Root Tensile Force and Resistance of Several Tree and

Croat. j. for. eng. 39(2018)2 255

Originalscientificpaper

Root Tensile Force and Resistance

of Several Tree and Shrub Species of Hyrcanian Forest, Iran

Ehsan Abdi

Abstract

Shallow landslides are a frequently recurring problem in some parts of Iran, including the Hyrcanian forest. In addition to traditional civil engineering measures, a potential solution for this problem is the application of soil bioengineering techniques. The mechanical reinforce-ment effect of plant roots is one of the major contributions of vegetation to the mitigation of shallow landslides. Given the lack of information on the mechanical properties of common Hyrcanian forest species, the present study assessed the root strength of 10 common species of this forest. Eight tree species occurring in natural regeneration sites (Carpinusbetulus, Fagusorientalis, Parrotiapersica and Quercuscastaneifolia) and plantations (Acer ve-lutinum, Alnusglutinosa, Fraxinusexcelsior and Piceaabies) and two shrub species (CrataegusmicrophyllaandMespilusgermanica) were selected. Fresh roots were col-lected and mechanical tests were carried out on 487 root samples. The ranges of root diameter, tensile force, and root resistance were 0.29–5.90 mm, 3.80–487.20 N, and 2.41–224.35 MPa, respectively. Two different algorithms, including the nonlinear least square method and log-transformation, were used to obtain power regressions for diameter-force and diameter-resis-tance relationships. The results of the two algorithms were compared statistically to choose the optimal approach for soil bioengineering applications. The nonlinear least square method resulted in lower Akaike information criteria and higher adjusted R2 values for all species, which means that this model can more efficiently predict tensile force and resistance based on root diameter. Log-transformation regressions generally underestimate tensile force and re-sistance. Significant differences were found among mean root tensile force (ANCOVA; F=37.36,p<0.001) and resistance (ANCOVA; F=34.87, p<0.001) of different species. Also, root diameter was significant as a covariate factor in tensile force (F=1453.77, p<0. 001) and resistance (F=274.26, p<0.001). Shrub species and trees in natural regeneration sites had higher tensile force and resistance values, while trees from plantation stands had lower values. The results of this study contribute to the knowledge on the root force and resistance charac-teristics of several shrub and tree species of the Hyrcanian forest and can be used in evaluating the efficiency of different species for bioengineering purposes.

Keywords: landslides, log-transformation, nonlinear least square, power regression, soil bio-engineering, stability

slope-formingmaterials,withgravityandwaterastheprimarytriggers(Stokesetal.2014).IntheHyrcanianforest,landslidesposeaseverethreattoaccessinfra-structure,includingforestroads,resultinginsevereeconomicalconsequences.Amongthedifferentland-slide triggeringmechanisms in this forestenviron-ment,rainfall-induced(Abedietal.2010)andhuman-induced(SavadkoohiandHosseini2013)landslides

1. IntroductionShallowlandslidesareafrequentlyrecurringprob-

leminsomepartsofIran,includingtheHyrcanianforest.InIran,thetotalestimatedlossesasaresultoflandslidesareabout$50millionperyear(Ebrahimietal.2015).Landslidesaredefinedasprocessesthatre-sult in the downward and outwardmovement of

Ehsan Abdi Root Tensile Force and Resistance of Several Tree and Shrub Species of Hyrcanian Forest, Iran (255–270)

256 Croat. j. for. eng. 39(2018)2

are themost frequently reported.Human-inducedlandslidesusuallyoccurafterdisturbancessuchasroadconstruction,modifyingtheshapeoftheslope,removingvegetationcover,anddecreasingboththedensityandresistanceofroots(Verganietal.2016);suchprocessesmightalsocontributetotheconcentra-tionofflowandincreasepore-waterpressure,result-inginlocalinstabilities(Schwarzetal.2010b).Incon-trast, rainfall-induced landslides are generallytriggeredbyhigh-intensityrainfall,whichcausesasuddenincrease insoilmoisturecontentandade-creaseinsoilsuction,leadingtosoilstrengthreductionandpossiblefailure(Cislaghietal.2017,Hayatietal.2017).Ingeneral,highsoilmoisturecontents(orlowsuction)leadtoweakerapparentsoilcohesionandhigherlandsliderisks(Stokesetal.2014).Traditionalcivilengineeringmeasures(i.e.grey

solutions)withhighinitialcostsandincreasingmain-tenanceneedsover timeareunsuitable in the longterm(MorganandRickson1995),especiallyinnaturalresourceswithextensiveareas.Potentialsolutionsforthis problem are soil bioengineering measures orgreensolutions,whicharecharacterizedbytheuseofanyformofvegetation(grass,shrubs,ortrees)asma-terialstoperformengineeringfunctions(MorganandRickson1995)suchassoilreinforcement,erosioncon-trol,andthepreventionofshallowinstability.Inre-centyears,usingplantsforsoilbioengineeringmea-sureshasbeenrecognizedasaneco-friendlyandlowCO2emissionsolutionforsoilstabilization,ascom-paredtotheexistingtraditionalgreyor»hard«engi-neeringsolutions(Boldrinetal.2017).Plantsareself-regeneratingandresponddynamicallytochangesofsiteconditionswithoutlosingtheirengineeringprop-erties(MorganandRickson1995);inaddition,theycanimprovethestabilityofhillslopesandtheecolog-icalconditions(Bischettietal.2010).Vegetationcanincreaseslopestabilitybyprotect-

ingandholdingsoilparticlestogether,mechanicallyreinforcingthesoilandincreasingsoilmatricsuction(Capillerietal.2016)throughbothinterceptionofrain-fallanddepletionofsoilwatercontentviatranspira-tion(Hayatietal.2017).Thisisparticularlythecaseforforestenvironments,wheremechanicalandhydro-logicalmodificationsbytreesenhancethestabilityofhillslopes(Moosetal.2016).Themechanicaleffectofrootsystemshasbeenrecognizedasoneofthemajorcontributionsofvegetationtothemitigationofshal-lowlandslides(Verganietal.2016).Someresearchershavereportedanegativecorrelationbetweenthemag-nitudeofrootreinforcementandlandslidesusceptibil-ity(Hubbleetal.2013,Roeringetal.2003,Schmidtetal.2001,Moosetal.2016),and it isnowclear that

plantspositivelyinfluencethetriggeringmechanismsviarootstrength,hydrologicalregulation,androotanchorage(Cislaghietal.2017).Thickrootsactlikesoilnailsonslopes,reinforcingsoilinthesamewaythatsteelreinforcesconcrete.Thinandfinerootsactinten-sionduringfailureonslopes;iftheycrosstheslipsur-face,theyreinforcethesoilbyaddingadditionalcohe-sion to the soil cohesion (Stokes et al. 2014). Theefficiencyofrootsinreinforcingsoildependsonrootstrengthresistance,rootdistribution,andmorpholo-gy.Thegreaterthestrengthandthewiderthedistribu-tion,thebettertheplantswillreinforcethesoil(Stokes2002).Withincreasingrootstrength,largermassesofsoilareneededtoovercomeresistingforcesand,there-fore,thecriticallandslideareaincreases(Moosetal.2016).Itisworthmentioningthattheeffectivenessofroot reinforcement is limited toshallow landslideswithavolumeoflessthanabout1000m3(Giadrossichetal.2017),whichincludesmostoftheshallowland-slidesintheHyrcanianforest.Regardingtheimportantroleofplantrootsinsoil

bioengineering,manystudieshaveassessedrootten-sileresistance,whichvarieswidelyfromthousandstomillionsofPascal,dependingonthespeciesandtheenvironment (NilaweeraandNutalaya1999,Aber-nethyandRutherfurd2001,Schmidtetal.2001,Tosi2007,Genetetal.2008,Schwarzetal.2010a,Verganietal.2012,Giadrossichetal.2016).Tensileresistancedependsonavarietyoffactorssuchasplantspecies(Stokes2002),rootdiameter(Watsonetal.1999,Bisch-ettietal.2005),soilenvironment(GoodmanandEnnos1999),timeofyear(AbernethyandRutherfurd2000),managementtype(CoppinandRichards1990),testspeed,samplelengthanddiameter,rootmoistureandstorage(DeBaetsetal.2009,HalesandMiniat2016),chemicalcomposition(Genetetal.2005),orientationalongtheslope(Abdietal.2010),plantageandalti-tude(Verganietal.2014),andthemechanicalroleofthe root (Stokes 2002).Although some researchershavereportedthatvariationsinroottensileresistancearedependentonthespecies(e.g.Bischettietal.2009,Abdietal.2010,Verganietal.2012),manypreviousstud-ieshavefocusedononeorafewspecies(e.g.Watsonetal.1999,Tosi2007,Genetetal.2008,Abdietal.2009,Abdietal.2010).However,assessingandcomparingthemechanicalpropertiesofseveralspeciesprovidevaluabledataforrankingspeciesintermsoftheirpo-tential role inbioengineeringapplications (WatsonandMarden2004).Themagnitudeofsoil reinforcementdueto the

presenceofroots(Cr)canbemodeledusingdifferentmethods,e.g.theWumethod(Wuetal.1979),thefiberbundlemodelorFBM(PollenandSimon2005),the

Root Tensile Force and Resistance of Several Tree and Shrub Species of Hyrcanian Forest, Iran (255–270) Ehsan Abdi

Croat. j. for. eng. 39(2018)2 257

andBirchard2008,Packardetal.2011).Therefore,weusedtwopowerregressionmethods,theAkaikeinfor-mationcriteria (AIC)andadjustedR2,asstatisticalcriteriaforselectingtheoptimalmodel(Zuuretal.2007,Xiaoetal.2011,Laietal.2013).Inthiscontext,theobjectivesofthisstudywereas

follows:i)toinvestigatetowhichextentroottensileforceandresistancedependuponthespeciesandii)toinvestigatetheeffectsofdifferentregressionmeth-ods(nonlinearleastsquareandlog-transformation)onthepowerregressioncoefficientsfortherelationshipbetweenroottensileforceandtensileresistanceasafunctionofrootdiameter.

2. Material and Methods

2.1 Characteristics of the study siteTheHyrcanianvegetation zone, also called the

»Caspianforest«,isagreenbeltstretchingacrossthenorthernslopesoftheAlborzMountainRangesandcoveringabout1.9millionhectares.Theareaisrichinhardwoodspecies,withabout50treeand80shrubspecies.Broadleavedspeciesaredominant,andsomesmallstandsofsoftwoodshavebeenartificiallyintro-ducedtothisforestabout40yearsago.ThemaintreespeciesareFagus orientalis, Carpinus betulus, Parrotia persica, Acer cappadocicum, Acer velutinum, Alnus gluti-nosa, Ulmus glabra and Quercus castaneifolia.Thestudywasconductedintheeducationaland

experimentalforestoftheUniversityofTehran(Kheyrud Forest),withatotalareaofabout7000ha.Thefirstdistrict,namedPatom(Fig.1),coversanareaof900haandwaschosenasthestudyareabecauseofthehighoccurrenceofinstabilitiescomparedtootherdistricts.Elevationrangesfrom40to930mabovesealevel,whilethegradientrangesfrom0to70degrees.Theparent rock is composedofhard calcareous layerswithalargenumberofcracks.AccordingtotheUni-fiedSoilClassificationSystem(USCS),themostfre-quentsoiltypesareCH(claywithhighplasticity),CL(claywithlowplasticity),andML(siltwithlowplas-ticity).Averageannualprecipitationatthesiteisabout1200mm,withaveragesummerandwintertempera-turesof22.5and10°C,respectively.Themanagementsystemisthe»selectionsystem«,whichisfollowedtoensuresustainablemanagementandyields.Thefu-tureoftheforesthighlydependsonnaturalregenera-tion;insomeareas,treesareplantedingaps.Shallowlandslidesoccurinsomepartsofthisfor-

est(Fig.2)andaremorefrequentinareaswhereveg-etationhasbeenclearedfortheconstructionofroads.Theseslidesinvolvetheshallowlayersoftheslopes,

rootbundlemodelorRBM(Schwarzetal.2010a),andtherootbundlemodelWeibullorRBMw (Schwarzetal.2013).Apartfromthemodeltype,allmodelscon-siderCrasafunctionofroottensileresistance(inWuandFBMmodels)ortensileforce(inRBMandRBMwmodels)andofrootdistributionwithinthesoil.Concerning the engineering applications, the

quantificationofthetensileforceandtheresistanceofrootsarekeyparametersforseveralfieldsofapplica-tion,includingslopestability(Verganietal.2012);rootreinforcementestimation(Verganietal.2014);erosioncontrolmeasures(Giadrossichetal.2016),andsoilbioengineeringtechniquedesign(Bischettietal.2010).Forexample,insoilbioengineeringapplicationsinfor-estengineering,suchasbrushlayeringforthestabili-zationofroadcutsandfill-slopes(Bischettietal.2010),wattle fences for stabilizing uphill cut-slopes, andbrushwattlesforroadsideslopestabilization(Schiechtl 1980),plantrootsplayanimportantengineeringrole,androottensileforceandresistancevaluesareneededtoestimatethemagnitudeofthebioengineeringef-fectiveness(Bischettietal.2010).Inthecurrentstudy,roottensileforceandresis-

tanceof10typicalspecies(twoshrubandeighttreespecies)oftheHyrcanianforestwereinvestigatedtoexpandourknowledgeofthevaluestypicalofthisenvironmentandtocompareandassessthevariabil-ityamongspecies.Althoughsomestudieshavere-portedthetensileresistanceofHyrcanianforestspe-cies(e.g.,Abdietal.2009,Abdietal.2010),sofar,nostudyhasassesseddifferentspecies,especiallytreeswithdifferentstandorigin(i.e.naturalorartificialre-generation)andshrubs.Also,mostofthepreviousstudieshaveused log-transformationregressionasoneofthemostcommonpatternsinbiology(Xiaoetal.2011),whichwasintroducedapproximatelyacen-turyago (Packard2012) toexpress therelationbe-tween root tensile forceand tensile resistanceas afunctionof rootdiameter (e.g.Bischettietal.2005,Bischettietal.2009,Verganietal.2012,Verganietal.2014).However,Schwarzetal.(2013)andGiadrossichetal.(2017)reportedthatdifferentregressionmethods(log-transformationandnonlinearleastsquare)forfit-tingoftherootdiameter–forceandresistancecurveleadtoquitedifferentcoefficientsoftheequation,andthesechangesleadtovariationsintheestimatedrein-forcementeffectofvegetation.Also, inseveralbio-logicalstudies,theuseoflog-transformationpowerregression has been criticized, suggesting that theanalysisonlogarithmicscalesisflawedandthatin-stead,analysesshouldbecarriedoutontheoriginalmeasurementscale,usingnonlinearregression(e.g.Fattorini2007,Packard2009,Packard2011,Packard


258 Croat. j. for. eng. 39(2018)2

wherevegetationcanhaveabeneficialeffectonthestabilitythroughthereinforcingactionoflateralrootsandtheactionofcoarsetaproots.In1994and2004,a

seriesoflandslideshaveoccurredthatcausedtheclo-sureoftheroadnetworkinthePatomdistrict,result-ingincostsofabout$47000forthemaintenanceofa

Fig. 2 A natural shallow landslide; lateral root reinforcement can be seen along the tension crack (a) and failure of forest road cut slope (b) in the study area

Fig. 1 Location of the study area in the Kheyrud forest, northern Iran (the black point)


Croat. j. for. eng. 39(2018)2 259

damaged segment and the construction of gabionwallsinsomepartsoftheroadnetwork.Also,gullyerosionbyconcentratedflowfromroaddrainagecanbeseenalongsomepartsoftheroad(Fig.3).

2.2 SamplingEighttreeandtwoshrubspecieswereselectedfor

resistanceinvestigationsduetotheirdominanceandfrequentdistributionintheroadedgezone(Table1).Sixspecimenswereselectedrandomlyfromeach

speciestoconsiderintra-speciesvariability,andliverootsampleswerecollectedrandomlyfromthesoilbyexcavatingpitsbesidethetreesatadepthofabout30cmbelowthesoilsurface(CofieandKoolen2001,Abdietal.2010).Topreventpre-stresseffects,rootswerecutwithsharpscissorsandplacedinplasticbags.Inmostpreviousstudies,therootsamplesweretreat-edwitha15%alcoholsolution (e.g.Bischettietal.2005,Bischettietal.2016)topreservethemfromdete-riorationprior to the tensile tests. Therefore, theirmoisturecontent ishigher than thatoffield-testedroots(Verganietal.2016),andtherootisslightlyswol-len,resultinginahigherrootdiameter(Boldrinetal.2017)andpossibleerrorsources.HalesandMiniat(2016)foundthatrootswith50%lessmoistureweremorethantwiceasstrongasfreshroots.Toovercomethisproblemandpreventseverechangesofrootmois-turecontent,weonlysprayedtherootswitha15%alcoholsolutioninsteadofaddingthesolutiontotherootbags.Rootswerepreservedat4°Cforafewdaysto avoid an impact on the measured parameters(Bischettietal.2005).

2.3 Resistance testsInthelaboratory,rootswerecarefullyinspectedfor

possibledamage.Priortotheexperiment,rootdiam-eterwasmeasuredatthreedifferentpositionsalongthemiddlelengthoftheroottoobtainarepresentativevalue(Bischettietal.2005,Verganietal.2012,Abdietal.2014).TensiletestswerecarriedoutusingaFloorModel4486computer-controlled InstronUniversalTestingMachine(UK),equippedwitha5kNmaxi-mum-capacityreversibleloadcell.Therootendswereclampedandastrainrateof

10mm/min(Bischettietal.2005,Mattiaetal.2005,Pollen2007,Abdietal.2014)wasselected,similartotheapproachusedinpreviousstudies,toallowcom-parison.DeBaetsetal.(2008)reportedvelocitiesrang-ingbetween1and300mm/minforrapidlandslides.Asmostshallowinstabilitiesinthestudyareaareclas-sifiedasrapidandoccurduringandafterheavyrain-falls,thistestspeedwasconsideredadequate.Onlysamplesrupturednearthemiddleoftherootbetween

Fig. 3 Gully erosion formed by concentrated flow from a road side ditch

Table 1 List of studied plant species

Tree species Shrub species

IDBotanical

nameEnglish name

IDBotanical

nameEnglish name

1Acer velutinum

Persian maple

9Crataegus microphylla

Hawthorn

2Alnus glutinosa

Black alder 10Mespilus germanica

Medlar

3Carpinus betulus

Common hornbeam

4Fagus orientalis

Oriental beech

5Fraxinus excelsior

Common ash

6Parrotia persica

Persian ironwood

7 Picea abiesNorway spruce

8Quercus castaneifolia

Chestnut-leaved oak


260 Croat. j. for. eng. 39(2018)2

theclampswereconsidered.Atotalof487rootsam-plesweretested.Thetensileforce(N)atthepointofrupturewas

takenasthepeakload(FMax),andtherelatedtensileresistance(MPa)wascalculatedbydividingthebreak-ingforcebythecross-sectionalareaofeachtestedroot(mm2),see(Eq.1):

π

=

×

Max

2

4

FTR

d (1)

here:TR tensile resistanceFMax maximumforcetobreaktherootd rootdiameter.Thediametersoftherootsamplesrangedfrom0.29

to5.90mm.Thickerrootsweredifficulttotestbecauseofclampingconstraints(DeBaetsetal.2008).

2.4 Statistical analysesTherelationshipbetweenroottensileforce,F(N),

andtensileresistance,TR(MPa),asafunctionofrootdiameterd(mm),wasinterpretedthroughpowerre-gressions.Toobtainthepowerregressioncoefficients(i.e.,aandb),twodifferentmethodswereused:non-linearleastsquareandlog-transformationmethods,usingRsoftware.Thesuitabilityoftheregressionsandgoodnessoffit(efficiencyofthemodel)wereevalu-atedusingtheAkaikeinformationcriterion(AIC)andtheadjustedR2valuesasstatisticalcriteriaformodel

selection(Zuuretal.2007,Xiaoetal.2011,Laietal.2013).Tocompareroottensileforceandresistanceval-uesbetweenspeciesandtotakediameterintoconsid-erationasacovariatefactor,ANCOVAwasused(Abdietal.2010,Verganietal.2014,Verganietal.2016).TheKolmogorov–Smirnovtestwasusedtocheckthenor-malityofthedatabeforeproceedingtheANCOVA;duetothenon-normalityofthedata(forceandresis-tancevalues),thevalueswerelog-transformedtoen-surehomogeneousresidualvarianceandnormality.Tukey´stestwasusedtocomparemeanroottensileforceandresistanceofdifferentspecies.

3. ResultsDescriptivestatisticsoftestedrootsandtheircor-

respondingforceandresistancevaluesareshowninTable2.RegardingTable2,thenumberofvalidtensiletestsrangedbetween30and64,basedonthespecies.Rootdiameterrangedfrom0.29to5.90mm,andmeanrootdiameterforeachspeciesvariedbetween1.53and2.45mm.

3.1 Tensile ForceAsshowninTable2,thevariabilityofforceand

resistanceamongandevenwithinagivenspecieswashigh.Theminimumforcevaluesrangedbetween3.80and12.10Nforhardwoodtreespecies,4.30Nfortheonlysoftwoodtreespecies,andbetween8.90Nand15.30Nforthetwoshrubs.Consideringthemaximumvalues,therangeswere203.80o398.10Nforhard-woodtreespecies,198.70Nfortheonlysoftwoodtree

Table 2 Descriptive statistics of tested roots including diameter, force, and resistance

Species n Diameter, mm Force, N Resistance, MPa

Mean SD Max. Min. Mean SD Max. Min. Mean SD Max. Min.

Acer velutinum 56 1.72 1.26 4.45 0.29 64.15 75.11 291.30 7.11 30.77 25.03 135.87 3.97

Alnus glutinosa 59 1.71 1.25 4.68 0.38 60.25 64.84 251.80 7.20 26.12 16.79 108.43 4.52

Carpinus betulus 32 1.63 0.80 3.17 0.35 95.36 83.23 349.50 8.30 43.31 23.55 124.39 13.65

Fagus orientalis 33 1.69 0.94 4.00 0.52 74.92 61.68 237.40 8.30 30.47 12.34 66.47 12.92

Fraxinus excelsior 50 2.39 1.12 4.71 0.52 54.29 42.62 203.80 3.80 12.74 6.61 30.60 3.32

Parrotia persica 58 1.72 1.15 4.77 0.49 84.59 91.29 398.10 12.10 36.41 24.12 123.76 10.91

Picea abies 47 2.41 1.18 4.85 0.40 66.51 51.97 198.70 4.30 15.75 15.51 108.10 2.41

Quercus castaneifolia 30 1.53 0.93 4.02 0.60 83.59 69.87 249.00 9.90 42.67 21.79 104.47 15.83

Crataegus microphylla 64 2.08 1.08 5.90 0.40 135.86 100.06 441.90 8.90 44.94 29.77 224.35 11.01

Mespilus germanica 58 2.45 1.34 5.00 0.50 143.25 123.94 487.20 15.30 32.69 18.28 89.95 7.74

https://en.wikipedia.org/wiki/Akaike_information_criterion


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Fig. 4 Tensile force as a function of root diameter. Nonlinear least squares approximation (dashed line) and log-transformation method (con-tinues line)


262 Croat. j. for. eng. 39(2018)2

species,andbetween441.90Nand487.20Nforthetwoshrubs.Meantensileforcevaluesforhardwoods,softwood,andshrubspecieswere54.29–95.36,66.51,and135.86–143.25N,respectively.TherelationshipbetweenroottensileforceF(N)

androotdiameterd(mm)throughpowerregressions(nonlinearleastsquareandlog-transformation)ispre-sentedinFig.(4).AsshowninFig.(4), log-transformationregres-

sionsgenerallyunderestimatethesituation.Excep-tionswerefoundforF. orientalis,Q. castaneifolia, and

C. microphylla, forwhichthenonlinear leastsquareregressionswerebelowthecurvesforthelog-transfor-mation.Generally,themaindifferencesoccurredatthetopofthecurves(thickerrootdiameters).Theregressioncoefficients(aandb),AIC,andad-

justedR2fortensileforceregressionmodelsarepre-sentedinTable(3).The rangesofa andbwere 13.02<a<45.41 and

1.26<b<1.55forlog-transformationand14.16<a<61.83 and1.07<b<1.66fornonlinearleastsquarepowerre-gressionsregardingallspecies(Table3).

Table 3 Regression coefficients, adjusted R2 and AIC for tensile force of different species

Log-transformation Nonlinear least square

Species a b Adj. R2 AIC a b Adj. R2 AIC

Acer velutinum 25.02 1.26 0.70 575.96 20.71 1.61 0.79 556.85

Alnus glutinosa 22.98 1.41 0.85 547.63 25.48 1.38 0.86 543.35

Carpinus betulus 40.64 1.40 0.60 346.39 42.17 1.49 0.63 344.30

Fagus orientalis 29.77 1.52 0.92 280.37 32.96 1.41 0.93 278.59

Fraxinus excelsior 13.02 1.45 0.62 470.92 14.16 1.45 0.63 468.79

Parrotia persica 33.44 1.34 0.87 573.50 26.95 1.66 0.93 527.34

Picea abies 13.62 1.55 0.64 458.97 19.90 1.32 0.66 455.64

Quercus castaneifolia 38.01 1.50 0.73 302.22 50.77 1.15 0.80 293.58

Crataegus microphylla 45.41 1.33 0.61 711.91 61.83 1.07 0.65 705.95

Mespilus germanica 34.38 1.39 0.65 663.95 43.50 1.28 0.67 661.17

Fig. 5 Root tensile force (mean±SE). Means with different letters are statistically different (p<0.05)


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Fig. 6 Root resistance as a function of root diameter. Nonlinear least squares approximation (dashed line) and log-transformation method (continues line)


264 Croat. j. for. eng. 39(2018)2

AsshowninTable(3),nonlinearleastsquarere-gressionresultedinlowerAICandhigheradjustedR2 valuesinallcases;therefore,lowerresidualsandbet-tergoodnessoffit,indicatingtheadvantageofnonlin-earleastsquaremodelsandtheircoefficientsfortherelationshipbetweenforceanddiameter.TheresultsoftheANCOVAshowedthatmeanroot

tensileforcesweresignificantlydifferentamongspe-cies(F=37.36,p<0.001)withregardstorootdiameterascovariate factor (F=1453.77,p<0.000).TheresultsofTukey’stestformeancomparisonsarepresentedinFig.5.Thetwoshrubspecies,alongwithC. betulus, P.

persica, and Q. castaneifolia, are categorized as thestrongestspeciesorasthespecieswiththehighestten-sileforceamongthestudiedspecies(groupAinFig.5). ThespeciesA. velutinum andA. glutinosaareinterme-diate(groupB),whileP. abies andF. excelsior aretheweakestspeciesregardingtensileforce(groupCinFig.5).TheexceptionwasF. orientalis,which,althoughthesampleswereobtainedfromanaturalregenera-tionstand,wasnotamongthestrongestspeciesre-gardingtensileforce.

3.2 Root resistanceAs shown inTable 2,minimum root resistance

rangedbetween3.32and15.83MPaforhardwoodtreespecies,2.41MPafortheonlysoftwoodtreespecies,andbetween7.74and11.01MPaforthetwohard-woodshrubs.Consideringthemaximumvalues,therangeswere30.60to135.87MPaforhardwoodtreespecies,108.10MPafortheonlysoftwoodtreespecies,

and 89.95 to 224.35MPa for the twohardwoodshrubs.Meantensileresistancevaluesforhardwoods,softwood,andshrubswere12.74–43.31,15.75,and32.69–44.94MPa,respectively.The relationshipbetween root resistance (MPa)

androotdiameterd(mm)throughpowerregression(nonlinearleastsquareandlog-transformation)ispre-sentedinFig.(6).AsshowninFig.(6), log-transformationregres-

sionsgenerallyunderestimatethesituation,especiallyinsmallerrootsizes.TheexceptionswereA. velutinum, A. glutinosa, and P. persica, where nonlinear leastsquareregressionsarebelowthelog-transformationcurvesinrootsgreaterthan1mmindiameter.Theregressioncoefficients(aandb),AIC,andad-

justedR2forrootresistanceregressionmodelsarepre-sentedinTable(4).The ranges of a and b in resistance were

16.58<a<57.85and–0.66<b<–0.44forlog-transforma-tionand17.56<a<62.44and–1.08<b<–0.43fornonlin-earleastsquarepowerregression.AsshowninTable(4),nonlinearleastsquarere-

gressionresultedinlowerAICandhigheradjustedR2 valuesforallspeciesand,therefore,lowerresidualsandbettergoodnessoffit,indicatingtheadvantageofnonlinearleastsquaremodelsandtheircoefficientsfortherelationshipbetweenresistanceanddiameter.Theresultsof theANCOVA revealedthatmean

root tensile resistance was significantly differentamongthetestedspecies(F=34.87,p<0.001)withre-gardstorootdiameterascovariate(F=274.26,p<0.000).TheresultsofTukey’stestformeancomparisonsarepresentedinFig.(7).

Table 4 Regression coefficients, adjusted R2, and AIC for resistance of different species

Log-transformation Nonlinear least square

Species a b Adj. R2 AIC a b Adj. R2 AIC

Acer velutinum 28.59 –0.73 0.66 461.68 28.34 –1.08 0.79 434.86

Alnus glutinosa 26.27 –0.59 0.53 458.25 26.87 –0.82 0.58 450.79

Carpinus betulus 46.45 –0.59 0.37 280.30 50.05 –0.55 0.39 279.16

Fagus orientalis 34.02 –0.48 0.41 244.15 35.47 –0.48 0.42 243.52

Fraxinus excelsior 16.58 –0.54 0.35 310.97 17.56 –0.49 0.38 308.93

Parrotia persica 38.22 –0.65 0.58 484.82 40.63 –0.94 0.67 470.90

Picea abies 17.34 –0.44 0.08 389.31 23.49 –0.62 0.15 385.61

Quercus castaneifolia 43.44 –0.49 0.17 266.45 46.79 –0.43 0.20 265.49

Crataegus microphylla 57.85 –0.66 0.33 591.87 62.44 –0.65 0.35 590.32

Mespilus germanica 43.80 –0.60 0.44 470.42 48.38 –0.64 0.47 467.32


Croat. j. for. eng. 39(2018)2 265

RegardingFig.(7),thespeciescanbeclassifiedinthreegroups(A,B,andC)basedonroottensileresis-tance.Theweakestspecies(classifiedasgroupC)wasrepresentedbyF. excelsiorinthe40-year-oldplanta-tion.Intermediatespecies(groupB)wereA. velutinum, A. glutinosa, and P. abies inplantationstands.TwoshrubspeciesaswellasC. betulus,F. orientalis,P. per-sica,andQ. castaneifoliawerethestrongestspeciesre-gardingrootresistance(groupA).

4. DiscussionRoottensileforceandresistanceareimportantfac-

torsthatinfluencesoilreinforcementandtreeanchor-ageandprovideessentialdataaboutusinglivemateri-alsforbioengineeringtechniques(Bischettietal.2010).Similartoseveralpreviousstudies(e.g.,Bischettietal.2005,Mattiaetal.2005,Schwarzetal.2013,Verganietal.2014),wefoundalargevariabilityinroottensileforceandresistancebasedonspeciesandrootdiame-ter.Accordingtopreviousstudies(e.g.,NilaweeraandNutalaya1999,Bischettietal.2005,Abdietal.2010,Verganietal.2014),thisrelationshipiswelldescribed,intermsofbothforceandresistance,bypositiveandnegativepowerlawregressions,respectively,confirm-ingthestrongdependenceofrootstrengthonrootsize.Genetetal.(2005)justifiedthisrelationshipbydifferentcellulose to ligninratios,withsmaller rootshavinghigherratios.AlsoYeetal.(2017)attributedthisrela-tionshiptothechemicalcompositionofroottissues

andshowedthattensileforcewassignificantlynega-tivelycorrelatedwithcelluloseandholocelluloseandsignificantlypositivelycorrelatedwithligninandthelignintocelluloseratio,whilefortensileresistance,op-positecorrelationshavebeenreported.Inthecurrentstudy,tworegressionmethods(non-

linearleastsquareandlog-transformation)wereusedtoobtainthecoefficientsofthepowerregressions(i.e.,aandb).Basedontheresults, thepowerequationparametersweredifferentinthetworegressiontypes(Tables3and4).RegardingtheAICvaluesandtheadjustedR2valuesformodelselection,thenonlinearleastsquaremethodresultedinlowerAICandhigheradjustedR2values,makingitthepreferredmodelforbothforceandresistancepowerregressions.ThisisconsistentwithChangyongetal.(2014),whoreportedthat log-transformationmay inaccurately estimatemodelparameters.Zuuretal.(2007)statedthatthemodelwiththelowestAICandthehighestadjustedR2valuescanbeselectedastheoptimalmodel,whichindicatestheimprovedfitofthemodeltothedataand,therefore,alowerresidualsumofsquare(lackoffit).Thismaybeduetothebasisofthenonlinearleastsquaremethod,whichapproximatesthemodelfirstandthenrefinestheparametersbysuccessiveitera-tions(Hesse2006).ThisisconsistentwiththeresultsofSchwarzetal.(2013)andGiadrossichetal.(2017),whoindicatedthatdifferentalgorithmsleadtodif-ferentcoefficientsoftheequation,althoughtheydidnotreporttheoptimalmodel.Previousstudies(e.g.,

Fig. 7 Root resistance (mean±SE). Means with different letters are statistically different (p<0.05)


266 Croat. j. for. eng. 39(2018)2

Verganietal.2014,Giadrossichetal.2017)reportedthatsmallchangesinthefittingcurveoftherootdi-ameter–forceorresistancerelationshipleadtochang-esintheoutputofrootreinforcementmodelsasanimportantfactorinefficiencyassessmentofbioengi-neeringmeasures(Bischettietal.2010).Ithas,there-fore,beensuggestedthatrootresistanceandforcearecriticalfactorsinslopestabilityevaluations.Giadrossich etal.(2016)reportedthatusingthelog-transformationmethodresultsinanunderestimationofforce.OurresultsagreewiththefindingsofGiadrossichetal.(2016)andshowedthatusingthelog-transformationmethodleadstoanunderestimationofbothtensileforce andresistance.Therefore,usingvaluesoftheequationparametersoflog-transformationwillunderestimatetheeffectofrootreinforcementinstabilityanalyses.From a statistical point of view, Packard et al.

(2011)pointoutthatlog-transformedmodelspredictthegeometricmeanfortheresponsevariable,andthatlog-transformationinherentlydistortstherelationshipbetweenvariables.Theauthors,therefore,recommendthatanalysesshouldbeperformedonthearithmeticscalevianonlinearregression,andthiswasreportedastheadvantageofnonlinearregression.Also,Packard(2012)showedthatlog-transformationofdatacreatednewdistributionsthatactuallyobscuredtherelation-shipsbetweenpredictorandresponsevariablesandledtobias.Theauthorconcludedthatlog-transforma-tionisnotagenerallyreliablewaytoestimateparam-etersinasimplepowerfunctionontheoriginalscale.Theotheradvantageofnonlinearregressionisthattheuse of nonlinearmodel fitting is facilitated by theavailabilityofeasy-to-useadvancedstatisticalpack-ages(Laietal.2013),whichwerenotavailableatthetimewhenlog-transformationpowerregressionap-peared(Packard2012).Regardingtheadvantagesofnonlinearregression

andaspowermodelcoefficientsareimportantfactorsinrootreinforcementassessmentsinsoilbioengineer-ingmeasures,itissuggestedthatlog-transformationmodelsbereplacedbynonlinearleastsquaremodelstoobtainmorerealisticestimatesbasedonobserva-tions(testedrootsamples).Regardingtensileforcecoefficients,Verganietal.

(2012)obtainedthefollowingaandbrangesforsevencommon European tree species: 8.31<a<19.66 and1.49<b<1.85.Inthecurrentstudy,aandbcoefficientsfortheforce-diameterrelationshiparegenerallyoutoftherangeforEuropeantreespeciesinbothlog-trans-formationandnonlinearleastsquaremethods.Inthisstudy,therangesforeightcommontreespeciesofHyr-canianspecieswere13.2<a<40.64and1.26<b<1.55forthelog-transformationmethodand14.16<a<50.77and1.15<b<1.66forthenonlinearleastsquaremethod.The

differencebetweenthecoefficientsmaybearesultofdifferentspeciesandvaryingenvironmentalcondi-tions(Verganietal.2012,Boldrinetal.2017).Regardingrootresistancecoefficients,Nilaweera

(1994)suggestedthefollowingaandbrangesforhard-woodtreespeciesroots:29.1<a<87.0and–0.8<b<–0.4. Inthisstudy,therangesforeightcommontreespeciesof the Hyrcanian forest were 16.58<a<46.45 and–0.73<b<–0.44forthelog-transformationmethodand17.56<a<50.05 and –1.08<b<–0.43 for thenonlinearleastsquaremethod.Inthecurrentstudy,theaandb coefficientsweregenerallyintherangeobtainedforthelog-transformationmethod(exceptaforF. excelsior andP. abies).However,forF. excelsiorandP. abies,theavalueswereintherangeobtainedfromthenonlinearleastsquaremethod,whileforA. velutinum,A. glutinosa,andP. persica,thebvalueswereoutsideofthisrange(below–0.8).AsrangesreportedinNilaweera(1994)arebasedonforestsinThailand,theymayneedtobereconsideredandmodifiedbasedonmorerecentstud-iesindifferentforestzones.Inthecurrentstudy,themeasuredmeantensile

forcesfortheHyrcanianforestspecies(A. velutinum 64.15N,A. glutinosa60.25N,C. betulus93.36,F. orientalis 74.92N,F. excelsior54.29N,P. abies66.51N)weresimilartothoseobtainedbyVerganietal.(2016)forsomeEuropeanspecies(Acer pseudoplatanus65N,Ostrya carpinifolia56N,Fagus sylvatica 84N,Fraxinus excelsior47N,Picea abies46N).However,theywerelowerthanthosereportedbyChiaradiaetal.(2016)forFagus sylvatica(122.46N)andPicea abies(70.68N).Thedifferencesbetweenthevaluespresentedinthisstudyandthoseintheliteraturemaybeexplainedbythedifferentresponsesofplantstodifferentenvironmen-talconditions(plasticity)tominimizeabioticandbi-oticstresses(Boldrinetal.2017).Themeasuredmeantensileresistancevaluesinthe

currentstudy(A. velutinum30.77MPa,A. glutinosa 26.12MPa,C. betulus43.31MPa,F. orientalis30.47MPa,F. excelsior12.74MPa,P. persica36.41MPa,P. abies 15.75MPaandQ. castanefolia42.67MPa)arecompa-rabletothosereportedinStokes(2002),includingAlnus incana(22MPa),Fraxinus excelsior(26MPa),Acer platanoides(27MPa),Picea abies(28MPa),Quercus rubra (32MPa)andAlnus japonica(41MPa).However,ourvalueswerelargerthanthemeanresistancevaluesreportedbyBoldrin et al. (2017) for re-establishedsmalltrees(7.1–23.2MPa).ThismaybeexplainedbythereportofGenetetal.(2006),whoshowedthatten-sileresistancewaslowerintheearlygrowthstageandincreasedinolderplants.Comparisonsofforce–diameterrelationshipsfor

different species (ANCOVA, Fig. 5) confirmed that


Croat. j. for. eng. 39(2018)2 267

therearestatisticallysignificantdifferencesinrootten-sileforcebetweenspecies.Inthisregard,F. excelsior andP. abiescanbeconsideredastheweakestspecies,whileC. microphylla andM. germanica (shrubs) areamongthestrongestones.Broadleavedspeciesgener-allyhaveahighertensileforcethanconiferspecies(exceptF. excelsior species),whichisconsistentwiththeresultsofVerganietal.(2012).StokesandMattheck(1996)justifiedthiswiththedifferentrootanatomyofbroadleavesandconifers,asbroadleavesgenerallyhavelargercellsandthinnercellwalls.Wefoundstatisticallysignificantdifferencesinroot

tensileresistancebetweenspecies,withmeanvaluesrangingbetween12.74and44.49MPa.Severalauthorsattributedthesedifferencestogeneticandenviron-mentalfactorsandtherootsystemtissuecomposition(Genetetal.2005,DeBaetsetal.2008,Chiaradiaetal.2016,Yeetal.2017).Inthisregard,F. excelsiorwastheweakestspecies,

whileC. microphyllaandM. germanica(shrubs)wereamongthestrongestones.ThisisincontrastwithMorganandRickson(1995),whostatedthattherangeofrootresistanceofshrubspeciesisnotsignificantlydifferent from that of trees, although our resultsshowedthattheymayevenhavehighervaluesthansometreespecies. ThisisinagreementwithBuryloetal.(2011)andBoldrinetal.(2017),whorevealedthattherootsofshrubsweremoreresistantthanthoseoftreespecies.Themeanresistancevaluesofthetwoshrubspeciesinourstudy(i.e.,44.94and32.69MPa)arecomparabletotheresultsofMattiaetal.(2005)(Pistacia lentiscus55.0,Atriplex halimus57.2MPa),andarehigherthanthosefoundbyTosi(2007)(Rosa canina 22.95,Inula viscosa18.72,andSpartium junceum29.93MPa)andCominoandMarengo(2010)(Rosa canina 42.9 Cotoneaster dammeri 18.7and Juniperus horizontalis 14.8MPa).Concerning the resultsofBoldrin et al.(2017),Crataegus monogynahadthehighesttensilere-sistance(23.2MPa)amongthe10shrubsandsmalltreesofEurope,butinourstudy,theresistanceofCra-taegus microphylla(44.94MPa)wasabouttwiceashighas that reportedbyBoldrinet al. (2017).ThismaypartlybeexplainedbythedifferentenvironmentsandthefactthatthespeciesassessedbyBoldrinetal.(2017)wereintheearlystageofestablishment.Watsonetal.(1999)reportedadirectrelationshipbetweenplantgrowthandincreasedreinforcementeffect;similarly,Genetetal.(2006)reportedincreasingtensileresis-tancewithplantgrowthandattributedthisphenom-enontohigherquantitiesofcelluloseinolderplants.Previousstudieshavereportedrangesforcoeffi-

cientsofrootresistancepowerregressionsinshrubspecies(e.g.,Mattiaetal.2005,DeBaetsetal.2008,CominoandMarengo2010,Buryloetal.2011).They

reported the following ranges: 73.0<a<91.2 and–0.60<b<–0.45fortwoshrubspecies(Atriplex halimus and Pistacia lentiscus) in Mattia et al. (2005);4.41<a<45.59and–1.77<b<–0.45for14MediterraneanshrubspeciesinDeBaetsetal.(2008);14.79<a<37.77and−1.28<b<−0.83forthreespecies(Rosa canina, Coto-neaster dammeri and Juniperus horizontalis)inCominoandMarengo(2010);and4.4<a<91.2and−1.75<b<−0.52fortwoshrubbyspecies(Genista cinereaandThymus serpyllum)inBuryloetal.(2011).Theresultsofthecur-rentstudyareconsistentwiththoseofBuryloetal.(2011),butdonotfallintotherangeofDeBaetsetal.(2008)foracoefficientsandDeBaetsetal.(2008)andCominoandMarengo(2010)forbothaandbcoeffi-cients.Again,thedifferencebetweencoefficientsmaybetheresultofdifferentspeciesandenvironmentalconditions(Verganietal.2012,Boldrinetal.2017).Theweakestspecies,basedonresistance,werespe-

ciesin40-year-oldplantations(Fig.7).Thisisconsis-tentwiththeresultsofWatsonandMarden(2004),whoreportedlowerresistancevaluesforplantationsofradiatepine(17MPa)andDouglasfir(25MPa)com-paredto11NewZealandindigenousriparianplantspecies.Themeanresistancevaluesofplantationspe-ciesinourstudy(rangingfrom54–66MPa)weresig-nificantlyhigherthanthosereportedbyWatsonandMarden(2004)forradiatepine(17MPa)andDouglasfir(25MPa)inplantationsandbyGenetetal.(2008)forthreedifferentagestagesofCryptomeria japonica plantations,i.e., 22.6,25.3,and31.7MPaforthejuve-nile,intermediate,andmaturestands,respectively.Consideringthatmostlandslidesinthestudyarea

arerainfall-induced,hydrologicaleffectsofvegetationmightnotsignificantlyaffectsoilstabilityinseasonswithheavyrainfall(autumnandwinter).Inthesesea-sons,themechanicaleffectsofvegetationorrootrein-forcementcanplayanimportantroleinsoilstabiliza-tion.Rootscanmobilizetheirtensileresistanceduringfailuresalongtensioncracks(Verganietal.2017)andatthelateralsurfaceofthelandslide(Fig.2a)andin-crease the resisting force against thedriving force,therebyimprovingthestabilityoftheslope.Forthisreason,specieswithhigherrootresistance(groupAinFig.7)arepreferred(Stokes2002)insoilbioengineer-ingsystems(Bischettietal.2010),andthemagnitudeoftherootresistancecaninfluencetheperformanceofthesespecies.Theresultsofourstudyshould,there-fore,beconsideredbyforestmanagerswhenselectingsuitablespeciesforthereestablishmentofvegetationoncutandfillslopesafterroadconstruction.Inaddi-tionto tensileresistance,surcharge isan importantcriterionforsoilbioengineeringmeasures,especiallyon forest roadcutandfill slopes thatusuallyhavehigherslopeanglesthannaturalslopes.Inthisstudy,


268 Croat. j. for. eng. 39(2018)2

shrubsshowedhighrootresistance,andasthenega-tiveeffectofsurchargeonslopestability(Chiaradiaetal.2016)isnegligibleforthemcomparedtotreespecies(MorganandRickson1995),theycanbegoodchoicesforforestroadstabilization.Thismayrepresentanad-vantageofshrubscomparedtotreesinsoilbioengi-neeringapplications,especiallyonforestroadcutandfillslopes.

5. ConclusionsWeinvestigatedtherootmechanicalbehaviorof

eightcommontreeandtwoshrubspeciesoftheHyr-canianforest.Twoalgorithms(nonlinearleastsquareandlog-transformation)wereusedtoestimatetheco-efficientsofthepowerregressionsforforce-diameterandresistance-diameter.Rootmechanicalbehaviorisdependentonrootdi-

ameterandcanbewelldescribedbypowerlawrela-tionshipsasafunctionoftherootdiameterforbothforceandresistancefunctions.Ourresultsshowedthataandbnotonlydependonthespecies,butalsoonthestatisticalmethodapplied.Thenonlinearleastsquaremethodwasselectedastheoptimalmodelandcanbet-terexplain therelationshipbetweendiameter-forceanddiameter-resistance.Also,usingthelog-transfor-mationmodelunderestimatespowerregressionsofrootforceandresistanceandthereforewillunderesti-matetherootreinforcementmagnitude.Roottensileforceandresistancedifferedsignificantlyamongspe-cies(ANCOVA)andweregroupedintothreestrengthclasses;intermsofbothforceandresistance,shrubsconstitutedthestrongestclass.Furthermore,thisstudyshowedthattreesinplantationstandshadalowerre-sistancethantreesinnaturalstands.Roottensileforceandresistanceareimportantinputsofrootreinforce-mentmodelstoestimatethequantityofincreasedsoilcohesionandcalculateslopestability,consideringthepresenceofplantroots.Thesedatamaybeusedforthereestablishmentofvegetation incutandfillslopes,withtheaimtoreducetheriskofinstabilities.Furtherstudiesontheuseofplantsinbioengineer-

ingstrategiesshouldconsideradditionalfactorsthatmightinfluencerootmechanicalbehavior,suchassitetype,soiltype,plantage,andelevation.

AcknowledgementsTheauthorwouldliketoacknowledgethefinan-

cialsupportofthe»IranNationalScienceFoundation(INSF)«undertheprojectnumber93022486.AlsoIwouldliketothankthetwoanonymousreviewersfortheirdetailedcommentsandsuggestionsthatledtoanimprovementofthework.

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Received:November02,2016Accepted:February06,2018

Authors’address:

Assoc.prof.EhsanAbdi,PhD.*e-mail:abdie@ut.ac.irUniversityofTehranDepartmentofForestryP.O.BOX31585-4314,KarajIRAN

*Correspondingauthor

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Root Tensile Force and Resistance of Several Tree and