Increased competitive ability and herbivory tolerance in the invasive plant <Emphasis Type="Italic">Sapium sebiferum</Emphasis>

The evolution of increased competitive ability (EICA) hypothesis predicts that release from natural enemies in the introduced range favors exotic plants evolving to have greater competitive ability and lower herbivore resistance than conspecifics from the native range. We tested the EICA hypothesis in a common garden experiment with Sapium sebiferum in which seedlings from native (China) and invasive (USA) populations were grown in all pairwise combinations in the native range (China) in the presence of herbivores. When paired seedlings were from the same continent, shoot mass and leaf damage per seedling were significantly greater for plants from invasive populations than those from native populations. Despite more damage from herbivores, plants from invasive populations still outperformed those from native populations when they were grown together. Increased competitive ability and higher herbivory damage of invasive populations relative to native populations of S. sebiferum support the EICA hypothesis. Regression of biomass against percent leaf damage showed that plants from invasive populations tolerated herbivory more effectively than those from native populations. The results of this study suggest that S. sebiferum has become a faster-growing, less herbivore-resistant, and more herbivore-tolerant plant in the introduced range. This implies that increased competitive ability of exotic plants may be associated with evolutionary changes in both resistance and tolerance to herbivory in the introduced range. Understanding these evolutionary changes has important implications for biological control strategies targeted at problematic invaders.     

Decreased resistance and increased tolerance to native herbivores of the invasive plant Sapium sebiferum

Release from natural enemies may favor invasive plants evolving traits associated with reduced herbivore‐resistance and faster‐growth in introduced ranges. Given a genetic trade‐off between resistance and tolerance, invasive plants could also become more tolerant to herbivory than conspecifics in the native range. We conducted a field common garden study in the native range of Sapium sebiferum using seeds from native Chinese populations and invasive North American populations to compare their growth and herbivory resistance. We also performed a cage‐pot experiment to compare their resistance and tolerance to Bikasha collaris beetles that are specialist feeders on S. sebiferum trees in China. Results of the common garden study showed that Sapium seedlings of invasive populations relative to native populations were more frequently attacked by native herbivores. Growth and leaf damage were significantly higher for invasive populations than for native populations. Growth of invasive populations was not significantly affected by insecticide spray, but insecticide spray benefited that of native populations. In the bioassay trial, beetles preferentially consumed leaf tissue of invasive populations compared to native populations when beetles had a choice between them. Regression of percent leaf damage on biomass showed that invasive populations tolerated herbivory more effectively than native populations. Our results suggest that S. sebiferum from the introduced range had lower resistance but higher tolerance to specialist herbivores. Both defense strategies could have evolved as a response to the escape from natural enemies in the introduced range.     

Ecological impacts of the invasive grass Sorghum halepense on native tallgrass prairie

Invasive plants frequently have competitive advantages over native species. These advantages have been characterized in systems in which the invading species has already become well established. Surprisingly, invader impacts on native communities currently undergoing invasion are lacking from most ecological studies. In this work we document and quantify shifting patterns in plant community structure in a native ecosystem (remnant tallgrass prairie) undergoing invasion by the invasive exotic Sorghum halepense (Johnsongrass). Further, we use manipulative field and greenhouse studies to quantify impacts of potential allelochemicals contained in whole-plant S. halepense leachates on growth of the dominant native grass, Schizachyrium scoparium (Little Bluestem), and tested the inhibitory effects of the potential soil legacy of S. halepense on the native grass in the greenhouse. Plant diversity indices revealed three distinct plant communities within the remnant prairie: a native community, a densely S. halepense invaded area, and a transitional zone between the two. Dominance of the native grass, determined by relative percent cover, significantly declined with increased S. halepense invasion via rhizomatous growth. Annual global positioning system monitoring of the S. halepense invasion front was used to quantify advancement into native prairie, documented at an average rate of 0.45 m year^?1. In the manipulative field and greenhouse studies, native S. scoparium treated with invasive S. halepense leachate had significantly less biomass and fewer inflorescences than control plants. These findings indicate the prolific clonal growth in conjunction with the plant chemistry of S. halepense play a significant role in displacement of the native grass.     

Abiotic constraints on the competitive ability of exotic and native grasses in a Pacific Northwest prairie

In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated. The objective of our study was to use paired field and greenhouse experiments to evaluate the competitive dynamics between two native (Danthonia californica and Deschampsia cespitosa) and two exotic (Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions in an upland prairie in the Willamette Valley, Oregon. We hypothesized the two invasive, exotic grasses would be more competitive under high-nutrient, moderate-moisture conditions, resulting in the displacement of native grasses from these environments. In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum, produced more aboveground biomass when competition was reduced. In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses. Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses. L. multiflorum dominated competitive interactions with all other grasses across treatments. In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica. D. californica, D. cespitosa, and S. arundinaceus all produced more biomass in high-moisture, high-nutrient environments, and D. cespitosa, L. multiflorum, and S. arundinaceus allocated more biomass belowground in the low nutrient treatment. Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but, contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient-rich, moderately wet environments. Laurel Pfeifer-Meister and Esther M. Cole contributed equally to this work.     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

Feeding preferences of Melanoplus femurrubrum grasshoppers on native and exotic grasses: behavioral and molecular approaches

Generalist insect herbivores, such as grasshoppers, may either avoid feeding on exotic plants, potentially enabling these plants to become invasive in the introduced range, or insects may incorporate exotic plants into their diet, contributing to the biotic resistance of native communities and potentially preventing plant invasions. Accurate determination of insect diet preferences with regard to native and exotic plants can be challenging, but this information is critical for understanding the interaction between native herbivores and exotic plants, and ultimately the mechanisms underlying plant invasions. To address this, we combined behavioral and molecular approaches to accurately compare food consumption of the polyphagous red‐legged grasshopper, Melanoplus femurrubrum (De Geer) (Orthoptera: Acrididae), on native [Andropogon gerardii Vitman and Bouteloua curtipendula (Michx.) Torr.] and exotic, potentially invasive grasses [Miscanthus sinensis Andersson and Bothriochloa ischaemum (L.) Keng] (all Poaceae). We found that M. femurrubrum grasshoppers demonstrated strong feeding preferences toward exotic grasses in experiments with intact plants under both field and greenhouse conditions, but they showed no preference in experiments with clipped leaves. Additionally, we sampled the gut contents of M. femurrubrum collected in the field and identified the ingested plant species based on DNA sequences for the non‐coding region of the chloroplast trnL (UAA) gene. We found that exotic plants were prevalent in the gut contents of grasshoppers collected at study sites in Ohio and Maryland, USA. These results suggest that the generalist herbivore M. femurrubrum does not avoid feeding on exotic grasses with which they do not share coevolutionary history. In addition, by demonstrating greater food consumption of exotic plants, these grasshoppers potentially provide biotic resistance should these grasses escape cultivation and become invasive in the introduced range.     

Induction of extrafloral nectar depends on herbivore type in invasive and native Chinese tallow seedlings

Although induced defenses are widespread in nature, and a potentially important strategy used by invasive plants, it is unclear how induced defenses vary among populations and whether the intensity and duration of induced defenses depends on herbivore type. For invasive plants, low herbivore loads in their introduced ranges can lead to differences in herbivore defense compared to their native ranges, but we currently know little about how induced defenses vary among native and invasive populations. We conducted a greenhouse experiment to examine variation in one type of induced defense, extrafloral nectar (EFN) production, among native and invasive populations of Chinese tallow tree, Triadica sebifera. We experimentally manipulated herbivory from an exotic generalist scale insect, a native generalist caterpillar, both herbivores, or neither and then examined EFN production by Triadica. Damage from leaf-chewing caterpillars resulted in strongly induced EFN in both native and invasive populations while damage from phloem-feeding scales did not. Extrafloral nectar production and dissolved solute content peaked 4 days after caterpillar herbivory for both native and invasive populations. Number and proportion of leaves producing EFN, EFN volume and concentration of dissolved solutes were similar among native and invasive populations. These results suggest that selection for indirect defenses may be different than selection for other defenses in the introduced ranges of invasive plants, as constitutive and induced EFN production is retained in invasive populations.     

Response of Native and Exotic Grasses to Increased Soil Nitrogen and Recovery in a Postfire Environment

Native plant recovery following wildfires is of great concern to managers because of the potential for increased water run‐off and soil erosion associated with severely burned areas. Although postfire seeding with exotic grasses or cultivars of native grasses (seeded grasses) may mitigate the potential for increased run‐off and erosion, such treatments may also be detrimental to long‐term recovery of other native plant species. The degree to which seeded grasses dominate a site and reduce native plant diversity may be a function of the availability of resources such as nitrogen and light and differing abilities of native and seeded grasses to utilize available resources. We tested the hypothesis that seeded grasses have higher growth rates than native grasses when nitrogen and light availability is high in a greenhouse experiment. To determine how differing resource utilization strategies may affect distribution of native and seeded grasses across a burned landscape, we conducted botanical surveys after a wildfire in northern New Mexico, U.S.A., one and four years after the fire. In the greenhouse study we found seeded grasses to produce significantly more biomass than native grasses when nitrogen and light availability was high. Seeded grasses increased in cover from 1–4 years after the fire only in areas where total soil nitrogen was higher. Increased cover of seeded grasses did not affect recovery of native grasses, but it did lead to reduced native species richness at small scales. The potential negative long‐term consequences of seeding with exotic grasses should be considered in postfire rehabilitation treatments.     

California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.     

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.     

Reduced herbivory during simulated ENSO rainy events increases native herbaceous plants in semiarid Chile

El Niño Southern Oscillation (ENSO) events have profound consequences for the dynamics of terrestrial ecosystems. Since increased climate variability is expected to favour the invasive success of exotic species, we conducted a field experiment to study the effects that simulated rainy ENSO events in combination with herbivores and shade have on the composition of a semiarid herbaceous community in north-central Chile. We hypothesized that water pulses, such as those associated with rainy ENSO events could trigger significant changes in the relative abundance of exotic and native herbaceous species. Specifically, we predicted an increase in native grasses and a reduction in the abundance of exotic species, especially prostrate forbs, if water pulses were combined with reduced herbivory. We found that herbivory by small mammals, especially introduced European rabbits (Oryctolagus cuniculus) and hares (Lepus europaeus), have an overwhelming effect on species abundance and composition in this semiarid herbaceous community. Herbivore exclusion produced an overall increase in herb density and biomass mostly due to the extraordinary growth of tall native grasses (especially Bromus berterianus) that outcompeted small prostrate forbs (both native and exotic ones), and small exotic grasses (Koeleria pleoides, Schismus arabicus). Our results suggest that it might be possible to enhance the recovery of native grasses by applying efficient herbivore control during rainy years such as those associated with ENSO events although a negative consequence would be the loss of small native forbs, which greatly contribute to the richness of herbaceous communities in semiarid ecosystems.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Differences in growth and herbivory damage of native and invasive kudzu (Peuraria montana var. lobata) populations grown in the native range

The invasion success of exotic plants is often attributed to escape from natural enemies in their introduced ranges and subsequent evolutionary change in resource allocation from defense to growth and reproduction. We tested this idea by comparing resistance, tolerance, and growth between native (China) and invasive (US) populations of kudzu (Peuraria montana var. lobata) exposed to natural herbivores in the native range. The percentage of foliar damage was much higher in invasive populations than in native populations, indicating that plants from invasive populations had lower resistance to herbivory. Regression of total mass on percentage of foliar damage showed no significant differences in tolerance to herbivory between native and invasive populations. However, stem diameter and mass were significantly greater in invasive populations than in native populations. Our results may suggest geographic variation in herbivory damage and plant growth among kudzu native and invasive populations, but the role of herbivores influencing kudzu invasion requires further investigation.     

Can commercial soil microbial treatments remediate plant–soil feedbacks to improve restoration seedling performance?

Seedling performance is often a limiting factor in ecological restoration. Changes in the soil microbial community generated by invasive plants contribute to seedling failure. A method to remediate invasive species‐induced changes to the soil microbial community that results in increased native species seedling performance and decreased invasive species seedling performance could have a large impact on the success of many restoration efforts. In a greenhouse experiment, we first examined the changes in the soil microbial community created by invasive compared to native grasses. Then, we investigated four microbial treatments (bacterial inoculant, fungal inoculant, fungicide, and bactericide/fungicide) to remediate microbial plant–soil feedbacks (PSFs) created by invasive species Bromus inermis and Poa pratensis and increase the performance of natives Andropogon gerardii, Elymus canadensis, Pascopyrum smithii, and Schizachyrium scoparium. We found that the PSF mitigation treatments had some context‐dependent utility for restoration. For example, all of the treatments decreased the performance of B. inermis and fungal inoculant decreased the performance of P. pratensis. However, no single treatment increased the performance of all natives. Fungicide increased the performance of A. gerardii and E. canadensis in soil previously occupied by B. inermis and the performance of S. scoparium in soil previously occupied by P. pratensis. If validated in the field, PSF mitigation treatments may have utility for restoration practitioners.     

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.     

Allelopathy: a tool for weed management in forest restoration

Forest restoration uses active management to re-establish natural forest habitat after disturbance. However, competition from early successional species, often aggressively invasive exotic plant species, can inhibit tree establishment and forest regeneration. Ideally, restoration ecologists can plant native tree species that not only establish and grow rapidly, but also suppress exotic competitors. Allelopathy may be a key mechanism by which some native trees could reduce the abundance and impact of exotic species. Allelopathy is a recognized tool for weed management in agriculture and agroforestry, but few studies have considered how allelopathic interactions may aid restoration. Here we introduce the “Homeland Security” hypothesis, which posits that some naïve exotic species may be particularly sensitive to allelochemicals produced by native species, providing a tool to reduce the growth and impacts of invasive exotic species on reforestation. This article explores how exploiting allelopathy in native species could improve restoration success and the re-establishment of natural successional dynamics. We review the evidence for allelopathy in agroforestry systems, and consider its relevance for reforestation. We then illustrate the potential for this approach with a case study of tropical forest restoration in Panama. C₄ grasses heavily invade deforested areas in the Panama Canal watershed, especially Saccharum spontaneum L. We measured the effect of leaf litter from 17 potential restoration tree species on the growth of invasive C₄ grasses. We found that leaf litter from legume trees had a greater inhibitory effect on performance of S. spontaneum than did litter from non-legume trees. However, allelopathic effects varied greatly among species within tree functional groups. Further evaluation of intra- and inter-specific interactions will help to improve our selection of restoration species.     

Severe plant invasions can increase mycorrhizal fungal abundance and diversity

Invasions by non-native plants can alter ecosystem functions and reduce native plant diversity, but relatively little is known about their effect on belowground microbial communities. We show that invasions by knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula, hereafter spurge)—but not cheatgrass (Bromus tectorum)—support a higher abundance and diversity of symbiotic arbuscular mycorrhizal fungi (AMF) than multi-species native plant communities. The higher AMF richness associated with knapweed and spurge is unlikely due to a co-invasion by AMF, because a separate sampling showed that individual native forbs hosted a similar AMF abundance and richness as exotic forbs. Native grasses associated with fewer AMF taxa, which could explain the reduced AMF richness in native, grass-dominated communities. The three invasive plant species harbored distinct AMF communities, and analyses of co-occurring native and invasive plants indicate that differences were partly driven by the invasive plants and were not the result of pre-invasion conditions. Our results suggest that invasions by mycotrophic plants that replace poorer hosts can increase AMF abundance and richness. The high AMF richness in monodominant plant invasions also indicates that the proposed positive relationship between above and belowground diversity is not always strong. Finally, the disparate responses among exotic plants and consistent results between grasses and forbs suggest that AMF respond more to plant functional group than plant provenance.     

The effects of elevated ozone and CO2 on growth and defense of native,exotic and invader trees

Aims Elevated ozone and CO₂ can differentially affect the performance of plant species. Variation among native, exotic and invader species in their growth and defense responses to CO₂ and ozone may shape CO₂ and ozone effects on invasions, perhaps in part also due to variation between native and invasive populations of invaders.Methods We manipulated ozone (control or 100 ppb) and CO₂ (ambient or 800 ppm) in a factorial greenhouse experiment in replicated chambers. We investigated growth and defense (tannins) of seedlings of Triadica sebifera from invasive (USA) and native (China) populations and pairs of US and China tree species within three genera (Celtis, Liquidambar and Platanus).Important findings Overall, ozone reduced growth in ambient CO₂ but elevated CO₂ limited this effect. T. sebifera plants from invasive populations had higher growth than those from native populations in control conditions or the combination of elevated CO₂ and ozone in which invasive populations had greater increases in growth. Their performances were similar in elevated CO₂ because native populations were more responsive and their performances were similar with elevated ozone because invasive populations were more susceptible. Compared to other species, T. sebifera had high growth rates but low levels of tannin production that were insensitive to variation in CO₂ or ozone. Both China and US Platanus plants reduced tannins with increased CO₂ and/or ozone and US Liquidambar plants increased tannins with the combination of elevated CO₂ and ozone. The growth results suggest that intraspecific variation in T. sebifera will reduce the effects of CO₂ or ozone alone on invasions but increase their combined effects. The tannin results suggest that defense responses to CO₂ and ozone will be variable across native and exotic species. The effects of CO₂ and ozone on growth and defense of native and exotic species indicate that the benefit or harm to species from these global change drivers is an idiosyncratic combination of species origin and genus.     

Synergistic impact of water level fluctuation and invasion of Glyceria on Typha in a freshwater marsh of Lake Ontario

The effects of multiple stressors on the native Typha marsh community (mainly Typha latifolia) were examined using historical records of water levels, human census population, and field vegetation maps. Percent cover of the major plant species was estimated in a GIS, and the percent cover of Typha was related to changes in water level, human population growth, and percent cover of exotic Glyceria maxima and invasive Phragmites australis. Water level fluctuation was the major natural disturbance and it alone accounted for 88% of the variation in Typha. After partitioning out the effect of water level, both human population growth and the presence of exotic species were still significantly related to the decline of native Typha. We suggest that multiple stressors interact with each other to influence changes in native Typha community and cause greater detrimental impact. An important implication of our results is that projected water level decline due to climate change may not necessarily favor the restoration of a desirable native marsh because of the presence of other disturbances such as exotic and invasive species and altered nutrient regime.     

不同光照和水分下三叶鬼针草与本地种金盏银盘生长特征的比较研究

为探讨三叶鬼针草(Bidens pilosa)成功入侵机制,利用盆栽试验对不同光照和水分条件下三叶鬼针草与本地种金盏银盘(B.biternata)的生长特征进行了比较研究。结果表明,两物种对光和水的变化均具有较大可塑性,但与金盏银盘相比,在有利环境下三叶鬼针草具有较大的株高、叶面积、生物量;在不利环境下,三叶鬼针草具有较大的比叶面积和叶面积比;除叶面积比和叶生物量比外,三叶鬼针草各生理参数的可塑性指数均高于本地种。这说明三叶鬼针草具有较强的表型可塑性和入侵适应性,这些特性促进了其成功入侵。