短脚锦鸡儿灌丛对植物群落和土壤微生物群落的促进效应研究

以内蒙古荒漠区短脚锦鸡儿灌木为研究对象,采用野外调查法分析灌丛对植物群落的影响,采用传统培养法,结合分子鉴定法分析灌丛对土壤微生物群落的影响。结果表明:(1)灌丛内植物群落多度和总生物量显著大于灌丛外,但是物种丰度和Shannon-Wiener指数灌丛内外无显著差异;(2)随土壤深度的增加,表层土与深层土的土壤可培养细菌丰度和真菌多度差异不显著,其余土壤微生物群落多样性特征均表现为:表层土显著大于深层土;(3)灌丛对土壤微生物群落具有正效应,且表层土正效应最大;(4)灌丛对植物群落的促进作用大于对土壤微生物群落的作用。     

Plant community dynamics in arid lands: the role of desert ants

Ants (Formicidae, Hymenoptera) play an important role in seed bank, seedling establishment andplant composition of arid ecosystems. Thus, knowing plant-ant interaction provides useful information formanagers to design restoration and conservation plans. In this study, the roles of desert harvester ants (Messorintermedius and Messor melancholicus) and scavenger ants (Cataglyphis nodus and Lepisiota semenovi) on plantcommunities were investigated in arid ecosystems of southeastern Iran. Two vegetation types weredistinguished in the study area and the nest density of ant species was determined. Furthermore, plantcomposition and soil seed bank were estimated at different distances from the ant nests. Results showed thatthe density of M. intermedius and M. melancholicus nests was higher in dwarf shrub-shrub vegetation type and thedensity of C. nodus and L. semenovi nests was higher in dwarf shrub vegetation type. The harvester and scavengerants had enhanced the seed bank to 55% and 70%, respectively. Therefore, the role of scavenger ants on theplant communities' seed bank was greater than that of harvester ants. Although the scavenger ants were moreinfluential on the annuals and the invasive plant species, the radius impact of the harvester ants on theperennials was greater, i.e., a positive interaction existed between the perennial plants and the harvester ants. C.nodus and L. semenovi played an important role in enhancing the ecosystem's potential for restoration throughestablishment of pioneer species in early stage of succession. The activity of M. intermedius is crucial for thedevelopment and maintenance of climax plant communities in arid ecosystems through assisting the plantspecies' establishment in late stage of succession. It is essential to preserve the diversity of these key ant speciesfor the maintenance and sustainability of shrubs in arid ecosystems.     

巴丹吉林沙漠植被群落特征与土壤水分关系研究

通过对巴丹吉林沙漠周边及腹地进行野外调查采样,归纳出该区植被的生活型和群落类型,分析其植被多样性、优势度及均匀度指数,进而探讨该地区植被群落特征对土壤水分的响应.结果表明:(1)本次调查共记录到20科52属56种植物,分属于乔木、小乔木、灌木、小灌木、半灌木、草质藤本、多年生草本、一年生草本8类生活型,群落多以灌木和多...     

Exploration of unconventional fodder for arid land rehabilitation

Exploring new wild plant species of economic importance with tolerance to harsh environmental conditions in arid countries may be a viable option to improve forage yield. The present study was conducted during two consecutive years and aimed to investigate the nutritional values of eleven selected wild fodder species, six annuals, and five perennials. Detailed analyses of the nutritional values during two different seasons for perennials and two different growth stages for annuals are presented. The results showed that most studied plants have high nutritional values compared to earlier studies on taxonomically related species growing in different arid areas. The mean content of dry matter, ash, crude protein and lipids was 27, 13, 14, and 3.5%, respectively. Neutral detergent fiber, acid detergent fiber, lignin, cellulose and hemicellulose were 44, 26.5, 6.2, 20.2, and 13.7%, respectively. The studied species showed means of 43 and 49% for in vitro dry matter disappearance (IVDMD) for perennials and annuals, respectively. In general, there were slight differences between the 2 years for both perennials and annual species and significant differences among different seasons for perennials and between growth stages for annuals. The study strongly recommends further detailed experimental studies on A. graecizans, Boerhavia diffusa and Cymbopogon schoenanthus, which showed more crude protein, a low fiber content and high IVDMD, as unconventional forage crops in arid lands.     

古尔班通古特沙漠不同类型生物结皮对草本植物多样性影响

选择古尔班通古特沙漠的北部(一号点)、中部(二号点)、南部(三号点)3个不同样点的裸沙和藻结皮、地衣结皮与苔藓结皮3种生物结皮类型,对比研究了草本植物多样性的差异性及其主要环境影响因素.结果表明:(1)不同生物结皮类型的土壤理化性质有明显差异,土壤有机质、全氮、全磷、全钾含量以及黏粒、粉粒和细沙的含量随生物结皮演替显著...     

冷荒漠草本植物数量特征对不同水分输入和气象因子的响应

草本层是古尔班通古特沙漠植被的重要组成部分,在维持沙漠稳定、养分循环等方面具有重要意义。水分通常以积雪和降雨形式输入到土壤,输入方式及数量变化对荒漠植物的存活和生长有重要影响。本文以古尔班通古特沙漠南缘草本层为研究对象,利用人工增减积雪的方法,共设置4个积雪处理(0,50%,100%,200%积雪,其中100%积雪为对照),于2009—2016年春季调查草本植物的数量特征和物种丰富度,分析草本植物数量特征对水分变化的响应;同时结合气象数据,分析不同年份一年生荒漠草本植物数量特征与主要气象因子的关系。结果表明：表层土壤含水量与积雪厚度成正比,相应的草本植物的幼苗密度与积雪水当量、表层土壤含水量成正比关系;2009—2016年,年内积雪量的变化对物种丰富度没有显著影响,各积雪处理间物种丰富度差异不显著;2009—2016年自然处理下年际间物种丰富度差异显著,2015年物种丰富度显著小于其余年份。结合气象数据分析发现,物种丰富度年际变化主要受幼苗建成期的降雨调控,并且干旱年份过后的湿润年份物种丰富度不受前一年降水的影响,说明荒漠草本植物层片具有较强的自我调节能力;自然处理下,不同年份草本植物旺盛期的存活数量与大气干旱程度(空气饱和差)呈显著负相关关系（R²值为0.611,P<0.05）,与生长季的降雨量呈正相关关系（R²值为0.162, P>0.05）,说明大气干旱程度更能表征荒漠草本植物生长峰值期的存活情况。     

乌拉特荒漠草原群落物种多样性和生物量关系对放牧强度的响应

以内蒙古乌拉特荒漠草原灌丛和草本植物群落为对象,研究这2种植物群落的物种组成、物种多样性及其与地上生物量关系对不同放牧处理(对照、中牧、重牧)的响应。结果表明:①随着放牧强度的增加,灌丛群落中红砂(Reaumuria songarica)和碱韭(Allium polyrhizum)的优势度增大,沙生针茅(Stipa glareosa)的优势度减小;重牧降低草本群落中沙生针茅的优势度,增加了蒙古韭(Allium mongolicum)和碱韭的优势度。②不同放牧处理显著降低了灌丛和草本群落的盖度、高度以及灌丛群落的密度,消除了灌丛和草本群落之间盖度的差异(P> 0. 05)。中牧降低灌丛群落的Pielou均匀度指数,重牧降低灌丛群落的物种丰富度,灌丛和草本群落的其他多样性指数在放牧处理之间无显著差异(P> 0. 05)。除物种丰富度外,其他多样性指数在灌丛和草本群落之间差异显著(P <0. 05)。③不同放牧处理降低了灌丛和草本群落的地上生物量和凋落物量,导致灌丛和草本群落之间地上生物量的显著差异(P <0. 05)。④地上生物量与Simpson优势度指数负相关,与Shannon-Wienner多样性指数和Pielou均匀度指数正相关;放牧处理下地上生物量与密度、物种丰富度正相关。放牧处理改变了荒漠草原植物群落组成、结构和功能,进而改变了群落结构和功能的重要关系。     

荒漠植物群落特征及其与气候因子的对应分析

2002-2008年,采用样方法对内蒙古典型荒漠区禁牧样地的植物群落进行了调查。以7年的调查结果和数据分析了荒漠植物群落的季节和年度特征,并利用除趋势对应分析(DCA)和典范对应分析(CCA)对植物群落特征与气候因子的相关性进行了排序分析。结果表明:灌木和草本植物的高度、盖度、密度和生物量的季节特征均为夏季和秋季间差异均不显著(P>0.05),而秋季显著高于春季(P<0.05)。灌木植物与草本植物群落特征的年动态具有差异,草本植物群落特征的年间波动大于灌木植物。DCA排序表明荒漠植物群落结构的季节性和年度间差异不明显。CCA排序表明草本植物特征因子与气候因子的相关性大于灌木植物。草本密度与年降水量的相关性较大,草本高度与年均温度的相关性较大,草本生物量与年降水量、年均温度的相关性均较大。     

Annual-perennial plant relationships and species selection for desert restoration

Exotic plant invasion is a growing concern in the conservation and management of indigenous arid land ecosystems.By creating areas of ameliorated microclimates and fertile soil below their canopies,perennial plants might influence exotic annual plant invasions.We conducted a quantitative literature review of studies that compared exotic annual plant abundance among native perennial plant species and interspace(open areas)microsites in North America's Mojave Desert,where exotic plant invasion has corresponded with increasing extent of wildfire and broad-scale ecosystem transformation.Ten studies compared exotic annual plant abundance between interspaces and below a total of 36 native perennial species.These studies revealed that:(1)With few exceptions,most native perennial species supported a greater abundance of exotic annuals than interspaces,indicating overall facilitation of exotic species by native perennials.(2)Exotic species abundance varied by orders of magnitude among native perennial species,with some perennial species harboring amounts of exotics similar to interspaces.(3)Distributions of dominant exotic species varied,where Bromus rubens displayed a greater affinity for below-perennial microsites than did Schismus spp.and Erodium cicutarium that often were most abundant in interspaces.Results suggest that the degree of facilitation of exotic plants warrants consideration when selecting native perennial species for revegetation and restoration projects.     

荒漠区一年生植物层片的组织格局与生态适应模式

在阿拉善荒漠及其邻近地区 ,有多种一年生植物种群可组成荒漠与绿洲生态系统的恒有性植物层片。本项研究利用样地调查与统计资料 ,取得了多种一年生植物层片物种组成的研究结果 ;在吉兰泰盐湖地区、腾格里沙漠东南部、雅布赖低山区、孪井滩地区、祁连山山麓、额济纳绿洲等地 ,对一年生植物种群生物产量的季节与年度间的波动性进行了调查和测定 ,并与相关的气象资料做了比对分析 ,获得了气候年、季变化与一年生植物层片动态的相关性认识。据此 ,划分了一年生植物层片的生态类型。荒漠与绿洲生态系统中 ,主要的一年生植物层片有 :.荒漠夏雨型一年生禾草类植物层片 ,荒漠一年生杂草类植物层片 ,沙漠一年生沙生植物层片 ,盐漠一年生盐生植物层片 ,绿洲草甸一年生植物层片 ,绿洲胡杨林一年生植物层片 ,柽柳灌丛一年生植物层片 ,绿洲农田一年生杂草植物层片等。多数一年生植物种是在荒漠优势植物株丛间的裸斑土壤种子库中 ,有丰富的种子储备 ,利用夏季较为集中的降雨迅速萌发生长到结实 ,完成一个短暂的生活周期。也有些一年生植物在沙质土、盐渍土、草甸土等特异性生境中各自具有有效的生态适应机制 ,组成沙漠、盐漠、绿洲林地与草地的一年生植物层片。这些一年生植物层片的组织格局 ,生态适应模式 ,生态功能     

新疆北屯绿洲弃耕农田的植被变化特征

在北屯绿洲选择了3组不同年限的次生盐渍化弃耕农田,分别计算了样地内每种植物的重要值,采用Shannon-Wiener指数、Pielou指数、Simpson指数测定了生物多样性.结果表明:3组群落组成存在显著差异,优势种更替明显.3组群落构成依次为,一年生草本、一年生草本 多年生小半灌木、多年生草本 灌木.物种多样性表现为2年群落＞5年群落＞13年群落;群落均匀度表现为2年群落＞5年群落,而5年与13年群落相当,但是生态优势度却表现出与物种多样性相反的趋势,即2年群落＜5年群落＜13年群落.     

Species-specific traits determine shrub-annual interactions during a growing season

Stress gradient hypothesis predicted that facilitative interactions usually increase in intensity and are importance with abiotic stress.By contrast,facilitation may be lost in time,when it involves the growth of benefactors or beneficiaries.Less is known about which response pattern is more common in arid desert.We present an empirical study to explore shrub-annual interactions at the community and individual level along the course of a single growing season in a desert steppe in northwest China.Here the severity of drought stress may increase in time due to uneven precipitation during plant growing season.We assessed growth responses of annuals in understory where two dominant shrubs were removed.Annuals responses showed a switch from weakly positive to more strongly positive beneath Calligonum mongolicum,whereas from positive to negative beneath Nitraria sphaerocarpa during the growing season.Additionally,annual species with contrasting functional traits showed distinct growth responses to canopies removal.There was evidence of an increase in soil moisture below the canopy of shrubs,but a decrease in potential evaporation rate and photosynthetically active radiation,which can partly explain these species-specific responses.We conclude that the balance between competitive and facilitative effects in shrub-annual interactions is not only governed by the severity of stress but also determined by plant traits,such as canopy structure of shrubs and functional traits of their understory annuals.     

西鄂尔多斯天然荒漠灌丛光合生态适应性

为了探究西鄂尔多斯地区荒漠灌丛的光合生态适应性,以4种天然荒漠灌丛半日花(Helianthemum songaricum)、红砂(Reaumuria songarica)、沙冬青(Ammopiptanthus mongolicus)、霸王(Sarcozygium xanthoxylon)为研究材料,利用Li-6400便携式光合测定仪野外原位测定荒漠灌丛日光合动态和季节动态规律。结果表明:①不同季节灌丛光合作用适宜的温度存在差异,高温环境使灌丛净光合速率最适温度升高,低温环境使其降低。②不同季节4种灌丛的净光合速率、蒸腾速率、水分利用效率和气孔导度日变化曲线趋势为"双峰形",胞间CO2浓度日变化曲线趋势为"W形",但不同季节各指标平均日变化趋势表现各异。③不同季节灌丛净光合能力表现为:沙冬青>霸王>红砂>半日花,说明沙冬青具有较强的生态适应性。④净光合速率和蒸腾速率日变化趋势几乎保持同步状态,表明4种灌丛叶片气孔对外界环境变化反应敏感。⑤水分是限制荒漠灌丛生长发育的关键因素。为了保证荒漠植物的群落稳定性和生物多样性,在夏、秋季需对沙冬青、半日花、红砂和霸王进行水分管理。     

放牧与刈割对科尔沁草甸植被演替的影响

本文研究了放牧与刈割两种草地利用方式对科尔沁草甸植被演替的影响。结果表明:1)在自由放牧干扰下,优势度最大的前五个种为虎尾草(27.8)、蒲公英(3.52)、狼毒(3.42)、野韭菜(3.03)、鹅绒委陵菜(2.67);在秋季割草干扰下,优势度最大的前五个种为大油芒(9.57)、狗尾草(7.36)、野古草(6.36)、洽草(4.85)、芦苇(4.74)等五种禾本科植物;2)在自由放牧干扰下,一年生植物在群落中占据重要地位,其优势度为35.0,小于多年生植物的优势度55.9;在割草干扰下,多年生植物在群落中占据绝对优势,其优势度为82.9,大于一年生植物优势度10.6;3)在放牧扰动下,草地群落物种替代表现在不同植物生活型之间,主要是多年生草本为一年生草本和灌木所替代;表现在同一生活型不同物种之间,主要是多年生优质牧草为多年生劣质牧草或有毒有害杂草替代。     

Desert vegetation distribution and species-environment relationships in an oasis-desert ecotone of northwestern China

Environmental heterogeneity significantly affects the structure of ecological communities. Exploring vegetation distribution and its relationship with environmental factors is essential to understanding the abiotic mechanism(s) driving vegetation succession, especially in the ecologically fragile areas. In this study, based on the quantitative analysis of plant community and environmental factors in 68 plots at 10 different transects in the Minqin oasis-desert ecotone(ODE) of northwestern China, we investigated desert vegetation distribution and species-environment relationships using multivariate analysis. Two-way indicator species analysis(TWINSPAN), detrended correspondence analysis(DCA), and canonical correspondence analysis(CCA) methods were used. A total of 28 species, belonging to 27 genera in 8 families, were identified. Chenopodiaceae, Zygophyllaceae, Gramineae, and Leguminosae were the largest families. Annual and perennial herbs accounted for 28.60% of the total number of plants, while shrubs(42.90%) were the most dominant. Nitraria tangutorum was the constructive species of the desert plant community. We divided the 68 plots surveyed in this study into 7 community types, according to the results of TWINSPAN. The distribution of these 7 communities in the DCA ordination graph showed that species with a similar ecotype were clustered together. Results of CCA indicated that groundwater was the dominant factor influencing vegetation distribution, while distance between plot and oasis(Dis) and soil electrical conductivity(EC) were the local second-order factors. Our study suggests that optimizing the utilization of groundwater in oases is key to controlling the degradation of desert vegetation. The favorable topographic conditions of sand dunes should be fully utilized for vegetal dune stabilization, and the influence of soil salinity on the selection of afforestation tree species should be considered.     

Effect of pest management systems on foliage- and grass-dwelling spider communities in an apple orchard in Hungary

Spider communities (Araneae) inhabiting the canopy, the herbaceous layer and the borders, as well as the populations overwintering on the tree trunks of different aged IPM and conventional apple orchards were investigated in Hungary. Abundance and species richness of entire spider communities in IPM plots were significantly higher than in conventional plots, probably owing to the lower toxicity of pesticides used and higher prey densities. In the case of abundance, similar tendencies were observed in web-building and hunting spider guilds. Age of plantations can significantly influence spider density in the canopy, acting through the prey density. In young plantations, where size of the canopy was smaller and density of the pear lace bug (Stephanitis pyri L.) higher, significantly higher numbers of hunting spider communities were present than in old plantations which had been similarly treated. This relationship was not observed for web-building spiders. Diversity of canopy-inhabiting spider communities was higher in old plots, regardless of the treatments. The effect of the border of the orchard on spider communities was investigated and it was found that when selective insecticides were used, migration of spiders into orchards was increased significantly. When broad-spectrum insecticides were applied, spider densities in the canopy did not differ between outer and inner rows of the orchards. The effect of the treatments and orchard age, both on abundance and species richness of overwintering spider communities on the trunk, showed the same result as for canopy spiders, namely significantly larger spider communities were found in IPM plots and in the young plantation than in conventionally-treated plots and in the old plantation. Broad-spectrum insecticides reduced abundance and species richness of spider communities in the herbaceous layer of the conventionally-treated plot. At the same time, the spider communities of the herbaceous layer of the IPM plot did not differ significantly from adjacent herbaceous plants. A significant overlap exists between spider communities of the canopy and herbaceous layer. Despite chemical treatments, migration from the herbaceous layer into the canopy occurs. The effects of chemical treatments on the dominant species are discussed. There were no significant differences between the differently treated plots in abundance of one of the dominant species Oxyopes heterophthalmus Latreille. However, the other dominant species, Cheiracanthium mildei L. Koch, was more abundant in the IPM plots.     

Food partitioning between sympatric gerbils (genus Meriones) in arid conditions of the western part of the Caspian Depression

There are many patterns of coexistence in communities, including partitioning based on spatial, temporal, or food niches. This study considered the partitioning of food between two ecologically similar and sympatric species, the tamarisk gerbil and the midday gerbil, in the Terek-Kuma Lowland of the Republic of Daghestan. The qualitative and quantitative characteristics of the diets of both gerbils were revealed. The species have relatively wide diets; the tamarisk gerbil’s diet consisted of forty-six plant species, and the midday gerbil’s diet consisted of fifty plant species within the study areas. Thirty plant species were present in the diets of the two gerbils. The dietary breadths of both gerbils were similar. The dietary overlap of the two gerbils ranged from low to moderate (average Pianka’s dietary overlap index, 0.476?±?0.08). Finally, under conditions in which food resources are sufficient, the moderate abundance of gerbils and the high diversity of habitats and microhabitats in the study areas allow these two species of gerbils to coexist in a common location.     

Delayed seed dispersal species and related traits in the desert of the United Arab Emirates

The ability of plants to safely retain seeds in the mother plant is an adaptive mechanism described in many desert plants. However, research about delayed seed dispersal species in the desert of the United Arab Emirates (UAE) is lacking. This study aims to identify these delayed seed dispersal species and assess the relationships of the presence of delayed seed dispersal with plant growth form, habit, spatial dispersal, antitelechoric mechanism, and seed release time. The relationships between the presence of delayed seed dispersal and the above studied traits were assessed by using the Pearson Chi-square test and Nonlinear Principal Components Analysis (NLPCA). Results showed that a total of 46 delayed seed dispersal species were recorded (15.0% of 307 studied species) and the highest incidence occurred in the Fabaceae family (17.4%). Delayed seed dispersal species were predominantly perennial plants (73.9%) with spatial restricted dispersal (67.4%), which released seed in the dry season (45.7%). The dominant groups of delayed seed dispersal species were persistent fruits species and synaptospermy (28.3%). All graminoids showed persistent lignified fruits, while prostrate annuals were basicarpic species with myxospermy. Sandy habitats had the highest number of delayed seed dispersal species (54.3%), whereas salt flats had the lowest (23.9%). In the desert of the UAE, delayed seed dispersal species spread seeds until the end of the dry and windy season, thus breaking seed dormancy at this time and ensuring seed germination in the next arrival of the rainy season. This morphological and ecological adaptation of delayed dispersal species is essential to the survival and sustainable development of vegetation in desert environments.     

Seasonal changes in the water-use strategies of three herbaceous species in a native desert steppe of Ningxia,China

Frequent periods of drought conditions are known to limit plant performance,primary production,and ecosystem stability in arid and semi-arid desert steppe environments.Plants often avoid competition by shifting their water use seasonally,which affects the water-use patterns of dominant species as well as the composition and structure of plant communities.However,the water-use strategies of dominant herbaceous species,which grow under natural field conditions in the desert steppe region of Ningxia Hui Autonomous Region,China,are poorly known.Here,we explored the possible sources of water uptake and water-use efficiency(WUE)of three dominant herbaceous plant species(Stipa breviflora,Agropyron mongolicum,and Glycyrrhiza uralensis)in a native desert steppe in the semi-arid area of Ningxia through an analysis of multiple parameters,including(1)the stable isotopic oxygen and hydrogen(δ18O andδ2H)compositions of precipitation,soil water,and stem water,(2)the carbon isotope(13C)composition of leaves,and(3)the soil water contents,based on field sampling across varying water conditions from June to September,2017.Frequent small precipitation events replenished shallow soil water,whereas large events only percolated down to the deep soil layers.Changes in soil water availability affected the water-use patterns of plants.Generally,during light precipitation periods,the deep root system of G.uralensis accessed deeper(>80 cm)soil water,whereas S.breviflora and A.mongolicum,which only have shallow roots,primarily absorbed water from the shallow and middle soil layers.As precipitation increased,all three plant species primarily obtained water from the shallow soil layers.Variation in soil water uptake between the dry and wet seasons enabled plants to make better use of existing satoil water.In addition,theδ13C values of G.uralensis and S.breviflora were higher than those of A.mongolicum.Theδ13C values of the three plant species were significantly negatively correlated with soil water content.Therefore,G.uralensis and S.breviflora maintained a higher WUE through their conservative and water-saving strategies across the entire growing season.In contrast,A.mongolicum,with a relatively low WUE in the wet season but a high WUE in the dry season,exhibited a more flexible water-use strategy.The different water-use strategies of these dominant plant species demonstrated the mechanisms by which plant communities can respond to drought.     

荒漠草原不同放牧制度群落多样性研究

在内蒙古苏尼特右旗荒漠草原,通过划区轮牧与自由放牧的比较试验,研究不同放牧制度下群落α多样性和β多样性。结果表明:①对照区、轮牧区和自由放牧区的植物种类丰富度和多样性均较前一年均有所增加,优势度指数除对照区有所增加,轮牧区和自由放牧区基本不变化;均匀度指数轮牧区和自由放牧区均略有下降。②物种丰富度指数与均匀度指数呈负相关,荒漠草原植物群落多样性更多地受丰富度的影响。③对照区、轮牧区和自由放牧区群落β多样性指数随取样面积增加而降低;荒漠草原在不放牧、自由放牧和划区轮牧下,轮牧区β多样性指数最高,其次为对照区和自由放牧区;④2004年轮牧区与自由放牧区的相似性系数为最高,其次为轮牧区与对照区;自由放牧区与对照区差异最大。到了2005年,除轮牧区与自由区的相似性系数降低外,轮牧区与对照区、自由区与对照区的相似性系数均有所增加。