Tree assemblages and diversity patterns in Tropical Juri Forest, Bangladesh

Swapan Kumar Sarker?Muhammad Nur-Un-Nabi?Md.Mohasinul Haque?Mahmuda Sharmin?Sanjay Saha Sonet?Sourav Das?Niamjit Das

Tree assemblages and diversity patterns in Tropical Juri Forest, Bangladesh

Swapan Kumar Sarker?Muhammad Nur-Un-Nabi?Md.Mohasinul Haque?Mahmuda Sharmin?Sanjay Saha Sonet?Sourav Das?Niamjit Das

Juri is a biodiversity-rich primary forest in Bangladesh,which remains ecologically unexplored.We identified tree species and examined the richness,alpha(a) diversity and floristic similarity patterns within the identified communities.Vegetation and environmentaldata were sampled in 120(0.04 ha)study plots.Tree communities were delimited by two-way indicator species analysis (TWINSPAN).In total,78 tree species of 35 families and 58 genera were identified.TWINSPAN identified six tree communities:A—Tricalysia singularis;B—Kydia calycina-Castanopsis tribuloides;C—Polyalthia simiarum-Duabanga grandiflora;D—Ficus roxburghii;E—Artocarpus lacucha;F—Artocarpus lacucha.Mean richness,Shannon and Gini-Simpson indices were highest for the Polyalthia simiarum-Duabanga grandiflora community,while Ficus roxburghii showed lowestdiversity.Significantdifferences (p=0.05)in three diversity indices were recorded between Polyalthia simiarum-Duabanga grandiflora and Ficus roxburghii.Tree compositional similarity was greatest between Kydia calycina-Castanopsis tribuloides and Polyalthia simiarum-Duabanga grandiflora(0.712).

Two-way indicator species analysis (TWINSPAN)Richness indexShannon indexGini-Simpson indexTropicalforestConservation

Introduction

Distinguishing plant communities has been central to vegetation science for decades(Jongman et al.1995; Kashian et al.2003;Habib et al.2011).There is growing interest in describing plant community structure and diversity patterns in fragmented and degraded tropical forests(Laurance and Bierregaard 1997;Myers etal.2000; Gillespie et al.2004;da Silva et al.2011)because this has implications for conservation of species and habitats(John et al.2007;Toledo et al.2011).Current knowledge of vegetation distributions in species-rich communities is almost exclusively derived from neo-tropical forests (Condit et al.2002;Li et al.2009;Condit et al.2011), while Oriental forests have received less attention.As a result,spatial structure,species-habitat associations, diversity patterns and the mechanisms of species coexistence in Oriental forest ecosystems remain poorly understood despite their richness of floristic diversity and their vulnerability to intense anthropogenic disturbance and climate change(Toledo etal.2011).

The forests of Bangladesh are now subject to unprecedented threat,notonly from deforestation and fragmentation, butalso from climate change(FAO 1998;Barua etal.2010; Sarker et al.2011).Thus,the issue of conserving the remaining remnantforests has become a priority.Bangladesh government,a signatory to the convention on biological diversity(CBD)has assigned high conservation priority to the remaining naturalforests in the country.But,successful conservation requires detailed knowledge of floristic composition and diversity,ecology,and distribution of plant communities(Sarker et al.2013a).Unfortunately,there is limited knowledge oftree species distribution in the remnant primary forests in north-eastern Bangladesh.Greater understanding of patterns of tree distribution in forest landscapescan guide habitatmapping and design ofconservation strategies such as identification and protection ofrare or severely degraded tree communities.

Our study area,Juri Natural Forest(JNF),is a biodiversity-rich primary forest in northeast Bangladesh,and represents both tropical evergreen and deciduous vegetations.It is an inaccessible forest situated along the Indo–Burma Biodiversity Hotspot(Myers et al.2000;Sarwar and Araki 2010).Tree species composition of this forest has been severely degraded by both anthropogenic disturbances(e.g.,agricultural expansion,illegal felling,industrial logging,human settlement,fire and grazing)and environmental stresses(e.g.,prolonged drought,soil erosion,periodic flooding and catastrophic events)(Dewan 2009;Barua etal.2010;Rahman etal.2010).Moststudies (IUCN 2004;Uddin and Hassan 2011)of the north-eastern hill forests of Bangladesh have yielded qualitative rather than quantitative descriptions of floristic composition and tree species diversity.In the present study,we identified tree communities,and examined the species richness,alpha (a)diversity and floristic similarity patterns within and between these communities in relation to habitat characteristics with implications for conservation.

Materials and methods

Study site

JNR is a remnantnaturalforestin the Patharia Hill Reserve, northeast Bangladesh(Islam etal.2008).The forestis under the jurisdiction ofMoulivibazarForestRange 1 underSylhet ForestDivision and represents the naturalvegetations ofthe Lathitila forest beat(2433055900N and 9213038600E, 2,279.91 ha).The topography ofthe area variesfrom medium to steep slopes and hillocks of varying elevation with undulating valleys and streams.Severalchannels with many tributaries drain the forest(IUCN 2004).The hilly areas of JNF are composed ofuppertertiary rocks.Sediments are generally wellweathered and limestone isalso found athigh elevations, sometimes cemented with secondary ironstones(Islam etal. 2008).Soilsvary from clay to sandy-loamy with low pH.The soil texture consists of yellowish to dark grey granules. Accumulation of humus on the topsoil is limited by rapid decomposition ofdebrisundermoistwarmtropicalconditions (Hassan 1994;IUCN(InternationalUnion forConservation of Nature)2004).JNF lieswithin the monsoon climate zone.The annualaverage temperature is maximum 33.2C and minimum 13.6C and total rainfall is 3,334 mm.Relative humidity remains high(75–90%)most of the year and is highest from May to October.Humidity dips below 80% from Novemberto April(BMD 2012).The forestisgenerally classified as tropicalsemi-evergreen(Muzaffar etal.2007). The most common evergreen trees of this forest are Cinnamomum tamala,Amoora wallichi,Vitex pinnata,and Palaquium polyanthum.Common deciduous trees are Syzygium ramosissimum,Cleistocalyx operculatus,Albizia procera, Anthocephalus chinensis,and Bombax insigne.(Islam et al. 2008).

Sampling strategy

Field work was conducted during October–December 2012.During early reconnaissance we observed that forest species composition varied along an elevation gradient (18–100 m).Therefore,sampling plots were demarcated in two steps.At first,hills of the forest were stratified into elevation categories:foothillto mid-hill,mid-hill;and midhill to hill-top.Elevation percentage ranging 0–30%; 30–70%and 71–100%were used to denote foothill to mid-hill,mid-hill;and mid-hill to hill-top,respectively. After that,sampling was arbitrary but without preconceived bias(McCune and Grace 2002)in each elevation range.Species-area curve was used to determine the number of plots sampled in each elevation range.Sampling plots measured 20 m 9 20 m and numbered 120 in totalto representthe tree vegetation in JNF.

Tree and environmentaldata collection

Trees of diameter at breast height(DBH,1.3 m)[7.6 cm were identified to species level.A representative voucher was collected from each species in each plotfor identifi cation and deposition in the Forestry and Environmental Science Laboratory in Shahjalal University of Science and Technology,Sylhet,Bangladesh.Environmentaldata were recorded in each plot.Soil pH,moisture content(%)and elevation(masl)data were considered as environmental variables and measured.Measurements of soil pH and moisture were taken by digital soil pH and moisture meter from the fourcornersofeach plot,and mean valuesofsoilpH and soil moisture content were recorded.Elevations were estimated by hand-held digital GPS(Global Positioning System).

Data analysis

Tree community classification

Tree community types were delimited by two-way indicator species analysis(TWINSPAN)using the software Win-TWINS 2.3(Hill 1979;modified by ter Braak 1987).This toolwas chosen mainly fortwo reasons;first,itis based on ordination,i.e.,correspondence analysis,which should result in a classification based on floristic composition. Second,due to the noise-reducing effectof ordination,thedivisive algorithm should be less noise-sensitive than agglomerative algorithms(Gauch 1982).This was desirable since much of the floristic variation in the data could be considered noise(Ejrn?s and Bruun 2000).

TWINSPAN has been widely criticized,but it is still used by many ecologists,partly because there is no clearly improved and accepted alternative(Kent 2006).It continues to be used for the classification of ecologicaldata sets (Klinka et al.2002;Carter et al.2006)despite problems found in experiments using artificially produced data (Belbin and McDonald 1993).In this study,TWINSPAN was applied using default options for minimum group size for division(5);maximum number of indicator species per division(7);pseudo-species cut levels 0,0.05,0.1,0.2. Four levels of division were used.The first two indicator species were used to name the respective community.

Diversity and composition

Alpha diversity(Whittaker 1972)was calculated for each community by using three indices:species richness index, Shannon diversity index(H0)and Gini-simpson diversity index(D0).Species richness was calculated as the number of species found in each plot.The Shannon diversity index accounts forboth common and rare species,while the Ginisimpson index is more sensitive to common species(Hill 1973;Magurran 2004).Indices were calculated by using Biodiversity R package(Kindt and Coe 2005)in R software version 2.10.1(R Development Core Team 2010). Post hoc pair wise Tukey’s HSD mean separation test and homogeneity of variance test(p=0.05)were used to assess the significance of differences between diversity indices.This analysis was performed by using SPSS 11.5. The Morisita-Horn index based on a two-way probabilistic approach(Chao et al.2008)was used to determine the compositional similarity between communities.Despite its greater sensitivity to common species,this abundancebased index gives the most satisfactory results to deal with bias(Chao etal.2008).A bootstrap approach was taken by using the software SPADE(Chao and Shen 2010)to calculate the 95%confidence interval based on 200 simulations to measure compositional similarity.

Results

Community classification

Fig.1 Tree communities identified by TWINSPAN.Sample plots(N),indicator species and respective eigenvalues(k)of each division are indicated.The numeric values mentioned in the box are the total sample plots for a certain community

In total,78 tree species of 35 families and 58 genera were identified(Appendix Table 5).TWINSPAN classified 120 sample plots into six tree community types(Fig.1):A—Tricalysia singularis;B—Kydia calycina-Castanopsis tribuloides;C—Polyalthia simiarum-Duabanga grandiflora;D—Ficus roxburghii;E—Artocarpus lacucha;and F—Artocarpus lacucha.Summary of the communities is given in Table 1.

Community type A:Tricalysia singularis

Tricalysia singularis wasthe community atlowerelevations in JNF.Itwassampled by 8 sample plotsand 15 tree species were recorded,mostly on the foothill.Mean elevation ofthe study plots of this community was 20.5 masl.The soilwas moderately acidic with a mean pH 4.40(±0.33).This community had the lowest tree density(181 ha-1)of all communities.Common speciesincluded Artocarpuschama, Michelia oblonga,Pterocarpus indicus,Bombax insigne, Tricalysia singularis and Gurea paniculata.

Community type B:Kydia calycina-Castanopsis tribuloides

This was the characteristic community of the mid-hill sites and had the highest number of sample plots(50)and tree species(48).This community also contained the maximum number of unique species(12).Examples include,Garcinia cowa,Bixa orellana,Cynometra polyandra,Syzygium praecox,Baccaurea ramiflora,and Eclipta alba.Mean elevation and soilpH ofthis community were 40 masland 4.41,respectively.Kydia calycina,Lophopetalum wightianum,Carallia brachiata,Ficus roxburghii,Gurea paniculata,and Castanopsis tribuloides were common species of this community type.

Community type C:Polyalthia simiarum-Duabanga grandiflora

This community harbored 47 tree species in 30 sample plots and contained 4 indicator species(Polyalthia simiatum,Duabanga grandiflora,Syzygium cumuni and Acacia catechu).The plots were distributed from mid-hill to hilltop atelevations up to 100 masl.A distinguishing attribute ofthis community was the highestsoilacidity(mean pH of 4.37±0.38)of all sampled communities.Common species included Lophopetalum wightianum,Polyalthia simiarum,Duabanga grandiflora,Vitex pinnata,Kydia calycina,and Gurea paniculata.

Community type D:Glochidion lanceolarium

Glochidion lanceolarium community(18 sample plots) occurred in the drier foothill to mid-slope habitat of JNF and had very low average soil moisture content(45%). Mean soil pH was 4.53.Elaeocarpus floribundus,Duabanga grandiflora,Illex godajam,Gurea paniculata, Glochidion lanceolarium,and Garcinia xathocymus were common trees in this community.

Table 2 Descriptive statistics and multiple comparisons of different diversity indices in the forest communities

Community type E:Ficus roxburghii

Ficus roxburghii community included 10 sample plots and supported the lowest number of species.It occurred on eroded foothillsites along stream channels.Mean elevation of this community type was 19.66 masl.Common species were Syzygium fruticosum,Ficus roxburghii,Spondias pinnata,Terminalia bellirica,and Elaeocarpus floribundus.

Community type F:Artocarpus lacucha

This community occurred at high elevations with moist soil.It had the lowest number of sample plots(4).This community appeared at25–71 masland was comprised of nine species.Cleistocalyx operculatus was the only unique species.Carallia brachiata,Polyalthia simiarum,Glochidion lanceolarium,Ficus roxburghii,Vitex pinnata,and Palaquium polyanthum were common species.

Tree diversity and composition

Mean richnessand Shannon and Gini-Simpson indiceswere highestfor the Polyalthia simiarum-Duabanga grandiflora community while Ficus roxburghii showed lowest alpha diversity(Table 2;Fig.2).Differences in all diversity indices were significant(p=0.05)between Polyalthia simiarum-Duabanga grandiflora and Ficus roxburghii community types.The homogeneity of variance test confirmed the results of Tukey’s HSD test(Table 3).

Greatestpair-wise compositionalsimilarity wasrecorded between communities Kydia calycina-Castanopsis tribuloides and Polyalthia simiarum-Duabanga grandiflora(0.712) followed by Polyalthia simiarum-Duabanga grandiflora and Glochidion lanceolarium(0.631)(Table 4).Average similarity between allcommunities was 0.34.

Fig.2 Mean intervals of richness index(a),Shannon(b)and Gini-Simpson(c)diversity indices

Discussion

Tree communities

Several studies have documented the influence of topography and soil heterogeneity on species composition anddiversity in tropicalforests(Valencia etal.2004;John etal. 2007;Jones et al.2008).JNF is a complex hillecosystem in Bangladesh with remarkable variation in soilproperties (Hassan 1994).Thus,despite a narrow elevation gradient,a high degree of variability in tree species composition is expected between different landforms in the forest.Previous studies(IUCN 2004;Sarker et al.2013b;Uddin and Hassan 2011)mentioned the influence of topography in structuring plant assemblages in the northeastern hill forests of Bangladesh.We also observed that the identified tree communities varied by topography.T.singularis community occurred on foothill sites with moderate soil acidity.K.calycina—C.tribuloides community occurred on mid-slopes with moderate soilacidity.P.simiarum—D. grandiflora community spread over mid-hills to hill-tops where soils were strongly acidic.G.lanceolarium community comprised tree species that naturally occurred between drier foot-mid hill sites.F.roxburghii was the characteristic community of eroded foothills near stream channels.A.lacucha community occurred on shady midslopes with moist soils.

Table 3 Homogeneity of variance test

Table 4 Pair wise(C22)and simultaneous(C26)Comparison of communities by Morisita-Horn index(Bootstrap SE in parentheses is based on 200 replications)

Almost all tree communities contained several unique species and a numberofspecies common in the southeastern hill forests of Bangladesh.K.calycina—C.tribuloides community had the highest number of unique species and several tree species of this community such as Bombax insigne,Gurea paniculata,Michelia oblonga,G.lanceolarium,are also common in the moderately sloped hill forests of the southeastern zone(Ahmed and Hassan 2008).G. lanceolarium community included three unique species namely Amoora wallichi,Lagerstroemia indica and Albizia procera.These are evergreen trees of lowland forest (Ahmed and Hassan 2008).But these species can also survive at mean elevation of 40.25 m(±21.56)in JNF.F. roxburghii community experiences flush flood during the monsoon and this community mostly supports tree species thatcan survive waterlogged conditions(Ahmed and Hassan 2008;Sarker etal.2013b).Examples include F.roxburghii, Syzygium fruticosum and A.chaplasha.Soil acidity of the six communities usually increased with increasing elevation (Smith et al.2002).Differences in tree species composition were detectable along this pH gradient.Both pH andelevation can influence community composition by affecting species richness(Lobo 2001)as wellas ground-levelspecies composition and diversity(Stehn et al.2011).

Community diversity and compositional similarity

Alpha diversity patterns were associated with topographical variation.For example,tree alpha diversity was higheston the mid-hill to uphill sites(dominated by P.simiarum-D. grandiflora community)while diversity was lowest on the eroded foothillsites(dominated by T.singularis community) (Table 2).Apartfrom topographicalvariation,severalother factors(e.g.,dispersal,biotic interaction,and disturbance) mightbe responsibleforvariationsin diversity asistypicalin tropicalforest(Hubbell2001;Vellend 2010).

The abundance-based Morisita-Horn index,which is more sensitive to mostcommon species(Chao etal.2008), revealed thatcommunities ofthe shady moistsites(e.g.,K. calycina—C.tribuloides and P.simiarum—D.grandiflo ra) were compositionally most similar.In contrast,foothill communities(e.g.,T.singularis and F.roxburghii)were dissimilar.Nevertheless,we observed low averages in pairwise compositional similarity(Table 4),suggesting tree communities in the study area were highly diverse and require highest conservation priority.

Conservation implications

Classification of vegetation communities has become increasingly an important tool for conservation managementin the tropics(Whittaker 1962;Cowardin etal.1979; Pinto et al.2005).The results of this study have implications for the conservation and restoration of remnant naturalvegetation of Juri Forest.Our classification of the tree communities based on topography will enhance Bangladesh Forest Department’s ability to identify the most important sites for implementing future conservation works.

We recorded many national red-listed species(17) (Table 5 in‘‘Appendix’’section)(Bangladesh National Herbarium 2001)and these plant species were found throughout the forest.Of these four species(Alstonia scholaris,Intsia bijuga,Lophopetalum wightianum and Pterocarpus indicus)are globally threatened(IUCN 2013).Mid-hill and mid-hill to hill-top were the priority sites for conservation because they supported highly species-rich communities(e.g.,K.calycina–C.tribuloides and P.simiarum–D.grandiflora).These two communities together accounted for 85%of tree species and 65%of red listed tree species in the forest.In comparison,communities of the low elevation sites were comparatively species-poor.These sites have been subjected to frequent human disturbance(e.g.,grazing,fire,settlement,and illegal felling)and habitat alteration(Sarker et al.2013b). Hence,these sites require immediate restoration.Our information on the communities and sites can be used as reference for future management,restoration and conservation of JNF.

Conclusion

JNF is a conservation priority area in Bangladesh and houses numerous nationally and globally red-listed tree species.We identified six tree community types in JNF and they occupied distinct topographical locations.Tree diversity patterns within the communities were also associated with topographical variation.In terms of composition,communities of the shady moist sites were highly similar and foothill communities were dissimilar.Low overallaverage pair-wise compositionalsimilarity between the communities indicates that the tree communities were diverse.Communities of the mid-hill to uphill sites are priority sites for conservation because they support high diversity.Eroded foot hill sites with low diversity are subjected to frequent human and environmental perturbations and require immediate restoration.

Ecological information on tree communities and tree diversity patterns among and within communities are required for successful ecological restoration and conservation.Hence,the vegetation and environmental data collected,and the findings of this study can be used by conservation agencies(e.g.,Bangladesh Forest Department,IUCN)in implementing site specific conservation strategies for JNF.Above all,the numerical research approach could be a basis for vegetation mapping,monitoring and assessing site qualities.

AcknowledgmentsWe thank forestofficials and localexperts those helped during field data collection.Department of Forestry and Environmental Science,Shahjalal University of Science and Technology provided the logistic supportand the study was funded by the University Grant Commission(UGC),Bangladesh.

Appendix

See appendix Table 5.

Table 5 Name of the tree species with vernacular,scientific,family and genus name in six communities

Table 5 continued

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22 April 2013/Accepted:16 July 2013/Published online:9 January 2015

The online version is available athttp://www.springerlink.com

Corresponding editor:Yu Lei

Department of Forestry and Environmental Science,Shahjalal University of Science&Technology,Sylhet 3114,Bangladesh

e-mail:swapan_sust@yahoo.com