Lina Karlinasari?Suhada Andini?Descarlo Worabai?Prijanto Pamungkas?Sri Wilarso Budi?Iskandar Z.Siregar

Introduction

Many planting trials of economically and ecologically important tree species have concentrated on growth,yield and phenotypic characteristics (such as stem form,branching habits,crown shape,and pest and disease resistance).However,additional data beyond these traits are needed by silviculturists,breeders and others to improve the selection criteria for the best tree species or genotypes.Howe et al.(2006),Cherry et al.(2008)and Moore et al.(2009)all noted that not only are growth and form traits important for breeders and silviculturists,but so is wood-quality including density and wood stiffness[i.e.,modulus of elasticity(MOE)].

Wood stiffness,or MOE,is one of the most important properties of structural wood products in addition to wood density.Wood stiffness is a key mechanical property of timber especially in structural applications.Non-destructive testing/evaluation methods based on acoustic techniques offer an in situ means to assess MOE and thus wood quality(Mochan et al.2009;Wang et al.2001).Acoustic velocity(an indirect measure of MOE)can be used as a surrogate for bending stiffness because it is highly correlated with direct estimates of MOE obtained from bending tests(Wang et al.2001;Dzbenski and Wiktorski 2007;Cherry et al.2008).Non-destructive techniques have been used to investigate the effects of different factors on wood stiffness.For example,Wang et al.(2001)used a stress wave-based method to investigate the effects of silvicultural practices on wood stiffness of young-growth western hemlock(Tsuga heterophylla(Raf.)Sarg.)and Sitka spruce(Picea sitchensis(Bong.)Carrie`re).Acoustic velocity determined through non-destructive testing in standing trees has also been used for assessing wood quality of young trees,often in a tree-breeding context(Kasal et al.2007;Horvath et al.2010;Urhan et al.2014;Lenz et al.2013).

Limited information exists on early screening of the mechanical properties of tropical wood species by nondestructive means.Karlinasari et al.(2008)investigated the use of a non-destructive ultrasonic method for evaluating wood strength and the stiffness of 6-year Gmelina(Gmelina arborea Roxb.)from several positions in the tree,both vertically and horizontally.The results showed that there was good correlation between ultrasonic velocity and static bending test values.Chauhan and Kumar(2014)studied a 9-year plantation of Melia dubia Cav.and concluded that acoustic velocity in trees and basic wood density were not significantly related,nor were tree height and diameter at breast height related to wood density.However,significant negative correlations were found between wood density and tree height growth in tropical mixed-species plantations(Nguyen et al.2014).More generally,indirect measurements of wood stiffness to evaluate and genetically improve trees have been done for temperate wood species.Low positive correlations were found between acoustic velocity and tree height by Lenz et al.(2013)for white spruce[Picea glauca(Moench.)Voss]and by Cherry et al.(2008)for Douglas-fir(Pseudotsuga menziesii(Mirb.)Franco).The relationship between acoustic velocity and growth-related traits was weak to moderate for young(6-to 12-year)Douglas-fir and western hemlock(Urhan et al.2014),while the correlation between wood density and stem volume was moderately negative in Douglas-fir(Cherry et al.2008).Moore et al.(2009)investigated Sitka spruce at the stand level and found a weakly negative statistical relationship between dynamic MOE and diameter at breast height(dbh),stand density and growth rate.In addition,unfavourable negative correlations were found between tree diameter and wood density for Scots pine(Pinus sylvestris L.)(Hong et al.2014)and Douglas-fir(Cherry et al.2008).

Manii,or African wood(Maesopsis eminii),and Shorea leprosula Miq.,S.mecistopteryx Ridl.,S.stenoptera Burck,S.pinanga Scheff.,and S.palembanica Miq.(collectively known as meranti)are six economically and ecologically important species that are grown in Indonesia.Shorea leprosula is catagorised as a fast-growing Shorea species,but reliable reports on the growth rates of the other four Shorea species used in our present study are still lacking.Phillips et al.(2002)categorised S.leprosula(and S.johorensis Foxw.)as very fast growing,large trees.Also,Widiyatno et al.(2013,2014)studied a logged-over area in the tropical rainforest at Central Kalimantan and found that 6-year-old trees of S.leprosula had a diameter(dbh)of 0.14 m,tree height of 8.30 m and a survival rate of about 90%compared with other Shorea spp.(i.e.,S.platyclados Slooten ex Endert,S.parvifolia Dyer,S.virescens Parijs and S.johorensis).In contrast,Schwarzwaller et al.(1999)reported that the mean annual diameter increase of S.leprosula at 4–10 mm a-1was similar to that of S.pinanga(9–11 mm a-1)but was generally higher than for S.mecistopteryx(6–7 mm a-1).Philipson(2009)reported that S.leprosula appears to be very adaptable since it had almost identical growth in mid-and high-light treatments.

Manii(M.eminii)is a fast-growing species grown mainly on private lands in Java.It is mostly found in very small-scale plantations,but its role in supplying timber to meet local needs is well recognised.Because it is a multipurpose tree species(Hanum and Maesen 1997),improved planting stock material is needed.A cooperative tree breeding programme for genetic improvement has been initiated between the Korea Forest Research Institute and Ministry of Forestry,Indonesia(Kang and Son 2012).However,no assessments of wood quality have yet been reported.

In contrast to manii,the five Shorea sp.(Dipterocarpaceae)studied here are common in natural forests.They are categorised as fast-to slow-growing,and conservation action has been initiated to avoid depleting wild populations.Although these trees are shade tolerant,Shono et al.(2007)and Millet et al.(2013)reported that meranti(Shorea spp.)tree species can adapt to open and semi-open lands.These species possess moderate wood strength according to the strength class of the Indonesian standard(Seng 1990)and are a mainstay for structural wood in the region.Therefore,information on wood traits is necessary,particularly with respect to different growth rates.The recommended improvement approach for meranti is to establish plantation forests in the form of enrichment planting,a silvicutural technique to increase commercially preferred timber species in natural forest,logged-over forest areas or degraded forest to supplement existing seedlings.It can be done via line planting or inserting in former skidding yards(Pinard et al.1998;Sheikh Ali 2006).

Data on wood traits from standing tree evaluations are not yet available,and information is limited on whether tree growth is negatively associated with wood stiffness and wood density for these species.Better information may be obtained by utilising data on variations in growth from specifically designed planting trials in which environmental effects are minimised.Representative specific gravity data for wood from some Indonesian tree genera(three or more species)were reported by Slik(2006)who found a good correlation between specific gravity of wood by genus and wood by species.Based on these results,we considered that combining data from several different Shorea species in the present study was appropriate.Because non-destructive assessment has contributed to obtaining early information on wood traits of standing trees,we assessed wood traits in two separate planting trials to determine the relationship between tree growth and wood quality,particularly in terms of the MOE and density of wood.

Materials and methods

Growth and yield measurement

In two separate planting trials in West Java six common tree specieswere grown:maniiwood(or M.eminii),S.leprosula,S.mecistopteryx,S.stenoptera,S.pinanga,and S.palembanica(Table 1).Trees were categorised based on growth rate(i.e.,slow,moderate and fast)as reported by Suzuki(1999),Philips et al.(2002)and Fleury et al.(2005).Manii tree seed planted in the plantation trial originated from three locations and had been collected to build a progeny test seedling seed orchard.Zulfahmi(2007)reported that the manii progenies from the three locations were genetically different.Tree growth categorisation in the study was based on information from a genetic improvement programme for manii wood in Cirangsad at Jasinga subdistrict,Bogor,West Java(Kang and Son 2012).In addition,the experimental design for manii species followed common garden trials by applying randomized complete block design(RCBD)arranged in eight blocks comprising 100 open-pollinated familieswith fourindividualsperfamily plot(Kang and Son 2012).On the other hand,characteristics of tree species growth for Shorea speciesormerantiwood atGunung Walat Educational Forest(GWEF),Cibadak subdistrict,Sukabumi,West Java(Faculty of Forestry 2005)were also assessed from a species trials with no special experimental design due to limited space.In thisstudy,Shorea standswere planted following randomly species block arrangement between existing plantations of trees that had diameters of more than 40 cm,and the intensity of light received may have varied.Diameter at breast height(dbh 1.3 m)and height were measured on all trees growing in the trials.Diameter and height of individual trees from the two trials were measured(n=800 individuals for manii;n=161 individuals for Shorea spp.).Fornon-destructive evaluation,48 trees for manii and 35 trees for Shorea spp.were sampled as described in Table 1.

Non-destructive assessment

Wood stiffness can be predicted by non-destructive assessments in standing trees using commercial non-destructive testing tool based on ultrasonic wave propagation(Sylvatest-Duo?with frequency 22 kHz,cbs-cbt Timber Construction Co.,St-Sulpice,Switzerland)was used.

The longitudinal measurements were carried out by mounting two transducers(a transmitter and a receiver)as shown in Fig.1 about 1-m apart vertically on the same side of the tree.The transducers were driven 20 mm into the tree to measure ultrasonic velocity from two opposite sides,three times per side.The values were then averaged to obtain a mean velocity per tree.

Wood density

Green wood samples were taken transversally using an increment borer with a 5-mm diameter on the north side ofeach tree at breast height.The samples were then wrapped in plastic and shipped to the laboratory.Green density was obtained by calculating the ratio of the sample’s bulk mass to its volume.Moisture content was determined using a gravimetric method by weighing the wood sample,then drying it at 103± 2°C for at least 24 h to a constant mass;the sample was ad then reweighed.Wood stiffness can be estimated as dynamic MOE using two components,the wood density and velocity of transmitted sound through the wood,and Christoffel’s fundamental equation(Bucur 2006;Horvath et al.2010):

Fig.1 Measurement in standing trees using nondestructive testing tool

where MOEdis the dynamic modulus of elasticity(Pa),ρ is the green density(kg m-3),V=velocity of sound propagation(m s-1),and g is the gravitational constant(9.8 m s-2).

Data were analysed to determine the influence of growth rate categories on growth performance traits as well as the correlation for each data pair(x,y)of growth rate categories of the species and tree volume through box plots and univariate scatter plots.Linear regression analyses were performed to evaluate the relation between wood properties and tree growth.

Results

Growth performance

The average tree(total)volume of M.eminii and the five Shorea species combined was 0.024 and 0.032 m3,respectively(Tables 2,3).Statistical analysis using Duncan’s new multiple range test(MRT)showed a significant difference for each growth performance category for manii wood(Table 2).Further,these significant differences were con firmed also in the box plot analysis(Fig.2).

Significant differences in the tree height,diameter and volume of S.leprosula compared with other species of Shorea were apparent(Table 3).In contrast to manii wood as found in the box plot analysis(Fig.2),the values for the five Shorea species still showed overlapping error bars,indicating large variation in terms of growth.The growth performance in both diameter and height for S.leprosula was in line with research by Phillips et al.(2002)and Widiyatno et al.(2013,2014).

Box plots provided more detailed information on the distribution of observed data values.For fast tree growth,the mean stand yield of tree volume was to be higher than for moderate and slow growth(Fig.2).Growth performance at this stage was higher in Shorea spp.than in M.eminii.Nevertheless,the best average growth rate of M.eminii still had a larger diameter than that of the best Shorea sp.,i.e.,S.leprosula.Although the study was conducted with limited data,it appeared that Shorea spp.had greater variance for each growth trait compared with M.eminii as shown in Tables 2 and 3.Shorea spp.at the seedling stage are highly dependent on the light conditions(Ashton 1998).Sukendro and Sugiarto(2012)reported that 60%shading treatment(light intensity of 40%)gave the best growth response to the growth of in S.leprosula and S.mecistopteryx.In this study,Shorea stands were planted between old plantations with trees that had diameters of more than 40 cm,and the intensity of light received mayhave varied.Philipson(2009)reported that only S.leprosula appears to be very adaptable since it had almost identical growth in mid to high light treatments.

Table 2 Means(±SD)for growth variables of Maesopsis eminii(at 7 years of age)according to growth rate of trees

Table 3 Mean values of Shorea spp.growth performance(at 9 years of age)

Wood characteristics

Fast-growing trees tend to have poor wood quality,while slow-growing trees have high wood density(Evans 1992).The average green-wood density for M.eminii was 780 kg m-3.In Shorea spp.,the average green-wood densities for S.leprosula,S.mecistopteryx S.stenoptera,S.pinanga and S.palembanica were 820,680,700,760 and 680 kg m-3,respectively.The mean ultrasonic wave velocity(Vus)value for M.eminii was 3266 m s-1.This value was lower than that of Shorea spp.as estimated for each Shorea spp.(from 4104 to 4447 m s-1).The average dynamic MOE for M.eminii was 8.53 GPa.Meanwhile,the mean of dynamic MOE values for S.leprosula,S.mecistopteryx,S.stenoptera,S.pinanga and S.palembanica was 14.15,13.19,12.65,15.19,and 13.86 GPa,respectively,at a moisture content of approximately 70%.

Statistical analysis showed there was no significant difference in the respective wood traits for each growth rate category for manii wood(Table 4).Significant differences were found in wood density of Shorea species,while other wood characteristics for each growth rate category were not significantly different(Table 5).

A highly positive relationship was found between stiffness properties of dynamic MOE and ultrasonic velocity for all species(Figs.3,4).A low to moderate positive correlation existed between the dynamic MOE and greenwood density in M.eminii and Shorea spp.Very low negative correlations were found between ultrasonic velocity and wood density for both M.eminii and Shorea spp.For all species together,the correlation between tree volume and wood density and between dynamic MOE and ultrasonic velocity were very weak and mostly negative,respectively(Figs.3,4).

Discussion

Indirectly measured using acoustic velocity for trees and genetic improvement programmes,MOE has been effectively used as a selection criterion for Douglas-fir in comparison with bending stiffness,which involves costly and destructive testing(Johnson and Gartner 2006;Vikram et al.2011).Chiu et al.(2013)used a non-destructive ultrasonic wave propagation method on Taiwan incense cedar(Calocedrus formosana(Florin)Florin)to assess the relationship between characteristics of standing trees and wood properties.

Fig.2 Box plot showing the range of tree volumes for each growth rate in(a)Maesopsis eminii and(b)Shorea spp.(open circle:outliers;asterisk:extremes;superscript numbers indicate corresponding data lines)

For breeding programmes,selecting for increased growth may significantly influence wood quality(Howe et al.2006).Cherry et al.(2008)found that improvement in growth could have a small positive impact on bending stiffness.Tree stands that grow fast in the early stages of development were linked to mature xylem structure in terms of wood quality.From a production perspective,an early assessment of wood traits enables the silviculturist or tree breeder to manage tree-improvement programmes more effectively.Results from the current study suggest that there are no significant differences in wood stiffness with growth rate category(Tables 4,5).However,the plantation trials in this study were initially designed to focus only on evaluating growth performance,not on wood traits.By combining two separate field trials for comparison,we were able to obtain general trends in terms of growth and wood stiffness quality for designing better field experiments in the future.Also,it may be important to consider a destructive sampling for confirming the results of the non-destructive testing(NDT)in this study.So,integration of wood quality assessment into on-going trials is recommended for breeding programs of the species of interest.

Previous studies on tropical wood properties using nondestructive testing have focused mainly on timber and small specimens(Oliveira et al.2002,2006;Karlinasari et al.2005,2008;Teles et al.2011;Baara et al.2015).Indirect estimates of mechanical properties in standing trees can be obtained by measuring green-wood density and the velocity of acoustic waves travelling through the wood,which can then be used to calculate the stiffness properties of the dynamic MOE.The highly positive relationship between dynamic MOE and ultrasonic velocity showed that the elastic and viscoelastic properties of a material were major factors that affected wave propagation(Bucur 2006).Our study found that green-wood density had a positive relationship with dynamic MOE.In some of the studies on temperate tree species mentioned already,basic wood density was positively correlated with MOE determined using other indirect tools(e.g.ST300 or HM200,Fibre-gen,Christchurch,New Zealand)(Cherry et al.2008;Hong et al.2014).Increasing density is not always correlated with increasing wood stiffness,since the influence of microfibril angle can be more important in determining wood strength characteristics(Chauhan and Walker 2006).A negative relationship was found between tree growth in terms of height(Nguyen et al.2014),diameter and volume(Cherry et al.2008)with wood density.Also,Urhan et al.(2014)found that squared acoustic velocity had weak genetic correlations with growth traits(height,diameter at breast height,andvolume)in young Douglas-fir,but moderately positive correlations in young western hemlock.Lenz et al.(2013)showed a trend toward a positive correlation between acoustic velocity and tree height for young trees of white spruce(P.glauca),which provided an opportunity to improve both wood quality and growth,while avoiding the negative correlation between growth and wood density especially in young trees.In the current study,tree volume was negatively correlated with wood density and acoustic velocity,but stiffness properties were not significantly different based on growth rate.

Table 4 Average values for wood traits of Maesopsis eminii growing at different rates

Table 5 Average values for wood traits of Shorea spp.growing at different rates

Fig.3 Fitted models of Maesopsis eminii for each pair of variables

Fig.4 Fitted models of Shorea spp.for each pair of variables

Conclusion

Early screening and selecting of individuals by means of non-destructive ultrasonic-based testing offers a cost-effective way to identify trees that will yield more value to provide information on wood quality at a young age.Studies conducted using two separate plantation trials found that M.eminii from a progeny trial of fast-growing trees had significantly different volumes for each growth rate(i.e.,fast,moderate,slow),likely due to genetic variation.For the Shorea spp.tested,the fast-growing S.leprosula had the highest tree volume values,which differed from the growth rates of moderately growing species(S.mecistopteryx and S.stenoptera)and slow-growing species(S.pinanga and S.palembanica).However,ultrasonic velocity and wood stiffness properties derived from the dynamic MOE values revealed no significant differences for any growth rate in either M.eminii or Shorea spp.The results of this study show that negative correlations were found between tree volume and wood quality traits,i.e.,wood density,dynamic MOE,and ultrasonic velocity.In a breeding programme context,it should be noted that increased growth in young trees will lead to poor wood quality.However,the wood traits did not differ signif icantly different based on growth rate for the young trees studied here.

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