Allelopathy of a native grassland community as a potential mechanism of resistance against invasion by introduced plants

Successful plant invasions depend, at least partly, on interactions between introduced plants and native plant communities. While allelopathic effects of introduced invaders on native resident species have received much attention, the reverse, i.e. allelopathic effects of native residents on introduced plants, have been largely neglected. Therefore, we tested whether allelopathy of native plant communities decreases their invasibility to introduced plant species. In addition, we tested among the introduced species whether the invasive ones are more tolerant to allelopathy of native plant communities than the non-invasive ones. To test these hypotheses, we grew nine pairs of related (congeneric or confamilial) invasive and non-invasive introduced plant species (i.e. 18 species) in the presence or absence of a native grassland community, which consisted of three common forbs and three common grasses, with or without activated carbon in the soil. Activated carbon reduced the survival percentage and growth of introduced plants in the absence of the native plant community. However, its net effect on the introduced plants was neutral or even slightly positive in the presence of the native community. This might suggest that the native plant community imposed allelopathic effects on the introduced plants, and that these effects were neutralized or reduced by activated carbon. The invasive and non-invasive introduced plants, however, did not differ in their tolerance to such allelopathic effects of the native plant community. Thus, although allelopathy of native plant communities might increase their resistance against introduced plants, there was no evidence that tolerance to allelopathy of native plant communities contributes to the degree of invasiveness of introduced plants.     

Allelopathy and exotic plant invasion

The primary hypothesis for the astonishing success of many exotics as community invaders relative to their importance in their native communities is that they have escaped the natural enemies that control their population growth – the `natural enemies hypothesis'. However, the frequent failure of introduced biocontrols, weak consumer effects on the growth and reproduction of some invaders, and the lack of consistent strong top-down regulation in many natural ecological systems indicate that other mechanisms must be involved in the success of some exotic plants. One mechanism may be the release by the invader of chemical compounds that have harmful effects on the members of the recipient plant community (i.e., allelopathy). Here, we provide an abbreviated compilation of evidence for allelopathy in general, present a detailed case study for Centaurea diffusa, an invasive Eurasian forb in western North America, and review general evidence for allelopathic effects of invasive plants in native communities. The primary rationale for considering allelopathy as a mechanism for the success of invaders is based on two premises. First, invaders often establish virtual monocultures where diverse communities once flourished, a phenomenon unusual in natural communities. Second, allelopathy may be more important in recipient than in origin communities because the former are more likely to be naïve to the chemicals possessed by newly arrived species. Indeed, results from experiments on C. diffusa suggest that this invader produces chemicals that long-term and familiar Eurasian neighbors have adapted to, but that C. diffusa's new North American neighbors have not. A large number of early studies demonstrated strong potential allelopathic effects of exotic invasive plants; however, most of this work rests on controversial methodology. Nevertheless, during the last 15 years, methodological approaches have improved. Allelopathic effects have been tested on native species, allelochemicals have been tested in varying resource conditions, models have been used to estimate comparisons of resource and allelopathic effects, and experimental techniques have been used to ameliorate chemical effects. We do not recommend allelopathy as a `unifying theory' for plant interactions, nor do we espouse the view that allelopathy is the dominant way that plants interact, but we argue that non-resource mechanisms should be returned to the discussion table as a potential mechanism for explaining the remarkable success of some invasive species. Ecologists should consider the possibility that resource and non-resource mechanisms may work simultaneously, but vary in their relative importance depending on the ecological context in which they are studied. One such context might be exotic plant invasion.     

化感——外来入侵植物的“Novel Weapons”

综述了与外来植物入侵相关的理论假说。并主要介绍了从化感角度提出的“N ove lW eapons”假说(NW假说)。某些外来入侵植物能产生一些化学物质。这些物质可以发挥较强的化感作用,或者成为植物和土壤微生物之间相互作用的调节者。由于不同区域的植物群落共同进化的轨迹不同,被入侵群落的植物对这些化学物质缺乏适应性,因此,这些物质成为了外来种入侵的利器。而对外来种发源地的群落来说,由于长期的共存关系,这些物质对其伴生植物的影响相对较小。NW假说为化感作为外来植物入侵的一种机制奠定了理论基础,为外来入侵植物的防治提供了理论依据,但作为一种较新的理论假说,还需要更多的实验研究加以支持。     

Allelopathy and plant invasions: traditional,congeneric, and bio-geographical approaches

A relatively small subset of exotic plant species competitively exclude their neighbors in invaded “recipient” communities but coexist with neighbors in their native habitat. Allelopathy has been argued as one of the mechanisms by which such exotics may become successful invaders. Three approaches have been used to examine allelopathy as a mechanism for invasion. The traditional approach examines exotic invasives in the same way that other native plants also suspected of allelopathic activities are studied. In this approach dose, fate, and replenishment of chemicals can provide powerful evidence for allelopathic processes. The bio-geographical approach often does not provide as much mechanistic evidence for allelopathy, but comparing the allelopathic effects of exotic invasives on species from their native and invaded communities yields stronger evidence than the traditional approach for whether or not allelopathy actually contributes to invasive success. The congeneric, or phylogenetic, approach involves comparative studies of exotic species with natives in the same genus or that are as closely related as possible. Congeneric approaches are limited in inference and have been used to study the role of natural enemies in exotic invasion, but this approach has not been widely used to study allelopathy and invasion. We discuss these three approaches and present a data set for congeneric Lantana and Prosopis to illustrate how the congeneric approach can be used, and use Centaurea maculosa and (±)-catechin to demonstrate experimentally how traditional and bio-geographic approaches can be integrated to shed light on allelopathy in exotic plant invasions.     

Influence of Pluchea lanceolata (Asteraceae) on selected soil properties

Allelopathic interference may operate simultaneously, sequentially, and/or in combination with other mechanisms of interference such as nutrient interference. It is hypothesized that under field conditions, allelopathic plants may cause changes in chemical characteristics of soils in addition to qualitative and quantitative changes in the allelochemical status of soil infested with the allelopathic plant. To test this hypothesis, the perennial allelopathic weed Pluchea lanceolata was selected. A comparative study of P. lanceolata-infested soils, and soils 10, 20, 30, and 40 m away from the weed was undertaken to examine soil characteristics and quantitative and qualitative variation in soil phenolics. Impact of seasonal weather on the biotic and chemical characteristics of P. lanceolata, and quantitative variation in phenolics of weed-infested soils was also studied. Growth experiments were conducted to study the seasonal impact on allelopathic interference of P. lanceolata toward certain crop plants. Results indicate that P. lanceolata influences soil properties in addition to causing variation in soil phenolics. Two-way tests (i.e., analyzing allelopathic and nutrient interference) should be run regardless of whether one is studying allelopathy or nutrient interference and it is important to test allelopathy in all studies dealing with nutrient interference.     

Inference of allelopathy is complicated by effects of activated carbon on plant growth

Allelopathy can play an important role in structuring plant communities, but allelopathic effects are often difficult to detect because many methods used to test for allelopathy can be confounded by experimental artifacts. The use of activated carbon, a technique for neutralizing allelopathic compounds, is now employed in tests for allelopathy; however, this technique also could produce large experimental artifacts. In three independent experiments, it was shown that adding activated carbon to potting media affected nutrient availability and plant growth. For most species tested, activated carbon increased plant biomass, even in the absence of the potentially allelopathic agent. The increased growth corresponded to increased plant nitrogen content, likely resulting from greater nitrogen availability. Activated carbon also affected nitrogen and other nutrient concentrations in soil media in the absence of plants. The observed effects of activated carbon on plant growth can confound its use to test for allelopathy. The detection of allelopathy relies on the difference between plant growth in medium with carbon and that in medium without carbon in the presence of the potentially allelopathic competitor; however, this difference may be biased if activated carbon alters soil nutrient availability and plant growth even in the absence of the focal allelopathic agent.     

外来植物火炬树水浸液对土壤微生态系统的化感作用

引种自北美的外来植物火炬树(Rhus typhina L.)是中国北方主要造林树种之一.然而,近年来分布区的不断扩大暗示着该树种的潜在入侵性.以火炬树为对象,研究火炬树不同浓度(0、0.005、0.025、0.1g/mL)的鲜枝叶水浸液对土壤微生物群落结构、酶活性、土壤养分含量及土壤矿化的影响.研究结果表明:随着水浸液浓度的提高,火炬树增加了细菌和真菌的数量;火炬树对所测土壤酶活性产生了不同程度的影响,脲酶和磷酸酶均有随着水浸液浓度的提高而增大的趋势,而蔗糖酶活性受影响不明显;随水浸液浓度升高,火炬树显著提高了土壤全碳、全钾、速效氮、有效磷、速效钾的含量,对土壤含水量、pH值、全氮与全磷没有显著影响;同时,火炬树通过促进微生物的矿化速率,提高了土壤无机氮的供给.以上结果表明,火炬树可以改变土壤的微生物组成和土壤酶活性并影响土壤相关营养元素循环,从而为自身的入侵创造有利条件.本研究揭示外来植物火炬树水浸液对土壤微生态系统的影响,从化感间接作用角度为火炬树的潜在入侵性提供进一步的数据支持.     

Enhanced allelopathy and competitive ability of invasive plant Solidago canadensis in its introduced range

Aims Why invasive plants are more competitive in their introduced range than native range is still an unanswered question in plant invasion ecology. Here, we used the model invasive plant Solidago canadensis to test a hypothesis that enhanced production of allelopathic compounds results in greater competitive ability of invasive plants in the invaded range rather than in the native range. We also examined the degree to which the allelopathy contributes increased competitive ability of S. canadensis in the invaded range.Methods We compared allelochemical production by S. canadensis growing in its native area (the USA) and invaded area (China) and also by populations that were collected from the two countries and grown together in a 'common garden' greenhouse experiment. We also tested the allelopathic effects of S. canadensis collected from either the USA or China on the germination of Kummerowia striata (a native plant in China). Finally, we conducted a common garden, greenhouse experiment in which K. striata was grown in monoculture or with S. canadensis from the USA or China to test the effects of allelopathy on plant–plant competition with suitable controls such as adding activated carbon to the soil to absorb the allelochemicals and thereby eliminating any corresponding allopathic effects.Important findings Allelochemical contents (total phenolics, total flavones and total saponins) and allelopathic effects were greater in S. canadensis sampled from China than those from the USA as demonstrated in a field survey and a common garden experiment. Inhibition of K. striata germination using S. canadensis extracts or previously grown in soil was greater using samples from China than from the USA. The competitive ability of S. canadensis against K. striata was also greater for plants originating from China than those from the USA. Allelopathy could explain about 46% of the difference. These findings demonstrated that S. canadensis has evolved to be more allelopathic and competitive in the introduced range and that allelopathy significantly contributes to increased competitiveness for this invasive species.     

Soil biota reduce allelopathic effects of the invasive Eupatorium adenophorum

Allelopathy has been hypothesized to play a role in exotic plant invasions, and study of this process can improve our understanding of how direct and indirect plant interactions influence plant community organization and ecosystem functioning. However, allelopathic effects can be highly conditional. For example allelopathic effects demonstrated in vivo can be difficult to demonstrate in field soils. Here we tested phytotoxicity of Eupatorium adenophorum (croftonweed), one of the most destructive exotic species in China, to a native plant species Brassica rapa both in sand and in native soil. Our results suggested that natural soils from different invaded habitats alleviated or eliminated the efficacy of potential allelochemicals relative to sand cultures. When that soil is sterilized, the allelopathic effects returned; suggesting that soil biota were responsible for the reduced phytotoxicity in natural soils. Neither of the two allelopathic compounds (9-Oxo-10,11-dehydroageraphorone and 9b-Hydroxyageraphorone) of E. adenophorum could be found in natural soils infested by the invader, and when those compounds were added to the soils as leachates, they showed substantial degradation after 24 hours in natural soils but not in sand. Our findings emphasize that soil biota can reduce the allelopathic effects of invaders on other plants, and therefore can reduce community invasibility. These results also suggest that soil biota may have stronger or weaker effects on allelopathic interactions depending on how allelochemicals are delivered.     

Allelopathic invasive tree (<Emphasis Type="Italic">Rhamnus cathartica</Emphasis>) alters native plant communities

Many plants release allelopathic chemicals that can inhibit germination, growth, and/or survival in neighboring plants. These impacts appear magnified with the invasion of some non-native plants which may produce allelochemicals against which native fauna have not co-evolved resistance. Our objective was to examine the potential allelopathic impact of an invasive non-native shrub/tree on multiple plant species using field observation and experimental allelopathy studies. We surveyed and collected an invasive, non-native tree/shrub (Rhamnus cathartica) at Tifft Nature Preserve (a 107-ha urban natural area near Lake Erie in Buffalo, NY). We also surveyed understory plant communities in the urban forest to examine correlations between R. cathartica abundance and local plant community abundance and richness. We then used experimental mesocosms to test if patterns observed in the field could be explained by adding increased dosages of R. cathartica to soils containing five plant species, including native and non-native woody and herbaceous species. In the highly invaded urban forest, we found that herbaceous cover, shrubs and woody seedlings negatively covaried with R. cathartica basal area and seedlings density. In the mesocosm experiments, R. cathartica resulted in significant decreases in plant community species richness, abundance, and shifted biomass allocation from roots. Our results provide evidence that R. cathartica is highly allelopathic in its invaded range, that R. cathartica roots have an allelopathic effect and that some plant species appear immune. We suggest that these effects may explain the plant’s ability to form dense monocultures and resist competitors, as well as shift community composition with species-specific impacts.     

Invasive forbs differ functionally from native graminoids, but are similar to native forbs

? Exotic plant invasions can alter ecosystem processes, particularly if the invasive species are functionally different from native species. We investigated whether such alterations can be explained by differences in functional traits between native and invasive plants of the same functional group or by differences in functional group affiliation. ? We compared six invasive forbs in Europe with six native forbs and six native graminoids in leaf and whole-plant traits, plasticity in response to nutrient supply and interspecific competition, litter decomposition rate, effects on soil nutrient availability, and allelopathy. All traits were measured in a series of pot experiments, and leaf traits additionally in the field. ? Invasive forbs differed from native forbs for only a few traits; they had less leaf chlorophyll and lower phosphorus (P) uptake from soil, but they tended to have a stronger allelopathic effect. The invasive forbs differed in many traits from the native graminoids, their leaves had lower tissue densities and a shorter life span, their litter decomposed faster and they had a lower nitrogen-use efficiency. ? Our results suggest that invasive forbs have the potential to alter ecosystem properties when invading graminoid-dominated and displacing native graminoids but not when displacing native forbs.     

Allelopathy and its coevolutionary implications between native and non‐native neighbors of invasive Cynara cardunculus L.

Invasive plants apply new selection pressures on neighbor plant species by different means including allelopathy. Recent evidence shows allelopathy functions as remarkably influential mediator for invaders to be successful in their invaded range. However, few studies have determined whether native and non‐native species co‐occurring with invaders have evolved tolerance to allelopathy. In this study, we conducted germination and growth experiments to evaluate whether co‐occurring native Juncus pallidus and non‐native Lolium rigidum species may evolve tolerance to the allelochemicals induced by Cyanara cardunculus in Australian agricultural fields. The test species were germinated and grown in pots filled with collected invaded and uninvaded rhizosphere soil of C. cardunculus with and without activated carbon (AC). Additionally, a separate experiment was done to differentiate the direct effects of AC on the test species. The soil properties showed invaded rhizosphere soils had higher total phenolic and lower pH compared with uninvaded soils. We found significant reduction of germination percentage and seedling growth in terms of above‐ and belowground biomass, and maximum plant height and root length of native in the invaded rhizosphere soil of C. cardunculus, but little effect on non‐native grass species. Even soil manipulated with AC showed no significant differences in the measured parameters of non‐native except aboveground biomass. Taken together, the results indicate allelochemicals induced by C. cardunculus exert more suppressive effects on native than non‐native linking the coevolved tolerance of those.     

Comparison of phenolic compounds and the effects of invasive and native species in East Asia: support for the novel weapons hypothesis

One prediction of the novel weapons hypothesis (NWH) for the dominance of exotic invasive plant species is that the allelopathic effects of successful invaders will, in general, be more biochemically inhibitory to native species and microbes in invaded regions than the native plants themselves. However, no study has compared biochemical concentrations, compositions, or effects of large numbers of native species to those of large numbers of invasive species. In this context we tested the allelopathic and antimicrobial potentials of nine native plant species and nine invasive species in East Asia by comparing their broad phenolic contents and the effects of extracts made from each of the species on target plants and soil fungi. Three of the invasive species, including Eupatorium rugosum, had higher concentrations of total phenolic compounds than any of the native species, and the mean concentration of total phenolics for invasive species was 2.6 times greater than the mean for native species. Only scopoletin was novel to the invasive species, being found in all of nine invasive species, but not in the native species. More importantly, the effects of the total suites of phenolic compounds produced by invasive species differed from the effects of phenolics produced by natives. Extracts of invasive species reduced radicle growth of the three test plant species by 60–80%, but extracts of native species reduced radicle growth by only 30–50%. Extracts of invasive species reduced shoot growth of the three test species by 20–40%, but the overall effect of native species’ extract was to stimulate shoot growth. The antimicrobial activity of invasive species was also significantly higher than that of native species. It should be noted that phenolics are just one component of a plant’s potential allelopathic arsenal and non-phenolic compounds are likely to play a role in the total extract effect. For example, extracts of P. americana contained the lowest levels of phenolic compounds, but exhibited the strongest inhibition effect. We could not determine whether the greater inhibitory effects of the extracts from invasive species were due to novel combinations of chemicals or higher concentrations of chemicals, but our results are consistent with the predictions of the NWH.     

Allelopathy in the natural and agricultural ecosystems and isolation of potent allelochemicals from Velvet bean (Mucuna pruriens) and Hairy vetch (Vicia villosa).

We have studied on allelopathy of plants and developed methods to identify the effective substances in root exudates, leaf leacheate, and volatile chemicals emitted from plants. We found traditional cover plants that show allelopathic activity are useful for weed control. It could eliminate the use of synthetic chemicals for this purpose. Allelopathy is a natural power of plants to protect themselves by producing natural organic chemicals. Some endemic plants in Asia, already known by farmers in the region, as either cover crops used in intercropping, hedgerow, or agroforestry, were found to possess strong allelopathic abilities. Our group identified several allelochemicals from these plants. These allelopathic cover crops, mostly leguminous plants, provide protein rich food, and grow easily without artificial fertilizers, herbicides, insecticides and fungicides. In this regards, these allelopathic cover crops could save food shortage in rural area, and are useful for environmental conservation. Screenings of allelopathic plants by specific bioassays and field tests have been conducted. Hairy vetch (Vicia villosa) and Velvet bean (Mucuna pruriens) are two promising species for the practical application of allelopathy. An amino acid, L-DOPA, unusual in plants, plays an important role as allelochemical in Velvet bean (Mucuna pruriens). Hairy vetch is the most promising cover plant for the weed control in orchard, vegetable and rice production and even for landscape amendment in abandoned field in Japan. We have isolated "cyanamide", a well known nitrogen fertilizer, from Hairy vetch. This is the first finding of naturally produced cyanamide in the world.     

Shallow moving structures promote marine invader dominance

Global increases in urban development have resulted in severe habitat modification in many estuaries. Most are now dominated by artificial structures, which might have a myriad of effects on native species. The provision of extra hard substrata presents additional free space, and recent research suggests non-indigenous epifauna may be able to exploit these artificial structures (particularly pontoons) more effectively than native species. The early development of fouling assemblages was compared on settlement plates attached to fixed or moving experimental structures at depths of 0.5 m and 2 m. Invertebrate invaders as a group were disproportionately more numerous on shallow, moving plates (essentially floating surfaces) than on deeper plates, whereas native epifauna were less numerous than invaders in all treatments. Importantly, however, individual invasive species showed differing effects of movement and depth. Future management strategies should take into account the potential for shallow, moving structures to enhance invader dominance and strongly consider using fixed structures to reduce opportunities for invaders.     

Competitive and allelopathic effects of garlic mustard (<Emphasis Type="Italic">Alliaria petiolata</Emphasis>) on American ginseng (<Emphasis Type="Italic">Panax quinquefolius</Emphasis>)

The purpose of this study was to test competitive and allelopathic effects of invasive garlic mustard on American ginseng (Panax quinquefolius) seedlings under natural conditions. For comparative purposes, we also examined the potential effects of the native striped violet (Viola striata). In order to partition effects of resource competition and chemical suppression via allelopathy, field soils were amended with activated carbon or left unamended. Activated carbon positively affected ginseng growth as well as biomass of competitors. Ginseng mortality tended to increase with garlic mustard presence, though activated carbon alleviated this response. Garlic mustard had no significant effect on ginseng seedling growth, while striped violet suppressed shoot length in the absence of activated carbon. Our results showed a surprising effect of activated carbon on plant growth, a potential allelopathic effect of the native striped violet and suggest that newly invaded ginseng populations with low densities of garlic mustard may be able to withstand its effects. However, recruitment within invaded populations may decline.     

Invasive success and the evolution of enhanced weaponry

A key hypothesis that has been proposed to explain plants’ invasive success suggests that some invasive species produce allelochemicals that are novel against naïve neighbours at the introduced range and therefore provide an advantage there (novel weapons hypothesis – NWH). However, a seldom‐studied hypothesis suggests that invasive populations could not only possess novel weapons, but might also evolve their enhanced production. Moreover, so far no study has examined both the novelty and evolution of allelopathic effects. Here, we examined these two hypotheses in a set of experiments with the highly invasive plant Impatiens glandulifera. In the first experiment, we examined the evolution of allelopathic ability by comparing the inhibitory effects of leaf extracts from native versus invasive I. glandulifera on the germination success of its dominant neighbour Urtica dioica. In the following experiments, we examined the NWH by comparing the germination success of U. dioica seeds collected at the native versus invasive range of I. glandulifera, in response to either leaf extracts or soil trained with invasive I. glandulifera. The results of the first experiment indicate that invasive I. glandulifera exert a stronger inhibitory effect on the germination of U. dioica compared to their native counterparts, providing support for the hypothesis that allelopathic ability can evolve at the invasive range. However, the results of the two following experiments reveal no difference in the response of U. dioica from the native versus invasive range of I. glandulifera, to the allelopathic effects of either the leaf extracts or the trained soil. These results therefore do not provide support for the NWH, and suggest that increased allelopathy in invasive I. glandulifera might have been selected for by other processes. The results of this study call for biogeographical experiments that will examine not only the novelty but also the evolution of allelopathic effects in invasive plants.     

Predation contributes to invasion resistance of benthic communities against the non-indigenous tunicate <Emphasis Type="Italic">Ciona intestinalis</Emphasis>

Marine anthropogenic structures offer novel niches for introduced species but their role in the subsequent invasion to natural habitats remains unknown. Upon arrival in new environments, invaders must overcome biotic resistance from native competitors and predators if they are to establish successfully in natural habitats. We tested the hypotheses that (1) artificial structures (e.g., suspended aquaculture installations) present a niche opportunity for invasive species by providing a refuge from native benthic predators, and (2) native predators in natural benthic habitats suppress successful colonization by invaders. A recruitment experiment showed that the ascidians Pyura chilensis (native) and Ciona intestinalis (invasive) could recruit to both suspended artificial structures and natural benthic habitats. Ciona, however, was only able to establish adult populations on artificial structures. In natural benthic habitats Ciona only recruited and grew in predator-exclusion cages, because without this protection predation prevented its establishment. In predation experiments, native invertebrate and fish predators removed all invasive ascidians (recruits and adults) in benthic habitats, which contrasted with the high adult survival of the native ascidian P. chilensis. The refuge from a number of benthic predators facilitates the establishment of large populations of invasive species on suspended structures. We present a conceptual model of the invasion processes that includes the anthropogenic structures as a transitional stepping-stone that facilitates invasion by enhancing and prolonging propagule supply to surrounding natural communities. Those established invaders might then overcome biotic resistance during time periods when populations of consumers or competitors are weakened by natural or anthropogenic disturbances. Our results suggest that the conservation of natural habitats with a high diversity of native predators can be an effective means to prevent the spread of invasive species growing on suspended structures.     

Low allelopathic potential of an invasive forage grass on native grassland plants: a cause for encouragement?

Tall fescue (Festuca arundinacea Schreb.), a highly competitive European grass that invades US grasslands, is reportedly allelopathic to many agronomic plants, but its ability to inhibit the germination or growth of native grassland plants is unknown. In three factorial glasshouse experiments, we tested the potential allelopathic effects of endophyte-infected (E+) and uninfected (E−) tall fescue on native grasses and forbs from Midwestern tallgrass prairies. Relative to a water control, at least one extract made from ground seed, or ground whole plant tissue of E+ or E− tall fescue reduced the germination of 10 of 11 species in petri dishes. In addition, the emergence of two native grasses in potting soil was lower when sown with E+ and E− tall fescue seedlings than when sown with seeds of conspecifics or tall fescue. However, when seeds of 13 prairie species were sown in sterilized, field-collected soil and given water or one of the four tall fescue extracts daily, seedling emergence was lower in one extract relative to water for only one species, and subsequent height growth did not differ among treatments for any species. We conclude that if tall fescue is allelopathic, its inhibitory effects on the germination and seedling growth of native prairie plants are limited, irrespective of endophyte infection. On the other hand, the apparent inability of these plants to detect tall fescue in field soil could hinder prairie restoration efforts if germination near this strong competitor confers fitness consequences. We propose that lack of chemical recognition may be common among resident and recently introduced non-indigenous plants because of temporally limited ecological interactions, and offer a view that challenges the existing allelopathy paradigm. Lastly, we suggest that tall fescue removal will have immediate benefits to the establishment of native grassland plants.     

薇甘菊化感物质对土壤氮素矿化的影响及其化感利己作用

【目的】研究入侵植物薇甘菊提取物对土壤氮素矿化的影响及化感利己作用,为薇甘菊通过释放化感物质促进自身生长并排斥本地植物生长提供实验证据,为揭示薇甘菊的入侵机制提供理论依据。【方法】采用经典的化感生测实验、化感物质添加实验以及盆栽控制实验,对比研究薇甘菊及其本地伴生植物水提液和粗提物的化感生测效应、水提液和粗提物添加对土壤氮素的影响以及水提液添加对薇甘菊生长的影响。【结果】与本地伴生种相比,薇甘菊水提液的化感抑制作用较强且促进了土壤硝态氮的生成;而薇甘菊粗提物的化感作用较弱且抑制了土壤铵态氮和硝态氮的生成。水提液盆栽控制实验结果表明,与本地伴生种火炭母相比,薇甘菊水提液具有显著的化感利己作用,这与薇甘菊水提液处理后薇甘菊生长土壤中的蛋白酶活性增强和有效氮含量增加相一致。【结论】薇甘菊可通过水溶性化感物质促进有效氮的生成来实现化感利己作用。