Identification of volatile compounds released by roots of an invasive plant, bitou bush (Chrysanthemoides monilifera spp. rotundata), and their inhibition of native seedling growth

Allelopathy has been suggested as a mechanism promoting the monoculture formation of some invasive exotic plants. Previous studies have shown that hydrophobic extracts of the roots and soil of exotic bitou bush (Chrysanthemoides monilifera spp. rotundata (DC.) T. Norl.) inhibited the seedling growth of five Australian native plants, including the dominant acacia (Acacia longifolia var. sophorae (Labill.) F. Muell.). Based on this finding, we compared the hydrophobic root and soil chemical profiles of bitou bush and acacia to determine whether bitou bush roots release allelopathic compounds that are novel to the invaded system. We detected three compounds that were exclusive to the bitou bush root and soil, and seven compounds that were common to the bitou bush and acacia roots but only present in the bitou bush soil. The compounds unique to the bitou bush invaded soil were all sesqui- and diterpenes. Several of these compounds were found to inhibit the seedling growth of a native sedge, Isolepis nodosa (Rott.) R. Br. Of particular interest are the sesquiterpenes: β-maaliene, α-isocomene, β-isocomene, δ-cadinene, 5-hydroxycalamenene and 5-methoxycalamenene which were found in high concentrations in the bitou bush root and soil extracts and exhibited phytotoxic activity. Therefore, we present evidence to suggest that bitou bush exudes low molecular weight volatile compounds into the soil which inhibit native plant seedling growth. The reduced establishment of native plants via allelopathy is likely to create space and contribute to the invasion of bitou bush on the eastern Australian coast.     

Impacts of a woody invader vary in different vegetation communities

The impact of an exotic species in natural systems may be dependent not only on invader attributes but also on characteristics of the invaded community. We examined impacts of the invader bitou bush, Chrysanthemoides monilifera ssp. rotundata , in fore and hind dune communities of coastal New South Wales, Australia. We compared invader impacts on vegetation structure, richness of both native and exotic growth forms and community variability in fore and hind dunes. We found that impacts of bitou invasion were context specific: in fore dune shrublands, functionally distinct graminoid, herb and climber rather than shrub growth forms had significantly reduced species richness following bitou invasion. However, in forested hind dunes, the functionally similar native shrub growth form had significantly reduced species richness following bitou invasion. Density of vegetation structure increased at the shrub level in both fore and hind dune invaded communities compared with non-invaded communities. Fore dune ground-level vegetation density declined at invaded sites compared with non-invaded sites, reflecting significant reductions in herb and graminoid species richness. Hind dune canopy-level vegetation density was reduced at invaded compared with non-invaded sites. Bitou bush invasion also affected fore dune community variability with significant increases in variability of species abundances observed in invaded compared with non-invaded sites. In contrast, variability among all hind dune sites was similar. The results suggest that effects of bitou bush invasion are mediated by the vegetation community. When bitou bush becomes abundant, community structure and functioning may be compromised.     

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.     

Novel technique shows different hydrophobic chemical signatures of exotic and indigenous plant soils with similar effects of extracts on indigenous species seedling growth

Changes to ecosystem abiotic parameters are regarded as possible mechanisms facilitating plant invasion and community composition shifts. This study compared the hydrophobic chemical signatures of soil from exotic bitou bush (Chrysanthemoides monilifera spp. rotundata) invaded, indigenous acacia (Acacia longifolia var. sophorae) dominated and bare sand (unvegetated) habitats using a novel, rapid, capturing technique which utilised Amberlite® XAD4 resin filled bags that were placed in situ. The hydrophobic chemical signature of the bitou bush soil extract was significantly different to the acacia soil and bare sand extracts. High concentrations of 18 sesquiterpenes dominated the hydrophobic signature of the bitou bush extract. Low concentrations of all three extracts did not significantly affect the seedling growth of three indigenous test species under laboratory conditions, however, at higher concentrations, the extracts from soil inhabited by plants, whether exotic or indigenous, similarly inhibited the seedling growth of two species, while seedling growth of the third species was inhibited by extracts from all three soil types. These results do not support the hypothesis that exotic invasive species are more likely to exhibit allelopathic effects than indigenous plant species.     

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.     

Unintended Consequences of Invasive Predator Control in an Australian Forest: Overabundant Wallabies and Vegetation Change

Over-abundance of native herbivores is a problem in many forests worldwide. The abundance of native macropod wallabies is extremely high at Booderee National Park (BNP) in south-eastern Australia. This has occurred because of the reduction of exotic predators through an intensive baiting program, coupled with the absence of other predators. The high density of wallabies at BNP may be inhibiting the recruitment of many plant species following fire-induced recruitment events. We experimentally examined the post-fire response of a range of plant species to browsing by wallabies in a forest heavily infested with the invasive species, bitou bush Chrysanthemoides monilifera. We recorded the abundance and size of a range of plant species in 18 unfenced (browsed) and 16 fenced (unbrowsed) plots. We found the abundance and size of bitou bush was suppressed in browsed plots compared to unbrowsed plots. Regenerating seedlings of the canopy or middle storey tree species Eucalyptus pilularis, Acacia implexa, Allocasuarina littoralis, Breynia oblongifolia and Banksia integrifolia were either smaller or fewer in number in grazed plots than treatment plots as were the vines Kennedia rubicunda, Glycine tabacina and Glycine clandestina. In contrast, the understorey fern, Pteridium esculentum increased in abundance in the browsed plots relative to unbrowsed plots probably because of reduced competition with more palatable angiosperms. Twelve months after plots were installed the community structure of the browsed and unbrowsed plots was significantly different (P = 0.023, Global R = 0.091). The relative abundance of C. monilifera and P. esculentum contributed most to the differences. We discuss the possible development of a low diversity bracken fern parkland in Booderee National Park through a trophic cascade, similar to that caused by overabundant deer in the northern hemisphere. We also discuss its implications for broad scale fox control in southern Australian forests.     

Competition between Lemna minuta and Lemna minor at different nutrient concentrations

We investigated the differential responses of invasive alien Lemna minuta and native Lemna minor to nutrient loading as well as the mechanism of competition between the species. The role of nutrients, species identity, species influence in determining the outcome of competition between the species was estimated using the Relative Growth Rate Difference (RGRD) model. The two species differed in their response to nutrient loading. The native L. minor responded indifferently to nutrient loading. The species Relative Growth Rate (RGR) was 0.10 d⁻¹, 0.11 d⁻¹ and 0.09 d⁻¹ in high, medium and low nutrients, respectively. On the other hand, the invasive L. minuta responded opportunistically to high nutrient availability and had an RGR of 0.13 d⁻¹, 0.10 d⁻¹ and 0.08 d⁻¹ in high, medium and low nutrients, respectively. As a result, the invasive species was dominant in high nutrient availability but lost to the native species at low nutrient availability. The invader formed approximately 60% and less than 50% of the stand final total dry biomass in high and low nutrient availability, respectively. Species RGR were reduced by both intra- and interspecific competition but intraspecific effects were stronger than interspecific effects. On the overall, the species significantly differed in their constant RGR. These differences in RGR between the species (species identity) and the differential response to nutrient loading were the main determinant of change in final biomass composition of these species in mixture. Species influence (competition) only had a small influence on the outcome of competition between the species. The observed species response to nutrient loading could be targeted in management of the invasive species. Lowering nutrients can be proposed to reduce the impact of the invasive L. minuta.     

Evidence for allelopathy as a mechanism of community composition change by an invasive exotic shrub, Chrysanthemoides monilifera spp. rotundata

Chemical interference is increasingly suggested as a mechanism facilitating exotic plant invasion and plant community composition. In order to explore this further, we employed a comprehensive extract-bioassay technique that facilitated detection and demarcation of phytotoxicity, direct allelopathy and indirect allelopathy of bitou bush (Chrysanthemoides monilifera spp. rotundata) compared to an indigenous dominant of the invaded system, acacia (Acacia longifolia var. sophorae). Extracts of the leaves and roots of both species exhibited phytotoxic effects against five indigenous plant species. Evidence for allelopathy between co-evolved indigenous plants was detected between acacia and Isolepis nodosa. Allelopathy between bitou bush and four indigenous plant species was also detected. Therefore we propose that both the acacia and bitou bush have the potential to chemically inhibit the establishment of indigenous plants. Eventual dominance of bitou bush is predicted, however, based on more ubiquitous effects on seedling growth.     

High competitiveness of a resource demanding invasive acacia under low resource supply

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via ¹⁵N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H. halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.     

Plant-induced changes in soil nutrient dynamics by native and invasive grass species

Alteration of soil nutrient dynamics has recently garnered more attention as both a cause and an effect of plant invasion. This project examines how nutrient dynamics are affected by native (Elymus elymoides, Pseudoroegneria spicata, and Vulpia microstachys) and invasive (Aegilops triuncialis, Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae) grass species. This research questions whether natives and invasives differ in their effects on nutrient dynamics. A greenhouse study was conducted using two field-collected soils. Effects on nutrient dynamics were compared using an integrated index that evaluates the total nutrients in soil and in plant tissue compared to an unplanted control. With this index, we evaluated whether soil nutrients increased or decreased as a result of plant growth, controlling for plant uptake. We found no consistent support for our hypothesis that invasive grass species as a group influence nutrient dynamics differently than native grass species as a group. Our results indicate species-specific effects on nutrient dynamics. Alteration of nutrient dynamics is not a trait shared by all of the invasive grass species in our study. However, alteration of nutrient dynamics may be a mechanism by which some individual species increase their invasive potential.     

The potential for indirect effects between a weed, one of its biocontrol agents and native herbivores: A food web approach

The safety of biological control is a contentious issue. We suggest that constructing and analyzing food webs may be a valuable addition to standard biological control research techniques, as they offer a means of assessing the post-release safety of control agents. Using preliminary data to demonstrate the value of food webs in biocontrol programs, we quantified the extent to which a key agent has infiltrated natural communities in Australia and, potentially, impacted on non-target species. Using these data, we also demonstrate how food webs can be used to generate testable hypotheses regarding indirect interactions between introduced agents and non-target species. We developed food webs in communities invaded to varying degrees by an exotic weed, bitou bush, Chrysanthemoides monilifera ssp. rotundata, and a key biocontrol agent for this weed in Australia, the tephritid fly, Mesoclanis polana. Three food webs were constructed during springtime showing the interactions between plants, seed-feeding insects and their parasitoids. One food web was constructed in a plot of native Australian vegetation that was free of bitou bush (‘bitou-free’), another in a plot of Australian vegetation surrounded by an invasion of bitou bush (‘bitou-threatened’) and a third from a plot infested with a monoculture of bitou bush (‘bitou-infested’). The bitou-free web contained 36 species, the bitou-threatened plot 9 species and the bitou-infested web contained 6 species. One native Australian herbivore attacked the seeds of bitou bush. M. polana, a seed-feeding fly, was heavily attacked by native parasitoids, these being more abundant than the parasitoids feeding on the native seed feeders. A surprising result is that none of the three species of native parasitoids reared from M. polana were reared from any of the native herbivores. The food webs revealed how a highly host-specific biocontrol agent, such as M. polana has the potential to change community structure by increasing the abundance of native parasitoids. The webs also suggest that indirect interactions between M. polana and native non-target species are possible, these been mediated by shared parasitoids. The experiments necessary to determine the presence of these interactions are outlined.     

Influence of light and nutrient conditions on seedling growth of native and invasive trees in the Seychelles

Several recent studies have shown that plant invasions can occur in resource-poor and relatively undisturbed habitats. It is, therefore, important to investigate whether and how life-history traits of species invasive in such habitats differ from those of species that are only invasive in disturbed and resource rich habitats. We compared the growth of seedlings of native and invasive tree species from nutrient-poor secondary forests in the tropical Seychelles. We hypothesised that the relative performance of the two groups would change predictably along resource gradients, with native species performing better at low levels of resource availability and invasive species performing better at higher levels. To test this hypothesis, we performed a common garden experiment using seedlings of six invasive and seven native tree species grown under three levels of light (65, 11 and 3.5% of ambient light) and two of nutrients (low and high). Due to large variation among species, differences in growth rates (RGR) were not significant among seedlings of the native and the invasive species. However, seedlings of the invasive species showed higher specific leaf areas (SLA) and higher leaf nutrient contents than seedlings of the native species. They also exhibited greater plasticity in biomass and nutrient allocation (i.e., greater plasticity in LAR, RSR and leaf nutrient contents) in response to varying resource availability. However, differences between the mean values of these parameters were generally small compared with variation within groups. We conclude that successful invaders on nutrient-poor soils in the Seychelles are either stress-tolerant, possessing growth traits similar to those of the native species, or fast-growing but adapted to nutrient-poor soils. In contrast, the more typical, fast-growing alien species with no particular adaptations to nutrient-poor soils seem to be restricted to relative nutrient-rich sites in the lowlands. The finding—that some introduced species thrive in resource-poor habitats—suggests that undisturbed habitats with low resource availability may be less resistant to plant invasions than was previously supposed.     

Rapid nutrient cycling in leaf litter from invasive plants in Hawai’i

Physiological traits that contribute to the establishment and spread of invasive plant species could also have impacts on ecosystem processes. The traits prevalent in many invasive plants, such as high specific leaf areas, rapid growth rates, and elevated leaf nutrient concentrations, improve litter quality and should increase rates of decomposition and nutrient cycling. To test for these ecosystem impacts, we measured initial leaf litter properties, decomposition rates, and nutrient dynamics in 11 understory plants from the Hawaiian islands in control and nitrogen + phosphorus fertilized plots. These included five common native species, four of which were ferns, and six aggressive invasive species, including five angiosperms and one fern. We found a 50-fold variation in leaf litter decay rates, with natives decaying at rates of 0.2–2.3 year^–1 and invaders at 1.4–9.3 year^–1. This difference was driven by very low decomposition rates in native fern litter. Fertilization significantly increased the decay rates of leaf litter from two native and two invasive species. Most invasive litter types lost nitrogen and phosphorus more rapidly and in larger quantities than comparable native litter types. All litter types except three native ferns lost nitrogen after 100 days of decomposition, and all litter types except the most recalcitrant native ferns lost >50% of initial phosphorus by the end of the experiment (204–735 days). If invasive understory plants displace native species, nutrient cycling rates could increase dramatically due to rapid decomposition and nutrient release from invasive litter. Such changes are likely to cause a positive feedback to invasion in Hawaii because many invasive plants thrive on nutrient-rich soils.     

Soil‐mediated effects on germination and seedling growth of coastal wattle (Acacia sophorae) by the environmental weed,bitou bush (Chrysanthemoides monilifera ssp. rotundata)

Bitou bush (Chrysanthemoides monilifera ssp. rotundata; Asteraceae) is a major woody weed that competes with the native legume Acacia sophorae in coastal ecosystems of eastern Australia. Three glasshouse experiments examined whether litter or soil from beneath bitou bush or Acacia plants could influence seed germination and seedling growth of A. sophorae. The presence of litter decreased seed germinability and this effect was greater for bitou bush litter than for Acacia litter. Shoot growth was increased by the addition of Rhizobium after 40 days, irrespective of soil type. After 78 days, shoot and root biomass were significantly lower for seedlings grown in bitou bush soil than for those grown in Acacia soil. There was a non‐significant trend towards a lower median population of Rhizobium in the soil beneath bitou bush than in that beneath Acacia. The results demonstrated a slight effect of bitou bush on the growth of A. sophorae, which could, however, be overshadowed by the judicious use of herbicides or fire for weed control and revegetation.     

An invasive grass shows colonization advantages over native grasses under conditions of low resource availability

Increased or fluctuating resources may facilitate opportunities for invasive exotic plants to dominate. This hypothesis does not, however, explain how invasive species succeed in regions characterized by low resource conditions or how these species persist in the lulls between high resource periods. We compare the growth of three co-occurring C₄ perennial bunchgrasses under low resource conditions: an exotic grass, Eragrostis curvula (African lovegrass) and two native grasses, Themeda triandra and Eragrostis sororia. We grew each species over 12?weeks under low nutrients and three low water regimes differentiated by timing: continuous, pulsed, and mixed treatments (switched from continuous to pulsed and back to continuous). Over time, we measured germination rates, time to germination (first and second generations), height, root biomass, vegetative biomass, and reproductive biomass. Contrary to our expectations that the pulsed watering regime would favor the invader, water-supply treatments had little significant effect on plant growth. We did find inherent advantages in a suite of early colonization traits that likely favor African lovegrass over the natives including faster germination speed, earlier flowering times, faster growth rates and from 2?weeks onward it was taller. African lovegrass also showed similar growth allocation strategies to the native grasses in terms of biomass levels belowground, but produced more vegetative biomass than kangaroo grass. Overall our results suggest that even under low resource conditions invasive plant species like African lovegrass can grow similarly to native grasses, and for some key colonization traits, like germination rate, perform better than natives.     

Does disturbance,competition or resource limitation underlie Hieracium lepidulum invasion in New Zealand? Mechanisms of establishment and persistence,and functional differentiation among invasive and native species

The processes underlying plant invasions have been the subject of much ecological research. Understanding mechanisms of plant invasions are difficult to elucidate from observations, yet are crucial for ecological management of invasions. Hieracium lepidulum, an asteraceous invader in New Zealand, is a species for which several explanatory mechanisms can be raised. Alternative mechanisms, including competitive dominance, disturbance of resident vegetation allowing competitive release or nutrient resource limitation reducing competition with the invader are raised to explain invasion. We tested these hypotheses in two field experiments which manipulated competitive, disturbance and nutrient environments in pre‐invasion and post‐invasion vegetation. H. lepidulum and resident responses to environmental treatments were measured to allow interpretation of underlying mechanisms of establishment and persistence. We found that H. lepidulum differed in functional response profile from native species. We also found that other exotic invaders at the sites were functionally different to H. lepidulum in their responses. These data support the hypothesis that different invaders use different invasion mechanisms from one another. These data also suggest that functional differentiation between invaders and native resident vegetation may be an important contributing factor allowing invasion. H. lepidulum appeared to have little direct competitive effect on post‐invasion vegetation, suggesting that competition was not a dominant mechanism maintaining its persistence. There was weak support for disturbance allowing initial establishment of H. lepidulum in pre‐invasion vegetation, but disturbance did not lead to invader dominance. Strong support for nutrient limitation of resident species was provided by the rapid competitive responses with added nutrients despite presence of H. lepidulum. Rapid competitive suppression of H. lepidulum once nutrient limitation was alleviated suggests that nutrient limitation may be an important process allowing the invader to dominate. Possible roles of historical site degradation and/or invader‐induced soil chemical/microbial changes in nutrient availability are discussed.     

Disturbance governs dominance of an invasive forb in a temporary wetland

Dominance of invasive species is often assumed to be due to a superior ability to acquire resources. However, dominance in plant communities can arise through multiple interacting mechanisms, including disturbance. Inter-specific competition can be strongly affected by abiotic conditions, which can determine the outcome of competitive interactions. We evaluated competition and disturbance as mechanisms governing dominance of Phyla canescens (hereafter lippia), an invasive perennial forb from South America, in Paspalum distichum (perennial grass, hereafter water couch) meadows in floodplain wetlands of eastern Australia. Water couch meadows (in the study area) are listed under the Ramsar Convention due to their significance as habitat for migratory waterbirds. In the field, we monitored patterns of vegetation boundaries between the two species in response to flooding. Under controlled glasshouse conditions, we explored competitive interactions between the native water couch and lippia subject to different soil moisture/inundation regimes. We did this using a pairwise factorial glasshouse experiment that manipulated neighbor density (9 treatments) and soil moisture/inundation (4 treatments). In the field trial, inundation increased the cover of water couch. Under more controlled conditions, the invader had a competitive effect on the native species only under dry soil conditions, and was strongly inhibited by inundation. This suggests that dry conditions favor the growth of the invader and wetter (more historical) conditions favor the native grass. In this system, invader dominance is governed by altered disturbance regimes which give the invader a competitive advantage over the native species.     

Root plasticity of native and invasive Great Basin species in response to soil nitrogen heterogeneity

Soil nutrients are heterogeneously distributed in natural systems. While many species respond to this heterogeneity through root system plasticity, little is known about how the magnitude of these responses may vary between native and invasive species. We quantified root morphological and physiological plasticity of co-occurring native and invasive Great Basin species in response to soil nitrogen heterogeneity and determined if trade-offs exist between these foraging responses and species relative growth rate or root system biomass. The nine study species included three perennial bunchgrasses, three perennial forbs, and three invasive perennial forbs. The plants were grown in large pots outdoors. Once a week for 4 weeks equal amounts of ¹⁵NH₄ ¹⁵NO₃ were distributed in the soil either evenly through the soil profile, in four patches, or in two patches. All species acquired more N in patches compared to when N was applied evenly through the soil profile. None of the species increased root length density in enriched patches compared to control patches but all species increased root N uptake rate in enriched patches. There was a positive relationship between N uptake rate, relative growth rate, and root system biomass. Path analysis indicated that these positive interrelationships among traits could provide one explanation of how invasive forbs were able to capture 2 and 15-fold more N from enriched patches compared to the native grasses and forbs, respectively. Results from this pot study suggest that plant traits related to nutrient capture in heterogeneous soil environments may be positively correlated which could potentially promote size-asymmetric competition belowground and facilitate the spread of invasive species. However, field experiments with plants in different neighbor environments ultimately are needed to determine if these positive relationships among traits influence competitive ability and invader success.     

No evidence for evolutionarily decreased tolerance and increased fitness in invasive Chromolaena odorata: implications for invasiveness and biological control

Tolerance, the degree to which plant fitness is affected by herbivory, is associated with invasiveness and biological control of introduced plant species. It is important to know the evolutionary changes in tolerance of invasive species after introduction in order to understand the mechanisms of biological invasions and assess the feasibility of biological control. While many studies have explored the evolutionary changes in resistance of invasive species, little has been done to address tolerance. We hypothesized that compared with plants from native populations, plants from invasive populations may increase growth and decrease tolerance to herbivory in response to enemy release in introduced ranges. To test this hypothesis, we compared the differences in growth and tolerance to simulated herbivory between plants from invasive and native populations of Chromolaena odorata, a noxious invader of the tropics and subtropics, at two nutrient levels. Surprisingly, flower number, total biomass (except at high nutrient), and relative increase in height were not significantly different between ranges. Also, plants from invasive populations did not decrease tolerance to herbivory at both nutrient levels. The invader from both ranges compensated fully in reproduction after 50?% of total leaf area had been damaged, and achieved substantial regrowth after complete shoot damage. This strong tolerance to damage was associated with increased resource allocation to reproductive structures and with mobilization of storage reserves in roots. The innately strong tolerance may facilitate invasion success of C. odorata and decrease the efficacy of leaf-feeding biocontrol agents. Our study highlights the need for further research on biogeographical differences in tolerance and their role in the invasiveness of exotic plants and biological control.     

Eco-evolutionary litter feedback as a driver of exotic plant invasion

Many studies have examined positive feedbacks between invasive plant traits and nutrient cycling, but few have investigated whether feedbacks arise from introduction of pre-adapted species or from eco-evolutionary feedback that develops after introduction. Eco-evolutionary feedback could occur between an invader's leaf tissue C:N ratio and its response to litter accumulation. Previous modeling predicts that occurrence of this feedback would be reflected by: (1) field data showing higher litter:biomass ratios in the invasive range; (2) high C:N genotypes benefiting more from experimental litter additions than low C:N genotypes; (3) this beneficial effect on high C:N genotypes inducing a critical transition toward invader dominance when a critical amount of litter is added to a native species-dominated community experiencing low nutrient conditions. Here, we empirically tested these predictions for the invasive grass Phalaris arundinacea, which has undergone post-introduction evolutionary change toward attaining higher C:N ratios under high nutrient conditions. We performed a biogeographical comparison of litter:biomass ratios in the native (Europe) and invasive (USA) range, and an experiment with mesocosms from the invasive range under low nutrient conditions. Low and high C:N Phalaris genotypes were introduced into native-dominated and bare mesocosms, to which varying litter amounts were added. The biogeographical comparison revealed that litter:biomass ratios were higher in the invasive range. The mesocosm experiment showed that when grown in isolation, only high C:N genotypes responded positively to litter. This effect, however, was not strong enough to stimulate Phalaris when exposed to competition with native species. Our results suggest that eco-evolutionary feedback between Phalaris’ C:N ratio and litter accumulation could occur, but only under high nutrient conditions. Our experiments suggest that eco-evolutionary feedback may select for specialist rather than superior genotypes. Hence, genotypic variation induced by post-introduction admixture may be subject to context-dependent selection due to eco-evolutionary feedback, increasing trait variation within invasive populations.