Thai J. For. 33 (3) : 109-130 (2014) วารสารวนศาสตร์ 31 (1) : 1-8 (2556)

Original article

Effects of Past Burning Frequency on Woody Plant Structure and Composition in Dry Dipterocarp Forest

Kobsak Wanthongchai1* Jürgen Bauhus2 Johann G. Goldammer3

1Department of Silviculture, Faculty of Forestry, Kasetsart University, Bangkok 10900, Thailand *CorrespondingAuthor,E-mail:fforksw@ku.ac.th2InstituteofSilviculture,UniversityofFreiburg,Tennenbacherstr4,79085Freiburg,Germany3GlobalFireMonitoringCentre,FireEcologyResearchGroup,c/oTheUniversityofFreiburg,Tennenbacherstr4,79085Freiburg,Germany

Received:Sep2,2014 Accepted:Nov22,2014

ABSTRACT

AnthropogenicburningindrydipterocarpforestshasbecomeacommonphenomenonthroughoutThailand.Toofrequentfiresmayaffectplantspeciescomposition,soilpropertiesandprocesses,andnutrientdynamics,therebyimpactinguponecosystemproductivityandsustainability.Conversely,completefireexclusion,mayresultinchangestoecosystemcomponents,andanincreasedriskofhigh-intensitywildfire.Thisstudyaimedtoinvestigatetheeffectsofdifferentburninghistories(7,2,1or0firesinthepast10years)andprescribedfireonsiteswithdifferentpastburningregimesonwoodyplantstructureandcompositioninadrydipterocarpforestatHuaiKhaKhaengWildlifeSanctuary,Thailand.Eachburninghistorycomprisedofthree30m×30mplotsforthemeasurementofoverstoreyvegetation.Four4m×4msubplotsweresetupwithintheplottoassesssaplingsandseedlings.Theseinvestigationsweremeasuredbothpre-burnandone-yearpost-burn.Thestudyrevealedthatthestructureofstandsthathadbeenfrequentlyburnedinthepastdifferedfromthatofthelessfrequentlyburnedstands.Moreover,thespeciescompositionofthesmall-sizedoverstoreyvegetationinthefrequentlyburnedstandsalsodifferedsignificantlyfromthatoftheotherstands.Thespeciescompositionoftheunburnedsitedifferedfromthatoftheotherburningregimesonlymodestly.Frequentburningalsoresultedinareductionofthesproutingcapacityofseedlingsandsaplings,inhibitedregenerationand,consequently,resultedin a disproportionate representation of individuals of certain tree species at the various stages of development.Therefore,fire-freeintervalsofatleast6-7yearsshouldbeintroducedtofacilitatethe successful recruitment of young trees.

Keywords: Burningfrequency,Drydipterocarpforest,HuaiKhaKhaengWildlifeSanctuary,Prescribedfire,Woodyplant

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INTRODUCTION

Inmanypartsoftheworld,firemaintainstheexistenceofforestecosystems(Pyne2001).However,theresponseofvegetationtofireishighlyvariable.Crownfirescankillmaturetrees and initiate a recovery process that may take several decades or even centuries to complete. In grassland communities, recovery fromfiremaybesorapidthattheeffectsmaydisappearwithin amatter ofmonths. Firebehaviour, fire duration, the pattern of fuelconsumption,andtheamountofsubsurfaceheating all influence injury andmortalityof plants, and tree population subsequentrecovery (Miller 2000). Plant species vary greatly in termsof theirfire resistance andrecovery mechanisms, namely, their sprouting capacity, life history, timing of reproduction and recruitment, evolution of fire-survivaltraitssuchasathickbark,andflammability(BondandWilgen1996). Dry dipterocarp forest (DDF) is a fire-dependent ecosystem (Sabhasri et al. 1968,Kutintara1975,Mueller-DomboisandGoldammer 1990). The DDFs have a long historyoffiredisturbance,derivedfrombothnatural and anthropogenic sources. Fires are lit annually for various reasons, such as, for gatheringnon-timberforestproducts(NTFPs)and to facilitate hunting (Akaakara 2000). BurningusuallyoccursbetweenJanuaryandAprilwhentheannualavailablesurfacefuel,i.e., leaf litter is greatest, due to the earlier leaf fall by the deciduous species found inthese forests (Figure 1). The desiccation of grasses also provides additional dry fuels. SomeDDFs in protected areas by contrasthavelongbeenunburned,asaresultoffireprevention activities.

From an ecological perspective, the reductioninfirefrequencymaycausestructuralchanges to DDF stands, including increased treedensity,canopycover,successiontowardsmorefire-sensitiveandshadetolerantoverstoreyspecies.However,toofrequentburnsmayresultinaconsiderablelossofnutrientsandplantspecies, resulting in ecosystem degradation (SukwongandDhamanityakul1977).Therefore,anappropriateburning frequency iscrucialfor the maintenance of ecosystem functions inDDF.Inrecentdecades,theeffectsoffireon the vegetation of DDF in Thailand have beeninvestigatedbyanumberofresearchers(SukwongandDhamanityakul1977,Sunyaarch1989, Kanjanavanit 1992, Wachrinrat 2000, Samran and Tongtan 2002, Himmapan 2004). However, theafore-mentionedstudieswereonlyconcernedwithsinglefireevents.Theeffectofburningfrequency,whichiscrucialfor plant species persistence, composition and structure, is poorly understood and needs to beassessedtogainabetterunderstandingoftheroleoffire,especiallyburningfrequency,onvegetationdynamicsinthisfire-dependentecosystem. Thispaperpresentsthewoodyplantstructure and compositions associatedwiththedifferentpastburningregimesinDDFatHuai Kha Khaeng Wildlife Sanctuary (HKK), Thailand. Also, this paper attempts to determine theeffectsofprescribedfiresonthewoodyplant structure and composition of stands with different burning frequencyhistories.Specifically,thehypothesestestedwerethat,withincreasingburningfrequencies,theratesofestablishmentofwoodyplantsdeclineandmortalityincreases,andthatfrequentburningreduces the species richness and diversity of woodyplantspeciesinDDF.

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Figure 1 The dry dipterocarp forest in rainy season and dry season.

Figure 2 Location of the experimental study area in dry dipterocarp forest, Huai Kha Khaeng Wildlife

Sanctuary, Thailand.

Figure 1 The dry dipterocarp forest in rainy season and dry season.

Figure 2 Location of the experimental study area in dry dipterocarp forest, Huai Kha Khaeng Wildlife

Sanctuary, Thailand.

Figure 1 The dry dipterocarp forest in rainy season and dry season.

MATERIALS AND METHODS

Study area ThestudywasconductedintheHKK,locatedapproximately350kmnorthwestofBangkok inThailand.The study sitewaslocated in DDF, in the northeast region of the sanctuary (Figure 2). The region has a tropicalmonsoonalclimatewithdryseasonextendingfromNovembertoApril,andawetseason fromMay toOctober.The averageannual rainfall is 1348mmyear-1.Themeandaily temperaturevaries from23.2˚CinDecember to30.9˚C inApril.Themean

relativehumidityis89.1%,whichvariesfrom79%inAprilto93%inSeptember(ForestFireResearchCenter2006).Theelevationofthestudyarearangesfrom300-400ma.s.l.TheDDFisdominatedbydeciduoustreespeciesbelongingtotheDipterocarpaceae,suchas,Shorea obtusa, S. siamensis, Dipterocarpus tuberculatus, and to the Leguminosae, such as Xylia kerrii and Sindora siamensis. The understorey consists mainly of a grass layer dominated byHeteropogon contortus and Imperata cylindricaaswellasothershrubs,herbs,andsaplingsandseedlingsofoverstoreytree species (Forest Research Center 1997).

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Figure 1 The dry dipterocarp forest in rainy season and dry season.

Figure 2 Location of the experimental study area in dry dipterocarp forest, Huai Kha Khaeng Wildlife

Sanctuary, Thailand.

Figure 2Locationoftheexperimentalstudyareaindrydipterocarpforest,HuaiKhaKhaengWildlife Sanctuary, Thailand..

Fire history reconstruction ThepastburninghistorywasreconstructedforthestudyareabymappingburnedareasusingfirereportsfromtheNationalParks,WildlifeandPlant Conservation Department, and a series of satellite images spanning a 10-yr period from 1995to2004.Asaresult,4differentburninghistorieswereidentified.Theseincludedfirefrequenciesof7,2,1and0firesoverthepast10years,whicharesubsequentlyreferredtoasfrequent(FB),infrequent(IB),rare(RB)andunburned(CB),respectively.Thetimessincethelastfireinrelationtotheseburningregimeswere1,3,7,and10years,forFB,IB, RB, and CB, respectively. For experimental purposes, threereplicate plotswere established for eachburningregime(treatment).Itwasnotpossibletoreplicatepatchesofthesamefirefrequencyand time since last fire in the landscape,

so that treatmentswere not interspersed inspace. Therefore, the plots represent pseudo-replications(Hurlbert1984)ofthesamefireevent and history. Within each treatment, plots wereapproximately300-500mapart.Withinthemappedareasofdifferentfirefrequencies,allplotswereselectedtorepresentthesamegeologic parent material, soil unit, and forest type.Theywerealsolocatedatsimilarelevationand topography.

Burning protocol Threeexperimentalplots50m×50mwere set up in each treatment.These plotscontainedaninnerplotof30m×30mfordetailedmonitoring,anda10-mwidebufferstripthatpermittedmeasurements tobemadeacrosstheentireinnerplotwithoutedgeeffects.Firelineswereestablishedaroundthemarginsofeach50m×50mplot.Withtheexceptionofthe

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unburnedarea(CB),experimentalburningwascarried out in the nine plots during the period from 28thDecember2004to9thJanuary2005.Three-strip burning techniques (each stripapproximately16mwide)wereappliedfortheexperimentalburning.Theexperimental

burnswerelow-intensitysurfacefires,withflamelengthsofabout1.5m.Therateoffirespreadrangedfrom1.3-2.6mmin-1,whereasthefrontalfirelineintensitywasgenerallylessthan500kWm-1(Table1)(Wanthongchaiet al. 2008).

Table 1 Quantitativefirebehaviourcharacteristicsobservedfortheexperimentalfiresatsitesofdifferentfirefrequencyplots;frequentlyburnedsite(FB),infrequentlyburned(IB),andrarelyburned(RB).

Fire characteristicsFire frequency plots 1

RB IB FBRate of spread

(m min-1)1.3 a(0.2)

2.6b(0.3)

2.7b(1.0)

Flame length(m)

1.27 a(0.11)

1.53 a(0.09)

1.51 a(0.22)

Fireline intensity(kW m-1)

291 a(43)

467 a(62)

361 a(150)

Remarks: 1Different letters (a, b) indicate significant differences (ANOVA, P

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Data analysis 1. Vegetation structure Treemortalitywasexaminedoneyearafterthefire.Thecrownprojectionarea,cover,basalareaandstemdensitywerecomparedbetweentheburningregimes,andforpre-burnand post-burn.An estimated crown coverandleafareaindex(LAI)oftheoverstoreywasanalysedusing theHemiviewsoftware(Hemiview2.1,Delta-TDevicesLtd.,Burwell,Cambridge,UK). Therelativeabundanceoftreeswascalculatedfromstandbasalarea.Theimportancevalueindex(IVI)wasalsoevaluated.TheIVIintegrates three important phyto-sociological measures,namely,density,frequencyandbasalareaofspeciesabundance.Logisticregressionprocedureswere employed to evaluate theprobabilityoftreemortalityforsomeimportanttreespeciesasfunctionsoffirefrequencyandtreedbh,usingJMPINsoftware,version4.0.4(SAS Institute Inc 2001). A log-likelihood statisticwasusedtotestthenullhypothesisthatfirefrequencyandtreedbhdidnotaffecttheprobabilityoftreemortality.Waldstatisticswereusedtotestthestatisticalsignificanceofindividuallinearpredictors.PredictorvariableswereconsideredsignificantifP

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2005).Allofvariableswerefirst tested forhomogeneityofvariance(Levene’stest)andnormaldistribution(Kolmogorov-Smirnov),tomeettherequirementsofANOVA.Wheretheserequirementswerenotmet,thedataweretransformed using either common logarithm orsquarerootforms.Statisticallysignificantdifferences between past burning regimes(P

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Table 2 Overstorey tree characteristics of each burning regime; unburned (CB), frequentlyburned (FB), infrequentlyburned (IB) and rarelyburned (RB).Standard errors aregiven in parentheses.

Burning regime1P- value

FB IB RB CBTree density(indiv. ha-1)

667a

(54.7)1344b

(129.0)1311b

(108.2)1558b

(113.0)0.000

dbh(cm)

14.0(1.5)

11.1(0.6)

11.4(0.5)

11.6(0.5)

0.092

Basal area(m2 ha-1)

12.9a

(2.1)17.2ab

(1.4)19.4b

(1.4)23.6c

(1.7)0.000

Crownarea(m2 ha-1)

8221.7a

(1827.0)9984.4ab

(783.8)12590.1b

(965.2)16505.8c

(1289.5)0.000

Remarks: 1Different letters (a, b, c) indicate significant differences (ANOVA, P

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Table 3 Relative density (RD), relative dominance (RDo), relative frequency (RF) andimportancevalue index(IVI)of5 important treesspecieson the frequentlyburnedsite(FB),infrequentlyburned(IB),rarelyburned(RB),andunburned(CB)inthedrydipterocarp forest, Huai Kha Khaeng Wildlife Sanctuary.

Burning regime

No. Species RD (%) RDo (%) RF (%) IVI

FB 1 Shorea siamensisMiq. 43.65 54.99 15.00 113.642 Gardinia obtusifoliaRoxb. 7.18 1.85 11.25 20.283 Sindora siamensisTeijsm.ex.Miq. 5.52 5.81 6.25 17.594 Antidesma ghasembilla Gaertn. 7.18 2.38 7.50 17.065 Xylia xylocarpaTaub. 4.97 2.88 8.75 16.60

IB 1 Shorea obtusa Wall. 26.72 27.31 10.08 64.112 Shorea siamensisMiq. 13.50 21.75 8.40 43.653 Aporosa ficifolia Baill. 14.60 9.23 9.24 33.084 Dipterocarpus tuberculatusRoxb. 6.34 15.08 3.36 24.785 Buchanania latifoliaRoxb. 6.06 5.03 6.72 17.82

RB 1 Shorea obtusa Wall. 27.12 28.41 9.02 64.552 Shorea siamensisMiq. 15.25 13.98 6.02 35.253 Dipterocarpus tuberculatusRoxb. 5.93 11.64 3.76 21.334 Aporosa ficifolia Baill. 5.37 5.52 6.02 16.905 Dalbergia cultrataGrah.ExBenth. 3.39 3.90 4.51 11.80

CB 1 Shorea obtusa Wall. 31.73 32.90 7.73 72.362 Shorea siamensisMiq. 16.58 28.27 7.73 52.573 Xylia xylocarpaTaub. 6.24 4.01 5.80 16.044 Canarium subulatum Guill. 3.03 6.70 5.31 15.045 Buchanania latifoliaRoxb. 3.03 3.91 4.35 11.29

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Table 4 Characteristics of sapling populations in plots of different past burning regimes(unburned(CB),frequentlyburned(FB),infrequentlyburned(IB)andrarelyburned(RB)),beforeandafterexperimentalburning.Standarderrorsaregiveninparentheses.

Burning regime1

FB IB RB CBBefore After Before After Before After Before After

Density (indiv. ha-1)

573Aa(210)

833Aa(247)

2969Ab(746)

1927Aa(740)

3385Ab(862)

1875Ba(576)

4570Ab(705)

3906Ab(796)

D0 (mm)

25.1Aa(2.6)

25.5Aa(2.0)

35.1Ab(2.1)

37.0Aa(5.3)

38.2Ab(3.7)

32.1Aa(3.3)

31.3Aab(1.6)

34.1Aa(2.0)

Ht (m)

1.6Aa(0.2)

1.7Aa(0.1)

2.2Aab(0.2)

2.3Aa(0.3)

3.1Ac(0.3)

3.0Aa(0.4)

2.6Abc(0.2)

2.4Aa(0.2)

Basal area (m2 ha-1)

0.6Aa(0.3)

0.7Aa(0.3)

3.3Ab(0.9)

1.9Aa(0.4)

4.3Ab(0.9)

2.2Ba(0.7)

4.0Ab(0.6)

4.0Bb(0.9)

Cover(%)

4.3Aa(1.6)

7.2Aa(2.2)

20.2Aab(4.6)

11.6Aa(3.5)

45.1Abc(13.5)

22.3Aa(7.8)

61.4Ac(16.0)

63.3Ab(26.9)

Remark: 1 Identical lower case letters indicate no significant difference between treatmentswithin asampling period (ANOVA, P

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Figure 4 Diameter distribution (mean ± SE) of the tree (a) and diameter at ground level (d0) of young

trees (seedlings and saplings) (b) in the different past burning regimes.

0

20

40

60

80

100

120

4.5-10 10.1-15 15.1-20 20.1-25 25.1-30 30.1-35 35.1-40 >40

Num

ber o

f tre

es

dbh class (cm)

(a) Frequently burnedInfrequently burned

Rarely burned

Unburned

020406080

100120140160180200

>10 10.1-2020.1-3030.1-4040.1-5050.1-6060.1-7070.1-8080.1-90

Num

ber o

f ind

ivid

uals

d0 class (mm)

(b) Frequently burnedInfrequently burned

Rarely burned

Unburned

Figure 4Diameterdistribution(mean±SE)ofthetree(a)anddiameteratgroundlevel(d0)ofyoungtrees(seedlingsandsaplings)(b)inthedifferentpastburningregimes.

Effects of different past burning regimes on woody plant composition A total of 54 overstorey specieswas found in thisstudy(Table6),butonly25specieswerefoundontheFBsite,withthe number of species increasing to 33, 43and51speciesat theIB,RBandCBsites,respectively. The H′value,however,didnotdiffersignificantlybetweenthepastburningregimes. The MRPP revealed that the overstorey

speciescompositiondiddifferbetweenthepastburningregimes(A=0.32,P0.3,P40, P

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Forexample,S. obtusa, Canarium subulatum, Xylia xylocarpa and Aposula villosaweremostabundantontheCBsite.Bycontrast,the overstorey species found at all burningregimes (observed IV0.05)wereS. siamensis, Pterocarpus macrocarpus, Grewia eriocarpa, Terminalia alata, Dillenia spp. and Lannea coromandelica(Table8). A total of 37different specieswasrecordedassaplingspriortoburning.Only7ofthesewerefoundontheFBsite,whilethenumberofspeciespresentassaplingsonthe

IB,RBandCBsiteswas15,16and16species,respectively.Likewise,ofthetotalnumberof66speciespresentamongsttheseedlingspriortoburning,30wererecordedontheFBsite,whereas37,39,and42specieswerefoundon the IB, RB and CB sites, respectively. The pre-burnH′ofthesaplingswaslowestontheFBsite(0.6)andhighestontheRBsite(1.9)(P

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No. Species Family FB IB RB CB

24 Erythrophleum succirubrum Ganep. Caesalpiniaceae

25 Flacourtia indica (Burm.f.) Merr. Flacourtiaceae

26 Garcinia spp. Guttiferae

27 Gardinia obtusifoliaRoxb. Rubiaceae

28 Grewia eriocarpa Juss. Tiliaceae

29 Haldina cordifolia (Roxb.)Ridsdale Rubiaceae

30 Heterophragma sulfureumKurz Bignoniaceae

31 Hymenodictyon excelcum Wall. Rubiaceae

32 Lannea coromandelica Merr. Anacardiaceae

33 Mammea harmandii Kosterm. Guttiferae

34 Melanorrhoea usitata Wall. Anacardiaceae

35 Millettia leucanthaKurz Papilionaceae

36 Morinda coreia Ham. Rubiaceae

37 Ochna integerrima Merr. Ochnaceae

38 Pavetta tomentosa Roxb.ex.Sm. Rubiaceae

39 Phyllanthus embrica Linn. Euphorbiaceae

40 Pterocarpus macrocarpusKurz Papilionaceae

41 Schleichera oleosa (Lour.) Oken Sapindaceae

42 Shorea obtusa Wall. Dipterocarpaceae

43 Shorea roxburghii G. Don Dipterocarpaceae

44 Shorea siamensisMiq. Dipterocarpaceae

45 Sindora siamensisTeijsm.ex.Miq. Caesalpiniaceae

46 Spondias pinnata (L.f.)Kurz Anacardiaceae

47 Strychnos nux-vomica L. Strychnaceae

48 Terminalia alataHeyneexRoth Combretaceae

49 Terminalia bellericaRoxb. Combretaceae

50 Terminalia chebulaRetz. Combretaceae

51 Terminalia corticosaPierreexLaness. Combretaceae

52 Vitex glabrata R. Br. Verbenaceae

53 Vitex limonifolia Wall. Verbenaceae

54 Xylia xylocarpaTaub. Mimosaceae

Table 6 (Cont.)

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Table 7 Summary statistics for the multi-response permutation procedure (Sørensen distances) of the overstorey species compositions associated with the different past burningregimes;unburned(CB),frequentlyburned(FB),infrequentlyburned(IB)andrarelyburned(RB).Theresultsprovideacomparisonbetweenallburningregimes,aswellasmultiplepairwisecomparisonsoftheSørensendistances.

Sørensen distanceδ under null hypothesis

T P-value AObserved δ Expected δ Variance Skewness

Overall comparison 0.341 0.500 0.0020 -0.73 -3.55 0.003 0.318Multiple comparisons

FB vs IB 0.310 0.500 0.0051 -1.74 -2.67 0.024 0.381FB vs RB 0.333 0.500 0.0033 -2.33 -2.90 0.022 0.333FB vs CB 0.281 0.500 0.0039 -1.83 -3.49 0.010 0.439IB vs RB 0.532 0.500 0.0010 -0.39 1.03 0.850 -0.063IB vs CB 0.491 0.500 0.0014 -0.87 -0.23 0.357 0.017RB vs CB 0.481 0.500 0.0009 -0.17 -0.64 0.255 0.037

Table 8 TheMonteCarlotestofsignificanceofobservedmaximumindicatorvalues(IV) for certainselectedoverstoreyspecies(mostcommonspeciesandthespeciesmoreabun-dantunderspecificburningregimes),basedon1000randomisations.Themeansandstandard deviations of the IVfromtherandomisationsaregivenalongwithP-values forthehypothesisthattherewerenodifferencesthebetweenpastburningregimes.

Burning Regime Species

1Observed indicator value (IV)

IV from randomised groupsMean Std. dev. P-value

FB Antidesma ghasembilla * 65.8 40.9 14.18 0.048RB Dipterocarpus tuberculatus * 65.8 36.8 11.86 0.022CB Xylia xylocarpa* 65.2 35.6 12.48 0.037CB Canarium subulatum * 65.2 32.9 14.55 0.05CB Aporosa villosa * 65.2 38.5 12.95 0.02RB Erythrophleum succirubrum * 61.4 37.1 10.14 0.019CB Shorea obtusa * 40.4 34.2 4.35 0.048

Terminalia corticosa ** 31.5 48 18.25 0.775Shorea siamensis ** 31.4 31.7 2.26 0.537Grewia eriocarpa ** 31.3 35 17.96 0.576Lannea coromandelica ** 26.5 39.3 13.21 0.863Morinda coreia ** 23.8 35.3 10.76 0.875Flacourtia indica ** 21.4 31.6 15.95 0.85Pterocarpus macrocarpus ** 21.4 36.6 15.1 0.844Terminalia chebula ** 19.6 34.2 17.28 0.828Dillenia spp. ** 19.6 34.2 17.28 0.828Terminalia alata ** 16.7 31.9 15.43 0.851Strychnos nux-vomica ** 16.7 35.5 15.94 0.908

Remarks: 1 *Speciesspecificallyassociatedwithaparticularpastburningregime(P0.05)

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Effects of prescribed burning on woody plant mortality ItwasnotclearwhetherprescribedburningcausedtreemortalityinDDF.Mostofthedeadtreeswereinthelowerdbhclasses,irrespectiveoftheburningregime.MostofthedeadtreeswereeitherS. obtusa, S. siamensis or A. ficifolia.Therelativetreemortalitywas4,5,14and5%ontheFB,IB,RBandCBsites,respectively,thoughitwasnotsignificantbetweenpastburningregimes.Thelogisticregressionanalysisrevealedthatalthoughtheprobabilityoftreemortalityincreasedwithdecreasingdbh(P

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Effects of past burning regime on woody plant structure The results revealed that the overstorey structurewasclearlyloweronthefrequentlyburned site than thaton the less frequentlyburnedsites.TheoverstoreyspeciescompositionoftheFBsitealsodifferedsignificantlyfromthat of the others (IB, RB and CB). The species diversity (H′),ontheotherhand,exhibitednodifferencesbetweenthepastburningregimes.Thedifferencesinthestockingdensitiesbetweenthesitesofthedifferentpastburningregimesweremainlyduetovariationsinthenumberofsmalltrees,supportedbythewellknownfact that small trees are more readily killed byfirethanthelargeones.Thisimpliesthatthe establishment and thegrowthofyoungtrees into the higher diameter classes (i.e., progressing from sapling to tree stage) is a more continuous process during the longer fire-free intervalson lessfrequentlyburnedsites. Watson and Wardell-Johnson (2004) reported that, although species richness did not differ significantlywith either the timesincefireorthefirefrequency,bothofthesefactors affected community structure and composition in open forest andwoodlandvegetationinGirraweenNationalPark,South-East Queensland, Australia. The densities of saplings and seedlings were loweron theFBsite thanon the lessfrequently burned sites, an observationsimilar to that of Peterson and Reich (2001). ThisalsocorrespondedwiththefindingsofWachrinrat(2000),whoreportednumbersaslowas203saplingsha-1and3140seedlingsha-1 inDDFwherefireprotectionactivitieswere not conducted.By contrast, inDDFwherefiremanagementwasimplemented,theauthor recorded as many as 1292 saplings ha-1

and 9110 seedlings ha-1.The lower saplingand seedling densities on the FB site might havebeendueto thefact thatfireoccurredalmost every year, limiting the regeneration andsubsequentdevelopmentofseedlingstosaplingstoaveryshortfire-freeperiod(Sukwong1982).Thedevelopmentoffireresistanceiscloselylinkedtoheightanddiametergrowth.Thegrowth ratesof seedlings and saplingsinDDF-HKKwere studied extensivelybyHimmapan(2004),whocalculatedtheaveragediametergrowthofallspeciestobe1.1cmyr-1 for saplings and 1.2 cm yr-1forseedlings,withheightgrowthof65cmyr-1 for saplings and 38cmyr-1 for seedlings. Based on the height growthcalculatedbyHimmapan(2004)andtheflameheightobservedinthisstudy(about1.2-1.5m),theseedlingsmayrequireafire-freeintervalofabout5-6yearsbeforetheircrownsliftabovetheaverageflameheightsso they can develop into saplings. Theseedlingcoveragewassignificantlyhigher on the FB site than on the IB and RB sites.Thiscanbeattributed to the fact thatmorethan53%oftheseedlingsand80%ofthe saplings on the FB site had more than one main stem. The corresponding values on the IBsitewereonly18%and20%,respectively,and10%and7%ontheRBsite.

Absence of woody plant species as a result of different burning regime A disparity in the proportions of S. siamensis and S. obtusatrees,twoofthemostdominant species in DDF, in the different stages ofdevelopment,wasobserved.Thenumberofsaplings of these dominant species increased withthelengthofthefire-freeinterval(Figure6).Thissuggeststhatthesetwospeciesmayrequireafire-freeintervalofbetween4to7

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years for seedlings to reach the sapling stage, aswasthecaseontheRBsite.Thisinferenceis reinforcedby thefindingsof a studybySukwong(1982),whoreportedthatS.obtusaseedlings required afire-free interval of atleast7yearstoattainasizethatenablesthemtowithstandfiredamage.Ashorterfire-freeinterval, as observedon theFB site, is notsufficientlylongforthedevelopmentofthesespecies. Even though dominant on all sites, the disproportionate representation of these species amongst the various stages of development on the FB site may impact negatively upon their long-term development in the landscape. The species composition of the overstorey on the frequently burned sitesdifferedsignificantlyfromthatoftheothers(IB,RBandCB).Thisdifferencewascausedby the absence of certain species from theFBsite.Forexample,Diospyros ehretioides, Vitex limonifolia, Aporosa villosa, Schleichera oleosa and Buchanania latifoliawere notobservedontheFBsite,yettheywerepresenton the other three, resulting in the differing speciescompositions.TheabsenceofsomespeciesontheFBsiteraisedthequestionofcomparabilitybetweenpastburningregimes.Himmapan (2004) stated that the average diameter increment across all species in the DDF-HKKwasabout0.4cmyr-1. Therefore, overstoreytreeswithdbhover15cmcanbeconsideredoldtrees,andhaveprobablybeenalive for at least 40 years. The analysis of the species composition using MRPP revealed that the overstorey oldtree(dbh>15cm)speciescompositionbroadlyoverlappedbetweenthepastburningregimes (A=0.14, P=0.051).This suggeststhat differences in the species composition ofthevariouspastburningregimeswasdue

todifferencesinthenumberofyoungmaturetrees (dbh< 15 cm).Moreover, this alsoreveals that the species composition of sites wassimilarhistorically,andwas,therefore,comparable.The differences in the youngmature tree composition, therefore, may have beeninfluencedbythefrequencyofburning.Consequently,itisunlikelythatthelossoftheafore-mentionedspecieswasbroughtaboutby site differences.However, the absenceof these specieswas generally observed inDDFs throughout Thailand (Sunyaarch 1989, SahunaluandDhanmanonda1995,Wachrinrat2000, Samran and Tongtan 2002, Himmapan 2004).Thesespecieshavefireadaptivetraits,suchas,athickbarkandtheabilitytosprout(Kaitpraneet1987,RundelandBoonpragob1995), andmaybe consideredfire-tolerantspecies.Itispossible,however,thattheycannotwithstandfrequentburning,suchasontheFBsite,becausetheregenerationprocessistooseverelyimpeded.Thesespeciesmayalsobelesslight-demanding,andpossiblyfoundtheconditions under the relatively open canopy ontheFBsiteunsuitable.Thelesserdegreesof canopy openness on the other sites (IB, RB, and CB), on the other hand, may have served to favour less light-demanding species. However, theselessfrequentlyburnedsiteswerestillbrightenoughthatlight-demandingspecies could occur. S. siamensisappearstohavebeenanimportantoverstoreyspeciesinthefrequentlyburnedsites,asthisspecieswasalsoreportedtobethemostdominanttreebyHimmapan(2004),whostudiedfrequentlyburnedforestsites in the HKK. Gardinia obtusifolia, Sindora siamensis and Antidesma ghasembillawerealsodominantontheFBsite.Onlessfrequentlyburnedsites,however,S. obtusawasthemost

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abundantspecies,withS. siamensis, A. ficifolia and D. tuberculatus also dominant on the IB and RB sites. These species have adaptation traits suchasthickbarks,andre-sproutingcapacityfollowingdamagetothestem.Inaddition,the

fruits of Shorea spp. and Dipterocarpus spp. aredispersedbywindafterthepeakofthefireseason,andgerminateatthebeginningofthewetseason(Sukwonget al.1975).

Figure 6 Diameter distributions of selected tree species in the different burning regimes. Frequently burned, Infrequently burned, Rarely burned, Unburned.

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Effects of prescribed burning on woody plant Therewasnoevidencetoshowthatprescribedburningcaused treemortality inDDF.Indeed,thetreemortalityrateobservedontheunburned(CB)sitewassimilartothatontheburnedareas.Thelowincidenceoffire-relatedmortalitycanbeattributedtothickbark,whichreducesheateffectsonthecambium;thelow-intensitysurfacefires(approximately290-470 kW m-1)andlowflamelengthneverreached the tree crowns (flame lengthwasonly1.2-1.5m).Whelan(1995)suggestedthatmortalityafterfireisnotalwaysanindicationthatthefirehashadasignificantimpactonthecommunityasthedeadtreesmayhavebeensick,weakened,oldorinjured,andmayhavebeendestinedtodieevenwithoutthefire.Itispossiblethatthemortalitythatsetinafterthelow-intensitysurfacefireswasconfinedtosuppressedtrees,whichdevelopfireresistancecharacteristicsmuchmoreslowlythanvigoroustreesofthesameageandspecies.Exceptionaldrought conditions in the year 2004 may also haveinfluencedtreemortality,particularlythatof small trees. Total rainfall in the year 2004 wasonly996.4mm,comparedto1332.9mmand1453.7mmintheyears2003and2005,respectively (Forest Fire Research Center 2006).However,themortalityofsmalltreesasaresultoffrequentburningsuggeststhatifthis trend continues, future stand development and,consequently,theproductivityofthesespecies,couldbejeopardisedbecauseofthereduction of the regenerative capacity of the forest (Guinto et al. 1999). ItappearedthatburningresultedinanincreaseinthenumberofsaplingsontheFBsite,whereasfirelikelyreducedthenumberof saplings at the other sites.Thismaybeattributedtothefactthatthestemsofseedlings

maynotbeseriouslyaffectedbyfireandthattheycontinuetogrow,orthatwherethestemisdamagedanewshootcansprout rapidlyandreachthesaplingstage(>1.3mheight)inthefollowingyear.ExceptfortheRBsite,thefactthattherewasgreatvariationinthesaplingdensityatthesubplotlevelafterthefire,characterisedbybothincreasesanddecreases,meant that a statistical analysis (paired t-test) revealed no significant differences to thepre-burnsituation.Increasesintheseedlingdensitymaybe attributed to the improvedseedbedconditionsafterfire.Firestimulatesseed germination, and improves nutrient availability.ThisresultcorrespondswiththefindingsofSunyaarch(1989)andHimmapan(2004). A reduction in re-sprouting capacity in thefrequentlyburnedsite(Figure5)maybeattributedtobetterrootsystemdevelopmentand nutrient storage in the roots on the less frequently burned sites.Even thoughmostwoodyplants inDDFcan sprout after theabove-groundpartoftheplanthasbeeneitherkilledordamagedbyfire(Sukwonget al.1975,Kaitpraneet 1987, Stott et al. 1990, Rundel andBoonpragob1995),thecapacitytosproutdeclineswhenburningistoofrequent,asthenutrient and starch reserves stored in the root systemcannotbereplenished.Bycontrast,thelongerfire-freeintervalatthelessfrequentlyburned sites allowed for a recovery of thesproutingcapacity.Therefore,frequentburningmay jeopardise long-term tree population dynamics and, hence, the productivity of DDF.

CONCLUSION

This study revealed that the structure ofstandsthathadbeenfrequentlyburnedinthepastdifferedfromthatofthelessfrequentlyburnedstands.Moreover,thespeciescomposition

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of the small-sizedoverstoreyvegetation inthe frequently burned stands also differedsignificantly from that of the other stands.The species composition of the unburnedsitediffered from thatof theotherburningregimes onlymodestly. Frequent burningalso resulted in a reduction of the sprouting capacityofseedlingsandsaplings,inhibitedregenerationand,consequently,resultedinadisproportionate representation of individuals of certain tree species at the various stages of development.Therefore, fire-free intervalsofatleast6-7yearsshouldbeintroducedtofacilitate the successful recruitment of young trees.

ACKNOWLEDGEMENTS

TheauthorswishtothanktheNationalParks, Wildlife and Plant Conservation Department in Thailand for the permission to conduct thisstudyinHKK.Wewouldliketothankthe Geo-Informatics and Space Technology Development Agency (GISDA) in Thailand for the provision of satellite images. Special thanks to the Faculty of Forestry, Kasetsart University, Thailand for providing a vehicle andequipment.Wealsowishtothankthosestudentswho assisted in thefieldwork.WeexpressourgratitudetoNiphonSangaunyart,Soodchai Visuthipanit, Kraisorn Wiriya, and all their staff for the invaluable assistancethroughout thefieldwork.Many thanks toAlexanderHeld,formerstaffofGlobalFireMonitoring Center (GFMC), for technical assistance.Thisstudywaspartlysupportedby the InternationalPhDProgramme (IPP)at the Faculty of Forest and Environmental Sciences,UniversityofFreiburg,Germany.

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