, there is as yet no rigorous study examining thepotential role of seed production and seed dispersal limita-tion on the species coexistence pattern in DDFs and itremains unclear what factors or processes generate andmaintain the much finer-grained local variation in speciesdominance that is so evident at YokDon National Park andelsewhere.

Composition of the regeneration is decoupled fromcomposition of the overstorey

The compositional differences between the dominant spe-cies of the seedling and the tree layer suggest that the DDFat YokDon National Park may experience a shift in speciescomposition in the future, particularly for S. siamensisstands, for several reasons. First, relative to the abundanceof adult trees, the regeneration of S. siamensis is signifi-cantly less abundant than the regeneration of the threeother dipterocarp species (Table 1; Figure 5). Second,regeneration of S. siamensis is poor in all plots regardlessof whether S. siamensis is the dominant canopy species ornot. Third, species other than S. siamensis dominated theseedling layer in 87.5% of plots in which S. siamensis wasthe dominant tree species. In addition, in the field weobserved many S. siamensis-dominated stands with

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Figure 4. Weibull shape parameters for (a) DBH and (b) heightdistributions shown for the four deciduous dipterocarp species inthe 70 plots surveyed at YokDon National Park in the CentralHighlands of Vietnam.

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Figure 5. Frequency distribution of all seedlings and adults infive size classes of the four deciduous dipterocarp species sur-veyed in 70 plots at YokDon National Park in the CentralHighlands of Vietnam. All values are scaled to a per hectarebasis.

8 T.T. Nguyen and P.J. Baker

abundant individuals >20 cm DBH, but very few S. sia-mensis seedlings on the ground. If the recruitment ofseedlings and transition to saplings and adults in S. sia-mensis-dominated stands continues to fail, there is a riskthat the future composition of the tree layer in these standswill shift towards dominance by other (mostly non-dipter-ocarp) species.

Local and regional patterns of species richness

Species richness in the stands was relatively low. Acrossour seventy 0.04 ha plots, the number of tree species≥10 cm DBH per plot ranged from 1 to 11(mean ± SD = 4.5 ± 1.8), while that of trees ≥1 cmDBH ranged from 3 to 16 (mean ± SD = 7.0 ± 2.8).This is comparable to the species richness of the DDF insouthern India, which has 1–17 species per 0.04 ha plot(mean ± SD = 6.7 ± 3.3; Suresh et al. 2011). In studies thatused larger (1 ha) plots, the number of tree species (DBH≥10 cm) ranged from 24 to 64 in Thailand(Bunyavejchewin et al. 2011) and 8 to 28 in Vietnam(Tran et al. 2013). In addition to the similar patterns ofspecies abundance and the dominance of the dipterocarps,the composition of the non-dipterocarp component of oursites was nearly identical to those of previous studies inboth Thailand (e.g., Kutintara 1975; Bunyavejchewin1983) and Vietnam (e.g. Tran 1991; Dinh 1993). Speciescommon to all of these studies include: Aporosa spp.,Cratoxylum formosum, Dillenia spp., Phyllanthus emblica,Randia spp., Strychnos spp., Terminalia spp., Vitex spp.and Xylia xylocarpa in the canopy layer; and Cycas sia-mensis, Arundinaria ciliata, A. pusillia and Imperatacylindrica in the ground layer. These non-dipterocarptree species are also known to be associated with DDF inMyanmar (Brandis 1921; Troup 1921) as well as salforests in India and Nepal (Timilsina et al. 2007). Our

results highlight the broad geographical consistency ofthe species associations in DDF across large areas ofcontinental South and Southeast Asia.

Variation in stand structure

In our 70 plots, total stem density did not differ among theplots dominated by each of the four species and was onlyslightly higher (for trees ≥10 cm DBH) than in the DDFdescribed by Kutintara (1975) and Bunyavejchewin (1983)(Table 3). Notably, though, our plots had, on average,nearly twice as many stems ≥10 cm DBH as thosedescribed by Dinh (1993) (Table 3), which were estab-lished in DDF near ours. While total stem density did notdiffer significantly among plots dominated by the fourdipterocarp species, the distribution of tree sizes did. Inparticular, stands dominated by S. siamensis tended tohave more large trees and fewer small trees than standsdominated by the other three dipterocarps (Figures 4 and5). However, the large S. siamensis trees were similar insize to the large trees of the other dipterocarp species.Thus, the main difference between the size distributionsof S. siamensis and the other three dipterocarp species isthe lack of saplings and small trees relative to the numberof adults (Figure 5).

While long-term data on the dynamics of DDFs are notyet available, a comparison of the relative abundance ofseedlings, saplings and trees can provide some indirectinsights into the status of life-history stage transitionsand the abundance of seedling relative to their adults.The most notable feature across all four species and almostall of our plots was the distinct lack of individuals in thesmall sapling stage. This structural feature has not beenwell described in the other studies of DDF, althoughWanthongchai (2008) reported a similar absence of sap-lings in a burning experiment in a DDF in western

Table 3. Stem density and basal area of trees ≥10 cm DBH in the deciduous dipterocarp forests of continental Southeast Asia describedin previous studies.

Reference Species assemblages Stem density (ha−1) Basal area (m2 ha−1) No. of species

Kutintara (1975) Dipterocarpus tuberculatus – Shorea obtusa 514 26.96 –Dipterocarpus obtusifolius – Shorea obtusa 478 27.06 –Dipterocarpus tuberculatus – D. obtusifolius 510 32.22 –Shorea obtusa – Shorea siamensis 565 17.6 –Dipterocarpus tuberculatus – Pinus spp. 527 42.08 –

Bunyavejchewin (1983) Dipterocarpus tuberculatus – Shorea obtusa 470 ± 78 23.88 ± 8.09 19 ± 7Dipterocarpus obtusifolius – Shorea obtusa 438 ± 100 23.51 ± 7.96 19 ± 7Shorea obtusa 416 ± 144 16.74 ± 5.55 11 ± 4Shorea siamensis 486 ± 168 20.32 ± 7.48 18 ± 4Dipterocarpus spp. – Pinus spp. 462 ± 96 24.39 ± 5.24 10 ± 3

Dinh (1993) Dipterocarpus tuberculatus – Shorea obtusa 200–350 – –Dipterocarpus obtusifolius 100–400 – –Dipterocarpus tuberculatus 200–500 – –Shorea siamensis – S. obtusa 200–400 – –

Note: Number of species in Bunyavejchewin (1983) was per 0.2-ha plot.Kutintara (1975) and Bunyavejchewin (1983) described the deciduous dipterocarp forests in northern Thailand. Dinh (1993) described the deciduousdipterocarp forests in the Central Highlands of Vietnam.

Structure and composition of deciduous dipterocarp forest 9

Thailand that had many species in common with the DDFat YokDon National Park (e.g. Dipterocarpus tubercula-tus, Pterocarpus macrocarpus, Shorea obtusa, S. siamen-sis, Terminalia alata, T. corticosa, Xylia xylocarpa).Timilsina et al. (2007) observed abundant seedlings andsaplings in sal forests in northern India and Nepal, butfound lower tree densities in the 10–20 cm size class. Theobserved lack of small saplings in our study and DDFelsewhere is, however, consistent with the well-described‘bottleneck’ at the sapling stage that occurs globally inmany savannah ecosystems, in which the vegetation isalso characterised by a tree and a grass layer. Theseinclude savannahs in Australia (e.g. Williams et al. 1999;Prior et al. 2010), Africa (e.g. Hester et al. 2006;Archibald et al. 2005; Lehmann et al. 2009; Staver et al.2009) and America (e.g. Peterson and Reich 2001). It isbelieved that ‘top-down’ controls, such as fires and herbi-vores have caused this structural feature in many savannahecosystems (Bond and Keeley 2005; Staver et al. 2009,2012; Staver and Bond 2014).

There are several potential explanations for theobserved bottleneck at the sapling stage in the DDF atYokDon National Park. The first is the frequent fires thatoccur at the Park (Ho 2008), but also across much of theDDF-dominated areas of continental Southeast Asia eachyear during the dry season. Wanthongchai (2008) foundthat the densities of seedlings and saplings in DDF inwestern Thailand were significantly lower in frequentlyburnt sites than in less frequently burnt or unburnt sites.Timilsina et al. (2007) also suggested that frequent dryseason fires were associated with the absence of pole-sizetrees in sal forests. Indeed, while adult trees in the DDFare relatively immune to fires because of their thick bark,the majority of deciduous dipterocarp seedlings are top-killed by fire, but resprout within weeks or months of thefire (Stott 1986; Dinh 1993). While the actual mortality ofboth seedlings and adults is low, the repeated dieback ofseedlings and small saplings to the root collar will limit thetransition of seedlings and saplings to adult trees.

A second potential cause of the observed bottleneck atthe sapling stage is drought-induced dieback associatedwith the annual dry season. We have observed a largeproportion of seedlings of seed origin that die of droughtbefore the fires come. However, drought-induced mortalityin the regeneration of deciduous dipterocarp seedlings hasnot been studied, with all of the previous studies ofdrought effects on dipterocarps focusing on evergreenspecies (e.g. Cao 2000; Marod et al. 2002; Baltzer et al.2008). A third potential explanation for the regenerationbottleneck might be effects of herbivory, which is welldocumented in other savannah ecosystems around theworld (Moncrieff et al. 2009; Staver et al. 2009; Staverand Bond 2014). Due to the abundance of grass, the DDFharbours a wide range of large mammals, such as elephant,gaur, banteng and deer (Duckworth et al. 2005; Gray andPhan 2011). These large animals consume significantamounts of grass and plants as part of their diet(Fernando and Leimgruber 2011), which may directly or

indirectly impact seedling regeneration. However, mea-surements of browsing pressure on deciduous dipterocarpsdo not exist, so it is difficult to gauge the extent of thispotential source of damage.

To become an established adult tree in the DDF, newlyestablished individuals must overcome each of these poten-tially limiting factors. There are likely several life historystrategies to facilitate this process but, at a minimum, indivi-dual trees must either have thick enough bark to protectthemselves or must be able to grow rapidly enough in heightto escape fire and browsing pressures (Hoffmann et al. 2009;Lawes et al. 2011). This raises the important question of whatconditions are required – and for how long – for deciduousdipterocarp seedlings and saplings to safely reach fire-proofsizes (Bond and Keeley 2005). Based on previous savannahstudies, we expect that a period of relatively cool, moistconditions that create a fire-free period of several yearswould be a minimum requirement for the species to rapidlymove through this bottleneck stage. However, identifying aspecific fire return interval for the sustainable management ofDDF may be complicated. Early experiences with fire man-agement in sal forests of India and Nepal demonstrated thatincreasing fire frequency shifted the structure and speciescomposition of the DDF toward a more savannah type(Troup 1921); however, prolonged exclusion of fire led to amore species-rich forest (Troup 1921; Goldammer 2007).

Of the four dipterocarp species in our study, S. sia-mensis had the poorest regeneration. For stands dominatedby S. siamensis in the tree layer, the future stand composi-tion will likely be dominated by non-dipterocarp speciesdue to the near-complete absence of S. siamensis seedlingsand the relative abundance of non-dipterocarp seedlings. Itis not clear why S. siamensis has substantially poorerregeneration than the three other dipterocarps. Relativesensitivity to fire has not been assessed for the deciduousdipterocarp species, nor have fecundity, seed viability andseed dispersal patterns – all of which may contribute to theobserved absence of small saplings.

Conclusions

The DDFs that cover the landscape of YokDon NationalPark in Central Vietnam are dominated by four deciduousdipterocarp species. However, within individual stands orpatches of dipterocarp forest only one or two of thesespecies dominate the forest canopy. The four dipterocarpspecies rarely all co-occur, and in few instances that theydo, the species are not equally abundant. Stands dominatedby one dipterocarp species are recognisably distinct in thefield, sharing abrupt boundaries with stands dominated byother dipterocarp species, or other vegetation types. Insome instances, there are also structural differencesbetween stands that are dominated by the different dipter-ocarp species. The most notable structural feature of theDDF, irrespective of dominant species, is the absence ofsmall saplings, suggesting a bottleneck in the transitionbetween regeneration and successful establishment offuture canopy trees. The most notable compositional

10 T.T. Nguyen and P.J. Baker

feature of the DDFs is the apparent disconnect between thecomposition of the canopy trees and the regenerationbelow them. This was particularly evident for S. siamen-sis-dominated stands, which were typically dominated bynon-dipterocarp species regeneration. Our observationsraise important management questions regarding the futurecomposition of deciduous dipterocarp species and ecolo-gical questions concerning the mechanisms that may becontributing to the changing composition of theregeneration.

AcknowledgementsThe authors would like to acknowledge all of the field assistants whocontributed to the project: Oanh Bui, Toi Cao, Bon Trinh, Luong Ho,Huy Tran, Jonathan Ho. We thank Stuart Davies for comments on anearly draft of the article. Logistical support from the staff of theYokDon National Park and the Vietnamese Academy of ForestScience is greatly appreciated. TTN was supported by an AUSAIDLeadership Award and PJB was supported by an Australian ResearchCouncil Future Fellowship (FT120100715).

Disclosure statementNo potential conflict of interest was reported by the authors.

FundingThis work was supported by the AUSAID Leadership Award;Australian Research Council Future Fellowship; [FT120100715].

Notes on contributorsThuy T. Nguyen performed the fieldwork, analysed the data andwrote the article.

Patrick J. Bakera provided interpretation of the data analyses andassisted in writing the article.

ORCID

Thuy T. Nguyen http://orcid.org/0000-0003-3458-465X

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AppendixTree species occurrence across a network of forest study plots atYokDon National Park, Vietnam. For each species with at leastone stem in our 70 study plots, we have provided the total numberof individuals (N) and total basal area summed across all plots.

12 T.T. Nguyen and P.J. Baker

Family Species N Total basal area (m2)

Fabaceae Albizia lebbekoides 5 0.043Euphorbiaceae Aporosa dioica 120 0.767Bombacaceae Bombax ceiba 1 0.065Sapindaceae Boniodendron parviflorum 2 0.036Anacardiaceae Buchanania latifolia 79 0.428Burseraceae Canarium bengalense 8 0.064Hypericaceae Cratoxylum formosum 5 0.200Fabaceae Dalbergia oliveri 43 0.385Dipterocarpaceae Dipterocarpus obtusifolius 235 4.736Dipterocarpaceae Dipterocarpus tuberculatus 1425 20.52Theaceae Eurya nitida 7 0.036Flacourtiaceae Flacourtia indica 4 0.031Clusiaceae Garcinia poilanei 27 0.465Rubiaceae Gardenia panduriformis 19 0.027Phyllanthaceae Glochidion daltonii 1 0.027Lythraceae Lagerstroemia noei 26 0.513Fagaceae Lithocarpus harmandii 12 0.175Lauraceae Litsea cambodiana 2 0.009Anacardiaceae Melanorrhoea laccifera 1 0.013Melastomataceae Memecylon edule 13 0.032Calophyllaceae Mesua spp. 2 0.038Rubiaceae Mitragyna rontundifolia 8 0.033Rubiaceae Morinda tomentosa 15 0.054Rubiaceae Neonauclea sessilifolia 7 0.058Euphorbiaceae Phyllanthus emblica 3 0.001Fabaceae Pterocarpus macrocarpus 8 0.133Rubiaceae Randia dasycarpa 2 0.002Dipterocarpaceae Shorea obtusa 475 9.883Dipterocarpaceae Shorea siamensis 584 9.566Dipterocarpaceae Shorea thorelii 1 0.001Fabaceae Sindora siamensis 1 0.028Bignoniaceae Stereospermum cylindricum 21 0.082Apocynaceae Tabernaemontana corymbosa 2 0.055Combretaceae Terminalia alata 141 3.138Combretaceae Terminalia chebula 61 0.597Combretaceae Terminalia corticosa 14 0.491Unknown Unknown 1 1 0.0002Fabaceae Xylia xylocarpa 61 0.819

Structure and composition of deciduous dipterocarp forest 13