Discussion on Calculation and Optimization of Benefits of Carbon Sequestration of Plant Communities in Urban Parks
YI Lan , WANG Hongcheng
依兰 / 1992 年生 / 女 / 天津大学建筑学院 / 硕士研究生
YI Lan, born in 1992, female, is a Master of Landscape Architecture in Tianjin University.
王洪成 / 1965 年生 / 男 / 天津大学建筑学院教授 / 博士生导师 / 研究方向为风景园林理论与设计、低碳园林
通讯作者邮箱(Correspondent author E-mail): firstname.lastname@example.org
WANG Hongcheng, born in 1965, male, is a professor in the School of Architecture in Tianjin University. Research direction: landscape planning and design and low carbon garden.
摘要：城市公园植物群落的固碳作用是缓解城市环境问题的有效途径之一。量化植物在光合作用中固定的二氧化碳，估算出植物群落固碳效益，可使设计师对方案进行调整。本文以天津公园植物群落为研究对象，选取公园内12个相对典型的植物群落，利用 NTBC 估算城市公园植物群落的年固碳效益，归纳植物群落的固碳能力与不同影响因素之间的关系。探讨城市公园内植物群落的设计优化策略，既要考虑植物的固碳能力和生态效益，也要平衡植物群落的其他多种功能，营造和谐的城市生态环境。
Abstract: The carbon sequestration of plant communities in urban parks is one of the effective ways to alleviate urban environmental situations. Quantifying the carbon dioxide that plants fix in photosynthesis and estimating the carbon sequestration benefits of plant communities can allow designers to adjust the projects. Taking the plant communities in Tianjin park as the research object, selecting 12 typical plant communities in the park, and estimate the annual carbon sequestration efficiency of the urban park plant community by NTBC. Summarize the relation between the carbon sequestration ability of plant communities and different influencing factors. Finally, to explore the design optimization strategy of plant communities in urban parks, it is necessary to consider the carbon sequestration capacity and ecological benefits of plants, as well as balance other functions of plant communities and create a harmonious urban ecological environment.
Key words: plant community; plant community structure; effection of carbon fixation; NTBC; optimized strategy
自工业革命以来，人类活动造成大气中 CO2 浓度持续增加，导致环境问题日益严重，对城市气候以及生存环境带来了较大影响。因此，减排增汇成为可持续发展中的重要内容。城市绿地作为城市重要的自然生态系统之一，能够通过其自身的光合作用固碳释氧，从而减缓城市区域内气候恶化的趋势。城市公园植物群落的固碳效益与绿地的生态效益具有直接的联系。但由于城市公园植物群落具有种类多样化、群落结构复杂、植被破碎化程度高等特点，对其固碳作用的定量分析研究发展较为缓慢。
城市绿地固碳效益的研究起源于以生物量法为基础的城市区域内的植物生长研究。在1993年，美国 Nowak 最先发表了关于城市树木与其降低大气中碳含量的研究。Nowak 等（2002年）得出结论：在城市绿地群落中，生长期较长、生长速度较快的成年树的固碳释氧效益更高。德国 Strohbach（2012年）讨论了绿地内的树木死亡率对整体固碳效益的显著影响，如树木死亡率在增加0.5 %～4 %时，则该树木的总碳储量会相应减少70 %。在同一时期内，国内关于城市树木固碳效益的研究中也论述过，不同的植物群落特征因素对城市绿地的固碳效益的影响的区别性，使得所产生的固碳效益也有所差距。李辉等（1999年）在北京市居住区中选取不同结构的植物群落进行夏季固碳释氧的比较研究，分析到乔灌草类型的群落结构比灌草型及草坪型群落的整体生态效益更高。章银柯等（2013年）估算杭州西湖风景区不同生长时期的绿地的长期固碳量和分析不同典型绿地的碳储量能力。在以往文献中，生物量法、蓄积量法等传统方法在固碳效益研究的使用过程当中步骤烦琐、不易操作。因此，很多国家和地区对基于大数据库研究的植物固碳量计算系统进行研究，包括美国所研发的 City Green 计算系统或者树木效益计算器 Tree Benefit Calculator[5-7]。此计算方法是基于美国数据所得出，结论存在一定的相对误差，但软件中的计算模型是通过实地测量植物的基础数据，模拟出植物的生物量的计算公式，代入所得的生物量的计算数据的基础上得出相应的固碳效益值，与其他常用计算方法如平均生物量法、生物量转换因子法等都是相同的数学推理逻辑。因此将计算出的数据通过对比分析得到的参考取值依然具有一定的研究价值[8-10]。
天津市位于中国华北东部的华北平原海河流域下游，为东经116°43′～118°04′、北纬38°34′～ 40°15′之间，属于中纬度欧亚大陆东岸，为温带季风气候，主要受季风环流影响。年降雨量为544.3 mm且主要在夏季。天津市的地势以平原和洼地为主，地势最低处平均海拔3.5 m，是海拔最低的中国大城市。目前，天津市存在淡水资源匮乏、土壤盐碱化等的问题。
研究地点为天津市水上公园，位于市区的西南区域，建于1950年。公园占地面积约1 267 100 m2，其中陆地面积为500 000 m2，绿化面积达350 000 m2。公园内栽植花木近200个品种以及约20 000株的各类乔灌木种植。公园内植物群落的配置以及整体结构具有典型性，体现出天津中心城区的植物景观特色以及公园植物景观的特征。
植物群落样地采集点主要位于水上公园的北部区域，共选定了12个植被生长情况良好、群落相对稳定、兼具美学特征和天津地区特色的代表性植物群落，面积约在400 m2～600 m2（图1、2）。对群落内的树木进行数据采集，包括树木名称、叶形、树木胸径、生长状态、层次结构、植物配置以及群落景观效果等。利用美国 NTBC（National Tree Benefit Calculator）软件，通过输入单株树木的树种、胸高直径和所在区域及所在地土地利用方式，计算出样地中单株树木的单位年固碳量。该软件是基于美国的基础数据而研发，本研究选取天津市经纬度和气候环境接近的堪萨斯城（Kansas City, Missouri）作为地理位置参考，如软件内缺少对应树种，则选择相似或同科属的树种作为替代。计算得出所选取区域的植物群落年固碳效益值。
Figure 1 Distribution Map of Tianjin Water Park
Figure 2 Sampling Sites of Tianjin Water Park
经调查测量，所选取植物群落均为人工种植群落，整体处于较为稳定的生长环境中。在所调查的区域内，由针阔叶混交乔木层和阔叶灌木层组成的群落类型占50 %，这类植物群落更易形成有层次感的林下空间、增强人群视角下的群落观赏性。其他常见群落类型有针阔叶混交乔木层及针阔叶混交灌木层。样地群落内的树木多数为天津乡土树种，能够在该地区的土壤环境中较好生长。大乔木的平均胸径为33.26 cm，平均冠幅为4.5 m；小乔木平均胸径为13.62 cm，平均冠幅为2.7 m。公园内树木健康状况中等，大乔木多数生长良好、树形饱满、具有一定的观赏价值；小乔木以及灌木等生长状态参差不齐，其中有栽植密度过大而影响光照条件以及幼年树木缺乏生机等问题。
调查的12个样地植物群落种类的丰富度高，包括不同叶形以及季相的乔灌木草等植物（表1）。共调查记录有37种植物，其中分别属于15科27属，包括有蔷薇科（Rosaceae）有5属10种、豆科（Fabaceae）4属6种。在公园的植物群落中出现的阔叶类乔木占总数量88 %，具有不同叶形、四季色相树种搭配以及呈现出不同群落结构。在所调研的植物群落中，出现频率超过25 %的乔木有刺槐、圆柏、国槐、臭椿、绒毛白蜡与毛白杨。其中，频率最高的为刺槐共出现8次，应用频率达66.7 %；其次，圆柏共出现7次，应用频率达58.3 %。在小乔木和灌木中紫叶李、忍冬、西府海棠、女贞、龙爪槐、金枝槐等树种应用频繁，紫叶李的应用频率最高，为41.7 %。
通过 NTBC 软件对样地数据的计算，天津市水上公园中所测定的植物群落平均年固碳量可达36254.43 kg/hm2。由图3可知，12个样地内年固碳量最高的为11号样地，该群落是针阔叶混交乔木层搭配针阔叶混交灌木层结构，其主要组成树种为国槐、圆柏、龙柏、金枝槐、皂荚等，多数为天津市的乡土树种；年固碳量最低的是10号样地，是针阔叶混交乔木层搭配阔叶灌木层的群落结构，以银杏、油松、紫叶李、桃、柿树为主要树种。不同植物群落固碳效益的差异较大，多数群落的固碳效益在20000 kg/hm2～40000 kg/hm2范围内。其中，影响群落固碳效益的高低的影响因素众多，在群落内单个树木的种类，规格以及整体栽植密度的变化中呈现不同的结果。
通过相关性分析可得到图6所示，在10 cm～20 cm群落平均胸径范围内的植物群落年固碳量更高，达36723.40 kg/hm2，略高于平均胸径在20 cm～30 cm的植物群落以及30 cm～40 cm范围内的植物群落，另外两个胸径范围内群落固碳量分别有36083.60 kg/hm2、36116.30 kg/hm2，相差大约有600 kg/hm2。总体上，不同群落平均胸径范围的单位固碳量差异不大，平均单位固碳量基本持平。因此，群落树木的平均胸径对整体植物群落的固碳效益的影响不是很明显，其主要作用在于单个树种的固碳能力。因此选择栽植树木时，适当调配不同大小胸径的树木的比例，能够提高植物群落的美景度以及群落固碳效益。
按照不同的群落栽植密度，对样地中植物群落的总年固碳量进行比较如图7所示。在12处样地内，不同群落栽植密度范围的植物群落在其整体固碳效益上差距较大。其中，300 株/hm2～450 株/hm2栽植密度范围内植物群落的固碳效益最高，达49582.20 kg/hm2；栽植密度在0 株/hm2～150 株/hm2范围时，群落年固碳效益最低，为29407.80 kg/hm2。由此可知，具有中等栽植密度的植物群落的年固碳效益明显高于高栽植密度与低栽植密度的植物群落。另外，中等密度植物群落之间在年固碳量上并没有显著的差异。群落结构以及树木规格适中，对群落对固碳效益产生正向的影响；在低栽植密度的群落内树木数量较少，相应固碳效益降低；而在高密度植物群落中，树木多数为胸径较小的中小乔木，植物生长空间拥挤而导致树木长势被限制，因此植物群落碳汇作用相对下降。
Figure 3 Annual Quantity of Carbon Sequestration of Plant Communities in Tianjin Water Park图4 随固碳效益增加的树木规格变化
Figure 4 Changes in the Tree Sizes with Increasing Benefits of Carbon Sequestration图5 随固碳效益增加的群落栽植密度变化
Figure 5 Changes of Community Planting Density with Increasing Benefits of Carbon Sequestration图6 不同范围的平均胸径的植物群落年固碳量
Figure 6 Annual Quantities of Carbon Sequestration of Plant Communities with Different Ranges of Average Diameters at Breast Height图7 不同范围的栽植密度的植物群落的年固碳量
Figure 7 Annual Quantities of Carbon Sequestration of Plant Communities with Different Ranges of Planting Densities
在选择高固碳能力的树种来提高植物群落的碳效益以外，应通过对植物群落结构进行合理的规划设计，使树木植物之间能够相互和谐共生，发挥整体的植物固碳能力的最大化[13-14]。首先，对植物群落的垂直结构中，应采用乔、灌、草模式的复层景观结构，如乔木树种中的垂柳、国槐和圆柏，灌木以及地被植物应选长势较好、固碳能力强的树种，包括珍珠梅、紫叶槐、铺地柏等木本植物。搭配不同色彩以及四季的景观效果，使植物群落具有层次感、色彩感以及本地特色。其次，控制植物群落的栽植密度，应在250 株/hm2～450 株/hm2的范围内，种植密度不应过于密集，树木应当有一定的植株距离，形成适当的郁闭度以及给予群落内的树木良好的生长空间，以使树木枝叶得到充分生长提高自身固碳能力。
Since the industrial revolution, human activities have resulted in the continuous increase of the concentration of carbon dioxide in the atmosphere, leading to increasingly serious environmental problems, which have had a significant impact on the urban climate and living environment. Therefore, reducing the emission of carbon dioxide and increasing the carbon sequestration have become an important part of sustainable development. As one of the most important natural ecosystems in a city, the urban green space can take in carbon and release oxygen through its own photosynthesis, thus slowing down the trend of climate deterioration in urban areas. The benefits of carbon sequestration of plant communities in urban parks are directly related to the ecological benefits of green space. However, since the plant communities in urban parks have features of species diversity, complex community structure and high degree of vegetation fragmentation, the development of quantitative analysis and research on carbon sequestration is relatively slow.
The research on the benefits of carbon sequestration of urban green space originates from the research on plant growth in urban areas based on the biomass method. As early as 1993, Nowak published a study on urban trees and their effects on the reduction of carbon content in the atmosphere. Nowak et al. (in 2002) concluded that in the communities of urban green space, full-grown trees with longer growth period and faster growth rate have higher benefits of carbon sequestration and oxygen release . Strohbach, from Germany (in 2012), discussed the significant impact of the tree mortality rate of the green space on the benefits of carbon sequestration of overall trees. If the tree mortality rate increases by 0.5-4%, the total carbon storage of the trees will decrease by 70% . During the same period, in the domestic research on the benefits of carbon sequestration of urban trees, the differences in the effects of different characteristic factors of plant communities on the benefits of carbon sequestration of urban green space have led to differences in the benefits of carbon sequestration. Li Hui et al. (in 1999) conducted a comparative study on carbon sequestration and oxygen release in summer by selecting plant communities with different structures in residential areas in Beijing. It was found that the community structure of arbor, shrub and grass types is higher than that of shrub and grass types and the lawn type in overall ecological benefits . Zhang Yinke et al. (in 2013) estimated the long-term quantities of carbon sequestration of green space in Hangzhou West Lake scenic area at different growth periods and analyzed the capacity of carbon sequestration of different typical green space . In the past literature, traditional methods such as the biomass method and the accumulation method have complicated steps and are not easy to operate in the use process of research on the benefits of carbon sequestration. Therefore, many countries and regions have conducted research on the calculation systems of quantities of plant carbon sequestration based on the research on the large database, including the City Green calculation system developed by the United States or the Tree Benefit Calculator [5-7]. This calculation method is based on American data, and the conclusion has certain relative errors. However, the calculation model of the software simulates the calculation formula of plant biomass by measuring the basic data of plants on the spot, and obtains the corresponding benefit value of carbon sequestration on the basis of substituting the calculation data of biomass into the calculation formula. It is the same mathematical reasoning logic as other commonly used calculation methods such as the average biomass method and the biomass expansion factor method. Therefore, the reference value obtained by comparing and analyzing the calculated data still has certain research value [8-10].
Based on the existing research on the benefits of carbon sequestration of plant communities in urban parks, this paper selects the plant communities in Tianjin urban parks to carry out the research. By analyzing the capacity of carbon sequestration of plant communities in urban parks and the influencing factors for the benefits of carbon sequestration of plant communities in Tianjin urban parks, it will finally summarize the influencing factors of the benefits of carbon sequestration of plant communities in urban parks on the design stage and the corresponding optimization strategies, providing reference for the subsequent research on carbon sequestration of urban green space.
1 Research Contents and Methods
1.1 Basic Data Collection
Tianjin is located in the lower reaches of the Haihe River Basin in the North China Plain in the eastern part of North China, between 116°43'E-118°04'E and 38°34'N-40°15'N. It belongs to the east coast of mid-latitude Eurasia and has a temperate monsoon climate, which is mainly affected by monsoon circulation. The annual rainfall is 544.3 mm, mainly in summer. The terrain of Tianjin is dominated by plains and depressions, with the lowest terrain at an average altitude of 3.5 m, which makes Tianjin the large city with the lowest altitude among the large cities in China. At present, Tianjin has problems such as shortage of fresh water resources and salinization of soil.
The research site is Tianjin Water Park, which is located in the southwest of the city and was built in 1950. The park covers an area of 1,267,100 m2, of which the land area is 500,000 m2 and the green space is 350,000 m2. Nearly 200 varieties of flowers and trees and about 20,000 arbors and shrubs are planted in the park . The distribution and overall structure of the plant communities in the park are rich and typical, reflecting the characteristics of the plant landscape in the central urban area of Tianjin and the characteristics of the plant landscape in the parks.
1.2 Research Methods
Plant community sampling sites are mainly located in the northern area of the water park, and a total of twelve representative plant communities with good vegetation growth, relatively stable communities, aesthetic characteristics and characteristics of Tianjin have been selected, with an area of about 400~600 m2 (figure 2). Data collection is carried out on trees in the communities, including the tree name, leaf shape, diameter at breast height, growth situation, hierarchical structure, plant disposition and landscape effect of the community. Using the American software NTBC (National Tree Benefit Calculator), the annual quantity of carbon sequestration per tree in the sampling sites is calculated by inputting the variety of the tree, the tree’s diameter at breast height, its location and the land use pattern of the area. The software is developed based on the basic data of the United States. In this study, Kansas City, Missouri, which is close to Tianjin in latitude and longitude and climatic environment, is selected as the geographical location option. If there is no corresponding tree species in the software, trees of similar or same family and genus will be selected as the replacement. The annual quantities of the benefits of carbon sequestration of plant communities in the selected areas are calculated.
2 Results and Analyses
2.1 Survey Results of Plant Communities
After investigation and measurement, the selected plant communities are all planted artificially, and all of the plant communities are in the relatively stable growth environment. In the investigated area, the community type composed of the tree layer being the mixed coniferous broad leaved forest and the shrub layer being the broad leaved forest accounts for 50%, and this kind of plant community is more likely to form the forest space of sense of depth and enhance community appreciation from the perspective of people. Other common community type includes the tree layer being the mixed coniferous broad leaved forest and the shrub layer being the mixed coniferous broad leaved forest. Most of the trees in the communities of sampling sites are native trees in Tianjin, which can grow well in the soil environment of the area. The average diameter at breast height of large arbors is 33.26 cm, and the average crown breadth is 4.5 m. The average diameter at breast height of small arbors is 13.62 cm and the average crown breadth is 2.7 m. The trees in the park are in moderate health condition. Most of the large arbors grow well and have full tree forms, which have certain ornamental value. The growth situation of small arbors and shrubs is varied, among which there are problems such as too high planting density affecting light conditions and lack of vitality of young trees.
There are a great many species of plant communities in the twelve sampling sites surveyed, including the arbor, shrub, grass and other plants with different leaf shapes and seasonal aspects (table 1). A total of thirty-seven species of plants are recorded and investigated, including fifteen families and twenty-seven genera, including five genera and ten species of Rosaceae and four genera and six species of Fabaceae. Broad-leaved arbors, which account for 88% of the total number of plant communities in the park, have different leaf shapes and different seasonal aspects, arrangements of tree species and show different community structures. Among the investigated plant communities, the arbors with a frequency of more than 25% are Robinia pseudoacacia, Sabina chinensis, Sophora japonica, Ailanthus altissima, Fraxinus velutina and Populus tomentosa. Among them, a Robinia pseudoacacia has the highest frequency for eight times, with the application frequency reaching 66.7%. Secondly, Sabina chinensis appears seven times with an application frequency of 58.3%. Among small arbors and shrubs, purple-leaf plum, honeysuckle, Malus micromalus, privet, Chinese pagoda tree, Sophora japonica and other tree species are frequently planted, with the purpleleaf plum having the highest application frequency of 41.7%.
2.2 Benefits of Carbon Sequestration of Plant Communities
According to the calculation of the data of sampling sites by using the NTBC software, the average annual quantity of carbon sequestration of the plant communities measured in Tianjin Water Park can reach 36254.43 kg/hm². As can be seen from figure. 3, the No.11 sampling site has the highest annual quantity of carbon sequestration in the twelve sampling sites. And its community structure is the tree layer being the mixed coniferous broad leaved forest and the shrub layer being the mixed coniferous broad leaved forest, and its main constituent tree species are Sophora japonica, Sabina chinensis, Dragon juniper, Sophora japonica, Gleditsia sinensis, etc., most of which are native tree species in Tianjin. The lowest annual quantity of carbon sequestration is the No.10 sampling site, which is a community structure of the tree layer being the mixed coniferous broad leaved forest and the shrub layer being the broad leaved forest, with ginkgo tree, Pinus tabulaeformis, purple-leaf plum, peach tree and persimmon tree as the main tree species. The benefits of carbon sequestration of different plant communities are quite different, and the benefits of carbon sequestration of most communities are in the range of 20000-40000 kg/hm². Among them, there are many factors that affect the benefits of carbon sequestration of the communities. Different results are shown in the changes of the species, sizes and overall planting density of the individual trees in the communities.
2.2.1 Comparison of Species Differences by Actual Measurement
Plants are the main body of benefits of carbon sequestration in urban parks, and there are many factors affecting their capacities of carbon sequestration. Plant species have direct influence on carbon sequestration effect of trees and communities. Therefore, fourteen kinds of arbors applied at medium and high frequencies in the sampling sites are selected as the comparative objects, and the capacities of carbon sequestration of the fourteen common tree species are analyzed as follows: Salix babylonica > Sophora japonica > Robinia pseudoacacia > ginkgo biloba > Gleditsia sinensis > Fraxinus velutina > Populus tomentosa > Ailanthus altissima > Eucommia ulmoides > Pinus tabulaeformis > Platycladus orientalis > Albizzia julibrissin > Dragon juniper > Sabina chinensis. This shows that among the arbors, deciduous arbors have stronger capacities of carbon sequestration and oxygen release due to their high indexes of leaf area, among which Salix babylonica, Sophora japonica and Ginkgo biloba have significantly higher capacities of carbon sequestration than other tree species. The annual capacity of carbon sequestration of deciduous broad-leaved arbors is greater than that of evergreen conifer trees and evergreen broad-leaved trees of corresponding sizes, of which coniferous trees have the lowest quantity of carbon sequestration in the same size. It can be seen that the larger the proportion of tree species with strong capacity of carbon sequestration, the higher the effect of carbon sequestration of the community.
2.2.2 Relevant Analyses of Size Changes
According to the correlation analysis, we can come to conclusions as shown in figure 6, the annual quantity of carbon sequestration of plant communities, whose trees are in the range of average diameter at breast height of 10~20 cm, is higher, reaching 36723.40 kg/hm², slightly higher than that of plant communities, whose trees are in the range of average diameter at breast height of 20~30 cm and plant communities whose trees are in the range of average diameter at breast height of 30~40 cm. The quantities of carbon sequestration of the latter two communities are 36083.60 kg/hm² and 36116.30 kg/hm² respectively, with a difference of about 600 kg/hm². In general, the average diameters at breast height of different communities have little difference in quantities of carbon sequestration per unit, and the average quantities of carbon sequestration per unit are basically the same. Therefore, the average diameter at breast height of the trees in the community has no obvious influence on the benefits of carbon sequestration of the whole plant community, and the main function is the capacity of carbon sequestration of individual tree species. Therefore, when selecting trees for planting, it would be better if properly adjusting the proportion of trees with different diameters at breast height, which can improve the scenic beauty of the plant communities and the benefits of carbon sequestration of the communities.
2.2.3 Benefit Effect of Community Density
According to the different planting densities of communities, the total annual quantities of carbon sequestration of plant communities in the sampling sites are compared as shown in figure 7. In twelve sampling sites, the plant communities with different planting density ranges have a large difference in their overall benefits of carbon sequestration. Among them, the benefit of carbon sequestration of the plant community within the planting density range of 300-450 plants/hm² is the highest, reaching 49582.20 kg/hm². When the planting density is in the range of 0-150 plants /hm², the annual benefit of carbon sequestration of the community is the lowest, 29407.80 kg/hm². Therefore, the annual benefits of carbon sequestration of plant communities with medium planting densities are significantly higher than those of plant communities with high planting densities and low planting densities. In addition, there is no significant difference in annual quantities of carbon sequestration among plant communities with medium planting densities. The moderate community structure and tree size have a positive impact on the benefits of carbon sequestration of the community. In the community with low planting density, the number of trees is less, and the corresponding benefits of carbon sequestration decrease. However, in plant communities with high planting densities, most of the trees are small and medium-sized arbors with small diameters at breast height. The growth of trees is limited due to the crowded growth space of plants. Therefore, the carbon sequestration effect of plant communities is relatively reduced. Therefore, in terms of selection of tree species, under the same conditions, broadleaved trees have a higher effect on the overall benefits of carbon sequestration of plant communities than coniferous trees. The carbon emission and short growth period of shrubs and ground cover plants caused by their plant characteristics and small sizes, which need frequent replacement and pruning, have no obvious effect on the benefits of carbon sequestration of the community. Salix babylonica, Sophora japonica, Robinia pseudoacacia, Ginkgo biloba, Gleditsia sinensis, Fraxinus velutina and other tree species with high capacities of carbon sequestration are the dominant species in the community. In terms of tree sizes, the larger the average diameter at breast height of the tree is, indicating that the tree is in the mature growth period, and the stronger the capacity of carbon sequestration is, which has a higher effect on improving the benefits of carbon sequestration of plant communities. In terms of the community structure, the benefits of carbon sequestration of two types of structures, i.e. the tree layer being the broad-leaved forest plus the shrub layer being the mixed coniferous broad leaved forest, and the tree layer being the mixed coniferous broad leaved forest plus the shrub layer being the mixed coniferous broad leaved forest, are higher. In the horizontal structure, when the community layout is well-organized and the planting density of each tree is reasonable, plants will have sufficient light conditions and more abundant growth environment, which can realize the maximization of the functions of carbon sequestration and oxygen release.
3 Optimization Strategies for High Benefits of Carbon Sequestration of Plant Communities in Urban Parks
3.1 Application of Tree Species with the Capacity of High Carbon Sequestration
First of all, according to the climate and environmental conditions of the region, consider choosing Tianjin local tree species with the strong capacity of carbon sequestration, including Salix babylonica, Sophora japonica, Sabina chinensis, Malus micromalus and other native tree species. Local plants have better adaptability, can adapt to the site environment and the local saline-alkali habitat in a short time, and the growth of trees can achieve better landscape effect. At the same time, compared with foreign tree species, local tree species adapt to the environment in a short time so as to reduce the frequency of maintenance and management, and reduce carbon emissions in transportation and management during the design and construction phase . Secondly, arbors have higher capacity of carbon sequestration and are mostly used. Shrubs and ground cover plants planted together should be tree species with the capacity of high carbon sequestration with good growth conditions and can reduce pruning frequency. Adjust the planting proportion of deciduous, evergreen, broad-leaved and coniferous trees; Select trees with small age and medium sizes to avoid reducing their own benefits of carbon sequestration due to over-age and short growth period, so as to optimize the selection of plant configuration .
3.2 Reasonable Community Structure
In addition to selecting trees with the capacity of high carbon sequestration to improve the benefits of carbon sequestration of plant communities, reasonable planning and design should be carried out on the structure of plant communities so that trees and plants can coexist harmoniously with each other and maximize the overall capacity of carbon sequestration of plants [13-14]. First of all, the vertical structure of the plant community should adopt a multi-layer landscape structure of the pattern of arbor, shrub and grass. For example, for the arbor species, we can choose Salix babylonica, Sophora japonica or Sabina chinensis. While for shrubs and ground cover plants, we should choose tree species with good growth conditions and strong capacity of carbon sequestration, including woody plants such as Sorbaria sorbifolia, Sophora japonica and procumbent juniper. With different colors and the landscape effect of the four seasons, the plant community has a sense of depth, color and local characteristics. Secondly, the planting density of the plant community should be controlled within the range of 250~450 plants /hm², the planting density should not be too dense, the trees should have a certain plant distance, form an appropriate canopy density and give the trees of the community a good growth space, so that the branches and leaves of the trees can fully grow and improve their capacities of carbon sequestration.
3.3 Optimize the Growth Environment and Management Mode of the Community
The growth environment and site conditions of the plant community should play the positive role in the functions of carbon sequestration and oxygen release of the plant community itself. First of all, try to reduce changes to the original site and earthwork, improve the original plant community on the basis of protection, and reduce carbon emissions from unnecessary mechanical operations and the impact on the soil environment. Secondly, trees should be planted appropriately according to the functions and conditions of the surrounding environment by using the principle of planting appropriate trees suitable for the land, so as to avoid affecting the original and newly planted trees. Finally, low-energy and high-efficiency operation methods and energy-saving technologies are adopted to carry out regular maintenance, pruning, fertilization, irrigation and other work on the plants of the plant community so as to prevent diseases and insect pests of the trees, keep the plants in a good growth state and give full play to their capacities of carbon sequestration and oxygen release for having a good growth environment [15-16].
4 Conclusion and Prospect
According to the results of investigation and study, it can be known that the plant community in Tianjin Water Park has certain influence on the benefits of carbon sequestration of the community in terms of its species, sizes, composition of hierarchical structure and planting density of the community. Take the planting density and species of trees in the community as fixed quantity, the higher the sizes of trees as independent variables are, the better the annual quantities of carbon sequestration of the community are. When the average diameters at breast height of trees and community density are fixed, the more native trees with strong capacities of carbon sequestration are, the higher the capacities of carbon sequestration of the community are. When the types and sizes of plants of the community are fixed, the plants growing in a better growth space in the community with medium planting density will make the community have higher benefits of carbon sequestration. Compared with plant communities with low benefits of carbon sequestration, communities with high benefits of carbon sequestration in Tianjin Water Park have less demand for later maintenance management and manual intervention, which can reduce additional carbon emissions generated in some maintenance management stages. For the plant community, the combination of tall arbors, low shrubs and ground cover plants can form a good landscape structure and spatial layout.
Therefore, it is necessary to ensure that the plant community planted in the park green space can achieve its ecological, recreational, landscape and other functions. According to the site conditions, we should select plants adapted to the local climate and soil conditions, not only consider the ecological benefits and benefits of carbon sequestration produced by different communities, but also continuously look for more optimal solutions to balance the different needs of park green space.
 Nowak D J, Stevens J C, Sisinni S M, et al. Effects of urban tree management and species selection on atmospheric carbon dioxide.[J]. Journal of Arboriculture,2002, 28(3):113-122
 Strohbach M W, Arnold E, Haase D. The carbon footprint of urban green space—A life cycle approach [J]. Landscape & Urban Planning, 2012,104(2):220-229
 李辉, 赵卫智, 古润泽, 李延明, 陈自新, 张新献. 居住区不同类型绿地释氧固碳及降温增湿作用.[J] 环境科学，1999, 11(6):41-44
Li Hui, Zhao Weizhi, Gu Runze, et al. Effects of Three Different Green-Lands in Plantation Structure on the O2-Emitting, CO2-Fixing, Heat-absorbing and Temperature-decreasing in Residential Quarters[J]. Environmental Science,1999, 11(6):41-44
 章银柯，马婕婷，王恩，包志毅. 基于碳储量测定的低碳高效城市园林绿化建设思路探讨——以杭州西湖风景名胜区为例[J]. 西北林学院学报，2013, 28: 221-226
Zhang Yinke, Ma Jieting, Wang En, Bao Zhiyi, et al. Discussion on Low Carbon Efficient Urban Landscape Construction Thought Based on Carbon Storage Determination - A Case Study of West Lake Scenic in Hangzhou[J]. Journal of Northwest Forestry University, 2013, 28: 221-226
 李薇. CITYgreen 软件在城市绿地生态效益评价中的应用——以奥林匹克森林公园规划方案为例[D]. 北京: 北京林业大学, 2007
Li Wei. The Application of CITYgreen in Urban Green-land Ecological Benefits Evaluation[D]. Beijing: Beijing Forestry University, 2007
 周坚华，胡永红，周一凡. 城镇绿地植被固碳量遥感测算模型的设计[J]. 生态学报,2010, 30(20):5653-5655
Zhou Jianhua, Hu Yonghong, Zhou Yifan, et al. A Design of Carbon Sink Model of Urban Landscape Vegetation Driven by Remote Sensing [J]. Acta Ecologica Sinica, 2010, 30(20):5653-5655
 冀媛媛，罗杰威，王婷. 建立城市绿地植物固碳量计算系统对营造低碳景观的意义[J].中国园林，2016,08: 31-35
Ji Yuanyuan, Paolo Vincenzo Genovese, Wang Ting. Development and Significance of Carbon Sequestration Calculation System of Green Plants in Urban Space[J]. Ecological Technology,2016,08: 31-35
 Casey Trees and Davey Tree Expert Co. National Tree Benefit Calculator[EB/ OL].[2018-04-09].http://www.eia.doe.gov/cneaf/electricity/st_profiles/toc.html
 孙楠，罗毅，李明翰. 在LAF 的“景观绩效系列(LPS)”计划指导下进行建成项目景观绩效的量化——以北京奥林匹克森林公园和唐山南湖生态城中央公园为例[C]. 中国风景园林学会论文,2011:601-606
Luo Nan, Luo Yi, Li Minghan. Quantification of Landscape Performance for Built Projects through LAF’s Landscape Performance Series[C]. Proceedings of CHSLA 2013,2011:601-606
 E. Gregory McPherson. Selecting Reference Cities for i-Tree Streets[J]. Arboriculture & Urban Forestry 2010. 36(5): 230-240
Tian Jin Water Park[EB/OL].[2018-04-09].http://www.tjwaterpark.com
 王洪成. 探索城市生态修复的低碳园林途径[J]. 风景园林, 2017, 11: 80-85
Wang Hongcheng. The Approach of Low Carbon Garden to Urban Ecological Restoration[J]. Landscape Architecture,2017, 11: 80-85
 包志毅，马婕婷. 试论低碳植物景观设计和营造[J]. 中国园林,2011(1):7-10
Bao Zhiyi, Ma Jieting. On the Design and Construction of Low-carbon Plant Landscape[J]. Landscape Architecture,2011(1):7-10
 赵艳玲. 上海社区绿地植物群落固碳效益分析及高固碳植物群落优化[D]. 上海: 上海交通大学. 2014
Zhao Yanling. Analysis on Carbon Fixation Effection and Optimizational Disposition about Plant Communities of Community Greespace in Shanghai[D]. Shanghai: Shanghai Jiao Tong University.2014
 冀媛媛. 可持续理念下低碳景观营造标准及策略研究[D]. 天津: 天津大学, 2015
Ji Yuanyuan. Research of the Evaluation System and Strategies of Low Carbon Landscape Based on the Sustainable Development Idea [D].Tianjin: Tianjin University. 2015
 张颖. 广州典型城市绿地碳足迹核算和评估研究[D]. 武汉：华中农业大学，2013
Zhang ying. The Carbon Footprint Accounting and Assessment Study of Guangzhou Typical Urban Green Space[D]. Wuhan: Huazhong Agricultural University.2013
 徐飞. 上海城市森林群落结构特征与固碳能力研究[D]. 上海：华东师范大学, 2010
Xu Fei. Study on Community Structure and Carbon Fixation of Urban Forest in Shanghai, China[D]. Shanghai: East China Normal University. 2010
 赵彩君， 刘晓明. 城市绿地系统对于低碳城市的作用.[J]. 中国园林, 2010,26(6):23-26
Zhao Caijun, Liu Xiaoming. The Role of Urban Green Space System in Lowcarbon City[J]. Chinese Landscape Architecture,2010, 26(6):23-26
 董延梅. 杭州花港观鱼公园57 种园林树木固碳效益测算及应用研究[D]. 杭州: 浙江农林大学，2013
Dong Yanmei. Research on the Measure of Carbon Fixation Benefit and Appliance of 57 Garden Specises in Hangzhou Huagangguanyu Park[D]. Hangzhou: Zhejiang A&F University.2013
 管东生，陈玉娟，黄芬芳. 广州城市绿地系统碳的贮存、分布及其在碳氧平衡中的作用[J]. 中国环境科学,2013,18(05):437-441
Guan Dongsheng, Chen Yujuan, Huang Fenfang. The storage and Distribution of Carbon in Urban Vegetation and Its Roles in Balance of Carbon and Oxygen in Guangzhou[J]. China Environmental Science,2013,18(05):437-441