Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.     

Impact of competition on plant performances along a flooding gradient: a multi‐species experiment

Question: 1. How do the competitive response and the importance of competition vary between species and along a flooding gradient? 2. How does the role of competition in constraining species distribution limits along the gradient vary between lower and upper limits? Location: A 1‐ha meadow within the Alzette floodplain in Luxembourg. Methods: Competitive response and importance of competition were assessed on seven meadow species differing in their tolerance to flooding. Species were cultured in monocultures and in mixtures, in three water treatments reflecting the wet, the middle and the dry end of a natural flooding gradient. We developed two models based on a multiple regression in order to express each component of competition as a function of the neighbour biomass. Results: Five species showed variations in their competitive response across water treatments; however, these species achieved either their highest or their worst competitive response in their optimal water treatment (i.e. the treatment in which the species had the highest biomass in monoculture). Competition was more important for the flood‐tolerant species in the dry treatment than for the flood‐intolerant species in the wet treatment. Conclusions: 1. Variations in species competitive responses along flooding gradients may be the result of either an amplified effect between competition and hydrological stresses, or a hierarchical effect of stress over competition. 2. The role of competition is more important in constraining the upper distribution limits of the flood‐tolerant species than the lower limits of the flood‐intolerant species along flooding gradients.     

Interactions between root and shoot competition vary among species

Understanding how the competition varies with productivity is essential for differentiating among alternative models of plant community organization. Prior attempts to explain shifts in root and shoot competition along gradients have generally assumed an additive interaction between the two competitive forms, using an experimental design which does not fully separate both above‐ and belowground processes. At the most basic level, few field studies have separated root and shoot competition, and we have limited knowledge about both the relative importance of these processes, and how they interact to affect plant growth in the field. Presented here are findings from a field study in which root and shoot competition were experimentally separated by using root exclusion tubes and neighbor tiebacks in an early successional community. Individuals of four species (Abutilon theophrasti, Amaranthus retroflexus, Rumex crispus, and Plantago lanceolata) were grown at two levels of fertilization with full competition, aboveground competition only, belowground competition only, or neither above‐ nor belowground competition. Competition was measured as competitive response, which is the natural log of the relative biomass of a target plant grown with competition compared to growth without competition. In contrast to predictions from current models of productivity‐competition relationships, but in agreement with other experimental studies, there was no change in the strengths or root, shoot, or total competition with a modest increase in productivity. Despite no effect of fertilization on the strength of competition, the form of interaction between root and shoot competition varied both as a function of species identity and fertilization. For both of the rosette forming species, the combined effects of root and shoot competition were less than predicted assuming no interaction (a “negative interaction”), with one species switching from a negative to an additive interaction with fertilization. The fact that fertilization caused a shift in the root‐shoot interaction, but not in the total strength of root and shoot competition, suggests that the root‐shoot interaction is itself a highly labile variable. If root‐shoot interactions are common in natural systems, then simply measuring the strength of one form of competition in no way provides any information about the overall importance of that competitive form to plant growth.     

Range size-abundance relationships in Australian passerines

Aim To investigate the relationship between geographical range size and abundance (population density) in Australian passerines. Location Australia (including Tasmania). Methods We analysed the relationship between range size and local abundance for 272 species of Australian passerines, across the whole order and within families. We measured abundance as mean and maximum abundance, and used a phylogenetic generalized least‐squares regression method within a maximum‐likelihood framework to control for effects of phylogeny. We also analysed the relationship within seven different habitat types. Results There was no correlation between range size and abundance for the whole set of species across all habitats. Analyses within families revealed some strong correlations but showed no consistent pattern. Likewise we found little evidence for any relationship or conflicting patterns in different habitats, except that woodland/forest habitat species exhibit a negative correlation between mean abundance and range size, whilst species in urban habitats exhibit a significant positive relationship between maximum abundance and range size. Despite the general lack of correlation, the raw data plots of range size and abundance in this study occupied a triangular space, with narrowly distributed species exhibiting a greater variation in abundances than widely distributed species. However, using a null model analysis, we demonstrate that this was due to a statistical artefact generated by the frequency distributions for the individual variables. Conclusions We find no evidence for a positive range size‐abundance relationship among Australian passerines. This absence of a relationship cannot be explained by any conflicting effects introduced by comparing across different habitats, nor is it explained by the fact that large proportions of Australia are arid. We speculate that the considerable isolation and evolutionary age of Australian passerines may be an explanatory factor.     

Species richness and endemism in the Western Australian flora

Aim Estimates of endemic and non‐endemic native vascular plant species in each of the three Western Australian Botanical Provinces were made by East in 1912 and Beard in 1969. The present paper contains an updated assessment of species endemism in the State. Location Western Australia comprises one third of the continental Australian land mass. It extends from 13° to 35° S and 113° to 129° W. Methods Western Australia is recognized as having three Botanical Provinces (Northern, Eremaean and South‐West) each divided into a number of Botanical Districts. Updated statistics for number of species and species endemism in each Province are based on the Census of Western Australian Plants data base at the Western Australian Herbarium ( Western Australian Herbarium, 1998 onwards). Results The number of known species in Western Australia has risen steadily over the years but reputed endemism has declined in the Northern and Eremaean Provinces where cross‐continental floras are common. Only the isolated South‐West Province retains high rates of endemism (79%). Main conclusions With 5710 native species, the South‐West Province contains about the same number as the California Floristic Province which has a similar area. The Italian mediterranean zone also contains about this number but in a smaller area, while the much smaller Cape Floristic Region has almost twice as many native species. The percentage of endemic species is highest at the Cape, somewhat less in south‐western Australia and less again in California. Italy, at 12.5%, has the lowest value. Apart from Italy, it is usual for endemism to reach high values in the largest plant families. In Western Australia, these mainly include woody sclerophyll shrubs and herbaceous perennials with special adaptations to environmental conditions. While those life forms are prominent in the Cape, that region differs in the great importance of herbaceous families and succulents, both of which are virtually absent from Western Australia. In California and Italy, most endemics are in families of annual, herbaceous perennial and soft shrub plants. It is suggested that the dominant factor shaping the South‐West Province flora is the extreme poverty of the area’s soils, a feature that emphasizes sclerophylly, favours habitat specialization and ensures relatively many local endemic species.     

Responses of Fraxinus excelsior seedlings to grass-induced above- and below-ground competition

The competitive interactions between woody seedlings and herbaceous vegetation have received increasing interest in recent years. However, little is known about the relative contributions and underlying mechanisms of above- and below-ground competition between species. We used a novel experimental approach to assess the responses of Fraxinus excelsior seedlings to different combinations of root and shoot competition imposed by the grass Dactylis glomerata under greenhouse conditions. Seedling growth was significantly reduced by competition for soil resources, but neither biomass nor height were significantly affected by shoot competition for light. Competitive response indices based on biomass confirmed that below-ground competition was more important than above-ground competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were almost all significantly affected by root competition; these responses varied depending on the trait examined. In contrast, morphological responses to shoot competition were limited. In the absence of root competition, seedlings showed a significant increase in the biomass allocated to leaves and a greater leaf area ratio in response to shoot competition. Our findings suggest that morphological modifications help to mitigate the negative effects of competition, but the expression of plasticity may be suboptimal due to resource constraints. The present study also highlights the importance of appropriate experimental controls and analysis to avoid confounding effects of experimental design and ontogeny on the interpretation of competitive responses.     

The symmetry of competitive interactions in mixed Norway spruce,silver fir and European beech forests

Questions

We aim for a better understanding of the different modes of intra‐ and inter‐specific competition in two‐ and three‐species mixed‐forests. How can the effect of different modes of competitive interactions be detected and integrated into individual tree growth models? Are species interactions in spruce–fir–beech forests more associated with size‐symmetric or size‐asymmetric competition? Do competitive interactions between two of these species change from two‐ to three‐species mixtures?

Location

Temperate mixed‐species forests in Central Europe (Switzerland).

Methods

We used data from the Swiss National Forest Inventory to fit basal area increment models at the individual tree level, including the effect of ecological site conditions and indices of size‐symmetric and size‐asymmetric competition. Interaction terms between species‐specific competition indices were used to disentangle significant differences in species interactions from two‐ to three‐species mixtures.

Results

The growth of spruce and fir was positively affected by increasing proportions of the other species in spruce–fir mixtures, but negative effects were detected with increasing presence of beech. We found that competitive interactions for spruce and fir were more related to size‐symmetric competition, indicating that species interactions might be more associated with competition for below‐ground resources. Under constant amounts of stand basal area, the growth of beech clearly benefited from the increasing admixture of spruce and fir. For this species, patterns of size‐symmetric and size‐asymmetric competitive interactions were similar, indicating that beech is a strong self‐competitor for both above‐ground and below‐ground resources. Only for silver fir and beech, we found significant changes in species interactions from two‐ to three‐species mixtures, but these were not as prominent as the effects due to differences between intra‐ and inter‐specific competition.

Conclusions

Species interactions in spruce–fir–beech, or other mixed forests, can be characterized depending on the mode of competition, allowing interpretations of whether they occur mainly above or below ground level. Our outcomes illustrate that species‐specific competition indices can be integrated in individual tree growth functions to express the different modes of competition between species, and highlight the importance of considering the symmetry of competition alongside competitive interactions in models aimed at depicting growth in mixed‐species forests.

     

Simulated herbivory counteracts the effects of grass competition on the invasive fireweed (Senecio madagascariensis)

Competition, herbivory and their interaction play a significant role in determining the competitive ability and survival of individual plant species. Understanding these processes and interactions can improve the efficacy of biocontrol programs against invasive weeds. Senecio madagascariensis (fireweed) is an invasive weed of South African origin that reduces pastoral productivity and poisons livestock in several countries, notably Australia. Although competitive pastures can suppress the weed’s growth in Australia, its competitive nature is poorly understood in relation to its invasion success. This greenhouse study assessed the growth and reproductive yield of fireweed growing in competition with six native and introduced grasses present in both South Africa and Australia. Since fireweed is a target for biocontrol in Australia, we examined whether its response to grass competition changed with herbivory (simulated by 40% leaf removal). The effect of grass competition and herbivory on the weed’s biomass and floral productivity was examined during a 12‐week pot trial in South Africa. Floral numbers were unaffected by both grass competition and herbivory. Biomass was used to calculate Relative Interaction Indices (RII) to quantify the weed’s competitive or facilitative response. This index compares a specific measurable trait, such as biomass, of fireweed growing alone, to fireweed growing with grass to determine the level of competitive suppression or facilitation resulting from the interaction. Despite the lack of species‐specific effects of grass competition, the presence of grass suppressed fireweed’s foliar, root and whole plant biomass the most when herbivory was absent. With herbivory, fireweed did not suffer from any measurable competitive suppression. This lack of competitive suppression may be due to an induced allelopathic response, given the levels of pyrrolizidine alkaloids common in many Senecio species. Since this result may weaken the case for biocontrol, the weed’s competitive responses should be verified in relation to actual insect herbivory.     

Disentangling Competition, Herbivory, and Seasonal Effects on Young Plants in Newly Restored Communities

Optimizing techniques of impact and consequence assessment are critical when faced with the challenges of reclamation within a damaged or altered ecosystem. Much debate has arisen over an appropriate index to evaluate herbivore and competition effects on restored communities. We assessed concurrent environmental pressures by means of repeated measurements using three common indices of plant performance (biomass, shoot extension, and survival) in conjunction with monitoring for number and timing of plants eaten. Our design incorporated 24 species, representing a range of taxonomic groups and growth forms, planted at low and high densities, inside and outside large‐scale mammal exclosures. We demonstrate that biomass and height measurements are correlated (at both the individual and the combined species levels), whereas the survival index often showed independent information. Using the most conservative measure (survival), we delineate between plant deaths attributed to seasonal effects, competition (some facilitation was apparent), and herbivory (both compensation and loss of fitness were demonstrated). Plant spacing effects depended on the index (response variable) and whether we measured individual or combined species. The survival index rarely showed competition effects. Due to counter facilitation effects, competition was not demonstrated for any index at the combined species level. The comparison of the relative order and magnitude of plants being eaten against impact identified vulnerable and compensating species. Once identified, compensating species may be used sacrificially to buffer damage in new reclamation systems, whereas deterrents may be used around known vulnerable species.     

ALTERNATIVE PATHS TO SUCCESS IN A PARASITE COMMUNITY: WITHIN‐HOST COMPETITION CAN FAVOR HIGHER VIRULENCE OR DIRECT INTERFERENCE

Selection imposed by coinfection may vary with the mechanism of within‐host competition between parasites. Exploitative competition is predicted to favor more virulent parasites, whereas interference competition may result in lower virulence. Here, we examine whether exploitative or interference competition determines the outcome of competition between two nematode species (Steinernema spp.), which in combination with their bacterial symbionts (Xenorhabdus spp.), infect and kill insect hosts. Multiple isolates of each nematode species, carrying their naturally associated bacteria, were characterized by (1) the rate at which they killed insect hosts, and by (2) the ability of their bacteria to interfere with each other's growth via bacteriocidal toxins called “bacteriocins.” We found that both exploitative and interference abilities were important in predicting which species had a selective advantage in pairwise competition experiments. When nematodes carried bacteria that did not interact via bacteriocins, the faster killing isolate had a competitive advantage. Alternatively, nematodes could gain a competitive advantage when they carried bacteria able to inhibit the bacteria of their competitor. Thus, the combination of nematode/bacterial traits that led to competitive success depended on which isolates were paired, suggesting that variation in competitive interactions may be important for maintaining species diversity in this community.     

基于引力模型的林木竞争分析

引力模型是否可以应用于森林群落林木竞争关系分析是值得研究的问题。基于引力模型建立林木相对活力圈能反映竞争木活力大小,基于竞争木的相对活力圈建立引力竞争指数能准确反映林木生长与林木竞争的关系。以浙江省天目山国家级自然保护区针阔混交林为研究对象,将V_Hegyi竞争指数、引力竞争指数分别与胸径进行相关分析,胸高断面积生长量分别与2种竞争指数进行相关分析,胸径生长率与2期引力竞争指数的比值(2021年与2006年的引力竞争指数之比)进行相关分析,此外,对相对活力圈直径与胸径进行相关分析,并比较分析了活立木与枯死木的竞争指数大小。结果表明: 2种竞争指数与胸径均呈显著负相关,且均服从幂函数关系。林木胸高断面积生长量与2种竞争指数均呈显著负相关,但引力竞争指数比V_Hegyi竞争指数更能反映林木生长与林木竞争的关系。相对于V_Hegyi竞争指数的比值,2期引力竞争指数的比值更能说明林木生长与林木竞争的关系。在针阔混交林中,阔叶树种的生长与竞争的相关性>针叶树种生长与竞争的相关性。林木枯损受竞争的显著影响。林木相对活力圈大小与林木胸径大小呈显著负相关。引力模型是反映空间相互作用的重要模型之一,可以应用于林木竞争关系的研究,且基于引力模型建立的引力竞争指数可以作为评价林木竞争和林木活力的一个空间结构指标,比V_Hegyi竞争指数更能反映林木生长与林木竞争的关系。     

Habitat-dependent competition and the coexistence of Australian heathland rodents

When competing species depress one another's fitness in the habitats that they occupy, their competitive effects will emerge in each species' pattern of density-dependent habitat choice. Thus, a regression analysis of joint densities, corrected by the habitat effect, should reveal the magnitude of interspecific competition. We tested this idea by 1) demonstrating the connection between removal experiments and regression estimates of competition with those obtained from isodars (regressions that implicitly incorporate evolutionarily stable strategies of habitat selection) and 2) evaluating whether interspecific competition inferred from isodars corresponded with the inferences emerging from regression and field experiments. Previous removal experiments on two herbivorous rodents occupying coastal wet heathlands in eastern Australia documented that competition between Rattus lutreolus and Pseudomys gracilicaudatus is asymmetrically biased in favor of the much larger Rattus . The asymmetry in competition was also revealed by regression estimates of competition. Isodar analyses illustrate a habitat-dependent mechanism for the asymmetry. Rattus compete effectively with Pseudomys in both 'wetter' and 'drier' patches of heath whereas Pseudomys appear to exert a competitive effect in only the drier sites. The magnitude of competition measured by a removal experiment in an area with more-or-less equal amounts of both habitats will be biased in favor of Rattus . More generally, one can use the isodar estimates to draw isolegs and isoclines of competitive coexistence. Isoclines for the two Australian rodents imply dynamic equilibria of stable competitive coexistence that vary with plant succession in fire-dominated heathland ecosystems.     

Sexual dimorphism in intra- and interspecific competitive ability of the dioecious herb Mercurialis annua

Males and females of dioecious plant species often show different responses to competition with individuals of the same or opposite gender, but almost no data are available on the outcome of competition with members of other species. Here, we show that male and female individuals of the wind-pollinated herb Mercurialis annua are sexually dimorphic in both their intraspecific and interspecific competitive abilities. In a controlled experiment, we found that both sexes of M. annua were negatively affected by interspecific competition, but the sensitivity of males and females depended on the identity of their competitor species, with females tending to suppress the aboveground growth of competitor species more than males. Further, we found that intrasexual and intersexual competition affected the aboveground growth of males but not that of females: only males showed a significant reduction in growth when growing with conspecific competitors (male or female). We discuss our results with reference to related studies that suggest that males and females of M. annua have different resource requirements for reproduction, which in turn affect their competitive abilities.     

Effects of neighbouring vegetation on eucalypt seedlings at two sites subject to different levels of abiotic stress

Abstract In spite of numerous studies on the effect of nutrient levels and/or standing crop on the intensity of resource competition the debate has not been resolved. Field studies that have used natural productivity gradients have generally supported the argument that competitive intensity and resource availability are positively correlated, whereas studies that have used artificial resource gradients have generally refuted the same argument. Here we report the results from study in which both approaches were used within the same system. We studied two species of eucalypt that occupy contrasting parts of the same landscape: Eucalyptus camaldulensis, found mostly along creek lines and in valleys with deep alluvial soils, and Eucalyptus microcarpa, found on hillsides and ridges with shallow soils. We studied the response of seedlings of the two species to the combined effects of competition and manipulated nutrient levels in a glasshouse experiment, and also investigated their responses to removal of neighbouring plants in the field. Eucalyptus microcarpa was less responsive to increased resource availability, which is consistent with one of the principal assumptions of Grime’s C‐S‐R model. In the glasshouse experiment both species of eucalypt responded in a qualitatively similar fashion to the combined effects of resource availability and competition: release from competition resulted in increased growth, but only in pots that received additional resources. In the field we found that neighbouring vegetation could severely limit the establishment of E. camaldulensis but the removal of neighbouring vegetation did not affect the performance of E. microcarpa seedlings. Eucalyptus camaldulensis seedlings suffered high levels of damage from herbivores. Our results thus generally support the predications of the C‐S‐R model, however, they indicate that the effects of competition and herbivory may be heavily confounded.     

Diversity and salt tolerance of native Acacia rhizobia isolated from saline and non‐saline soils

Re‐establishing native vegetation in stressed soils is of considerable importance in many parts of the world, leading to significant interest in using plant–soil symbiont interactions to increase the cost‐effectiveness of large‐scale restoration. However, effective use of soil microbes in revegetation requires knowledge of how microbe communities vary along environmental stress gradients, as well as how such variation relates to symbiont effectiveness. In Australia, shrubby legumes dominate many ecosystems where dryland salinity is a major issue, and improving plant establishment in saline soils is a priority of regional management agencies. In this study, strains of rhizobial bacteria were isolated from a range of Acacia spp. growing in saline and non‐saline soils. Replicates of each strain were grown under several salinity levels in liquid culture and characterized for growth and salt tolerance. Genetic characterization of rhizobia showed considerable variation among strains, with salt tolerance and growth generally higher in rhizobial populations derived from more saline soils. These strains showed markedly different genetic profiles and generic affiliations to those from more temperate soils, suggesting community differentiation in relation to salt stress. The identification of novel genomic species from saline soils suggests that the diversity of rhizobia associated with Australian Acacia spp. is significantly greater than previously described. Overall, the ability of some symbiotically effective strains to tolerate high salinity is promising with regard to improving host plant re‐establishment in these soils.     

The survival rate of Australian passerines

The Australian avifauna is composed largely of two groups–the 'old endemics', species that originated in Gondwana and radiated in Australia and New Guinea, and the 'new invaders', species that originated in Asia and invaded Australia during the Pleistocene. In addition, several species were introduced by Europeans during the last 200 years. The old endemics have clutch-sizes significantly smaller than those of the new invaders (Yom-Tov 1987). The aim of this paper was to study the survival rates of Australian passerines from the three groups.
The survival rates of 3 5 species of Australian passerines were calculated by using recapture data provided by the Australian Bird Banding Scheme for birds older than one year. Survival depends primarily on body-mass, with no difference between the old endemics and the new invaders. It is suggested that the survival rates of birds younger than one year is different between the two main groups.
Overall, the survival rates of Australian passerines is at least 1.2 greater than that of equal-sized British passerines. The survival rates of the introduced House Sparrow Passer domesticus and Blackbird Turdus merula were similar on the two continents, suggesting either that insufficient time had elapsed since their introduction to Australia for them to adapt to local conditions, or that the habitats occupied in Australia were so modified by human activities that selection did not favour long lives. The survival rate of the introduced Starling Sturnus vulgaris was lower in Australia, probably because it lives in natural habitats there.     

Competition between Australian native and introduced grasses along a nutrient gradient

Abstract Seven grass species were grown in monocultures and in multispecies mixtures along a gradient of total nutrient levels that ranged from 1/64 to 16× the normal level of nutrient solution. The seven grasses represented three ecological groups: (i) three perennial species native to Australia (Themeda triandra, Poa labillardieri and Danthonia carphoides); (ii) two introduced annuals (Vulpia bromoides and Hordeum leporinum); and (iii) two introduced perennials (Lolium perenne and Dactylis glomerata). We hypothesized that the native grasses would prove less competitive when grown at increased nutrient levels than those introduced from Europe. Results supported the hypothesis. The native species were unable to compete in mixtures even at the lowest nutrient level, where T. triandra was the most productive species in monoculture. Lolium perenne and Dactylis glomerata dominated mixtures at intermediate nutrient levels. The responses of the annual introduced grasses differed in that Vulpia bromoides showed an optimum at intermediate nutrient levels in both monoculture and in mixtures, whereas Hordeum leporinum dominated at the highest nutrient levels in mixture but was suppressed by V. bromoides, L. perenne and D. glomerata at intermediate levels. The results are discussed in terms of predicting species responses in mixtures from their performance in monocultures as well as in terms of previous observations on the sequential changes in botanical composition of south‐eastern Australian grasslands after 150 years of continuous grazing by sheep.     

Germination biology of selected central Australian plants

Abstract Risk spreading of germination may be particularly common in environments with unpredictable climates. Germinability, propensity to germinate at different temperatures and germination speed were classified for seeds of 105 species from the central Australian arid zone, and related to plant growth form, perenniality, seed size and seed dispersal mode. Almost all species had at least some seeds which were dormant, consistent with the idea that risk spreading is important in arid zones. Dispersal mode and plant perenniality were not found to be associated with germinability. Seeds of most species germinated rapidly relative to what has been recorded from higher-rainfall environments, as might be expected in an environment where wet soils are usually temporary. Faster germination tended to be associated with low germinability, suggesting a spectrum of strategies from species that risk a small number of their seeds in many rainfall events, to those that germinate only in large rainfall events but then risk large numbers of seeds.     

Common alien plants are more competitive than rare natives but not than common natives

Success of alien plants is often attributed to high competitive ability. However, not all aliens become dominant, and not all natives are vulnerable to competitive exclusion. Here, we quantified competitive outcomes and their determinants, using response‐surface experiments, in 48 pairs of native and naturalised alien annuals that are common or rare in Germany. Overall, aliens were not more competitive than natives. However, common aliens (invasive) were, despite strong limitation by intraspecific competition, more competitive than rare natives. This is because alien species had higher intrinsic growth rates than natives, and common species had higher intrinsic growth rates than rare ones. Strength of interspecific competition was not related to status or commonness. Our work highlights the importance of including commonness in understanding invasion success. It suggests that variation among species in intrinsic growth rates is more important in competitive outcomes than inter‐ or intraspecific competition, and thus contributes to invasion success and rarity.     

Trait values and not invasive status determine competitive outcomes between native and invasive species under varying soil nutrient availability

Invasion by exotic plants is often associated with nutrient enrichment of soils, particularly on soils of naturally low fertility. As a consequence, it is likely that the outcome of competitive interactions between native and invasive plants may be mediated by soil nutrient availability. We independently investigated competitive effect and response as well as the occurrence of asymmetric competition among native and invasive plants on soils of varying nutrient availability, using a glasshouse experiment. Seedlings of eight co‐occurring pairs of invasive and native species from low fertility Hawkesbury Sandstone‐derived soil were grown under low and high nutrient availability. We tested the hypotheses that native species would be competitively superior at low nutrient availability and have trait values associated with a resource conservation strategy while invasive species would be competitively superior at high nutrient availability and have trait values associated with a resource acquisition strategy. We found that nutrient availability did not mediate competitive interactions between invasive and native species. Instead, two invasive and one native species were always competitively superior irrespective of nutrient availability. Competitively superior species displayed a mixture of both resource conservation and acquisition strategies at low and high nutrient availability. In support of previous studies, we found that the a priori classification of invasive and native species does not predict competitive superiority at varying nutrient levels. Rather, species specific differences in trait values provide a competitive advantage in response to nutrient availability.