Exotic species display greater germination plasticity and higher germination rates than native species across multiple cues

Rapid germination or flexible germination cues may be key traits that facilitate the invasion of exotic plant species in new environments. We investigated whether robustness or plasticity in response to environmental cues were more commonly exhibited by exotic than native species during germination, evidenced by (1) exhibiting consistently greater germination rate under a variety of conditions (robustness), or (2) increasing germination rate more strongly than native species in response to favorable conditions (plasticity). We conducted growth chamber germination trials of 12 native and 12 exotic species common to coastal sage scrub, a shrub-dominated Mediterranean-type ecosystem in California. Time to germination and percentage germination were recorded in response to variation in three environmental cues: temperature, day length, and soil moisture. Exotic species, especially annuals, displayed consistently higher germination percentages and more rapid germination than native species. Exotic germination percentages also responded more strongly when conditions were favorable (warm temperatures and high soil moisture), and germinated earlier than natives when conditions were indicative of typical growing season conditions in Mediterranean ecosystems (short day length and cool temperatures). Exotic species had more rapid and prolific germination across a variety of environmental cues and in response to increased resource availability compared with native species, indicating both germination plasticity and robustness. These traits may enable colonization of novel environments, particularly if they allow exotic species to establish earlier in the growing season than native species, setting the stage for seasonal priority effects.     

Trade‐off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community

Ecological trade‐offs are fundamental to theory in community ecology; critical for understanding species coexistence in diverse plant communities, as well as the evolution of diverse life‐history strategies. Invasions by exotic species can provide insights into the importance of trade‐offs in community assembly, because the ecological strategies of invading species often differ from those present in the native species pool. Exotic annual species have invaded many Mediterranean‐climate areas around the globe, and often germinate and emerge earlier in the growing season than native species. Early‐season growth can enable exotic annual species to preempt space and resources, competitively suppressing later‐emerging native species; however, early‐emerging individuals may also be more apparent to herbivores. This suggests a potential trade‐off between seasonal phenology and susceptibility to herbivory. To evaluate this hypothesis, we monitored the emergence and growth of 12 focal species (six each native and exotic) in monoculture and polyculture, while experimentally excluding generalist herbivores both early and later in the growing season. Consistent with past studies, the exotic species emerged earlier than native species. Regardless of species origin, earlier‐emerging species achieved greater biomass by the end of the experiment, but were more negatively impacted by herbivory, particularly in the early part of the growing season. This greater impact of early‐season herbivory on early‐active species led to a reduction in the competitive advantage of exotic species growing in polyculture, and improved the performance of later‐emerging natives. Such a trade‐off between early growth and susceptibility to herbivores could be an important force in community assembly in seasonal herbaceous‐dominated ecosystems. These results also show how herbivore exclusion favors early‐active exotic species in this system, with important implications for management in many areas invaded by early‐active exotic species.     

The shifting balance of facilitation and competition affects the outcome of intra- and interspecific interactions over the life history of California grassland annuals

Trait-based resource competition in plants, wherein more similar plants compete more strongly for resources, is a foundation of niche-based explanations for the maintenance of diversity in plant communities. Alternatively, neutral theory predicts that community diversity can be maintained despite equivalent resource requirements among species. We examined interactions at three life history stages (germination, survival, and juvenile-adult growth) for three native and three exotic California annual species in a glasshouse experiment. We varied plant density and species composition in small pots, with pots planted with either intraspecific seeds or in a three species mix of intra- and interspecific neighbors. We saw a range of facilitative, neutral, and competitive interactions that varied significantly by species, rather than by native or exotic status. There were more competitive interactions at the emergence and juvenile-adult growth stages and more facilitative interactions for survival. Consequently, the relative strength of competition in intraspecific versus mixed-species communities depended on whether we considered only the juvenile-adult growth stage or the entire life history of the interacting plants. Using traditional analysis of juvenile-adult growth only, all species showed negative density-dependent interactions for final biomass production. However, when the net effect of plant interactions from seed to adult was considered, which is a prediction of population growth, two native species ceased to show negative density dependence, and the difference between intraspecific and mixed-species competition was only significant for one exotic species. Results were consistent with predictions of neutral, rather than niche, theory for five of six species.     

Can Carbon Addition Increase Competitiveness of Native Grasses? A Case Study from California

There is growing interest in the addition of carbon (C) as sucrose or sawdust to the soil as a tool to reduce plant‐available nitrogen (N) and alter competitive interactions among species. The hypothesis that C addition changes N availability and thereby changes competitive dynamics between natives and exotics was tested in a California grassland that had experienced N enrichment. Sawdust (1.2 kg/m) was added to plots containing various combinations of three native perennial bunchgrasses, exotic perennial grasses, and exotic annual grasses. Sawdust addition resulted in higher microbial biomass N, lower rates of net N mineralization and net nitrification, and higher concentrations of extractable soil ammonium in the soil. In the first year sawdust addition decreased the degree to which exotic annuals competitively suppressed the seedlings of Nassella pulchra and, to a lesser extent, Festuca rubra, both native grasses. However there was no evidence of reduced growth of exotic grasses in sawdust‐amended plots. Sawdust addition did not influence interactions between the natives and exotic perennial grasses. In the second year, however, sawdust addition did not affect the interactions between the natives and either group of exotic grasses. In fact, the native perennial grasses that survived the first year of competition with annual grasses significantly reduced the aboveground productivity of annual grasses even without sawdust addition. These results suggest that the addition of sawdust as a tool in the restoration of native species in our system provided no significant benefit to natives over a 2‐year period.     

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.     

Timing is everything: does early and late germination favor invasions by herbaceous alien plants?

Aims Plant invasions represent a unique opportunity to study the mechanisms underlying community assembly rules and species distribution patterns. While a superior competitive ability has often been proposed as a major driver of successful plant invasions, its significance depends crucially on the timing of any competitive interaction. We assess whether a mismatch in germination phenology can favor the establishment of alien species, allowing them to exploit vacant niches where competition is low. As well as having important effects on the survival, growth and fitness of a species, asymmetric competition and potential soil legacies resulting from early or late germination can also impact on species recruitment. However, early or late germination comes at a cost, increases the risks of exposure to unfavorable conditions and requires an enhanced abiotic resistance if it is to lead to successful establishment.Important findings While there are several anecdotal accounts of early and late germination for invasive species, there are limited comparative data with resident species growing under natural conditions. Available evidence from grassland communities indicates that a short-term germination advantage or priority (few days/weeks) provides invasive species with a strong competitive advantage over native species and is a critical factor in many invasions. While the exploitation of periods of low competition is a plausible mechanism for the successful establishment of many invasive plants, direct evidence for this strategy is still scarce. This is particularly true with regard to the exploitation of late germination niches. Consequently, long-term comparative monitoring of the germination phenology of invasive and native plants in situ is needed to assess its significance in a range of ecosystems and its impact on community dynamics.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Within‐Year Soil Legacies Contribute to Strong Priority Effects of Exotics on Native California Grassland Communities

Understanding priority effects, in which one species in a habitat decreases the success of later species, may be essential for restoring native communities. Priority effects can operate in two ways: size‐asymmetric competition and creation of “soil legacies,” effects on soil that may last long after the competitive effect. We examined how these two types of priority effects, competition and soil legacies, drive interactions between seedlings of native and exotic California grassland plants. We established native and exotic communities in a mesocosm experiment. After 5 weeks, we removed the plants from half the treatments (soil legacy treatment) and retained the plants in the other half (priority effect treatment, which we interpret to include both competition and soil legacies). We then added native or exotic seed as the colonizing community. After 2 months, we measured the biomass of the colonizing community. When germinating first, both natives and exotics established priority effects, reducing colonist biomass by 86 and 92%, respectively. These priority effects were predominantly due to size‐asymmetric competition. Only exotics created soil legacies, and these legacies only affected native colonizers, reducing biomass by 74%. These results imply that exotic species priority effects can affect native grassland restorations. Although most restorations focus on removing exotic seedlings, amending soil to address soil legacies may also be critical. Additionally, because native species can exclude exotics if given a head start, ensuring that natives germinate first may be a cost‐effective restoration technique.     

降雨年型变化及竞争对反枝苋和大豆生长的影响

为探索不同降雨年型及栽培方式下外来杂草与本地作物的竞争机制, 为未来全球变化背景下控制外来杂草提供理论依据, 本研究以广泛入侵东北农田生态系统的外来杂草反枝苋(Amaranthus retroflexus)和本地作物大豆(Glycine max)为研究对象, 在遮雨棚内人工模拟正常、欠缺、丰沛三种降雨年型, 采用盆栽实验的方法, 研究两种植物在单种和混种条件下的生长季节动态。结果表明, 降雨丰沛年两种植物的株高和总生物量均大于降雨正常年, 降雨欠缺年则均小于降雨正常年。生长季初期两种植物的根冠比均在降雨欠缺年最高, 说明两种植物均可通过增大根系的生物量分配, 减少地上生物量的分配来适应干旱环境。在三种降雨年型下, 混种时大豆的株高、相对生长速率及总生物量均显著小于单种大豆, 而反枝苋则相反, 尽管有时不显著, 说明种间竞争抑制大豆生长而促进反枝苋的生长, 两种植物之间的竞争是不对称竞争。总的来看, 降雨增加有利于提高大豆的竞争能力, 降雨减少有利于提高反枝苋的竞争能力, 随着生长发育的推移, 这种现象更明显。反枝苋可以在较广的降雨变化范围内保持较高的株高、相对生长速率及生物量, 这很可能是其成为全球范围成功入侵的外来杂草的重要原因之一; 干旱更有利于反枝苋入侵大豆田。     

Extreme drought alters growth and interactions with exotic grasses,but not survival,for a California annual forb

Climate change will not only alter mean conditions, but increase the frequency and intensity of extreme events such as severe droughts. Yet the consequences of extreme drought for plant demography are poorly understood. We compared phenology, demographic rates and effects of competition with exotic grasses for the California annual forb Phacelia distans between a year with slightly below average precipitation (2011–2012) and one in extreme drought (2012–2013). We also contrasted these demographic responses with changes in seedling emergence rates and cover in the annual plant community. Early Phacelia mortality actually fell in the extreme drought year, as low October rainfall shifted germination to cooler conditions in November. Survival from mid December to flowering did not change between years. In contrast to expectations, competition with early-emerging exotic annual grasses did not reduce Phacelia spring survival in 2012–2013. A shorter window for fall germination that reduced priority effects may explain this result. Yet the 2012–2013 growing season ended a month earlier than in 2011–2012, significantly reducing Phacelia size at flowering and inflorescence production. Community seedling emergence and cover changed only weakly between the 2 years. Our results demonstrate the importance of within season precipitation patterns in determining whether a climatically extreme year will lead to extreme ecological outcomes. This work also illustrates how annual plant drought responses depend critically on germination behavior and phenology. An important future question is whether seed banks can sustain native forb population persistence through an extended drought with multiple years of low or failed reproduction.     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

Exotic consumers interact with exotic plants to mediate native plant survival in a Midwestern forest herb layer

Consumer-facilitated invasions have been proposed as an alternative mechanism to direct competitive exclusion to explain the replacement of native plants by exotics. In a factorial field experiment manipulating competition from the exotic plant Alliaria petiolata and herbivory by exotic mollusks, we documented that mollusk herbivory significantly reduced the survival of two species of native palatable plants, but found minimal direct herbivore effects on less palatable species, including the invasive A. petiolata. These effects were evident after one growing season on younger juvenile plants of Aster cordifolius, but only after two growing seasons on older transplants of the same species, suggesting a greater vulnerability of young plants. In contrast to our expectations, A. petiolata competition alone had no effect on any of the six native species we tested. However, competition from A. petiolata did affect the survival of the most palatable native plant when mollusks were also present. While not significant for any other single species, this same pattern was observed for three of the five remaining native species tested. The selective grazing on palatable plants that we document provides novel evidence contributing to our understanding of observed shifts in the forest herbaceous layer towards the dominance of exotic plants and unpalatable species. More broadly, our results highlight the importance of the interactive effect of consumers and inter-specific competition in forest understories via its contribution to differential survival among regenerating species.     

Priority effects are affected by precipitation variability and are stronger in exotic than native grassland species

Exotic perennial grassland species often green up earlier than their native counterparts, allowing them to gain an advantage by dominating resources early (priority effects). Precipitation variability is expected to increase with climate change, and may alter the strength of priority effects. We hypothesized that exotics will have stronger priority effects than natives, precipitation variability will impact the strength of priority effects, and precipitation variability will impact the priority effects of native species more than those of exotics. We seeded one of five native or five exotic grassland species from the Central U.S. spanning multiple functional groups 28 days prior to a native seed mix. Priority effect strength was determined by how much establishment and diversity was reduced in the mix compared to controls (no species seeded before mix). We crossed these priority effect treatments with three water variability treatments, one low variability, and two high variability with alternate timing. Exotic species had stronger priority effects than natives, and decreased diversity and establishment from the seed mix. High variability precipitation when the growing season began dry significantly increased priority effects compared to low variability and high variability beginning wet. We found no significant evidence for a more pronounced impact of precipitation on native species, but trends suggest future studies may reveal significant interactions. Although future research in the field over multiple growing seasons is needed, our results suggest priority effects of exotics in Central U.S. grasslands are independent of precipitation timing and therefore likely to persist under changing climates.     

Native–Exotic Species Richness Relationships Across Spatial Scales in a Prairie Restoration Matrix

In highly invaded ecosystems, restoration of native plant communities is dependent upon reducing exotic species relative to native species. Even so, in monitoring, the native–exotic species richness ratio has been shown to be scale‐dependent. Measurement at small spatial scales (<1 m²) can reveal a negative native–exotic richness relationship, where niche occupation may prevent invasion. Conversely, at larger scales, a positive correlation may exist, where environmental heterogeneity and equally favorable conditions may drive native–exotic relationships. Here, we compare slopes of native–exotic relationships across spatial scales in a prairie undergoing active restoration. The observed native–exotic richness ratios varied considerably over scales ranging from 1 to 1,000 m², emphasizing the importance of choosing a measurement scale that is most pertinent to the treatment and ecological mechanism used to evaluate restoration success. Our native–exotic richness slopes were positive over all scales, but lower than would be expected in a random community assembly, suggesting the influence of niche‐based competition. Correspondingly, our native–exotic cover slope was more negative than a null model; however, areas of frequent fire treatments showed a significant deviation from null only for richness, indicating that burning may enhance native–exotic competitive dynamics for number of species but not cover. The negative native–exotic cover relationships appear to be driven in this system mainly by exotic graminoids, across burn treatments and native functional groups, supporting the concept that frequent burning can alter the dominant competitive mechanism from coverage of these exotic grasses to an improved environment for germination and dispersal of more native 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.     

Exotic tree seedlings are much more competitive than natives but show underyielding when growing together

Aims Invasive species continue to be a worldwide threat to ecosystems mainly as a cause for biodiversity loss. Forest ecosystems, for example, are subject to a change in species composition due to the invasion of exotic species. Specifying the attributes that cause the strong competitiveness of several exotic species may improve the ability to understand and effectively manage plant invasions in the future. In this study the following hypotheses were tested: (1) biomass production of below- and aboveground plant components of the exotic tree species is higher than that of the natives, resulting in a higher competitiveness of the exotics; (2) the exclusion of root competition has a positive effect on the biomass production of the inferior native species; and (3) mixtures of native and exotic species yield a higher biomass production than the respective monocultures.Methods A pot experiment, containing about 2000 tree seedlings, was established. We investigated the biomass productivity and growth reactions of two native (Quercus robur L., Carpinus betulus L.) and two exotic tree species (Prunus serotina Ehrh., Robinia pseudoacacia L.) in different intra- and interspecific, competitive situations with and without the influence of root competition.Important findings The biomass production of both exotic species was significantly higher and led to a strong competitive advantage, resulting in a biomass decrease of the less competitive native species. The high belowground biomass of both exotic species had a negative effect on the biomass production. The competitive pressure of exotic tree seedlings on the native ones was largely driven by root competition. Furthermore, mixtures of native and exotic tree species had a higher productivity than their growth in monocultures would have predicted. Competition was lower for exotic species in mixtures with the less productive native species compared to the competition in monocultures or in mixture with the other highly productive exotic species. Accordingly, both highly competitive exotic species produced less biomass in mixture with each other compared to monocultures. Despite the significantly higher biomass of P. serotina in all mixtures and in monoculture, R. pseudoacacia seemed to be the dominating species. Due to its strong root competition, R. pseudoacacia significantly reduced the biomass production of P. serotina .     

What matters most - Role of environment,arrival order and population fitness in invaded community assembly

High population fitness and early arrival in the community give exotic plants a competitive advantage over the natives in a novel environment. These processes are often influenced by environmental heterogeneity like habitat disturbance and seasonal impacts. These effects were experimentally investigated in this study with the hypothesis that environmental factors, arrival order and physiological fitness of the invading species determine the fate of the community assembly.Controlled regeneration experiments were conducted in a disturbed and undisturbed plot during the winter and monsoon periods. At a weekly interval, above ground biomass data of the invasive Mikania micrantha and the native species were collected for twenty weeks from the undisturbed plot in the winter and from the disturbed plot in both the seasons. Relative growth rate and carrying capacity were estimated using a logistic growth model and used as fitness metrics for the invasive and native species. During the winter season, priority effects were weakened. M. micrantha was late-arriving species but became community dominant due to the higher growth rate in the undisturbed habitat, whereas its stable growth rate and habitat disturbance reduced fitness of the native species in the disturbed habitat. Seasonal priority effect was established in monsoon when M. micrantha was the early-arriving species and due to seasonal influence on its rapid growth, it became a driver of community change leading to the regrowth failure of the annual species in the community. Based on the revegetation pattern in the experimental sites, season-specific management strategies and continuous monitoring were recommended for this invasive plant.     

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.     

Dynamic coexistence in a three-species competition–diffusion system

From the viewpoint of competitor-mediated coexistence, it is important to study the influence of an exotic species on other native species in terms of exogenous effects, in general, exotic species are weaker than native ones because they have evolved in a different environment. Even if an exotic species is weaker, however, it might cooperate with a native species, after which the competitive relations among native species may have reversed. This motivates us to consider the ecological situation whereby one exotic competing species invades the native system of two strongly competing species. Therefore, we discuss the problem of competitive exclusion or competitor-mediated coexistence using a three-species competition–diffusion system.     

The effect of soil nitrogen on competition between native and exotic perennial grasses from northern coastal California

The invasion of European perennial grasses represents a new threat to the native coastal prairie of northern California. Many coastal prairie sites also experience anthropogenic nitrogen (N) deposition or increased N availability as a result of invasion by N-fixing shrubs. We tested the hypothesis that greater seedling competitive ability and greater responsiveness to high N availability of exotic perennial grasses facilitates their invasion in coastal prairie. We evaluated pairwise competitive responses and effects, and the occurrence of asymmetrical competition, among three common native perennial grasses (Agrostis oregonensis, Festuca rubra, and Nassella pulchra) and three exotic perennial grasses (Holcus lanatus, Phalaris aquatica, and Festuca arundinacea), at two levels of soil N. We also compared the root and shoot biomass and response to fertilization of singly-grown plants, so we could evaluate how performance in competition related to innate plant traits. Competitive effects and responses were negatively correlated and in general varied continuously across native and exotic species. Two exceptions were the exotic species Holcus, which had large effects on neighbors and small responses to them, and competed asymmetrically with all other species in the experiment, and the native grass Nassella, which had strong responses to but little effect on neighbors, and was out-competed by all but one other species in the experiment. High allocation to roots and high early relative growth rate appear to explain Holcus’s competitive dominance, but its shoot biomass when grown alone was not significantly greater than those of the species it out-competed. Competitive dynamics were unaffected by fertilization. Therefore, we conclude that seedling competitive ability alone does not explain the increasing dominance of exotic perennial grasses in California coastal prairie. Furthermore, since native and exotic species responded individualistically, grouping species as ‘natives’ and ‘exotics’ obscured underlying variation within the two categories. Finally, elevated soil N does not appear to influence competition among the native and exotic perennial grasses studied, so reducing soil N pools may not be a critical step for the restoration of California coastal prairie.