松嫩草原榆树疏林对不同干扰的响应

榆树疏林是中国东北松嫩草原沙地植被演替的顶极群落,其对维持物种多样性,保持水土,涵养水源等都发挥着重要的功能。过去20a间,榆树疏林在人为干扰(农耕和放牧)下,其群落结构和物种组成发生了巨大变化。在1983年和2004年,对松嫩草原不同人为干扰下榆树疏林群落结构和物种组成等进行调查,结果显示:农耕和放牧一方面使松嫩草原榆树疏林群落多物种消失、生物量和多样性显著下降、植被结构简单化;另一方面,也是导致建群种——家榆低矮化,灌木化的主要因素。     

Intra- and interspecific hybridization in invasive Siberian elm

Hybridization creates unique allele combinations which can facilitate the evolution of invasiveness. Frequent interspecific hybridization between the Siberian elm, Ulmus pumila, and native elm species has been detected in the Midwestern United States, Italy and Spain. However, Ulmus pumila also occurs in the western United States and Argentina, regions where no native elm species capable of hybridizing with it occurs. We examined whether inter- or intraspecific hybridization could be detected in these regions. Nuclear markers and the program STRUCTURE helped detect interspecific hybridization and determine the population genetic structure in both the native and the two non-native ranges. Chloroplast markers identified sources of introduction into these two non-native ranges. No significant interspecific hybridization was detected between U. pumila and U. rubra in the western United States or between U. pumila and U. minor in Argentina and vice versa. However, the genetic findings supported the presence of intraspecific hybridization and high levels of genetic diversity in both non-native ranges. The evidence presented for intraspecific hybridization in the current study, combined with reports of interspecific hybridization from previous studies, identifies elm as a genus where both inter- and intraspecific hybridization may occur and help maintain high levels of genetic diversity potentially associated with invasiveness.     

Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy

In response to the first Dutch elm disease (DED) pandemic, Siberian elm, Ulmus pumila, was planted to replace the native elm, U. minor, in Italy. The potential for hybridization between these two species is high and repeated hybridization could result in the genetic swamping of the native species and facilitate the evolution of invasiveness in the introduced species. We used genetic markers to examine the extent of hybridization between these two species and to determine the pattern of introgression. We quantified and compared the level of genetic diversity between the hybrids and the two parental species. Hybrids between U. pumila and U. minor were common. The pattern of introgression was not as strongly biased towards U. pumila as was previously observed for hybrids between U. rubra and U. pumila in the United States. The levels of heterozygosity were similar between U. minor and the hybrids and both groups had higher levels of heterozygosity relative to U. pumila. The programs Structure and NewHybrids indicated the presence of first- (F₁) and second- generation (F₂) hybrids and of backcrosses in the hybrid population. The presence of healthy DED resistant U. minor individuals combined with the self-compatibility of U. minor could help explain the presence of F₂ individuals in Italy. The presence of F₂ individuals, where most of the variability present in the hybrids will be released, could facilitate rapid evolution and the potential evolution of invasiveness of U. pumila in Italy.     

A last stand in the Po valley: genetic structure and gene flow patterns in Ulmus minor and U. pumila

Background and Aims Ulmus minor has been severely affected by Dutch elm disease (DED). The introduction into Europe of the exotic Ulmus pumila, highly tolerant to DED, has resulted in it widely replacing native U. minor populations. Morphological and genetic evidence of hybridization has been reported, and thus there is a need for assessment of interspecific gene flow patterns in natural populations. This work therefore aimed at studying pollen gene flow in a remnant U. minor stand surrounded by trees of both species scattered across an agricultural landscape.Methods All trees from a small natural stand (350 in number) and the surrounding agricultural area within a 5-km radius (89) were genotyped at six microsatellite loci. Trees were morphologically characterized as U. minor, U. pumila or intermediate phenotypes, and morphological identification was compared with Bayesian clustering of genotypes. For paternity analysis, seeds were collected in two consecutive years from 20 and 28 mother trees. Maximum likelihood paternity assignment was used to elucidate intra- and interspecific gene flow patterns.Key Results Genetic structure analyses indicated the presence of two genetic clusters only partially matching the morphological identification. The paternity analysis results were consistent between the two consecutive years of sampling and showed high pollen immigration rates (∼0·80) and mean pollination distances (∼3 km), and a skewed distribution of reproductive success. Few intercluster pollinations and putative hybrid individuals were found.Conclusions Pollen gene flow is not impeded in the fragmented agricultural landscape investigated. High pollen immigration and extensive pollen dispersal distances are probably counteracting the potential loss of genetic variation caused by isolation. Some evidence was also found that U. minor and U. pumila can hybridize when in sympatry. Although hybridization might have beneficial effects on both species, remnant U. minor populations represent a valuable source of genetic diversity that needs to be preserved.     

ON THE NATURE OF INTRA- AND INTERSPECIFIC INCOMPATIBILITY IN ULMUS

Efforts to introduce Dutch Elm Disease resistance into the American elm (Ulmus americana L.) through breeding with Asian elms has been hampered by sexual incompatibility. Controlled pollinations of Ulmus americana and the Siberian elm (Ulmus pumila L.) were studied in detail to gain insight into the nature of this incompatibility. Microscopic observations revealed that germination and early pollen tube growth were inhibited on the stigmatic surface following both intra- and interspecific incompatible pollinations. Both qualitative and quantitative differences in pollen inhibition on the stigmatic surface indicated that the inhibition may involve the action of an inhibitory substance. Detailed observations on callose deposition indicated that this β-1,3 glucose polymer may implement the inhibition.     

Establishment of Ulmus pumila seedlings on steppe slopes of the northern Mongolian mountain taiga

The potential of Siberian elm (Ulmus pumila) to regenerate from seeds was experimentally studied on south-facing slopes in the northern Mongolian mountain taiga. These slopes are covered with a vegetation mosaic of different steppe communities and small, savanna-like, U. pumila open woodlands. The hypothesis is tested that the xeric microclimate and high herbivore densities limit the success of seedling establishment in U. pumila and thereby prevent elm from complete encroachment of the grassland-dominated slopes. Seeds were sown and 2-yr-old seedlings were planted prior to the growing season. The water supply was manipulated by irrigation, as was the feeding pressure by caterpillars with an insecticide. Large herbivores were excluded by fencing. Seeds germinated throughout the summer, but the emerged seedlings did not survive for more than 2 or 3 weeks. Germination rates increased with increasing soil water content and decreasing soil temperatures. Many seeds were consumed by granivores. Most planted 2-yr-old seedlings survived the two growing seasons covered by the study. However, the seedlings suffered from feeding damage by insects (gypsy moth, grasshoppers) and small mammals, from nitrogen deficiency and, to a lesser degree, from drought. The results suggest that high susceptibility of newly emerged seedlings to environmental stresses is a serious bottle neck for U. pumila that prevents them from the formation of closed forests on northern Mongolia's steppe slopes, whereas the probability for seedling survival after this early stage is high.     

New Data on the Distribution of the Flea-Weevil <Emphasis Type="Italic">Orchestes steppensis</Emphasis> Kor. (Coleoptera,Curculionidae: Rhamphini) in European Russia

An elm flea-weevil Orchestes steppensis Korotyaev, 2016 was found on Ulmus pumila L. in the southeastern corner of Voronezh Province; formerly the westernmost records of this species in European Russia were those in Saratov Province and at the western boundary of Stavropol Territory. New data on the distribution of this and allied species in European Russia are reported.     

Identification of RAPD markers linked to a black leaf spot resistance gene in Chinese elm

Black leaf spot (Stegophora ulmea) is a common foliage disease on Chinese (Ulmus parvifolia) and Siberian elms (U. pumila), two species which have been widely used as sources of Dutch-elm disease-resistance genes for interspecific elm hybrids. A dominant gene controlling resistance to black leaf spot was identified in a population derived from self-pollination of a single U. parvifolia tree. Using RAPD markers, in combination with bulked segregant analysis, we have identified three markers linked to this resistance gene. A survey of Chinese-elm hybrids revealed that the same gene is likely to confer a high level of resistance to black leaf spot in interspecific elm hybrids, although other genetic factors may also be involved in the determination of a disease phenotype.     

Interspecific hybridisation and interaction with cultivars affect the genetic variation of Ulmus minor and Ulmus glabra in Flanders

Interspecific hybridisation and gene flow from cultivated plants may have profound effects on the evolution of wild species. Considering the cultural history and past use of Ulmus minor and Ulmus glabra trees in Flanders (northern Belgium), we investigated the extent of human impact on the genetic variation of the remaining, supposedly indigenous elm populations. We therefore examined the rate of interspecific hybridisation, which is expected to be higher under human influence, the occurrence of clones within and among locations, the presence of cultivars and their possible offspring. Based on results produced using 385 amplified fragment length polymorphic (AFLP) markers, 46 % of the 106 investigated Flemish elms appeared to be F1 hybrids or backcrosses to one of the parent species, while no F2 hybrids (F1?×?F1 progeny) were found. Clonality was mainly found among U. minor and hybrids, which are more likely to form root suckers or sprouts as opposed to U. glabra. The majority of the studied locations (76 % of the locations with multiple samples) showed evidence of clonal reproduction. Several, sometimes distant, locations shared a multilocus lineage. We also found indications of gene flow from cultivated elms into native species. It is conceivable that reproductive material has been moved around extensively, obscuring the natural genetic structure of the elm populations. The results help guide the Flemish elm genetic resources conservation programme.     

浑善达克沙地榆树疏林幼苗更新空间格局

榆树疏林广泛分布于浑善达克沙地,是适应半干旱、半湿润气候的沙地植被类型.采用Ripley的K函数统计方法,分析了浑善达克沙地固定沙丘与丘间低地封育状态下的榆树幼苗空间分布及更新格局.结果表明:两种生境下榆树幼苗密度分别为88株/hm2和77株/hm2,丘间低地更适合幼苗的生长.固定沙丘和丘间低地榆树幼苗均呈聚集分布,但丘间低地的幼苗聚集强度更高.固定沙丘上榆树幼苗与成熟树种群在小尺度上呈负关联,而丘间低地幼苗与成熟树种群呈现较显著的正关联,但二者在大尺度上均无关联性.研究对揭示浑善达克沙地榆树幼苗分布、更新空间格局及其恢复保护具有重要意义.     

Range-habitat relationships of vascular plant species at the taiga forest-steppe borderline in the western Khentey Mountains,northern Mongolia

World distribution of 488 out of 619 vascular plant species known from an area of 500 km² within the western Khentey Mountains, northern Mongolia is analyzed. Most species belong to Eastern Asian (29%) or Asian, Eurasian, or circumpolar temperate species (24%) supporting the classification of the Khentey Mountains as part of the temperate zone of Eurasia. Seventeen percent of species are boreal plants. Circumpolar temperate-boreal (9%), Central (-Eastern) Asian (9%), Continental mountain species (5%), Middle-Central Asian (3%), arctic-alpine (3%) as well as Western Eurasian and western Siberian species are of lower significance for the flora of the western Khentey Mountains. Eastern Asian species occur in all types of habitats, whereas plants of other distribution types are focused on certain habitats. Boreal species preferably grow in the dark taiga, which prevails in the upper montane belt and on northern and eastern slopes of the most humid parts of the lower montane belt. Temperate and temperate-boreal species prefer subtaiga forests, which are found on northern and eastern slopes in drier parts of the lower montane belt as well as in upper parts of sun-exposed, southern and western slopes of the lower montane belt. Central (-Eastern) Asian and Middle-Central Asian species primarily inhabit forest steppe habitats, such as meadow and mountain steppes, Ulmus pumila open woodlands and dry Pinus sylvestris forests on steep, southern slopes.     

Adaptation to different host plant ages facilitates insect divergence without a host shift

Host shifts and subsequent adaption to novel host plants are important drivers of speciation among phytophagous insects. However, there is considerably less evidence for host plant-mediated speciation in the absence of a host shift. Here, we investigated divergence of two sympatric sister elm leaf beetles, Pyrrhalta maculicollis and P. aenescens, which feed on different age classes of the elm Ulmus pumila L. (seedling versus adult trees). Using a field survey coupled with preference and performance trials, we show that these beetle species are highly divergent in both feeding and oviposition preference and specialize on either seedling or adult stages of their host plant. An experiment using artificial leaf discs painted with leaf surface wax extracts showed that host plant chemistry is a critical element that shapes preference. Specialization appears to be driven by adaptive divergence as there was also evidence of divergent selection; beetles had significantly higher survival and fecundity when reared on their natal host plant age class. Together, the results identify the first probable example of divergence induced by host plant age, thus extending how phytophagous insects might diversify in the absence of host shifts.     

Transcriptome analysis of <Emphasis Type="Italic">Ulmus pumila</Emphasis> ‘Jinye’ responses to different shading involved in chlorophyll metabolism

A variety of colorful mutations exist in nature. However, shade stress will affect the color of ornamental plants. Ulmus pumila ‘Jinye’ is a cultivated species of elm with a yellow leaf color, which will become green under shade stress. In this paper, ‘Jinye’ samples were treated with different degrees of shade by using shading net to investigate the mechanism of greening of the yellow leaves through phenotype observation and high-throughput sequencing. The height of plants rose in slight shade stress then fell in heavy shade stress. The content of chlorophyll and carotenoid increased with the increase of shade degree, and the leaf size increased as well. On the other hand, the net photosynthesis capacity declined. About 32.17 Gb of clean data were obtained after the completion of sequencing. A total of 33,023 unigenes with annotated information were obtained after comparing ‘Jinye’ with other species. The number of differentially expressed genes increased, and the proportion of upregulated genes decreased with increasing shade. The differentially expressed gene (DEG) analysis showed that the porphyrin and chlorophyll metabolism of Ulmus pumila ‘Jinye’ is affected by light. This may be related to genes associated with pigment synthesis. This first sequencing result of U. pumila will provide a basis for further research into this species.     

Phenylalanine ammonia-lyase activity in suspension cultures of Ulmus pumila and U. campestris treated with spores of Ceratocystis ulmi

Summary Cell suspension cultures of a Ceratocystis ulmi-resistant (Ulmus pumila) and a -susceptible elm (U.campestris) were established from leaf callus tissue. Treatment of cultures with spores of C.ulmi induced a large increase in the activity of phenylalanine ammonialyase, only in the cells of the resistant species U.pumila with a maximum after 24 h. Inoculated U.pumila cells also excreted a red unidentified chemical into the culture medium. Neither responses were induced in inoculated U.campestris cultures. The results are discussed in relation to the development of the elm cell culture system as a model for studying the differential biochemical mechanisms of disease resistance in elms.     

Leaf litter age regulates the effect of native and exotic tree species on understory herbaceous vegetation of riparian forests

Decomposing litter is regarded as the most important source of allelochemicals released into soil. In this study, a greenhouse experiment was designed to assess the net effect of differently aged leaf litter from exotic (Ailanthus altissima, Robinia pseudoacacia and Ulmus pumila) and native riverine trees (Populus alba, Populus nigra and Ulmus minor) on the germination and growth of three herb species (Trifolium repens, Dactylis glomerata and Chenopodium album). We also characterized the chemical composition of litter samples at different litter ages (0, 1, 2 and 3 months) based on phenolic compounds, fibers and ergosterol (as a measure of fungal biomass) contents. Overall, litter from both native and exotic species had a negative effect on shoot and root growth of target species, indicating that phytotoxic effects of litter predominate over positive effects. The inhibition effect of the exotic species was similar or even lower than that of the natives, which does not support the Novel Weapons Hypothesis. Among exotic trees, U. pumila showed the highest inhibition effect on the growth of the target species. T. repens was the most sensitive target species. The importance of litter age varied with both target and donor species. In general, D. glomerata was more inhibited by fresh litter, C. album by half-decomposed litter of U. pumila and R. pseudoacacia and by fresh litter of A. altissima and T. repens was more inhibited by fresh litter of A. altissima and P. alba and by highly decomposed litter of U. minor. The concentration of total phenolics and flavonoids decreased while acid detergent fiber, lignin and ergosterol increased with increasing litter age. Hydroxybenzoic and protocatechuic acids and the flavonoid quercetin were detected in all litter species and at most of the litter ages, while gallic, chlorogenic, vanillic, coumaric and rosmarinic acids were species-specific and they were only detected in fresh litter. Ergosterol concentration appeared as the strongest constrictor of inhibitory effects of litter on understory species. The results of this study contribute to the understanding of the net effect of fresh and decomposed litter from exotic and native trees on the growth of understory species.     

Distribution and abundance ofErynniopsis antennata [Dipt.: Tachinidae] andTetrastichus brevistigma [Hym.: eulophidae], two introduced elm leaf beetle parasitoids in Northern California

Erynniopsis antennata (Rondani), an introduced larval and larval-adult parasitoid of the elm leaf beetle (ELB),Xanthogaleruca luteola (Müller), was found in 11 of 12 northern California cities whereUlmus procera Salisbury andUlmus pumila L. were sampled in 1986 and/or 1987. Maximum apparent parasitism was over 40% in 4 of the cities, but mean parasitism for all sites combined was 4.5%. An introduced pupal parasitoid,Tetrastichus brevistigma Gahan, was found in only 7 cities, with mean and maximum apparent parasitism of 1.2 and 22%, respectively.E. antennata may be of value in managing ELB populations, but more knowledge of its biology and improved sampling methods are required.      

Resistance to Dutch Elm Disease Reduces Presence of Xylem Endophytic Fungi in Elms (Ulmus spp.)

Efforts to introduce pathogen resistance into landscape tree species by breeding may have unintended consequences for fungal diversity. To address this issue, we compared the frequency and diversity of endophytic fungi and defensive phenolic metabolites in elm (Ulmus spp.) trees with genotypes known to differ in resistance to Dutch elm disease. Our results indicate that resistant U. minor and U. pumila genotypes exhibit a lower frequency and diversity of fungal endophytes in the xylem than susceptible U. minor genotypes. However, resistant and susceptible genotypes showed a similar frequency and diversity of endophytes in the leaves and bark. The resistant and susceptible genotypes could be discriminated on the basis of the phenolic profile of the xylem, but not on basis of phenolics in the leaves or bark. As the Dutch elm disease pathogen develops within xylem tissues, the defensive chemistry of resistant elm genotypes thus appears to be one of the factors that may limit colonization by both the pathogen and endophytes. We discuss a potential trade-off between the benefits of breeding resistance into tree species, versus concomitant losses of fungal endophytes and the ecosystem services they provide.     

Local adaptation and host race formation of a gall-forming aphid in relation to environmental heterogeneity

Summary The process of host race formation in the aphid Tetraneura yezoensis is examined in relation to its population structure. T. yezoensis induces pouch galls on new leaves of Ulmus davidiana and U. laciniata. Its populations on the two host species are often sympatric. Fundatrices found on one elm species, when reciprocally transplanted to the other, suffered greatly reduced average fitness. This shows that aphid populations associated with the two elm species are genetically differentiated in physiological traits. Individual trees of each elm species showed large differences in susceptibility to gall formation and in bud burst time, and such between-tree variations were consistent over years. Overwintered eggs taken in early spring from four trees (two from each species) were incubated under the same temperature conditions. The average hatching time differed significantly even between populations from conspecific trees, and the sequence of egg hatching paralleled that of the leafing of those four trees. This between-tree difference in hatching time was consistent over years and was found to be genetic, showing that gene flow between aphid populations on separate trees is often restricted. The heterogeneity in host traits may have promoted the evolution of philopatry in this aphid. Of the fundatrices that hatched on a tree of one elm species, a few precent were preadapted to gall formation on the other elm species. This suggests that the formation of a new host race proceeds parapatrically under disruptive selection and at a low level of gene flow. Evidence was actually obtained that a small fraction of Tetraneura alates are passively transported and land on non-host plants.     

干旱区绿洲城市树木健康评估及影响因素——以乌鲁木齐市为例

树木作为城市绿色基础设施重要组分,其健康直接影响城市生态系统服务及人居环境质量。城市化进程加快导致生态土地缺失,同时缺乏合理养护加之极端自然条件干扰对树木健康造成严重威胁。为揭示城市树木健康风险及影响因素,对乌鲁木齐市核心城区117个样地绿化树木进行健康评估,从4个维度12个指标构建树木健康分级体系,基于样地调查结果,分析树木组成、结构、关联性、健康状况及影响因素,揭示乌鲁木齐市不同功能区树木健康差异。旨在为研究区树木健康保护、城市绿地可持续经营及城市生态文明建设提供理论依据和数据支撑。结果表明：（1）乌鲁木齐市城市园林树木乔灌比为8.3 ： 1,树木多样性与种间关联程度较低。榆树占比高达43.87%,其中白榆占比22.62%,是主要优势树种;（2）不同树种健康程度呈现差异性,榆树类健康状况呈现圆冠榆 > 大叶榆 > 白榆 > 垂枝榆特征;（3）城市不同功能区树木健康也呈现异质性;以公园和居住小区树木健康分值较高,而行道树和学校庭院树木健康分值相对较低,且冗余分析进一步揭示地面铺装、林木配置结构、生长空间和胸径对树木健康影响程度最为显著。未来优化城市树木生境条件、提升物种丰富度、加强古树名木保育和提高园林绿化管理水平是维护和提升绿洲城市树木健康的重要举措。     

Coexistence and weak amensalism of congeneric gall-forming aphids on the Japanese elm

Closely related species of gall-forming aphids are often associated with a single host species. SixTetraneura species coexist on the Japanese elm,Ulmus davidiana, in Sapporo, northern Japan. This paper describes the probabilities of coexistence on macro- and microgeographic scales (i.e., on host trees and host leaves) and examines whether coexistence with conspecific or heterospecific galls on leaves or shoots has any effect on the fecundity of each aphid species using multiple regression. A Monte Carlo simulation showed that the frequency distribution of the numbers of species on individual host trees differed significantly from that expected from the null model. There were significantly positive or negative associations between species.Tetraneura radicicola andsorini always coexisted with other species on trees they infested. Multiple regression revealed that the coexistence of conspecific or heterospecific galls on individual leaves had no influence on the fecundity ofradicicola andsorini, but had a negative influence on that of sp. O. On average,radicicola andsorini produced a smaller number of offspring in galls than sp. O, and obviously consumed only a small part of resources available on the galled leaves. Evidence available suggests that although amensalism does arise between sp. O and other species, its influence in not so strong as to exclude sp. O competitively from theTetraneura community.