The impact of naturalized legumes on plant communities in Northern Taiwan: are we worrying too much?

Habitat transformation caused by naturalized legumes has been considered as a profound environmental threat worldwide. However, the weight of the impact on species diversity of local native and naturalized flora has yet to be revealed. In order to gain a better understanding of the phenomenon, we developed and tested the following hypotheses: (1) naturalized legumes promote local species diversity; (2) naturalized legumes increase local naturalized species diversity rather than native biodiversity; and (3) the impact of naturalized legumes varies with habitat type. Four counties in Northern Taiwan were selected to form the study site. Nine major habitat types were identified in 100 sampling sites (1 km²/each site) in northern Taiwan, and a total of 2,242 plots (1 m²/each plot) were sampled. Species, cover, and biodiversity indices of both native and naturalized floras were obtained, and soil samples were collected from plots with and without naturalized legumes analyzed. The biodiversity and cover of the whole and naturalized flora were increased significantly by naturalized legumes, while no effects on native flora were found. The significant increase in the species diversity and cover of the whole flora and naturalized flora disappeared when naturalized legumes were excluded from the data set; the same trend was observed when habitat type was considered. Soil nitrogen was marginally significantly higher in the plots with naturalized legumes. The effects of naturalized legumes on native and naturalized floristic composition are divergent. Although species diversity and cover were increased by naturalized legumes, the additional species were naturalized legumes per se, which were the only beneficiaries of the enriched soil. The naturalized legumes did not facilitate further invasion by other exotic species, nor did they have an impact upon the native community in terms of cover, species diversity, or composition.     

Effect of plant root symbionts on performance of native woody species in competition with an invasive grass in multispecies microcosms

The majority of terrestrial plants form mutualistic associations with arbuscular mycorrhizal fungi (AMF) and rhizobia (i.e., nitrogen‐fixing bacteria). Understanding these associations has important implications for ecological theory and for restoration practice. Here, we tested whether the presence of AMF and rhizobia influences the performance of native woody plants invaded by a non‐native grass in experimental microcosms. We planted eight plant species (i.e., Acacia acuminata, A. microbotrya, Eucalyptus loxophleba subsp. loxophleba, E. astringens, Calothamnus quadrifidus, Callistemon phoeniceus, Hakea lissocarpha and H. prostrata) in microcosms of field‐conditioned soil with and without addition of AMF and rhizobia in a fully factorial experimental design. After seedling establishment, we seeded half the microcosms with an invasive grass Bromus diandrus. We measured shoot and root biomass of native plants and Bromus, and on roots, the percentage colonization by AMF, number of rhizobia‐forming nodules and number of proteaceous root clusters. We found no effect of plant root symbionts or Bromus addition on performance of myrtaceous, and as predicted, proteaceous species as they rely little or not at all on AMF and rhizobia. Soil treatments with AMF and rhizobia had a strong positive effect (i.e., larger biomass) on native legumes (A. microbotrya and A. acuminata). However, the beneficial effect of root symbionts on legumes became negative (i.e., lower biomass and less nodules) if Bromus was present, especially for one legume, i.e., A. acuminata, suggesting a disruptive effect of the invader on the mutualism. We also found a stimulating effect of Bromus on root nodule production in A. microbotrya and AMF colonization in A. acuminata which could be indicative of legumes’ increased resource acquisition requirement, i.e., for nitrogen and phosphorus, respectively, in response to the Bromus addition. We have demonstrated the importance of measuring belowground effects because the aboveground effects gave limited indication of the effects occurring belowground.     

Phytoremediation in the Tropics—The Effect of Crude Oil on the Growth of Tropical Plants

Phytoremediation is a nondestructive, cost-effective in-situ technology to clean up contaminated soils. In the case of contamination with petroleum hydrocarbons, plants enhance microbial degradation of the contaminant in the rhizosphere. The potential of this technology for the tropics should be high due to prevailing climatic conditions favoring plant growth and stimulating microbial activity. Investigations of the potential of tropical plants for phytoremediation, however, are scarce. The present work studied two grasses and six legumes from the eastern savannah of Venezuela on their reaction to crude oil contamination in soil. Results shall help to identify plants with a potential for phytoremediation and subsequent studies. Seedling emergence and biomass production were determined for plants growing in soil contaminated with 0%, 3%, and 5% heavy crude oil. Contamination had, in general, a tendential but not significant negative influence on seedling emergence. Dry matter production was reduced by only a few percent to up to 85%. Furthermore, in some legumes inhibition of nodulation was observed. The grass Brachiaria brizantha and the legumes Centrosema brasilianum and Calopogonium mucunoides are promising for phytoremediation because in contaminated soil they combined high seedling emergence with least affected biomass production. Since they are cultivated forage/soil cover species also in other regions of the tropics, their potential for phytoremediation of petroleum contaminated soils extends beyond Venezuela.     

Phytoremediation in the Tropics—The Effect of Crude Oil on the Growth of Tropical Plants

Phytoremediation is a nondestructive, cost-effective in-situ technology to clean up contaminated soils. In the case of contamination with petroleum hydrocarbons, plants enhance microbial degradation of the contaminant in the rhizosphere. The potential of this technology for the tropics should be high due to prevailing climatic conditions favoring plant growth and stimulating microbial activity. Investigations of the potential of tropical plants for phytoremediation, however, are scarce. The present work studied two grasses and six legumes from the eastern savannah of Venezuela on their reaction to crude oil contamination in soil. Results shall help to identify plants with a potential for phytoremediation and subsequent studies. Seedling emergence and biomass production were determined for plants growing in soil contaminated with 0%, 3%, and 5% heavy crude oil. Contamination had, in general, a tendential but not significant negative influence on seedling emergence. Dry matter production was reduced by only a few percent to up to 85%. Furthermore, in some legumes inhibition of nodulation was observed. The grass Brachiaria brizantha and the legumes Centrosema brasilianum and Calopogonium mucunoides are promising for phytoremediation because in contaminated soil they combined high seedling emergence with least affected biomass production. Since they are cultivated forage/soil cover species also in other regions of the tropics, their potential for phytoremediation of petroleum contaminated soils extends beyond Venezuela.     

施氮对空心莲子草(Alternanthera philoxeroides)和莲子草(Alternanthera sessilis)种间关系的影响

自然界的氮素释放总是呈现出空间和时间上的异质性,但关于异质性氮释放对于入侵植物和本地植物种间关系影响的研究相对较少。将入侵植物空心莲子草(Alternanthera philoxeroides)和同属本地植物莲子草(Alternanthera sessilis)分别进行单种种植(12株,无种间竞争)和混种种植(每种6株,有种间竞争),模拟大气氮湿沉降设置由两种不同施氮总量(15g N m~(-2)a~(-1)和30g N m~(-2)a~(-1))和两种不同施氮频率(每5天1次和每15天1次)交叉组成的4种施氮处理,并以不施氮为对照。施氮总量的增加显著促进了两种植物的生长,但对两种植物的种间竞争关系没有显著影响。施氮频率对两种植物的生长以及种间竞争关系都没有显著影响。两种植物在面对竞争时表现出不同的生物量分配策略,空心莲子草将更多的生物量分配到茎,而莲子草将更多的生物量分配到根。在全球变化的背景下,大气氮湿沉降可能会改变两种植物的种群结构和动态,但可能对这两种植物的种间关系影响较小。     

Soil space and nutrients differentially promote the growth and competitive advantages of two invasive plants

Aims Invasive plants commonly occupy disturbed soils, thereby providing a stage for understanding the role of disturbance-enhanced resources in plant invasions. Here, we addressed how soil space and soil nutrients affect the growth and competitive effect of invasive plants and whether this effect varies with different invaders.Methods We conducted an experiment in which two invasive plants (Bromus tectorum and Centaurea maculosa) and one native species (Poa pratensis) were grown alone or together in four habitats consisting of two levels of soil space and nutrients. At the end of the experiment, we determined the total biomass, biomass allocation and relative interaction intensity of B. tectorum, C. maculosa and P. pratensis .Important findings Across two invaders, B. tectorum and C. maculosa, increased soil nutrients had greater positive effects on their growth than increased soil space, the effects of soil space on root weight ratio were greater than those of soil nutrients, and their competitive effect decreased with soil space but increased with soil nutrients. These findings suggest that changing soil space and nutrients differentially influence the growth and competitive advantages of two invaders. Bromus tectorum benefited more from increased soil resources than C. maculosa. Soil space and nutrients affected the biomass allocation of C. maculosa but not B. tectorum. The competitive effect of B. tectorum was unaffected by soil space and soil nutrients, but the opposite was the case for C. maculosa. Thus, the effects of soil space and nutrients on growth and competitive ability depend on invasive species identity.     

Current plant diversity but not its soil legacy influences exotic plant invasion

植物多样性而非其土壤遗留效应影响外来植物入侵 植物多样性可以影响外来植物入侵,然而植物多样性的土壤遗留效应是否能够影响外来植物入侵目前仍不清楚。植物多样性能够改变土壤微生物群落和土壤理化性质,这种遗留效应可能会对该土壤中外来植物的生长产生影响。因此,我们假设植物多样性的土壤遗留效应会影响外来植物的入侵。为了检验该假说,我们开展了一个两阶段的植物-土壤反馈实验。在土壤驯化阶段,我们将12个植物物种(4种禾草植物、3种豆科植物和5种杂类草植物)分别单独种植,或者随机选择8个物种(包含3个功能型)混合种植在土壤中。在反馈阶段,我们将入侵植物三叶鬼针草(Bidens pilosa)分别与本地禾草荩草(Arthraxon hispidus)、本地杂类草翅果菊(Pterocypsela indica)或者同时与荩草和翅果菊种植在被驯化过的土壤中。研 究结果显示,三叶鬼针草相对于其本地竞争植物的生长取决于驯化植物和竞争植物物种的功能型。驯化植物的多样性对三叶鬼针草与其本地竞争植物之间的生长差异没有显著影响。然而,随着本地竞争植物物种多样性的增加,三叶鬼针草相对于其本地竞争植物的生长显著降低。这些结果表明,当前的植物多样性可以通过增加入侵植物和本地植物之间的生长不平衡性来减少外来植物的入侵。但是,植物多样性的土壤遗留效应对外来植物入侵的影响可能很小。     

Native woody legumes respond more negatively to extreme drought than invasive ones

本地木本豆科植物比外来入侵木本豆科植物对极端干旱事件的负面响应更强 植物入侵可能会受到极端干旱事件增加和大气氮沉降的影响。然而,极端干旱、氮沉降及其相互作用如何影响外来木本豆科植物的成功入侵尚不清楚。本研究选取3种入侵木本豆科植物和3种本地木本豆科植物,开展温室控制实验。分别将这些植物种植在不同干旱处理(极端干旱和对照)和不同氮水平(低氮和高氮)下,然后对比分析两类植物的株高、叶片数、生物量以及根质量分数的差异。研究结果表明,极端干旱对本地木本豆科植物生长的抑制效应强于入侵木本豆科植物。尽管土壤氮素有效性的增加降低了植物的根质量分数,但这并不影响植物的整体表现。入侵木本豆科植物比本地木本豆科植物更能耐受长期极端干旱的负面效应。本研究加深了我们对气候变化导致的干旱事件如何影响外来木本豆科植物入侵的了解。     

Predicting invasiveness in exotic species: do subtropical native and invasive exotic aquatic plants differ in their growth responses to macronutrients?

We investigated whether plasticity in growth responses to nutrients could predict invasive potential in aquatic plants by measuring the effects of nutrients on growth of eight non-invasive native and six invasive exotic aquatic plant species. Nutrients were applied at two levels, approximating those found in urbanized and relatively undisturbed catchments, respectively. To identify systematic differences between invasive and non-invasive species, we compared the growth responses (total biomass, root:shoot allocation, and photosynthetic surface area) of native species with those of related invasive species after 13 weeks growth. The results were used to seek evidence of invasive potential among four recently naturalized species. There was evidence that invasive species tend to accumulate more biomass than native species ( P  = 0.0788). Root:shoot allocation did not differ between native and invasive plant species, nor was allocation affected by nutrient addition. However, the photosynthetic surface area of invasive species tended to increase with nutrients, whereas it did not among native species ( P  = 0.0658). Of the four recently naturalized species, Hydrocleys nymphoides showed the same nutrient-related plasticity in photosynthetic area displayed by known invasive species. Cyperus papyrus showed a strong reduction in photosynthetic area with increased nutrients. H. nymphoides and C. papyrus also accumulated more biomass than their native relatives. H. nymphoides possesses both of the traits we found to be associated with invasiveness, and should thus be regarded as likely to be invasive.     

Phosphorus addition reduces invasion of a longleaf pine savanna (Southeastern USA) by a non-indigenous grass (Imperata cylindrica)

Imperata cylindrica is an invasive C₄ grass, native to Asia and increasing in frequency throughout the tropics, subtropics, and southeastern USA. Such increases are associated with reduced biodiversity, altered fire regimes, and a more intense competitive environment for commercially important species. We measured rates of clonal spread by I. cylindrica from a roadside edge into the interior of two longleaf pine savannas. In addition, we measured the effects of fertilization with nitrogen and phosphorus on clonal invasion of one of these sites. Clonal invasion occurred at both sites and at similar rates. Older portions of an I. cylindrica sward contained fewer species of native pine-savanna plants. Clonal growth rates and aboveground mass of I. cylindrica were reduced by the addition of phosphorus relative to controls by the second growing season at one site. As a group, native species were not affected much by P-addition, although the height of legumes was increased by P addition, and the percent cover of legumes relative to native non-legumes decreased with increasing expected P limitation (i.e., going from P-fertilized to controls to N-fertilized treatments). Clonal invasion was negatively correlated with the relative abundance of legumes in control plots but not in P-fertilized plots. Species richness and percent cover of native plants (both legumes and non-legumes) were dramatically lower in N-fertilized plots than in controls or P-fertilized plots. Species richness of native plants was negatively correlated with final aboveground mass of I. cylindrica in control and P-fertilized plots, but not in N-fertilized plots. The results suggest that I. cylindrica is a better competitor for phosphorus than are native pine-savanna plants, especially legumes, and that short-lived, high-level pulses of phosphorus addition reduce this competitive advantage without negatively affecting native plant diversity. Ratios of soil P to N or native legume to non-legume plant species may provide indicators of the resistance of pristine pine savannas to clonal invasion by I. cylindrica.     

Soil-borne seed pathogens: contributors to the naturalization gauntlet in Pacific Northwest (USA) forest and steppe communities?

Soil-borne seed pathogens are omnipresent but are often overlooked components of a community’s biotic resistance to plant naturalization and invasion. Using multi-year greenhouse experiments, we compared the seed mortality of single invasive, naturalized, and native grass species in sterilized and unsterilized soils collected from Pacific Northwest (USA) steppe and forest communities. Native Pseudoroegneria spicata displayed the greatest seed mortality, naturalized Secale cereale displayed intermediate seed mortality, and invasive Bromus tectorum was least affected by soil pathogens. Seed mortality across all three species was consistently greater in soils collected from steppe than soils collected from forest; seeds sown into sterilized steppe soil experienced half the overall seed mortality compared to seeds sown into unsterilized steppe soil. Soil sterilization did not affect grass seed mortality in forest soils. We conclude that (1) removing soil-borne pathogens with sterilization does increase native and non-native grass seed survival, and (2) soil-borne pathogens may influence whether an introduced species becomes invasive or naturalized within these Pacific Northwest communities as a result of differential seed survival. Soil-borne pathogens in these communities, however, have the greatest negative effect on the survival of native grass seeds, suggesting that the native microbial soil flora more effectively attack seeds of native plants than seeds of non-native species.     

华南地区外来入侵和归化植物分析

为了解华南地区外来植物现状,通过野外调查和查阅文献等,确定华南地区有外来入侵和归化植物45科141属223种,以菊科(Asteraceae,38种)、豆科(Fabaceae,36种)、禾本科(Poaceae,19种)植物为主,其中草本植物和原产美洲的植物占优势,分别有204种(占总数的91.5%)和163种(占73.1%)。广东外来入侵植物有159种,归化植物23种;广西有入侵植物135种,归化植物16种;海南有入侵植物111种,归化种15种;香港有入侵植物105种,归化植物9种;澳门有入侵植物89种,归化种4种;华南5省区共有的外来入侵和归化植物为57种(55种为入侵植物,2种为归化植物)。广东是外来植物种类最多的地区,与其他4省区共有的外来入侵和归化植物种类也最多,说明广东同时是外来植物输入和输出大省。由于气候和生态生境的相似性,各省区都面临着其他植物从周边地区入侵的风险,因此在治理入侵植物时,应建立联合防控机制对外来入侵植物的联合监测和防控,以提升外来入侵植物的治理成效和降低其对人类健康和生态安全的威胁。     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Native and naturalized plant diversity are positively correlated in scrub communities of California and Chile

Abstract. An emerging body of literature suggests that the richness of native and naturalized plant species are often positively correlated. It is unclear, however, whether this relationship is robust across spatial scales, and how a disturbance regime may affect it. Here, I examine the relationships of both richness and abundance between native and naturalized species of plants in two mediterranean scrub communities: coastal sage scrub (CSS) in California and xeric-sloped matorral (XSM) in Chile. In each vegetation type I surveyed multiple sites, where I identified vascular plant species and estimated their relative cover. Herbaceous species richness was higher in XSM, while cover of woody species was higher in CSS, where woody species have a strong impact upon herbaceous species. As there were few naturalized species with a woody growth form, the analyses performed here relate primarily to herbaceous species. Relationships between the herbaceous cover of native and naturalized species were not significant in CSS, but were nearly significant in XSM. The herbaceous species richness of native and naturalized plants were not significantly correlated on sites that had burned less than one year prior to sampling in CSS, and too few sites were available to examine this relationship in XSM. In post 1-year burn sites, however, herbaceous richness of native and naturalized species were positively correlated in both CSS and XSM. This relationship occurred at all spatial scales, from 400 m² to 1 m² plots. The consistency of this relationship in this study, together with its reported occurrence in the literature, suggests that this relationship may be general. Finally, the residuals from the correlations between native and naturalized species richness and cover, when plotted against site age (i.e. time since the last fire), show that richness and cover of naturalized species are strongly favoured on recently burned sites in XSM; this suggests that herbaceous species native to Chile are relatively poorly adapted to fire.     

Are Mojave Desert annual species equal? Resource acquisition and allocation for the invasive grass Bromus madritensis subsp. rubens (Poaceae) and two native species

Abundance of invasive plants is often attributed to their ability ot outcompete native species. We compared resource acquisition and allocation of the invasive annual grass Bromus madritensis subsp. rubens with that of two native Mojave Desert annuals, Vulpia octoflora and Descurainia pinnata, in a glasshouse experiment. Each species was grown in monoculture at two densities and two levels of N availability to compare how these annuals capture resources and to understand their relative sensitivities to environmental change. During >4 mo of growth, Bromus used water more rapidly and had greater biomass and N content than the natives, partly because of its greater root-surface area and its exploitation of deep soils. Bromus also had greater N uptake, net assimilation and transpiration rates, and canopy area than Vulpia. Resource use by Bromus was less sensitive to changes in N availability or density than were the natives. The two native species in this study produced numerous small seeds that tended to remain dormant, thus ensuring escape of offspring from unfavorable germination conditions; Bromus produced fewer but larger seeds that readily germinated. Collectively, these traits give Bromus the potential to rapidly establish in diverse habitats of the Mojave Desert, thereby gaining an advantage over coexisting native species.     

内蒙古科尔沁沙地两种沙生植物冷蒿和差不嘎蒿竞争策略的生理生态学依据

研究了中国内蒙古科尔沁沙地两种优势灌木冷蒿(Artemisiafrigida Willd)和差不嘎蒿(Artemisia halodendronTurcz.ex Bess)在不同土壤水分状况条件下的气体交换、水分关系和叶片的化学特性.测定设置了5个土壤水分梯度:土壤最大含水量(体积含水量,30%)、田间持水量(对照,20%)、轻度水分胁迫(10%)、极端干旱(＜4%)和旱后复水(20%).冷蒿的净光合速率(Pn)、蒸腾速率(TR)和水势(ψw低于差不嘎蒿,相对水分亏缺(RWD)、束缚水含量(BWC)、束缚水与自由水的比值(BWC/FWC)和综合抗旱性指数(DI)高于差不嘎蒿.两种灌木对不同土壤含水量的响应不同,随着土壤含水量的逐渐下降,差不嘎蒿的ψw、BWC和BWC/FWC出现大幅度的波动,波动幅度远远的大于冷蒿,冷蒿则显示了一个较差不嘎蒿高的持水能力和抗旱能力.冷蒿和差不嘎蒿的脯氨酸和总可溶性糖含量随土壤含水量的降低均呈增加的趋势,冷蒿增加的幅度大于差不嘎蒿,说明冷蒿的渗透调节能力在干旱过程中有较大提高.在长期极端干旱条件下,两种灌木的ψw,RWD,BWC和BWC/FWC的终极值相近;有机物、脯氨酸和总可溶性糖含量大量累积;冷蒿的脯氨酸和总可溶性糖含量增加的幅度和可溶性蛋白质降解的幅度远远超过差不嘎蒿;我们认为此时累积的物质主要作为营养物质,以供植物的旱后恢复,此时冷蒿的恢复能力超过差不嘎蒿,这是极端干旱条件下差不嘎蒿死亡而冷蒿存活的主要原因之一.我们的研究结果显示,相对于差不嘎蒿而言,冷蒿在水分胁迫条件下的生理生态学特性更有利于其在固定沙地的生长,差不嘎蒿则由于对水资源微弱的竞争丧失了生存优势,是引起科尔沁沙地植被演替过程中冷蒿替代差不嘎蒿的主要原因.     

Effects of mycorrhizae on plant growth and dynamics in experimental tall grass prairie microcosms

Experimental microcosms (40 X 52 X 32 cm) containing an assemblage of eight tallgrass prairie grass and forb species in native prairie soil were maintained under mycorrhizal (untreated control) or mycorrhizal-suppressed (fungicide-treated) conditions to examine plant growth, demographic, and community responses to mycorrhizal symbiosis. The fungicide benomyl successfully reduced mycorrhizal root colonization in the fungicide-treated microcosms to only 6.4% (an 83% reduction relative to mycorrhizal controls). Suppression of mycorrhizas resulted in a 31% reduction in total net aboveground plant production and changes in the relative production of C4 and C3 plants. The C4 tallgrasses Andropogon gerardi and Sorghastrum nutans produced less plant biomass in the fungicide-treated microcosms, and had a greater ratio of reproductive to vegetative biomass. Cool-season C3 grasses, Koeleria pyramidata and Poa pratensis accumulated more biomass and were a significantly greater proportion of total community biomass in mycorrhizal-suppressed microcosms. Forbs showed variable responses to mycorrhizal suppression. The two legumes Amorpha canescens and Dalea purpurea had significantly lower survivorship in the fungicide-treated microcosms, relative to the controls. The results confirm the high mycorrhizal dependency and growth responsiveness of dominant prairie grasses, and indicate that differential growth and demographic responses to mycorrhizal colonization among species may significantly affect plant productivity and species relative abundances in tallgrass prairie.     

内蒙古科尔沁沙地两种沙生植物冷蒿和差不嘎蒿竞争策略的生理生态学依据(英文)

Gas exchange, water relations and leaf chemical characteristics were examined of twodominant psammophytes: Artemisia frigida Willd and A halodendron Turcz. ex Bess in Horqin sandy land, NeiMongol, China under different water regimes. The measurements were conducted by submitting the plantsto five different irrigation levels. A. fTigida was characterized by lower photosynthetic rate (Pn), lowertranspiration rate ( TR and lower shoot water potential (ψuw) relative to A. halodendron. Foliage of A frigdahad higher values of relative water deficit (RWD), bound water content (BWC), ratio of bound water contentto free water content (BWC/FWC) and integrated drought-resistant index (DI than that of A. halodendron.Water relations differed significantly between two species in response to soil water availability. ψw, BWCand BWC/FWC ratio of A halodendron exhibited large variation with gradual decrease of soil moisture.However, in terms of these parameters, A. fRIGIDA was characterized by higher capacity of water holding anddrought tolerance relative to A halodendron. Proline and total soluble sugar contents of A frigida and Ahalodendron tended to increase with decrease of soil moisture and the former had a larger increaseamplitude than the latter. This shows that A frigida has a higher osmotic regulation ability than A halodendron.Under the extreme drought conditions, ψw, RWD, BWC and BWC/FWC of two species were approximate,but soluble proteins degraded largely. A large amount of accumulation of organic matter, proline and totalsoluble sugars were observed in both A halodendron and A frigida. The increase in proline and total solublesugar contents and soluble protein degradation of A frigida far exceeded those of A. halodendron. Webelieve that the accumulated materials at this moment are mostly of nutrient substances available for therecovery of plants after the drought. This is one of the reasons why A halodendron plants died while Afrigida plants survived under extremely drought condition. Our results suggest that these ecophysiologicalfeatures of A frigida are favorable to its growth in the fixed sandy land compared with A halodendron, whichoften lost its dominance due to weak competition for water sources under lower soil water availability andare major factors resulting in replacement of A. halodendron by A. frigida in the later stage of sandyvegetation succession in Horqin.     

Independent effects of arbuscular mycorrhiza and earthworms on plant diversity and newcomer plant establishment

Questions: How do arbuscular mycorrhiza and earthworms affect the structure and diversity of a ruderal plant community? Is the establishment success of newcomer plants enhanced by these soil organisms and their interactions? Methods: We grew a native ruderal plant community composed of different functional groups (grasses, legumes and forbs) in the presence and absence of arbuscular mycorrhizal fungi (AMF) and endogeic earthworms in mesocosms. We introduced seeds of five, mainly exotic, plant species from the same functional groups after a disturbance simulating mowing. The effects of the soil organisms on the native ruderal plant community and seedling establishment of the newcomer plants were assessed. Results: After disturbance, the total above‐ground regrowth of the native plant community was not affected by the soil organisms. However, AMF increased plant diversity and shoot biomass of forbs, but decreased shoot biomass of grasses of the native plant community. Earthworms led to a reduction in total root biomass. Establishment of the introduced newcomer plants increased in the presence of AMF and earthworms. Especially, seedling establishment of the introduced non‐native legume Lupinus polyphyllus and the native forb Plantago lanceolata was promoted in the presence of AMF and earthworms, respectively. The endogeic earthworms gained more weight in the presence of AMF and led to increased extraradical AMF hyphal length in soil. However, earthworms did not seem to modify the effect of AMF on the plant community. Conclusion: The present study shows the importance of mutualistic soil organisms in mediating the establishment success of newcomer plants in a native plant community. Mutualistic soil organisms lead to changes in the structure and diversity of the native plant community and might promote newcomer plants, including exotic species.     

松嫩盐碱草地几种植物的土壤营养位分析

针对松嫩盐碱草地上的几种主要植物,以植物地上生物量和总和优势度（SDR）为土壤营养位效能指标,分别从多项土壤因子综合、可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量角度进行了土壤营养位分析。讨论了松嫩盐碱草地植物种群分布格局与土壤营养的关系,指出松嫩盐碱草地植物种群分布格局的形成是可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量5项主要土壤因子综合作用的结果,对于各植物种群土壤营养位的分化结果而言,没有一项土壤因子是起决定性作用的。