Comparison of life history traits between invasive and native populations of purple loosestrife (Lythrum salicaria) using nonlinear mixed effects model

We compared growth patterns of invasive and native populations of purple loosestrife (Lythrum salicaria) while varying water and nutrient levels. We examined three life-history traits (height, number of branches, and the size of largest leaf) during the growth period adopting a nonlinear mixed effects model. Invasive populations were found to be slower in shoot elongation but grew to be taller than native populations. Invasive populations produced more branches than natives only in the high water, high nutrient treatment. Invasive populations had a similar increase in the size of the largest leaf compared to natives, but ultimately produced a greater size of largest leaf than natives. Invasive populations were found to display a greater vegetative expansion, but this was not strongly affected by our treatments.     

Traits of an invasive grass conferring an early growth advantage over native grasses

Aims Invasive species often have higher relative growth rates (RGR) than their native counterparts. Nutrient use efficiency, total leaf area and specific leaf area (SLA) are traits that may confer RGR differences between natives and invasives, but trait differences are less prominent when the invasive species belongs to the same plant functional type as the dominant native species. Here, we test if traits displayed soon after germination confer an early size advantage. Specifically, we predicted that invasive species seedlings grow faster than the natives because they lack trade-offs that more strongly constrain the growth of native species.Methods We quantified plant morphological and physiological traits and RGR during early seedling growth at high and low nutrient levels in three dominant perennial native C₄ grasses: Panicum virgatum L. (switchgrass), Schizachyrium scoparium (Michx.) Nash (little bluestem) and Andropogon gerardii Vitman (big bluestem); and a perennial C₄ exotic invasive grass, Sorghum halepense (L.) Pers. (Johnsongrass).Important findings After 2 weeks of growth, Johnsongrass seedlings had greater biomass, SLA and photosynthetic nitrogen use efficiency, but lower leaf N concentrations (% leaf N) and root:shoot ratio than natives. As growth continued, Johnsongrass more quickly produced larger and thicker leaves than the natives, which dampened the growth advantage past the first 2 to 3 weeks of growth. Investment in carbon gain appears to be the best explanation for the early growth advantage of Johnsongrass. In natives, growth was constrained by an apparent trade-off between allocation to root biomass, which reduced SLA, and production of leaves with high N content, which increased carbon gain. In Johnsongrass, root:shoot ratio did not interact with other traits, and % leaf N was decoupled from RGR as a result of a trade-off between the positive indirect association of % leaf N with RGR and the negative direct association of % leaf N with RGR.     

Variation in phenotypic plasticity for native and invasive populations of Bromus tectorum

Phenotypic plasticity is often considered important for invasive plant success, yet relatively few studies have assessed plasticity in both native and invasive populations of the same species. We examined the plastic response to temperature for Bromus tectorum populations collected from similar shrub-steppe environments in the Republics of Armenia and Georgia, where it is native, and along an invasive front in California and Nevada. Plants were grown in growth chambers in either ‘warm’ (30/20 °C, day/night) or ‘cold’ (10/5 °C) conditions. Invasive populations exhibited greater adaptive plasticity than natives for freezing tolerance (as measured by chlorophyll a fluorescence), such that invasive populations grown in the cold treatment exhibited the highest tolerance. Invasive populations also exhibited more rapid seedling emergence in response to warm temperatures compared to native populations. The climatic conditions of population source locations were related to emergence timing for invasive populations and to freezing tolerance across all populations combined. Plasticity in growth-related traits such as biomass, allocation, leaf length, and photosynthesis did not differ between native and invasive populations. Rather, some growth-related traits were very plastic across all populations, which may help to dampen differences in biomass in contrasting environments. Thus, invasive populations were found to be particularly plastic for some important traits such as seedling emergence and freezing tolerance, but plasticity at the species level may also be an important factor in the invasive ability of B. tectorum.     

Introduced Brassica nigra populations exhibit greater growth and herbivore resistance but less tolerance than native populations in the native range

Rapid post-introduction evolution has been found in many invasive plant species, and includes changes in defence (resistance and tolerance) and competitive ability traits. Here, we explored the post-introduction evolution of a trade-off between resistance to and tolerance of herbivory, which has received little attention. In a common garden experiment in a native range, nine invasive and 16 native populations of Brassica nigra were compared for growth and defence traits. Invasive populations had higher resistance to, but lower tolerance of, herbivore damage than native populations. Invasive populations survived better and produced more seeds than native ones when released from herbivores; but fitness was equivalent between the regions under ambient herbivory. The invasive populations grew taller, and produced more biomass and lighter seeds than natives, irrespective of insecticide treatment. In addition to supporting the idea of post-introduction rapid evolution of plant traits, our results also contribute to an emerging pattern of both increasing resistance and growth in invasive populations, contrary to the predictions of earlier theories of resistance-growth trade-offs.     

Why only tetraploid Solidago gigantea (Asteraceae) became invasive: a common garden comparison of ploidy levels

Many studies have compared the growth of plants from native and invasive populations, but few have considered the role of ploidy. In its native range in North America, Solidago gigantea Aiton (Asteraceae) occurs as a diploid, tetraploid and hexaploid, with considerable habitat differentiation and geographic separation amongst these ploidy levels. In the introduced range in Europe, however, only tetraploid populations are known. We investigated the growth performance and life history characteristics of plants from 12 European and 24 North American (12 diploid, 12 tetraploid) populations in a common garden experiment involving two nutrient and two calcium treatments. Twelve plants per population were grown in pots for two seasons. We measured 24 traits related to leaf nutrients, plant size, biomass production and phenology as well as sexual and vegetative reproduction. Native diploid plants had a higher specific leaf area and higher leaf nutrient concentrations than native tetraploids, but tetraploids produced many more shoots and rhizomes. Diploids grown with additional calcium produced less biomass, whereas tetraploids were not affected. European plants were less likely to flower and produced smaller capitulescences than North American tetraploids, but biomass production and shoot and rhizome number did not differ. We conclude that a knowledge of ploidy level is essential in comparative studies of invasive and native populations. While clonal growth is important for the invasion success of tetraploid S. gigantea, its potential was not acquired by adaptation after introduction but by evolutionary processes in the native range.     

Phenotypic plasticity of introduced versus native purple loosestrife: univariate and multivariate reaction norm approaches

The plastic responses to environmental change by Lythrum salicaria (purple loosestrife) were compared between native plants derived from seeds collected in Europe and those introduced into North America. Plants from nine populations each were grown under two levels of water and nutrient conditions. At the end of the growing season, samples were evaluated for eight traits related to their life history, plant size/architecture, and reproduction. Genetic (G), environmental (E), and G × E interactions were assessed by restricted maximum likelihood (REML) analysis of covariance (ANCOVA) and multivariate analysis of covariance (MANCOVA). Both univariate and multivariate reaction norm analyses were used to test for differences in the magnitude and direction of phenotypic plasticity between introduced and native plants. Under high-nutrient conditions, introduced plants were taller and had more branches and greater aboveground biomass. They also exhibited significantly greater amounts of phenotypic plasticity for aboveground biomass than did the natives in response to changing nutrient levels in standing water. This difference in univariate plasticity contributed to the general contrast in multivariate plasticity between introduced and native plants. These results support the idea that introduced plants may successfully invade a habitat and grow better than native plants in response to increased resources.     

Transformation of detritus by a European native and two invasive alien freshwater decapods

Invasive alien species have the potential to alter biodiversity and ecosystem processes. In freshwaters, detritus decomposition is a major ecosystem service but it remains uncertain whether invasive alien decapods process detritus differently to natives. This study examined leaf litter processing, and cascading effects on biofilms, by the European native white clawed crayfish (Austropotamobius pallipes) compared to two invasive alien decapod species: the American signal crayfish (Pacifastacus leniusculus) and the Chinese mitten crab (Eriocheir sinensis). Invasive alien decapods were responsible for higher leaf litter decomposition than the native. In comparison with native crayfish, invasive alien crab and crayfish showed higher rates of litter consumption, increased production of smaller leaf fragments, fine particulate organic matter (FPOM) and dissolved organic carbon. Nutrients (ammonia and soluble reactive phosphorous) derived from excretion (measured separately in the absence of biofilms) varied among decapod species, being lower for P. leniusculus. However, nutrient concentrations did not vary among species in the detritivory experiments with biofilm, implying nutrients were utilised for biofilm production and respiration as no differences in biomass were evident among decapod treatments. These results show invasive alien decapods have the potential to increase the magnitude of detrital processing to FPOM in rivers, but indirect impacts on primary producers due to nutrient release are uncertain based on this experimental context.     

Reproductive output of invasive versus native plants

Aim Propagule size and output are critical for the ability of a plant species to colonize new environments. If invasive species have a greater reproductive output than native species (via more and/or larger seeds), then they will have a greater dispersal and establishment ability. Previous comparisons within plant genera, families or environments have conflicted over the differences in reproductive traits between native and invasive species. We went beyond a genus‐, family‐ or habitat‐specific approach and analysed data for plant reproductive traits from the global literature, to investigate whether: (1) seed mass and production differ between the original and introduced ranges of invasive species; (2) seed mass and production differ between invasives and natives; and (3) invasives produce more seeds per unit seed mass than natives. Location Global. Methods We combined an existing data set of native plant reproductive data with a new data compilation for invasive species. We used t‐tests to compare original and introduced range populations, two‐way ANOVAs to compare natives and invasives, and an ANCOVA to examine the relationship between seed mass and production for natives and invasives. The ANCOVA was performed again incorporating phylogenetically independent contrasts to overcome any phylogenetic bias in the data sets. Results Neither seed mass nor seed production of invasive species differed between their introduced and original ranges. We found no significant difference in seed mass between invasives and natives after growth form had been accounted for. Seed production was greater for invasive species overall and within herb and woody growth forms. For a given seed mass, invasive species produced 6.7‐fold (all species), 6.9‐fold (herbs only) and 26.1‐fold (woody species only) more seeds per individual per year than native species. The phylogenetic ANCOVA verified that this trend did not appear to be influenced by phylogenetic bias within either data set. Main conclusions This study provides the first global examination of both seed mass and production traits in native and invasive species. Invasive species express a strategy of greater seed production both overall and per unit seed mass compared with natives. The consequent increased likelihood of establishment from long‐distance seed dispersal may significantly contribute to the invasiveness of many exotic species.     

Responses to nitrogen pulses and growth under low nitrogen availability in invasive and native tree species with differing successional status

Invasive species are frequently found in recently disturbed sites. To examine how these disturbance-dependent invasive species exploit resource pulses resulting from disturbance, twelve physiological and morphological traits, including age-dependent responsiveness in leaf traits to nitrogen pulse, were compared between Bischofia javanica, an invasive tree species in Ogasawara islands, and three native Ogasawara species, each having a different successional status. When exposed to a nitrogen pulse, invasive B. javanica showed higher increases in photosynthetic capacity, leaf area, epidermal cell number and cell size in leaves of broad age classes, and root nitrogen absorption ability than two native mid-/late or late-successional species, but showed no particular superiority to a native pioneer species in these responses. Under low nitrogen, however, it showed the largest relative growth rate among the four species, while the native pioneer showed the lowest growth. From these results, we concluded that the combination of moderately high responsiveness to resource pulses and the ability to maintain steady growth under resource limitations may give B. javanica a competitive advantage over a series of native species with different successional status from early to late-successional stages.     

Evolutionary increases in defense during a biological invasion

Invasive plants generally escape from specialist herbivores of their native ranges but may experience serious damage from generalists. As a result, invasive plants may evolve increased resistance to generalists and tolerance to damage. To test these hypotheses, we carried out a common garden experiment comparing 15 invasive populations with 13 native populations of Chromolaena odorata, including putative source populations identified with molecular methods and binary choice feeding experiments using three generalist herbivores. Plants from invasive populations of C. odorata had both higher resistance to three generalists and higher tolerance to simulated herbivory (shoot removal) than plants from native populations. The higher resistance of plants from invasive populations was associated with higher leaf C content and densities of leaf trichomes and glandular scales, and lower leaf N and water contents. Growth costs were detected for tolerance but not for resistance, and plants from invasive populations of C. odorata showed lower growth costs of tolerance. Our results suggest that invasive plants may evolve to increase both resistance to generalists and tolerance to damage in introduced ranges, especially when the defense traits have low or no fitness costs. Greater defenses in invasive populations may facilitate invasion by C. odorata by reducing generalist impacts and increasing compensatory growth after damage has occurred.     

飞机草入侵种群和原产地种群生长和数量型化学防御物质含量差异的比较研究

增强竞争能力的进化假说认为,在入侵地外来植物逃离了原产地天敌的控制,把原来用于防御的资源分配到生长、生殖等,从而提高竞争力。为探讨进化在恶性外来入侵植物飞机草（Chromolaena odorata）入侵中的作用,在同质种植园中的两个养分条件下比较研究了飞机草原产地和入侵地各8个种群叶片单宁含量,茎和叶片总酚、半纤维素和纤维素含量以及总生物量的差异。结果表明,在两个养分条件下,飞机草入侵种群和原产地种群总生物量差异均不显著,入侵种群茎和叶片半纤维素含量均低于原产地种群;在高养分条件下,飞机草入侵种群叶片纤维素含量低于原产地种群;在低养分条件下,入侵种群茎和叶片总酚含量高于原产地种群。由此,我们得出结论：在入侵地,飞机草未发生加快生长的进化,但数量型化学防御物质发生了遗传变化;降低的半纤维素和纤维素含量可能是对入侵地专性天敌缺乏做出进化响应的结果,提高的总酚含量有利于飞机草防御入侵地的广谱天敌。     

Resource conservation strategy helps explain patterns of biological invasion in a low-N environment

Several studies have been conducted on the distribution of carbon (C) and nitrogen (N) in plant, however, there has been rare data linking Ecological Stoichiometry to plant invasion success, especially in a low-N environment. Accordingly, an experiment in this study was performed to explore the nutrition distribution and stoichiometry among various organs in plant, contributing to plant invasion. According to the results of this study, both invasive and native species exhibited a higher C, N concentration and lower C/N ratio in their leaf compared with stem and root overall (p < 0.05), the stem exhibited the lower N concentration, higher C concentration and C/N ratio compared with leaf and root overall (p < 0.05). In addition, C and N concentrations of invasive Solidago canadensis were significantly lower, while the C/N and Relative Growth Rate (RGR) were significantly higher than those of the native Artemisia argyi (p < 0.05). As revealed from the mentioned results, plant invasion success in low-N environmental conditions was dependent on the following: 1) Invasive used more conservative nutrients strategy (higher C/N ratio), instead of the high capacity to retain or resorb nutrients compared with natives; 2) Invasive species (higher RGR) had less N-limited growth as compared with natives. Furthermore, this study presented baseline information to describe the internal mechanism of plant invasion, thereby helping predict and control invasive species.     

Intraspecific and interspecific pair-wise seedling competition between exotic annual grasses and native perennials: plant–soil relationships

Few studies have examined plant–soil relationships in competitive arenas between exotic and native plants in the western United States. A pair-wise competitive design was used to evaluate plant–soil relationships between seedlings of the exotic annual grasses Bromus tectorum and Taeniatherium caput-medusae and the native perennial grasses Elymus elymoides and Pseudoroegneria spicata. Two soils were tested: an arid soil (argid) occupied by E. elymoides and presently invaded by B. tectorum and a high elevation, high organic matter, soil (aquept) where none of the tested species would typically occur. Plant growth proceeded for 85 days at which time above-ground biomass and tissue nutrient concentrations were quantified. Soil also was collected from the rooting zone beneath each species and analyzed for various nutrient pools. The exotic species had significantly greater above-ground biomass than the natives and grew far better in the aquept soil than the argid soil. Growth of B. tectorum, and to some degree, T. caput-medusae was suppressed in intraspecific competition and enhanced, especially in the aquept soil, when competing with the natives. Although not significant, biomass of natives strongly trended downward when competing with the exotic grasses. Overall, concentrations of tissue nutrients were minimally affected by competition, but natives tended to be more negatively affected by competition with exotics. Except for phosphorus (P), all species had significantly greater nutrient concentrations when growing in the aquept soil compared to the argid soil. In both soils, exotics had significant greater tissue concentrations of manganese (Mn), magnesium (Mg), and iron (Fe), while natives had significantly greater nitrogen (N). Species affects on soil nutrient pools occurred mostly in the aquept soil with exotic species significantly decreasing pools of available N, potentially available N, and soil-solution pools of calcium (Ca²⁺), potassium (K⁺), and magnesium (Mg²⁺) relative to natives. Overall, the data suggest that, in the seedling state, B. tectorum is a superior competitor. Moreover, when the natives compete intra- or interspecifically, particularly in the aquept soil, availability of N and other nutrients in their rooting zone is consistently greater than when they compete interspecifically with the exotic grasses. These data suggest the exotics are able to co-opt nutrients in the rooting zone of the natives and perhaps gain a competitive advantage.     

An invasive tree fern alters soil and plant nutrient dynamics in Hawaii

Invasive species that alter ecosystems are often successful competitors due to their effects on nutrient cycling. Sphaeropteris cooperi (Cyatheaceae; Australian tree fern) has been invading intact Hawaiian rainforests for decades and displacing the dominant native tree fern Cibotium glaucum (Cibotiaceae). S. cooperi produces more leaves that grow faster, contain more N and P, and decompose faster than C. glaucum leaves. Our experiment tested the effects of additions of leaf litter from native and non-native tree ferns on the growth and nutrient content of four native angiosperm species in forest (N-rich) and landslide (P-rich) soils. Both litter treatments inhibited growth initially in all species, but subsequent responses were species-specific. Compared to control treatments, the increase in biomass was highest in the fast-growing Carex wahuensis and Hibiscus arnottianus with S. cooperi litter in landslide soil. Leaf N in C. wahuensis was higher with S. cooperi litter and in forest soil, as expected, but other leaf nutrient responses showed some evidence of nutrient immobilization from litter addition. Several growth measures were higher with S. cooperi than C. glaucum litter and in forest than landslide soil, suggesting that N availability is the strongest driver of growth. Our results show that S. cooperi can alter nutrient cycling in Hawaiian plants, sometimes with positive effects on growth. However, under natural conditions, native plants must compete for these additional nutrients with S. cooperi and other invasive species. This study contributes to invasion biology as the first to examine the impact of leaf litter of an invasive fern on native species.     

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.     

Nutrient enrichment enhances hidden differences in phenotype to drive a cryptic plant invasion

Many mechanisms of invasive species success have been elucidated, but those driving cryptic invasions of non‐native genotypes remain least understood. In one of the most successful cryptic plant invasions in North America, we investigate the mechanisms underlying the displacement of native Phragmites australis by its Eurasian counterpart. Since invasive Phragmites’ populations have been especially prolific along eutrophic shorelines, we conducted a two‐year field experiment involving native and invasive genotypes that manipulated nutrient level and competitor identity (inter‐ and intra‐genotypic competition) to assess their relative importance in driving the loss of native Phragmites. Inter‐genotypic competition suppressed aboveground biomass of both native and invasive plants regardless of nutrient treatment (～ 27%), while nutrient addition disproportionately enhanced the aboveground biomass (by 67%) and lateral expansion (by > 3 × farther) of invasive Phragmites. Excavation of experimental plots indicated that nutrient addition generates these differences in aboveground growth by differentially affecting rhizome production in invasive vs native plants; invasive rhizome biomass and rhizome length increased by 595% and 32% with nutrient addition, respectively, while natives increased by only 278% and 15%. Regardless of nutrient level, native rhizomes produced twice as many roots compared to invasives, which field surveys revealed are heavily infected with mycorrhizal symbionts. These results suggest that native Phragmites competes well under nutrient‐limited conditions because its rhizomes are laden with nutrient‐harvesting roots and mycorrhizae. Invasive Phragmites’ vigorous aboveground response to nutrients and scarcity of lateral roots, in contrast, may reflect its historic distribution in eutrophic Eurasian wetlands and correspond to its prevalence in New England marshes characterized by elevated nutrient availability and relaxed nutrient competition. These findings reveal that discrete differences in phenotype can interact with anthropogenic modification of environmental conditions to help explain the success of cryptic invaders.     

Predatory functional response and prey choice identify predation differences between native/invasive and parasitised/unparasitised crayfish

Background

Invasive predators may change the structure of invaded communities through predation and competition with native species. In Europe, the invasive signal crayfish Pacifastacus leniusculus is excluding the native white clawed crayfish Austropotamobius pallipes.

Methodology and Principal Findings

This study compared the predatory functional responses and prey choice of native and invasive crayfish and measured impacts of parasitism on the predatory strength of the native species. Invasive crayfish showed a higher (>10%) prey (Gammarus pulex) intake rate than (size matched) natives, reflecting a shorter (16%) prey handling time. The native crayfish also showed greater selection for crustacean prey over molluscs and bloodworm, whereas the invasive species was a more generalist predator. A. pallipes parasitised by the microsporidian parasite Thelohania contejeani showed a 30% reduction in prey intake. We suggest that this results from parasite-induced muscle damage, and this is supported by a reduced (38%) attack rate and increased (30%) prey handling time.

Conclusions and Significance

Our results indicate that the per capita (i.e., functional response) difference between the species may contribute to success of the invader and extinction of the native species, as well as decreased biodiversity and biomass in invaded rivers. In addition, the reduced predatory strength of parasitized natives may impair their competitive abilities, facilitating exclusion by the invader.     

<Emphasis Type="Italic">Phragmites</Emphasis><Emphasis Type="Italic">australis</Emphasis>: How do genotypes of different phylogeographic origins differ from their invasive genotypes in growth,nitrogen allocation and gas exchange?

It has been suggested that in plant invasions, species may develop intrinsically higher gas exchange and growth rates, and greater nitrogen uptake and allocation to shoots, in their invasive range than in their native habitat under excess nutrients. In this study, native populations of two old world Phragmites australis phylogeographic groups (EU and MED) were compared with their invasive populations in North America [NAint (M) and NAint (Delta)] under unlimited nutrient availability and identical environmental conditions in a common garden. We expected that both introduced groups would have higher growth, nitrogen uptake and allocation, and gas exchange rates than their native groups, but that these enhanced traits would have evolved in different ways in the two introduced ranges, because of different evolutionary histories. Biomass, leaf area, leaf nitrogen concentrations (NH₄ ⁺ and NO₃ ^?) and transpiration rates increased in introduced versus native groups, whereas differences in SLA, leaf pigment concentrations and assimilation rates were due to phylogeographic origins. Despite intrinsic differences in the allocation of C and N in leaves, shoots and rhizome due to phylogeographic origin, the introduced groups invested more biomass in above-ground tissues than roots and rhizomes. Our results support the concept that invasive populations develop enhanced morphological, physiological and biomass traits in their new ranges that may assist their competiveness under nutrient-enriched conditions, however the ecophysiological processes leading to these changes can be different and depend on the evolutionary history of the genotypes.     

Pre-adaptation or genetic shift after introduction in the invasive species Impatiens glandulifera?

Invasive exotic plants often grow fast, reproduce rapidly and display considerable phenotypic plasticity in their invasive range, which may be essential characteristics for successful invasion. However, it remains unclear whether these characteristics are already present in native populations (pre-adaptation hypothesis) or evolve after introduction (genetic shift hypothesis).To test these hypotheses we compared means and phenotypic plasticity of vegetative and reproductive traits between populations of Impatiens glandulifera collected from either the invasive (Norway) or native range (India). Seeds were sown and the resulting plants were exposed to different experimental environments in a glasshouse. We also tested whether trait means and reaction norms harbored genetic variation, as this may promote fitness in the novel environment.We did not find evidence that invasive populations of I. glandulifera grew more vigorously or produced more seeds than native populations. Phenotypic plasticity did not differ between the native and invasive range, except for the number of nodes which was more plastic in the invasive range. Genetic variation in the slope of reaction norms was absent, suggesting that the lack of change in phenotypic plasticity between native and invasive populations resulted from low genetic variation in phenotypic plasticity initially harbored by this species. Post-introduction evolution of traits thus probably did not boost the invasiveness of I. glandulifera. Instead, the species seems to be pre-adapted for invasion.We suggest that differences in habitat between the native and invasive range, more specifically the higher nutrient availability observed in the new environment, are the main factor driving the invasion of this species. Indeed, plants in the more nutrient-rich invasive range had greater seed mass, likely conferring a competitive advantage, while seed mass also responded strongly to nutrients in the glasshouse. Interactions between habitat productivity and herbivore defense may explain the lack of more vigorous growth in the new range.     

Nitrogen allocation and partitioning in invasive and native Eupatorium species

There is a trade-off between nitrogen (N) allocation to photosynthesis and to defence. Invasive species may reduce N allocation to defence because of the absence of natural enemies. Thus, I hypothesised that invasive species may allocate a higher fraction of total leaf N to photosynthesis and have higher light-saturated photosynthetic rate ( P _max) and photosynthetic N-use efficiency (PNUE) than closely related native species. To test these hypotheses, invasive Eupatorium adenophorum and native E.   chinense and E.   heterophyllum were compared in a limestone shrub. Unlike expectation, the invader did not allocate a higher fraction of leaf N to photosynthesis than the natives. However, it was more efficient in photosynthetic N partitioning than the natives. It partitioned a higher fraction of the photosynthetic N to carboxylation and showed higher use efficiency of the photosynthetic N, while the natives partitioned a higher fraction of the photosynthetic N to light-harvesting components. Total leaf N content was not significantly different among the three studied invasive and native species. For the invader, the higher fraction of leaf N allocated to carboxylation resulted in the higher N content in carboxylation and in both carboxylation and bioenergetics, which led to higher P _max, and therefore to higher PNUE, water-use efficiency, respiration efficiency and apparent quantum yield. These physiological advantages of the invader and its higher leaf area ratio may contribute to its invasiveness.