Introduction

Forests store more carbon per unit area than any other terrestrial ecosystem(Daryaei and Sohrabi 2015;Temesgen et al.2015).However,exploitation of resources and conversion of forest to others land uses have contributed to the decline and degradation of temperate forests.Fortunately,reforestation projects have gained much interest worldwide:at present,about half of the overall increased area of tree plantations is on natural forest lands(Chen et al.2013).

Planted forests(plantations)can produce wood material for industrial uses and fuel(Bargali et al.1992a,b,1993;Bargali and Singh 1991,1995).During the last two decades,in response to worldwide increase in wood demand,fast-growing species in plantations have gained large interest(FAO 2010).According to theMillennium Ecosystem Assessment,plantations ful fi ll more than one third of the world demand for wood products.Furthermore,the establishment of plantations can be proposed as a tool for forest restoration of degraded lands(Phongoudome et al.2012).

In addition,plantations are ef fi cient sequesters of carbon and can mitigate the predicted rise in atmospheric CO2 concentration and future climate change(Zhang et al.2012).According to FAO(2010),total present C storage in forest plantations is approximately 11.8 Gt with an increase of 0.178 Gt a-1(Phongoudome et al.2012).

In Iran,initial reforestation projects were aimed at wood farming.These initial projects began as early as 1989.Because of the substantial number of monoclonal plantations in northern Iran,the nation was ranked tenth globally among other countries based on the size of planted forests(FAO 2006).Based on the of fi cial report of the Forests,Rangelands and Watershed Management Organization of Iran,to date,more than 158,650 hectares of plantations have been established.But still there is little information about the contribution of these plantations to carbon sequestration.

Numerous factors affect the magnitude and progress in C storage,such as tree species,which can strongly affect the C accumulation of an ecosystem(Gao et al.2014).The growth rate of different trees,biomass allocation to different tree parts,and varying rates of carbon sequestration in ecosystem components can all affect the rate of carbon sequestration and longevity of carbon storage.Moreover,the portion of sequestered C in different components of trees,such as in the leaf,stem,or branch(referred to as allocation of biomass)can affect the ecosystem carbon cycle and longevity of carbon storage.For this reason,selecting a suitable species for cultivation is an important part of plantation projects.

随着新课程改革对发展学生核心素养这一重要目标的提出，生物学核心素养已成为生物课程标准、学业评价标准、考试标准的重要依据。生物学核心素养包括生命观念、科学思维、科学探究、社会责任等四大组成要素，是学生在接受生物课程教学过程中逐步内化形成的。通过简析2018年中高考个别试题对核心素养的考查情况，对在中学生物课程中如何实现初高中生物学核心素养的衔接培养有所启示。

The objectives of this study were o characterize the storage and distribution of aboveground biomass and carbon in the bark,stems,branches and leaves of the trees.The results of this research will provide information for future site management strategies and species selection aimed to conserve site productivity or replenish soil fertility.

Materials and methods

滑坡稳定性计算及滑坡推力计算，其目的是为滑坡在不同工况条件下的稳定性评价及滑坡防治提供设计依据。计算荷载考虑滑体自重、地表荷载、暴雨、动荷载、地震等因素。按《滑坡防治工程设计与施工技术规范》(DZ/T0219-2006)，滑坡治理根据其危害对象程度及潜在经济损失，滑坡稳定性计算工况、荷载组合及抗滑稳定安全系数见表3。

The study was conducted in two regions with same climatic conditions as the southern coast of the Caspian Sea,Mazandaran Province,Northern Iran:Klodeh(in 36°35′N,52°10′E)and Chamestan(in 36°29′N,51°59′E).These regions have a temperate climate(Fig.1),with a mean annual temperature of 16.9˚C.The mean annual precipitation is 802–823.5 mm and most of precipitation falls between September and March.This region has a distinct dry season that stretches from April to August.The topography is characterized by fl at lands to low hills at an elevation of 5 and 100 m above sea level for the Klodeh and Chamestan sites,respectively.The site soil texture is silt-loam with poor drainage.The average pH ranges between 7.6 and 8.1.

拦水坝的修建反映了当地人民有抵御自然灾害的意识和修建防御工程的聪明才智，反映了少数民族应对复杂自然环境所具备的独特的智慧。

Fig.1 Location of study sites in northern Iran

Historically,the study sites were occupied by a temperatedeciduousforestdominated by Quercuscastaneifolia C.A.Meyer., Gleditschia caspica Desp.,Carpinus betulus L and Parrotia persica.Three tree species including Alnus subcordata C.A.Mey(three stands),Populus deltoides Bartr.Ex Marsh(three stands),and Taxodium distichum(L.)L.C.Rich(Two stands)were planted in 1992 after clear cutting with the aim of wood production.The plantations were established at an initial spacing of 625 trees ha-1.No thinning operations were made in these plantations.A description of the study sites is presented in Table 1.

Experimental design,sampling and laboratory analysis

Based on the DBH and height of the surveyed trees,we selected 36 trees(4+4+4 for A.subcordata and P.deltoides,and 6+6 for T.distichum)in different diameter classes,which were harvested during the summer.The trunks were marked in three parts(bottom,middle,and top),cut into 2 m sections,and weighed.The total weight of each stem was then calculated by adding up the component masses for all sections.At the end of each trunk section,we cut a 5 cm-thick disk.We then transferred samples to the laboratory and measured wood density,using the water replacement method.

Table 1 Site properties and three plantation stand characteristics in the study area

Plantation types A.subcordata P.deltoides T.distichum DBH(cm) 31.32 ± 7.68(15.92–50.96) 27.43 ± 6.34(12.42–43.94) 28.44 ± 6.83(17.83–47.04)Tree height(m) 21.98 ± 4.38(10.1–31.8) 29.79 ± 4.77(16.9–39.5) 17.74 ± 2.66(9.8–24.1)Crown height(m) 8.91±2.69 11.45±3.30 6.67±1.94 Crown diameter(m) 4.02±1.38 3.12±1.20 4.25±0.96 Stem density(trees ha-1) 459 556 556 Main understory vegetation Shrubs Ruscus hyrcanus,Smilax excels Ruscus hyrcanus,Smilax excels Herbaceous plants Carex remota,Carex sylvatica,Poa sp. Carex remota,Carex sylvatica,Poa sp.Soil particle Sand(%) 20.5 27 32 Silt(%) 59.2 54 51.8 Clay(%) 20.3 19 16.2

Mineral soil samples were taken from depths up to 100 cm in six plots from each stand.At each plot,three soil samples were extracted from fi ve depths(0–20,20–40,40–60,60–80 and 80–100 cm)using a soil corer.Soil samples from the same layer were mixed and one representative sample was taken to the laboratory.In addition,from each soil layer in the three plots of each stand,three samples were taken to determine the bulk density of soil.Soil samples were air dried and sieved with a 2 mm sieve and analyzed for total soil carbon concentration by Walkley–Black method.

Destructive tree sampling

In 2014,48 sampling plots(16×16 m)were set up in eight stands replication of the three species(6+6+6 for A.subcordata and P.deltoides,and 6+6 for T.distichum)based on a systematic random design.To estimate tree biomass,the diameter at breast height(DBH,at 1.3 m)and total height of all individual trees on plots was measured by caliper and Haglo f-VERTEX IV hypsometer,respectively.Three quadrants of shrubs(at 2×2 m each)and three quadrants of herbaceous vegetation and litter(at 1×1 m each)were established diagonally within each sampling plot(He et al.2013).In each quadrant,main species of shrubs and herbs were recorded based on canopy coverage and all were harvested.Additionally,all litter was were collected from the quadrants.Samples of litter and understory vegetation were taken to the laboratory and dried at 65°C to a constant weight for determination of dry matter and carbon fraction.The carbon concentrations of allsamples including aboveground components were measured by the dry combustion method.

现阶段我国的智能旅游仅仅停留在概念阶段，发展的初期，因此各种信息化建设的基础都没有正式实施。多数旅游景点有投入专项经费进行基础硬件购买，但网络以及收费管理等都较差。要想完全实现智能旅游，基础设施的建设依然要持续较长时间。旅游行业的产业要素也要得到有效的整体。平台的智能化、监管水平都较差。

降糖药物口服、胰岛素注射是对糖尿病治疗的主要方法，但是随着医疗技术水平的进步以及研究的深入，发现在糖尿病患者接受药物治疗的过程中，加强其饮食的控制，则可更好对其血糖水平进行控制[10]。黄永芳，雍恩华，王海燕等[11]研究显示，为精神科糖尿病患者实施心理干预以及饮食指导，可较好改善患者的心理状态，提高其对临床护理的配合程度，同时饮食指导的应用，能够对精神科糖尿病患者接受更加规范、合理的饮食进行保证，使得其血糖水平可以被控制在合理的范围内。

For each stem disk,we used hand tools to separate the bark and then measured fresh weight for the wood and bark of each stem disc to determine the portion of bark.We collected and separated branches,twigs,and leaves as well.We measured the fresh weight of each component(branch,twig,and leaf)to the nearest 0.1 kg in the fi eld.Approximately 300 g of fresh sample of each tree component was collected;this quantity was randomly collected for moisture and carbon content determination.Allsampleswerelabeledandtransportedinplasticbagsto the laboratory.Samples of each component were weighed the same day on an electronic balance(accuracy 0.1 g),and dried at 70°C until they reach a constant weight.The total dry biomass for each component was calculated by multiplying the fresh weight by the dry/wet ratio.Total abovegroundbiomassforeachtreewascalculatedbysummingthe biomass of its trunk,branch,twig,and leaf.

Data analysis

In determining the independent variable for allometric equations,we multiplied the square of DBH by the tree’s total height.We employed curve fi t analysis to drive the equations between biomass and an independent variable,and we selected optimum allometric equations to calculate the component biomass of other trees in the plots.We calculated carbon storage of both understory and litter by multiplying the carbon fraction with component biomass.We calculated soil carbon storage from the carbon fraction by multiplying by the bulk density,and the thickness of the soil layer.To assess the differences in carbon content and storage of among different tree components as well as different plantations,we applied a one-way analysis of variance(ANOVA),and we carried out multiple comparisons using Duncan’s method,with differences in the P＜0.05 signi fi cance level.

Results

Biomass and carbon storage of tree level

Tree biomass of different plantations was estimated based on the component allometric equations(Table 2).Interspeci fi c differences in tree biomass were signi fi cant(F=45.96,P＜0.001),and the order of individual tree biomass wereasfollows:P.deltoides＞A.subcordata＞T.distichum(Table 3).Annual biomass increments were 11.23±2.39, 7.77±2.75, 6.17±1.49 kg tree year-1,respectively.With all species,the trunk of the trees account for the largest proportion of aboveground tree biomass while the leaves represent the smallest proportion,particularly for P.deltoides,with the trunk and leaves representing 81.58% and 2.28 of the total biomass,respectively.

Study area

The results of a two-way analysis of variance showed thatthe carbon fractions among the tree species(F=30.58,P＜0.001)as well as the separate tree components(F=94.35,P＜0.001)were signi fi cantly different.Based on multiple comparisons of means,the carbon fraction of A.subcordata was signi fi cantly higher than others,but there were no signi fi cant differences between P.deltoides and T.distichum.Also,the carbon fraction of the different components decreased following the order of wood＞twig＞bark＞leaf(Table 4).

Figure 6 summarizes the carbon storage of the different components of each ecosystem and the total ecosystem carbon storage in the three plantations.Based on the results,carbon storage in ecosystem is ranked in the order of soil＞living aboveground biomass＞litter.Also,there were signi fi cant differences in forest ecosystem carbon storage among plantations(F=13.73,P＜0.001).The total ecosystem carbon storage of A. subcordata(626.57±80.96 Mg ha-1)was higher than P.deltoides(542.90±49.24 Mg ha-1)and T.distichum(486.76±59.66 Mg ha-1).

Table 2 Allometric equation for different component of the planted tree species

B,D and H are the biomass of component,diameter at breast height and total hight of the trees,respectively,R2 and SEE are r square and SE of the estimation of regression models,respectively and***represent statistical signi fi cance at level of P＜0.001

Tree component Allometric equation R2 SEE F A.subcordata Stem B=0.019(D2H)0.952 0.977 0.117 426.621***Wood B=0.017(D2H)0.952 0.977 0.116 429.330***Bark B=0.003(D2H)0.952 0.976 0.119 408.595***Foliage B=0.0005(D2H)1.096 0.911 0.274 102.459***Leaf B=0.003(D2H)1.112 0.91 0.28 101.103***Twig B=0.0002(D2H)0.990 0.84 0.346 52.339***Branch B=0.001(D2H)1.077 0.91 0.27 101.703***Branch wood B=0.0005(D2H)1.077 0.91 0.27 101.703***Branch bark B=0.0002(D2H)1.077 0.91 0.27 101.703***Aboveground tree B=0.018(D2H)0.978 0.987 0.088 786.794***P.deltoides Stem B=0.022(D2H)0.952 0.981 0.134 506.036***Wood B=0.015(D2H)0.976 0.981 0.136 518.603***Bark B=0.012(D2H)0.818 0.975 0.131 390.693***Foliage B=0.0003(D2H)1.022 0.83 0.464 48.841***Leaf B=0.0002(D2H)0.978 0.824 0.454 46.682***Twig B=0.0001(D2H)1.062 0.829 0.483 48.639***Branch B=0.0002(D2H)1.251 0.795 0.638 38.689***Branch wood B=0.0001(D2H)1.251 0.795 0.638 38.689***Branch bark B=0.00005(D2H)1.251 0.795 0.638 38.689***Aboveground tree B=0.017(D2H)0.995 0.983 0.132 568.930***T.distichum Stem B=0.037(D2H)0.876 0.975 0.14 389.965***Wood B=0.028(D2H)0.899 0.976 0.142 400.038***Bark B=0.038(D2H)0.600 0.921 0.175 117.301***Foliage B=0.005(D2H)0.826 0.899 0.277 88.760***Leaf B=0.001(D2H)0.894 0.876 0.336 70.775***Twig B=0.005(D2H)0.757 0.914 0.233 106.897***Branch B=0.009(D2H)0.840 0.904 0.274 94.039***Branch wood B=0.008(D2H)0.840 0.904 0.274 94.039***Branch bark B=0.001(D2H)0.840 0.904 0.274 94.039***Aboveground tree B=0.053(D2H)0.865 0.981 0.121 507.699***

Table 3 Biomass of component(kg per tree)of different tree species

Data represents the mean±SD.Signi fi cant differences among plantations are indicated with lowercase letters(P＜0.05).Values in parenthesis are percentages of component to the total tree biomass

Tree component A.subcordata P.deltoides T.distichum Stem 273.97±150.40(76.70) 335.86±176.43(81.58) 172.20±79.93(79.74)S.wood 245.13±134.56 292.98±157.50 163.15±77.52 S.bark 43.26±23.75 46.42±21.20 12.06±3.97 Foliage 31.45±19.83(8.80) 9.40±5.27(2.28) 14.28±6.29(6.61)Leaf 22.24±14.22 3.99±2.15 5.55±2.62 Twig 4.25±2.43 4.73±2.75 7.29±2.96 Branch 51.78±32.09(14.50) 66.43±45.12(16.14) 29.48±13.17(13.65)B.wood 25.89±16.04 33.22±22.56 26.20±11.71 B.bark 10.36±6.42 16.61±11.28 3.28±1.46 Total tree 338.49±190.79b 403.60±220.89a 221.55±101.66c

Biomass and carbon storage of understory

There was no signi fi cant difference between the biomass of understory vegetation(including shrubs and herbs)of A.subcordata and P.deltoides and there was no vegetation coverage observed beneath the T.distichum plantation.The biomass of litter signi fi cantly differed among the three plantations(F=18.24,P＜0.001).The T.distichum plantation had signi fi cantly higher litter biomass(8.49±3.71 Mg ha-1)than the other two plantations.The amount of biomass of litter was statistically the same for P.deltoides (2.27±1.11 Mg ha-1) and A.subcordata(3.12±0.70 Mg ha-1)(Fig.3).

路政闽还强调，看到中国酒业发展成绩的同时，也应看到酒类市场假冒伪劣乱象仍时有发生。民以食为天，食以安为先，打击侵权假冒酒品、保卫公众舌尖上的安全是政府义不容辞的责任。路政闽呼吁，打击酒业领域侵权假冒是一项系统工程，需要各界积极参与，共同努力。

Since the species composition of understory vegetation of P.deltoides and A.subcordata plantations were the same,we measured the carbon fractions together.Based on the results,there was no signi fi cant difference between the carbon fraction ofshrubs (45.2±0.02)and herbs(45.4±0.03).Litter carbon fraction was signi fi cantly different among plantations(F=97.7,P＜0.001)and their order deceased in this manner:P.deltoides＞A.subcordata＞T.distichum(Table 5).

杜家台分洪工程由汉江进洪闸（杜家台分洪闸）、行洪道、蓄洪区（汉南泛区）和长江泄洪闸（黄陵矶闸）等部分组成，是汉江中下游唯一的分洪控制工程，同时又被国务院确立为长江中游12个重点分蓄洪区之一。该工程自1956年4月建成至今已运用21次（分洪运用19次，分流运用2次），累计分泄汉江超额洪水196.68亿m3，有效地改善了汉江下游的防汛紧张局面。该工程在历次运用中，按实测洪峰水位与推算洪峰水位比较，降低仙桃站洪峰水位0.6～3.0 m，为保障汉江下游和武汉市的防洪安全发挥了巨大作用，防洪效益十分显著。

Understory plays a major role in forest fl uxes and stocks balances.Generally,development of understory vegetation biomass and biodiversity depends on various factors including plantation age,light intensities under different canopies,the undergoing operations,site fertility(nutrient availability),soil moisture,landscape position(alluvial vs.upland),and rotation length.Species selection can affect growth and diversity of understory plant biomass because different species have different growth rates and tree architecture that,in turn,may affect light availability in the understory(Fortier et al.2010).In our study,no understory vegetation was observed on the T.distichum plantation’s ground.Very low canopy openness,chemical properties,and secondary chemicals(high concentrations of tannins and phenolics in its leaves)and thickness of litter layer explain the cause of extremely poor development of vegetation understory biomass in the T.distichum plantation.

Table 4 Tree componentC fraction (%)in three plantation(mean±SD)

Signi fi cant differences among components or tree species are indicated with lowercase letters(P＜0.05)

A.subcordata P.deltoides T.distichum Mean Tree 48.0±1.6a 46.4±2.3b 46.2±2.0b Wood 50.0±0.05 49.6±0.03 48.8±0.04 49.5±0.5a Bark 47.6±0.02 45.6±0.14 44.7±0.02 46.0±1.3c Twigs 48.4±0.02 46.8±0.02 46.1±0.18 47.1±1.0b Leaf 45.8±0.38 43.6±0.04 44.3±0.54 44.9±0.1d

Fig.2 Tree biomass and carbon storage of different plantations.Note:signi fi cant differences in tree biomass or carbon storage among plantations are indicated with lowercase letters(P＜0.05).Vertical lines are SD

Fig.3 Component biomass of different plantation.Note:signi fi cant differences among plantations are indicated with lowercase letters(P＜0.05).Vertical lines are SD

Despite the high variability between the plantations in terms of carbon storage of different components of the understory,there was no signi fi cant difference among the plantations for the total carbon stored in the understory.Additionally,there was a common tendency among all species,that the litter represented a large proportion of the understory carbon storage;especially for T.distichum,where there was nothing but litter beneath the trees.P.deltoides and A.subcordata plantations displayed a similar pattern for the proportion of the stored biomass in the understory components(Fig.4).

3.2 检查调整。对整机上各紧固螺丝、螺帽进行检查，发现松动要及时紧固;对主离合器、插秧离合器、秧针与导轨的间隙、秧针与苗箱的间隙等进行检查调整。

Soil carbon fraction and storage

Per-hectare tree biomass and carbon accumulation of P.deltoides was also higher than other two species.The rate of this production is a multiplication of tree growth rate and survival.Since the survival rate of P.deltoides was 100%and its growth rate was higher than other two species,a higher rate of per hectare biomass and carbon accumulation could be expected.Previous research has shown advantages of poplar species for biomass and carbon storage over other species:poplars thrive in a large range of climate conditions and soil types(Zabek and Prescott 2006).

Forest ecosystem carbon storage

Based on analysis of variance,signi fi cant interspeci fi c differences were found between the stand tree biomass(F=15.62,P＜0.001)and carbon storage(F=14.81,P＜0.001).The biomass storage of P.deltoides(206.6±47.8 Mg ha-1)was higher than the others,while there was no signi fi cant difference between A.subcordata(134.5±55.0 Mg ha-1) and T.distichum (123.3±29.7 Mg ha-1).Tree carbon storage decreased in this order: P. deltoides＞A. subcordata＞T. distichum(Fig.2).

Table 5 Carbon fraction(%)of understory in three plantation(mean±SD)

Signi fi cant differences among plantations for carbon fraction of litter are indicated with lowercase letters(α=0.05)

A.subcordata P.deltoides T.distichum Understory vegetation Herb 45.4±0.03 45.4±0.03 –Shrub 45.2±0.02 45.2±0.02 –Litter 41.6±0.13b 43.5±0.93a 36.2±0.03c

Fig.4 Understory carbon storage of different plantations.Note:signi fi cant differences among plantations are indicated with lowercase letters(P＜0.05).Values in parenthesis are percentages of component to the total understory biomass.Vertical lines are SD

Discussion

Tree biomass and carbon at tree and stand level

Biomass production of fast-growing trees could be proposed as an economic and ecological solution to meet the demand for energy and shortage of raw materials for woodbased industries(Licht and Isebrands 2005).Also,they can be considered as a high potential for mitigation of greenhouse gases and for carbon sequestration(Baishya and Barik 2011).Since both area and budget for plantations are always limited,selecting the best species for producing more wood and for storing more carbon is a challenging topic.In our research,we tried to address this challenge to present the best fast-growth species for plantation in temperate ecosystems.

To calculate the biomass of standing trees,most scholars have used the variables DBH,tree height,or a combination for predicting the biomass(Rance et al.2012).Here,tree diameter and tree height were used for predicting the biomass.The goodness-of- fi t of the equations was satisfactory,because the power functions,using a combination of DBH and H,could explain more than 98% of the variability in the observed total aboveground tree biomass of all species.Arora et al.(2014)developed allometric equations to estimate biomass and biomass carbon in different tree components of Populus deltoides,which had adjusted R squares greater than 94%.

Because of inherent variation in growth rate(Fonseca et al.2012;Nelson et al.2012),interspeci fi c differences in tree biomass among different species grown in similar conditions can be expected(Gao et al.2014).In this study,tree biomass was signi fi cantly different among the three plantations in that P.deltoides had higher amount of tree biomass than other two species.

Fig.5 Changes of soil carbon fraction(a)and soil carbon(b)in different depth(0–100 cm)of different plantations.Note:vertical lines are SD

Fig.6 Ecosystem carbon storage of different plantations.Note:signi fi cant differences in aboveground live biomass,litter,soil and ecosystem carbon storage among plantations are indicated with lowercase letters(P＜0.05).Vertical lines are SD

The pattern of biomass allocation also varies among planted species.P.deltoeides allocated more biomass and carbon to the stem(81%).Since poplars usually do not produce large branches or a large amount of leaves,its main stem contributes most to the aboveground biomass.This characteristic also provides a good potential for industrial wood production.Similar results have also been reported by Mishra et al.(2010)is semi-arid zone of India.

The mean values of carbon fraction decreased with increasing of soil depth.The rate of this decline showed a sharp change in 0.5 m depth of the soil;the carbon fraction decreased by 50%from 20–40 to 40–60 cm.These patterns were the same for all plantations.The soil pro fi le indicated that the carbon fractions of soil were statistically similar for the different plantations(Fig.5).The amount of carbon stored per hectare was obtained considering soil depth(cm),bulk density(g cm-2)and carbon fraction of each depth.A.subcordata had the highest soil carbon storage(555.51±52.3 Mg ha-1),while there was no signi fi cant difference between T.distichum(425.17±44.2 Mg ha-1)and P.deltoides(440.12±25.7 Mg ha-1)plantation for carbon storage(Fig.6).

Carbon fraction and biomass allocation

Much of the literature usually estimates carbon content in biomass by using a standard carbon proportion of 50% of dry weight,butrecentresearchhasshownthattheCconcentration of tree components or tree species may be either above or below 50%(e.g.Herrero et al.2011;Fonseca et al.2012).

We analyzed the carbon content in all components and found a mean value of 48.0%(±1.6),46.4%(±2.3),and 46.2%(±2.0)forA.subcordata,P.deltoidesandT.distichum,respectively.Our results were consistent with Ragland and Aerts(1991),who reported concentrations of carbon in hardwood species of between 47 and 50% of dry weight.Tillmanetal.(1981)reportedaveragesof50.2%carbonfor11 hardwoods and 52.7%carbon for softwoods,while Lamlom and Savidge(2003)showed that the carbon content ranged from46.27to49.97%andforhardwoodand47.21–55.2%for softwood species.Mandre et al.(2012)determined C content ofPopulustremulaandP.tremuloidesonplantationsatRapla andKunda46.26and46.74%,respectively.Aroraetal.(2014)reported mean carbon concentration in the aboveground componentsofP.deltoidesvariedfrom39.7to51.7%inTarai ForestDivisioninUttarakhand,India.However,speciestype,analysis method,stand age,ecological conditions,section of the tree sampled,and the tree’s origin are considered as potential causes of this inconsistency.

（一）教师应引导学生观察校园生活。校园是学生是学生学习知识的重要地方，也是学生接触时间最长、最密集的地方，在教学过程中有意识地引导学生对自己的校园生活多加关注，不仅可以积累大量素材，更能够让学生用全新的视角观察自己习以为常的校园生活。

C storage and C fraction of vegetation understory

三是创新技术。着力研发有益膳食平衡、降低营养损失、消除有毒残留的饮料加工新技术；着力研发自动化、智能化、信息化的饮料新装备、新检测技术、新零售模式；着力研发生态环保、清洁卫生、安全可靠、质量保障的饮料生产新工艺。以创新创造，大力提升中国饮料工业的整体竞争力。

Based on our fi eld observations,this species produces a thicker canopy cover that limits light penetration.This stand characteristic can limit growth and development of understory layer.Since the decomposition rate of T.distichum layer is slow,the tree leaves of this species pile up and produce a very thick layer of litter which signi fi cantly limits the germination and growth of any vegetation.

In this study,understory vegetation does not signi ficantly contribute as an additional carbon pool(less than 3%).Generally,plantation understory particularly in younger stands is not well developed.

C storage and C fraction of litter layer

Decomposition of plant litter is closely determined by its litter quality(C/N ratio)(Thomas and Williams 2014).Because of the high concentration of soluble carbohydrates and low concentration of lignin,the decomposition rate of the deciduous leaf litter is greater than other trees.Based on the lower rate of decomposition of T.distichum’s leaves,higher amounts of litter carbon storage for T.distichum plantations could be expected.Berg(2000)stated that conifer litter contains more components that are diffi cult to decompose than broadleaf litter,resulting in a higher rate of litter accumulation on the forest fl oor.Sun et al.(2004)and Bradford et al.(2008)also reported that forest fl oor C accumulation of conifers is greater than that of broadleaf.

Also,our analysis highlights the fact that litter carbon does not contribute signi fi cantly to the overall aboveground carbon budget(1.94,1.03,and 4.99%for A.subcordata,P.deltoides and T.distichum,respectively).This is mainly because the stands are not still well developed.

The mean C concentrations of litter layer ranged from 36%in T.distichum stand to 43%in P.deltoids,which is within the range reported by earlier researchers.Therefore,applying standard C concentrations of 45–50%may cause signi fi cant errors in the upscaling of C pools.

采用SPSS 21.0统计学软件统计数据，计量资料以（±s）表示，采用 t检验，计数资料用[n(%)]表示，组间比用χ2检验，P＜0.05为差异有统计学意义。

C storage of soil

Litterfall and rhizodeposition are two main inputs for the increasing proportion of soil carbon,while the decomposition of soil organic matter mainly decreases soil carbon(Lee et al.2015).Carbon stocks are determined by the balance between these input or output patterns—which,in similar environmental conditions,are controlled mainly by tree species.Tree species determine the composition of the litter and the abundance and activity of soil microbes,fauna and fl ora.Also,the allocation strategy of different species can result in different patterns,rate,quality,and quantity of organic carbon input to the soil.

In our study,A.subcordata(519.1 Mg ha-1)had the highestrateofsoilcarbonstorage.Onereasonforthehighrate is the higher rate of leaf production in this species.Based on our fi ndings,itproducesfourto fi vetimesmoreleavesthanthe other two species.In addition,the carbon content of biomass for A.subcordata was higher in comparison to other species.

Forest ecosystem C storage

Based on our fi ndings,soil carbon(SC)storage was the largest carbon pool in the ecosystem throughout the three plantations(78–87% of ecosystem carbon storage).The differences in ecosystem C storage among plantations were mainly determined by the magnitude of SC pool.On the global scale,forest soils hold about twice as much carbon as tree biomass(Chen et al.2013).However,C storage in Forest Ecosystems,offer a lot of goods and services such as restoration/rehabilitation of degraded lands,wildlife habitat,watershed,soil protection,and scenic views beside timber production(Redondo-Brenes 2007).

Results of the present research demonstrate that changes in tree species can highly affect carbon-storage values.Also,we fi nd that broad leaves can store more carbon in soil and living biomass than conifers.But it should be pointed out the choice of species for plantation projects should generally not be made with reference to carbon storage alone,but instead also in consideration of other factors such as biodiversity and recreation values.

Acknowledgements The trees sampled were provided by Klodeh plantation in northern Hyrcanian forests.We thank Eng.Bahram Naseri and Ehsan Fakour for their valuable help in fi eldwork.Also,we greatly appreciate the help of Laura Clark Briggs(Middle Tennessee State University)for fi nal editing of the English text.

在小区门前，杜一朵碰见了一个熟人。熟人问她一大早忙什么去？杜一朵随口说，打牌去。都晓得杜一朵爱打牌，且打得一手好牌。熟人也没多想，点点头就过去了。后来熟人又遇见一个熟人，熟人惯性地问，你也打牌去？那人两眼通红大着嗓门说，清晨八早打个屁！打炮！

Author’s contribution JE Field works and collecting the data,the laboratory analysis,running the data analysis,and writing the paper.HS Designing the experiment,supervising the work,running the data analysis,and writing the paper.

Compliance with ethical standards

Con fl ict of interest The authors declare that they have no con fl ict of interest.

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