Md.Abdullah Al Mahmud?Mohammad Mahfuzur Rahman?Mohammed Kamal Hossain

Introduction

Teak(Tectona grandis Linn.f.)is a verbenaceous tree species that was first formally described by Carl Linnaeus the Younger in 1782(Derkyi et al.2009).It is mostly found across the tropical or subtropical regions of the world(Benthall 1946;Hedegart 1976).It grows well in welldrained,deep alluvial soil below 1000 m a.s.l.,with annual rainfall of 1250–3750 mm,minimum temperature of 13–17 °C,and maximum temperature of 39–43 °C.This species is highly light-demanding and it coppices and pollards vigorously.The primary factors affecting the growth of teak are soil depth,soil texture,drainage,moisture status,and fertility(Seth and Yadav 1959).

The species was naturalized in Java where it was probably introduced 400–600 years ago(Troup 1921;White 1991).Natural teak forests are estimated to cover 29.04 million ha across India,Lao PDR,Myanmar,and Thailand(Kollert and Cherubini 2012).Beyond its natural range,the total land area planted with teak plantations has greatly increased to 4.3 million ha across the world(Camino et al.2002;Kollert and Cherubini 2012;Kumar 2011;Pandey and Brown 2000),of which Bangladesh shares about 3.2%(Kumar 2005).Since the species produces a superior quality of wood(Jayaraman 1998;Kollert and Cherubini 2012;Pe′rez and Kanninen 2005)and has high resistance to diseases(Derkyi et al.2009),decay(Wehr et al.2010),and fire(Derkyi et al.2009;Ladrach 2009),it is the world’s most cultivated high-grade tropical hardwood species(Bermejo et al.2004).

Considering the global significance of teak,the first effort of raising a teak plantation in Bangladesh was attempted on only 4 ha area of land at the Sitapahar Range(now Kaptai Range)of Chittagong Hill Tracts(CHTs)in 1871,with seeds imported from Myanmar(Hossain et al.2008).The plantations were limited to the CHTs until 1920,when they were extended to Chittagong,Cox’s Bazar,and Sylhet districts in 1921(Hossain et al.2008);however,the total coverage of teak plantation in the country has been a matter of debate,with an area variously reported as between 40,000 and 216,994 ha(Haque 2000;Muhammed et al.2007).

More than 70%of the total plantation area in CHTs was occupied by teak(Rahman and Mustanoja 1978)but the more teak monoculture has expanded,the greater the ecological concerns for the region(Feroz et al.2014)because hill slopes are denuded,expediting soil erosion(Zaman et al.2010).A number of studies(Aborisade and Aweto 1990;Amponsah and Meyer 2000;Balagopalan 1995;Healey and Gara 2003;Mongia and Bandyopadhyay 1992;Myint and Dah 2001)reported that teak plantations are associated with soil deterioration.In addition,Carle et al.(2009)reported that teak plantations do not provide hydrological benefits in the plantation areas.

The continuous loss of surface soil in teak plantations results in reduced soil fertility(Gafur 2001).On many sites,teak production is limited by low nutrient availability,particularly in N(Zech and Drechsel 1991).Teak occurs naturally on fertile soils derived from limestone,basalt,and alluvium(Tanaka et al.1998);it also reflects a high nutrient requirement(Craven et al.2007;Zech and Drechsel 1991,1994).Soils of teak plantations tend to be acidic,potentially Al-toxic,and less fertile than under natural forests.Teak has a high requirement for N,P and Ca and pH＞4.7;therefore,teak sites need lime and fertilizer to ensure adequate establishment and growth of the planted teak trees(Craven et al.2007;Zech and Drechsel 1991,1994).

In general,nutrient accumulation increases with the age of teak stands mainly because biomass accumulation and nutrient uptake during early years is considered crucial to sustain high growth rates.Nevertheless,the rapid expansion of both crown and roots requires an appropriate soil nutritional status throughout the entire rotation period(Laclau et al.2003;Miller 1981).Given the knowledge in existing literature,we hypothesized that the rapid exposure of teak soils coupled with excessive nutrient uptake results in more or less barren soils.Though teak has been grown in plantations in Bangladesh for about 145 years,very little research has focused on the quality of the forest soils in these teak stands,and changes in the soil properties in the plantations have not been addressed adequately.Such studies are of great importance to sustain the expected teak yields and healthy forest soils.With a view toward achieving both goals,this study,the first of its kind in Bangladesh,assesses the soil physicochemical properties in teak plantations and evaluates the marginal effects of factors on these soil properties.The study is expected to provide vital suggestions on how to deal with the degraded soils under teak plantations in Bangladesh as a whole,especially in CHTs.

Materials and methods

Study area

This study was conducted in the Kaptai and Karnaphuli forest ranges in the southeastern hilly region of Bangladesh.Because teak was first introduced in 1871 to Bangladesh in these ranges(Hossain et al.2008),the study was expected capture the complete history of impacts of teak plantations on soil properties in the region.The forest ranges are located in the Sitapahar Reserve Forest,which is historically one of the oldest teak-dominated forest areas in Bangladesh.In addition,the ranges are a fair representation of the teak plantation stocks in the entire southeastern area of Bangladesh.The Kaptai and Karnaphuli ranges are in the Chittagong Hill Tracts(South)Forest Division(Fig.1).Both of the ranges are situated in the Kaptai upazila of the Rangamati Hill District.

The topography of the study area is diversified,from almost flat land to medium and high hillocks(Feroz et al.2014;Rahman et al.2013).Approximately 10%of the landscape is occupied by well-drained flood plains(Rahman et al.2013).The remaining area,owing to its steep slopes and exposed surface,is often subject to soil erosion and landslides(Biswas et al.2012).The area has a sandy to sandy loam soil texture(Table 1)developed on consolidated or semi-consolidated siltstone,sandstone,and clayey soil(Biswas et al.2012;Feroz et al.2014).The soil pH ranges from 4.5 to 6.0.Mean annual temperature is 29.6°C,and mean annual rainfall is 2540 mm(Feroz et al.2014).The mean relative humidity is over 90%throughout the year(Rahman et al.2013).The parent materials are poor in weatherable minerals(Biswas et al.2012).

Data collection and laboratory analysis

We selected teak plantations from 1960,1971,1979,and 2000 in the Kaptai range and of 1952 and 1989 in the Karnaphuli range.These plantations were established and managed by the Bangladesh Forest Department(BFD).Details on the study sites are illustrated in Table 1.

Five quadrats,20×20 m each,were randomly located in each of the six selected age series to collect plantation growth data.We calculated stems per ha(SPH)and basal area per ha(BAPH)for each of the plots.Of the five quadrats in each age class,three were randomly chosen,totalling 18 quadrats,for collecting soil samples.In each of the 18 quadrats,a single soil pit was dug using earth augur.Each pit contained three depths of soil profile(DSP):0–7 cm,＞7–15 cm,and＞15–30 cm.Thus,a total of 54 soil samples were collected for physicochemical analysis of soil moisture content(MC),moist bulk density(MBD),dry bulk density(DBD),particle density(PD),and soil texture and pH,organic carbon(OC),nitrogen(N),phosphorus(P),and potassium(K).

Fig.1 Map of study area in the Chittagong Hill Tracts,Bangladesh

Table 1 Plantation history,topographic structure and the physical features of soils in teak plantations in Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

In the laboratory,collected moist soil samples were first sieved through a 10-mm wire mesh to remove gravel,small stones,and coarse roots.Then the sieved soil was passed through a 2-mm sieve.After that,the sieved samples were mixed thoroughly and stored at room temperature.The Bouyoucos hydrometer method was used to determine soil texture using the dry soil samples(Huq and Alam 2005).Moist field soil samples were used for determining soil pH.In addition,some subsamples were dried in an oven at 105°C for 8 h to determine MC.Oven dry samples were used to determine OC,N,P,and K.Core soil samples were used to determine MBD,DBD,and PD.MC was determined from core samples collected for bulk density.Mass of field-moist and oven-dry soils in the core was divided separately by core volume to determine MBD and DBD,respectively.PD was determined taking two 10 mL cylinders,the first filled with 10 mL of water.In the second cylinder,2 g of soil burnt in a furnace was taken and water from the first cylinder poured until water reached the 10 mL mark.The remaining water in the first cylinder indicated the volume of soil particles.Then,the mass of soil divided by volume gave the soil PD.Soil OC was calculated by the loss using the ignition method according to Ball(1964).Soil pH was determined using a TOA pH meter in triplicate at 1:2 soil–water ratios(Jackson 1973).Total N was determined by the micro-Kjeldahl method(Jackson 1973).K concentration was quantified using a flame photometer(Jackson 1973).Available P was extracted by ammonium fluoride-hydrochloric acid(Bray and Kurtz-2)and determined according to the SnCl2-reduced molybdophosphoric blue color method(Jackson 1973).

Analytical framework

The variation of means and standard errors(SE)of the physical and chemical soil properties across plantation age,soil depth,topography,and forest ranges was examined.Since teak commercially matures at the age of around 40 years(Muhammed et al.2004,2007),we divided the stands into two age classes to examine whether immature stands differed from mature stands in altering soil properties in the plantations.We classified teak plantations＜40 years old as immature stands and those≥40 years old as mature.

Correlation matrix

To examine the degree of association between the plantation attributes and physical and chemical properties of the soils,a correlation matrix was estimated using the Hmisc package of R(version 3.2.2).This package provides both the correlation matrix and the significance level of each correlation between any two variables.

Multiple linear regressions

While correlation coefficient reveals the degree of association between two independent variables,the correlation matrix cannot provide the impact of a single explanatory variable on the response when more than one explanatory variable is present in the system.Unlike a correlation matrix,multiple linear regressions provide marginal impacts of each of the explanatory variables on the response variable.Thus,our explanatory variables were the soil physical properties and plantation attributes and the response variable was OC,total N,available P,or K concentration for each time the model was run;however,we estimated the multicollinearity between the regressors of all possible pair of regressors using the VIF package of R 3.2.2.Before running the regression,we dropped one of the variables from each of the correlated pair of variables.This change improved the predictability of the models.We estimated the following ordinary least square regressions:

where Yiis soil nutrients and i∈ ｛ OC，N，P，K｝,X1the dry bulk density(g/mL),X2the particle density(g/mL),X3the depth of soil pro file(1:0–7 cm,2:＞7–15 cm,3:＞15–30 cm),X4the stems/ha,X5the topography(1:hilltop,2:mid-slope,3:valley),X6the forest range(1:Kaptai range,2: Karnaphuli range), β0the model intercept,β1，β2，β3，β4，β5，β6are parameters to be estimated,and εiis normally distributed error terms with mean zero and constant variance.

Results and discussion

Descriptive statistics

Mean impacts across age groups and soil depths

Figure 2 illustrates the mean impacts of teak on different physical and chemical properties of the soil in the plantation area.We found no significant difference in any of the physical and chemical properties of mature versus immature teak stands(Fig.2a).Teak plantations affected physical and chemical properties at different depths along the soil profile(Fig.2b).In topsoil,the concentration of P was significantly higher than in lower soil layers.The K concentration at the＞7–15 cm depth did not differ signif icantly from that at＞15–30 cm.The results for N were similar to those for K content.A similar trend(decreasing concentration)was observed in the concentration of OC in the soil(Fig.2b).OC significantly decreased from top towards the bottom layers.

Unlike OC,soil MC and DBD increased from the top to the bottom layer of the soil profile(Fig.2b).Similarly,the bottom layer of soil had a significantly higher percentage of MC compared to the mid-layer.Rain water infiltration and solar heating of the exposed topsoil probably caused the MC decrease from the top to the bottom layers.Similar results were reported by Miah et al.(2014).DBD,as expected,had significantly increased its concentrations downward;however,the top and mid-layers did not differ in DBD concentrations.Chaudhari et al.(2013)clarified that soil bulk density increases with soil profile depth,due to changes in OC content,porosity,and compaction.In a similar study in teak plantations in Ghana(Amponsah and Meyer 2000),bulk density increased downward through the soil profile.

Fig.2 Mean values for soil properties in teak plantations in the Kaptai and Karnaphuli forest ranges of Chittagong Hill Tracts,Bangladesh

Mean impacts across topography and forest ranges

Figure 2c illustrates the soil properties at different positions on slopes of the hilly teak forest.All the physicochemical attributes of the forest soils in the teak plantations differed significantly at different hill slope positions except for P and DBD.N concentration in the soil on the hilltop significantly differed from that of the mid-slope and valley.Even though the K ion concentration on the hilltop was the same as that on the mid-slope of hilly terrain,it was significantly greater than that in the valleys but the mean value for soil N and K content was lower in teak plantations during the study.Thus,we can conclude that due to the conversion of natural forest to teak plantations,the concentrations of N and K decreased through leaching loss in the study areas.Similar findings were identified by Hase and Foelster(1983).They reported that the lower amounts of K storage in teak plantations were characterized due to leaching loss caused by replacement of the natural forest.The concentration of OC was the same at the hilltop and mid-slope of the study area.The valley had a significantly lower concentration of OC than in the upper slopes.The lower concentrations of OC in the valley might be contributed to the removal of litter by the local people for fuelwood.This finding contradicts the results of Lawal et al.(2014),who found a more OC on the lower hill slopes of teak plantations.Again,as expected,the MC was significantly higher in the valley compared to that of the hilltop and mid-slope of the mountainous teak forests,but the mid-slope and hilltop MC did not differ significantly.Figure 2d summarizes the differences in the soil properties in the two forest ranges under study—Kaptai range and Karnaphuli range.Of the soil properties,only OC and MC differed significantly between the ranges.The soils of Kaptai range had significantly greater OC concentrations compared to those in the soil of Karnaphuli range;however,the MC of the Karnaphuli soils was significantly greater than that of the Kaptai range.

Correlation between plantation attributes and soil properties

Table 2 presents the pairwise correlations between the soil properties and plantation attributes.The teak plantation clearly significantly impacted different physical and chemical properties of the forest floor.Plantation age and SPH were inversely correlated(r=-0.33 and-0.66,respectively)with N content,while the BAPH was positively correlated(r=0.57)with N.That means younger plantations added more N to forest soils than did the older ones,probably because the younger plantation still had some under growth that was adding N to the forest floor.With an increase in teak plantation age,the forest floor became exposed as the underbrush disappeared,contributing less N to mature and over-mature teak plantations.This latter finding agrees with the study by Farley and Kelly(2004),who reported a linear decline in N concentration with stand age;however,the results of other studies were in contrast.For example,Adekunle et al.(2011)found lower N content in young teak plantations than in older ones,but lower litter inputs and higher nutrient demands in younger stands act as a key driver of a reduction in nutrient status in the teak forest floor(Singh and Sharma 2007).Since older tree plots had greater BAPH than in younger plots,the positive correlation between N and BAPH was complemented by the negative correlation between N and SPH.Thus,we concluded that there was no universal and uniform relationship between teak plantation age and N content in the forest soils.The soil P content was negatively correlated with soil depth,plantation age,and BAPH(r=-0.34,-0.28,and-0.68,respectively).Thus,soil P content was higher in the surface soil layers than in the lower layers.Our resulted were similar to those of Ogundele(2015),Ali et al.(2010)and Zaman et al.(2010),who all reported that P content in plantation soils decreased downward through the soil profile.Release of P tends to be lower in those soils with high compaction at deeper soil depths(Brady and Weil 2002).Negative correlation between plantation age and soil P content indicates that,in the younger teak plantations,the soil P content was higher compared to that in the older plantations.Since the older plantation had greater BAPH,our observation that P-content being inversely related to plantation age was justified by the inverse relation between BAPH and soil P content.However,Adekunle et al.(2011)reported a direct relationship between P content and BAPH in teak plantation soils.This contrast might be attributed to the inherent differences in soil properties of the study area.

The exchangeable K content was significantly correlated with soil depth,plantation age,topography,forest range and BAPH.A negative correlation(-0.27)between the K content and soil depth suggests that soil K content decreased with the increase of soil depth,similar to results from Vadivelu et al.(1993)and Hossain(2002).We found a negative relationship(r=-0.18)between the soil K content and the age of the teak plantations suggesting that soil under younger teak plantations was richer in K than the older ones.Adekunle et al.(2011)also found similar results in the soil under teak plantations.Since BAPH increased with increasing plantation age,BAPH should also be negatively linked with the K+concentration in the soil.Thus,the negative relationship between K+and BAPH was intuitive.Soil K content also showed a negative correlation(-0.37)with topography indicating that,at the hilltop,there was higher K content in the soil of teakplantation than on lower slopes.Soil K content was found negatively correlated(-0.28)between our two study areas,with Kaptai richer than Karnaphuli.Soil of Karnaphuli range had a greater pH value compared to that of the Kaptai range and soil pH showed a positive association(r=0.69)with the SPH.The study area had greater density(SPH)in younger plantations,the older plantation sites contained lower pH level compared to the younger sites,similar to a study(Suzuki et al.2007)conducted in Myanmar.

Table 2 Correlation matrix of teak plantation attributes and soil properties in Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

Soil MC was significantly correlated with all plantation attributes except SPH.Soil MC was positively correlated(0.36)with soil depth suggesting that the topsoil in the teak plantation was relatively dry compared to the deeper soil layers.This outcome could be due to excessive litter collection from the forest floor by the local people,exposing the surface layer to direct sunlight,and exacerbated by slash burning during the summer season.Liao et al.(2012)found a similar result in their meta-analysis.MC offered a favourable environment for microbial activities that might contribute to increased N in the soils.Lipman and Sharp(1915)and Myers et al.(1982)reported that there is a linear relationship between N mineralization and soil MC.Again,higher soil MC in the older teak plantation was confirmed by a positive correlation(r=0.33)between soil MC and plantation age,but no similar results were found in other studies.

Topography was positively associated(r=0.52)with soil MC.As expected,the valley area contained greater moisture than the area with higher elevation.Further,BAPH was positively and strongly correlated with soil moisture content,which again confirms that MC in the soil increases with increasing plantation age.Both moist and dry bulk densities had similar relationships with a particular plantation’s attributes,but the association between MBD and each of the plantation attributes was not significant,while DBD was significantly negatively related to topography and study area.DBD decreased with increasing elevation of the plantation,and the sites in the Kaptai range has greater DBD.Again,soil PD was significantly associated with plantation age,SPH,and BAPH.There was a significant positive relationship of plantation age and BAPH with soil PD(r=0.28 and 0.70,respectively).This latter result was further complemented with the negative association(r=-0.60)between PD and SPH.OC content of the soil had significant negative associations with soil depth,topography,forest range,and SPH(r=-0.46,-0.59,-0.77,and-0.44,respectively),suggesting that topsoil was rich in OC contents than the soil beneath it.Binkley and Fisher(2012)and Usuga et al.(2010)also found the highest soil OC content in the surface soil in teak plantations.The lowest OC was found in the valley,a counter-intuitive result.Lawal et al.(2014)opined that the lower the hill slope,the higher the OC content in the teak plantations would be but only because the litter in the valleys was mostly removed by the local people as fuelwood.We found that the Karnaphuli range had signif icantly less OC than in the Kaptai range.Again,younger plantations added less OC than older plantations did,indicating that older plantations added more litter to the forest floor.Similarly,Suzuki et al.(2007)reported that soil OC contents were inversely associated with the age of teak plantations in Myanmar.

Marginal impacts of different variables on soil chemical properties

Organic carbon accumulation

We assumed that teak plantations significantly contributed to changing the physical and chemical properties of forest soils.DBD,DSP,SPH,and forest range significantly affected the OC contents in the soils of the teak plantations(Table 3).Each unit increase in the bulk density contributed to a 0.503%decrease of OC in the soil.Sakin et al.(2011)and Chaudhari et al.(2013)also reported that soil OC had a strong negative correlation(r=-0.89)with the soil bulk density.Again,with a unitary increase of DSP,OC concentration decreased by 0.191%,which resembled the findings of Binkley and Fisher(2012)and Usuga et al.(2010),who reported that the soil OC gradually decreased from the upper layer to lower layer of soil in teak plantations.The latter result indicates that penetration of OC into the soil profile was slow.Since DBD was negatively linked with OC,and DBD increased with an increase in soil depth,the density of OCreduction in the deeper soil horizon was justified.The parameter estimate for SPH was-0.001,which means that the plots with higher value of SPH,i.e.,plots containing younger teak trees,had lower OC contents.Suzuki et al.(2007)conducted a study in Myanmar and also reported lower OC in the soil of younger teak plantations.Finally,since the parameter estimate of forest range was-0.157,we conclude that OC was higher in the Karnaphuli range than in the Kaptai range.

Table 3 Factors affecting organic carbon accumulation in the soils of teak plantations in Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

Total nitrogen accumulation

We found DBD,DSP,SPH,topography and forest range significantly affected the status of N in the soils of the studied teak plantations(Table 4).The parameter estimate for DBD was-0.129,which was highly significant indicating that more compacted forest soils had low N content.The parameter estimates of DSP,plantation age,and SPH were all significant at 1%level and were negatively linked to N content.Thus,deeper soil,older plantations,and denser tree coverage were related to reduced soil N contents,which was also confirmed by Zaman et al.(2010)and Lawal et al.(2014).The parameter estimate for topography was 0.016 indicating that N concentration increased from the hilltop toward the valley where the soil was rich in litter accumulation,MC,and microbial activities.Lawal et al.(2014)also found lower total N in teak soil at the mid-slope position.Similarly,the beta estimate for forest range was positive(0.042)and significant,indicating that the Karnaphuli range had greater N concentration than the Kaptai range.

Table 4 Factors affecting nitrogen accumulation in the soils of teak plantations in the Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

Available phosphorus accumulation

Four variables,DSP,SPH,topography,and forest range,significantly affected P accumulation in the soils of the teak plantation in the study area(Table 5),and except for forest range,all variables had negative estimates.Like the N concentration,P concentration was also higher in the Karnaphuli range than in the Kaptai range,but beta estimates were-3.99,-0.005,and-0.175 for DSP,SPH,and topography,respectively.As expected,P concentration decreased with an increased soil depth.Ogundele(2015),Ali et al.(2010)and Zaman et al.(2010)reported similar results.Again,forest floors with more stems per ha(indicating more younger trees)had lower P accumulation.However,the opposite was found by Adekunle et al.(2011),who reported that soil P-concentration was higher in the older teak plantations than in the younger ones,but P concentration was elevated on the hilltop sites compared to low values in the valley.Lawal et al.(2014),on the other hand,found that the available P content was lower at midslope than in the valley.

Exchangeable potassium accumulation

All variables except PD significantly affected the K concentration at both study areas(Table 6).DBD was negatively linked to K concentration with a beta estimate of-0.5,indicating that,as expected,compacted soil had relatively low concentration of K.DSP had a parameter estimate of-0.056,which was significant at 1%level,suggesting that K concentration also decreased with increase of depth of the soil profile,similar to results from Vadivelu et al.(1993)and Hossain(2002).These latterstudies also identified a decreased soil K-concentration with profile depth because of the higher compaction in the soil underneath.Tree density per ha was negatively linked to K-concentration,which was also found for the soil under teak plantations by Adekunle et al.(2011).Topography had the parameter estimate of-0.142 indicating that K concentration decreased from hilltop downward to the valley.The parameter estimate for forest range was positive(0.268)showing that the Karnaphuli range was richer in K concentration than the Kaptai range.

Table 5 Factors affecting phosphorus accumulation in the soils of teak plantations in Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

Table 6 Factors affecting potassium accumulation in the soils of teak plantations in Kaptai and Karnaphuli forest ranges of the Chittagong Hill Tracts,Bangladesh

Conclusions

This study showed that the intensive monoculture teak plantation has adversely affected the soil properties in our study region.Exposure of the soil surface to the sun and gradual disappearance of undergrowth left these teak plantation floors with infertile,degraded soils.The N content in the older teak plantations was scarcer due to soil compaction.We concluded that teak monoculture has caused the soil to deteriorate,with reduced N,P,and K in the region.Thus,it is important to take steps to improve the soil quality of the study area by converting the monoculture to mixed tree farming and preparing the forest floor to support the growth of an understory composed preferably of leguminous herbs and shrubs.At the same time,litter removal must be reduced to create a mulch for the forest floor and favourable soil microclimatic conditions for the sustainable growth and development of diverse forest crops in the study area.Together,these steps could be expected to ameliorate the degraded soils on the hilly terrain of the teak plantations in the Chittagong Hill Tracts,Bangladesh.

AcknowledgementsWe acknowledge Dr.Ian Thompson from Canada and Ms.Carmeneisha Stallings from the United States for their support to improve the English of the manuscript.We also appreciate the Deputy Editor-in-Chief of the Journal of Forestry Research,Dr.Ruihai Chai and the anonymous reviewers who helped us improve the overall quality of this article.

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