3种本地植物与入侵植物紫茎泽兰的竞争

为筛选替代控制紫茎泽兰的本地植物及探讨提高替代控制效率的方法,通过盆栽实验并利用相对产量(RY)和竞争攻击力系数(A)衡量了3种本地植物与紫茎泽兰的竞争关系,同时评估了活性炭(AC)、杀真菌剂(FC)和二者联合(AC+FC)对它们竞争的影响。结果表明：(1)不添加任何物质条件下：紫茎泽兰与南酸枣混种时,紫茎泽兰的生物量明显高于单种(P<0.05),其RY和A分别显著大于1和0(P<0.05);紫茎泽兰分别与假地豆和狗尾草混种时,紫茎泽兰的株高和生物量均明显低于单种(P<0.05),其RY和A均分别显著小于1和0(P<0.05)。说明紫茎泽兰的竞争力强于南酸枣而弱于假地豆和狗尾草,假地豆和狗尾草可以在一定区域作为替代控制紫茎泽兰的潜在目标植物。(2)与不添加任何物质相比,紫茎泽兰与南酸枣混种时,AC、FC及AC+FC处理增加了紫茎泽兰的根冠比,降低了其地上生物量比,也降低了南酸枣的生物量(P<0.05);紫茎泽兰与假地豆混种时,AC和AC+FC处理增加了假地豆的株高和生物量,FC处理增加了紫茎泽兰的株高(P<0.05);紫茎泽兰与狗尾草混种时,AC和A...     

种间竞争对入侵植物和本地植物生长的影响

为了探讨入侵植物与本地同属和异属植物之间的关系,本研究以入侵植物喜旱莲子草(Alternanthera philoxeroides)、三裂叶蟛蜞菊(Wedelia trilobata)和本地植物莲子草(A.sessilis)、蟛蜞菊(W.chinensis)为试验材料开展了温室控制实验。实验设置了单种种植和混种种植两种方式,测定了每种植物生物量及其他生长指标,并分析了种间关系。结果表明:(1)对入侵植物而言,同属和异属种间竞争均增加了喜旱莲子草的生物量、根冠比和分节数,而降低了三裂叶蟛蜞菊的株高和分节数;(2)对本地植物而言,同属种间竞争降低了莲子草的株高和蟛蜞菊的分节数,异属种间竞争增加了莲子草的生物量、株高和分枝数,而降低了蟛蜞菊的总生物量和株高;(3)同属种间竞争对入侵种喜旱莲子草具有促进作用,异属种间竞争对本地种莲子草也具有促进作用,同属和异属种间竞争对其他两个物种的影响不大。未来应加强野外原位下入侵植物与本地植物研究,为入侵植物防控提供理论依据。     

红薯对薇甘菊的竞争效应

在外来入侵生物的生态管理中,利用具有较高经济价值的本地植物或伴生的本地物种进行替代控制是治理入侵植物的有效途径之一。本研究以本地作物红薯(Ipamoea batas)和入侵植物薇甘菊(Mikania micrantha)为试验对象,田间试验条件下采用deWit取代法,研究了红薯对薇甘菊的竞争效应。结果表明:薇甘菊单种时,主茎长、分枝长、节间长和分枝数大于红薯,而主茎节数、不定根节数、叶柄长、叶面积均明显小于红薯,使薇甘菊在与红薯争夺养分和光照时处于劣势;在薇甘菊与红薯共存条件下,各个混种比例中薇甘菊的株高、分枝、叶、茎节不定根和生物量等均受到显著抑制,且受到的抑制率普遍高于其对红薯的抑制率;薇甘菊和红薯的相对产量(RY)、相对产量总和(RYT)均显著小于1.0,说明2物种的种间竞争大于种内竞争;红薯对薇甘菊的竞争平衡指数,只有在薇甘菊所占比例高时(1:3)竞争平衡指数显著小于0,其余各个混种比例下竞争平衡指数均显著大于0,说明红薯与薇甘菊之间存在很强的竞争作用,红薯对薇甘菊的竞争力强于薇甘菊。本研究表明,无论是从形态特征还是生物量来看,红薯都可以作为一种对薇甘菊进行替代控制的理想竞争植物。     

三种牧草植物对黄顶菊田间替代控制

在田间条件下,采用替代试验对比研究了3种1年生牧草——高丹草、紫花苜蓿和欧洲菊苣与入侵我国华北地区的一种外来植物黄顶菊之间的相对竞争表现;通过设置不同牧草与黄顶菊组合的替代比例(牧草与黄顶菊分别单种及1∶1、1.5∶1和2∶1比例混种),建立了3种牧草与黄顶菊的田间替代试验区。结果表明:随着3种牧草替代比例的增加,其盖度也逐渐上升,均对黄顶菊表现不同程度的抑制,其中高丹草出苗较黄顶菊更早且具有更强的遮阴能力,在各混种替代组合中完全抑制了黄顶菊生长,抑制率达100%;紫花苜蓿和欧洲菊苣与黄顶菊的混种种群中,黄顶菊的生物量、株高等均极显著低于对照,且在中等替代比例(1.5∶1)下对黄顶菊的抑制效果为最佳,抑制率分别为87.7%和96.2%;在与3种牧草竞争的条件下,黄顶菊相对产量均显著1.0,表明黄顶菊种间竞争显著小于种内竞争,使该外来种生长受到有效抑制。在黄顶菊已入侵和易于入侵的生境建立牧草替代种群是进行生态重建和保持当地生物多样性的有效手段。     

氮肥和种植密度对紫茎泽兰生长和竞争的影响

紫茎泽兰（Ageratina adenophora）是我国西南热带亚热带地区危害最严重的外来入侵植物之一。本研究在野外设置不同氮肥供给水平和种植密度下的紫茎泽兰、拔毒散（Sida szechuensis）、伏生臂形草（Brachiaria decumbens）和非洲狗尾草（Setaria anceps）的单种和紫茎泽兰与其他3种植物混种的人工群落,从植被-土壤相互作用的角度分析紫茎泽兰的入侵机制。结果表明：单种时4种受试植物均表现了对氮营养的响应,随氮肥水平的升高,根冠比降低,比叶面积升高;通过比较总生物量增幅,氮响应能力非洲狗尾草>伏生臂形草>紫茎泽兰>拔毒散。混种时受试植物与紫茎泽兰的生物量比和株高也表明非洲狗尾草的竞争能力>伏生臂形草>紫茎泽兰>拔毒散;与拔毒散混种的紫茎泽兰以及与 紫茎泽兰混种的伏生臂形草和非洲狗尾草的种内竞争随种植密度的增大而增大,导致种间竞争能力下降,在高种植密度下对混种物种的抑制作用减弱。紫茎泽兰在氮响应能力、竞争能力和表型可塑性方面均强于本地种,可能与其入侵机制有关;牧草的氮响应能力和竞争能力强于紫茎泽兰,其中非洲狗尾草又强于伏生臂形草,可能更适宜于入侵地的替代控制。     

模拟氮沉降对鬼针草及其近缘本地植物的影响

【目的】理解氮素增加对入侵植物与近缘共存本地植物生长和竞争的影响,可为预测全球氮沉降背景下入侵植物的风险评价和本地植物的保护提供依据。【方法】为模拟氮沉降,设置对照(无氮添加)、低氮(1 g·m^-2·a^-1)和高氮(5 g·m^-2·a^-1)添加3种处理,将入侵植物鬼针草及其近缘共存的本地植物金盏银盘分别单种和混种,比较它们在各处理下的生长(株高、分枝数和生物量等)和竞争[相对优势度指数(RDI)与相对互作强度指数(RII)]情况。【结果】无论有无竞争,高氮添加均显著增加鬼针草的生物量,降低其根冠比;无竞争时,高氮添加还显著增加其株高和分枝数。但高氮添加仅显著增加金盏银盘无竞争时的生物量。在所有氮处理下,鬼针草的RDI和RII均显著大于金盏银盘。高氮添加显著增加鬼针草的RDI,但其RII在所有氮处理间无显著差异;高氮添加显著降低金盏银盘的RDI和RII。【结论】高氮更利于鬼针草的生长,其在所有氮素处理下均有较强的竞争力和竞争耐受性,且高氮处理下竞争力更强。因此,全球氮沉降增加会加剧鬼针草对近缘本地植物...     

紫茎泽兰与4种功能型草本植物根系生长特征和竞争效应

研究了紫茎泽兰与非洲狗尾草、本地狗尾草、佩兰和小藜在野外单、混种条件下根系形态特征,并对相对竞争能力进行了分析.结果表明： 紫茎泽兰与非洲狗尾草混种条件下,紫茎泽兰根系长度、体积和表面积比单种时减小,而非洲狗尾草增大;紫茎泽兰混种时生物量比单种时降低了77.1%,而非洲狗尾草增加了80.4%;非洲狗尾草相对产量和竞争平衡指数高于紫茎泽兰,相对产量总和约为1.0,说明非洲狗尾草地下竞争能力强于紫茎泽兰.紫茎泽兰与本地狗尾草混种条件下,两物种根系长度、体积和表面积低于单种,生物量分别比单种降低了45.3%和22.8%;竞争效应参数表明两物种具有较强的竞争效应.紫茎泽兰与佩兰混种时,两物种根系长度、体积和表面积与在单种时无显著差异,生物量分别低于单种;竞争效应参数表明紫茎泽兰具有竞争优势.紫茎泽兰与小藜混种时,紫茎泽兰根系形态和竞争能力均占优势.从地下根系竞争来看,可以利用非洲狗尾草替代控制紫茎泽兰以及对紫茎泽兰入侵迹地进行生态修复.     

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。     

不同养分水平对飞机草生长与生物量分配的影响

采用温室盆栽模拟实验研究了不同养分水平对外来入侵植物飞机草(Chromolaena odorata)的生长性状、生物量积累以及生物量分配格局的影响。实验共设置5 种养分浓度处理, 分别为Hoagland 标准营养液的10%、25%、50%、100%和200%溶液。结果表明: 养分水平对飞机草植株的生长性状以及生物量的积累、分配产生显著影响。随着养分水平的增加, 飞机草的总分枝数量、长度以及一级分枝数量、长度持续增加, 并且100%、200%处理还能促进二级分枝的萌发生长。飞机草的叶片数、总叶面积、总生物量以及茎、叶两器官生物量随养分水平的上升显著增加, 但株高、根生物量不受养分浓度变化的影响。根生物量比、根冠比随养分水平的提高显著下降, 叶生物量比则显著上升, 但茎生物量比在各养分浓度保持稳定。叶面积比、叶根比、RGR 亦随养分含量的上升显著增加。说明养分资源丰富的环境将促进飞机草的地上部生长, 而生物量分配格局的变化可能是其在入侵蔓延过程中适应养分异质性生境的重要生态策略。     

喜旱莲子草和接骨草竞争对模拟增温的响应

研究外来入侵植物与本地植物种竞争对气候变暖的响应, 对于预测未来气候变化背景下入侵植物的入侵趋势、理解其入侵机制以及筛选生态替代种具有重要的意义。以入侵我国的外来植物喜旱莲子草(Alternanthera philoxeroides)和本地植物种接骨草(Sambucus chinensis)为材料, 通过两种植物单栽、纯栽和混栽, 采用红外辐射加热器模拟增温, 研究了两种植物竞争对模拟增温的响应。结果表明: (1)在模拟增温期间(2013年5-12月), 增温组空气平均温度比不增温组提高了0.47 ℃, 相对湿度降低了1.87%; (2)混栽的喜旱莲子草除根冠比与单栽无显著差异外, 其余各生物量和根系形态指标均显著低于单栽喜旱莲子草; 无竞争、种间竞争和种内竞争三种竞争间, 接骨草除根冠比、细根与总根生物量比、比根长和比根表面积无显著差异外, 其余指标均呈现无竞争>种间竞争>种内竞争的趋势; (3)无竞争、种间竞争和种内竞争三种条件下, 喜旱莲子草各指标在增温和不增温处理间差异均不显著, 而接骨草总生物量和根生物量在无竞争和种间竞争条件下增温处理均显著低于不增温处理, 在种内竞争条件下则相反; (4)增温使接骨草的相对拥挤系数降低, 接骨草对温度升高反应敏感, 而喜旱莲子草则表现出一定的适应性。由此推测, 在中度遮阴陆生生境中, 接骨草有望成为喜旱莲子草生物替代控制的材料。     

Evolution of an invasive phenotype: shift to belowground dominance and enhanced competitive ability in the introduced range

In response to novel selection pressures in an introduced range, non-native species may evolve more competitive phenotypes unique from those of their native range. We examined the existence of an invasive phenotype in the herbaceous perennial Artemisia vulgaris, a frequent invader of the Northeast and Mid-Atlantic US. Populations from both the native (European) and the introduced (North American) ranges were grown in intra-specific competition (same population), inter-specific competition with the native perennial herb Solidago canadensis, and alone in a common garden to quantify shifts in resource allocation and neighbor effects on performance and competitive ability. Without competition, introduced A. vulgaris populations were much shorter than native populations, but germinated earlier, produced more ramets, more belowground and total biomass, and maintained higher root-to-shoot ratios. Under inter- and intra-specific competitions, introduced A. vulgaris populations were shorter, but produced more ramets, belowground, and total biomass than native populations. S. canadensis belowground and total biomass were more highly suppressed by introduced than native A. vulgaris. Our data suggest that since the introduction to North America, A. vulgaris has evolved a more competitive invasive phenotype characterized by many short ramets with more extensive root/rhizome networks. This rapid evolutionary shift likely benefits A. vulgaris in its introduced range by allowing establishment and subsequent dominance in dense stands of existing vegetation.     

Differences in Competitive Ability between Plants from Nonnative and Native Populations of a Tropical Invader Relates to Adaptive Responses in Abiotic and Biotic Environments

The evolution of competitive ability of invasive plant species is generally studied in the context of adaptive responses to novel biotic environments (enemy release) in introduced ranges. However, invasive plants may also respond to novel abiotic environments. Here we studied differences in competitive ability between Chromolaena odorata plants of populations from nonnative versus native ranges, considering biogeographical differences in both biotic and abiotic environments. An intraspecific competition experiment was conducted at two nutrient levels in a common garden. In both low and high nutrient treatments, C. odorata plants from nonnative ranges showed consistently lower root to shoot ratios than did plants from native ranges grown in both monoculture and competition. In the low nutrient treatment, C. odorata plants from nonnative ranges showed significantly lower competitive ability (competition-driven decreases in plant height and biomass were more), which was associated with their lower root to shoot ratios and higher total leaf phenolic content (defense trait). In the high nutrient treatment, C. odorata plants from nonnative ranges showed lower leaf toughness and cellulosic contents (defense traits) but similar competitive ability compared with plants from native ranges, which was also associated with their lower root to shoot ratios. Our results indicate that genetically based shifts in biomass allocation (responses to abiotic environments) also influence competitive abilities of invasive plants, and provide a first potential mechanism for the interaction between range and environment (environment-dependent difference between ranges).     

Species composition,functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of ‘invasion success’ in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non‐native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant‐soil fungi and plant allelochemical interactions on invasion success were species‐specific.     

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.     

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.     

Invasion Success in a Marginal Habitat: An Experimental Test of Competitive Ability and Drought Tolerance in Chromolaena odorata

Climatic niche models based on native-range climatic data accurately predict invasive-range distributions in the majority of species. However, these models often do not account for ecological and evolutionary processes, which limit the ability to predict future range expansion. This might be particularly problematic in the case of invaders that occupy environments that would be considered marginal relative to the climatic niche in the native range of the species. Here, we assess the potential for future range expansion in the shrub Chromolaena odorata that is currently invading mesic savannas (>650 mm MAP) in South Africa that are colder and drier than most habitats in its native range. In a greenhouse experiment we tested whether its current distribution in South Africa can be explained by increased competitive ability and/or differentiation in drought tolerance relative to the native population. We compared aboveground biomass, biomass allocation, water use efficiency and relative yields of native and invasive C. odorata and the resident grass Panicum maximum in wet and dry conditions. Surprisingly, we found little differentiation between ranges. Invasive C. odorata showed no increased competitive ability or superior drought tolerance compared to native C. odorata. Moreover we found that P. maximum was a better competitor than either native or invasive C. odorata. These results imply that C. odorata is unlikely to expand its future range towards more extreme, drier, habitats beyond the limits of its current climatic niche and that the species’ invasiveness most likely depends on superior light interception when temporarily released from competition by disturbance. Our study highlights the fact that species can successfully invade habitats that are at the extreme end of their ranges and thereby contributes towards a better understanding of range expansion during species invasions.     

Laboratory studies on the biology and host range of Dichrorampha odorata (Lepidoptera: Tortricidae), a biological control agent for Chromolaena odorata (Asteraceae)

Dichrorampha odorata (Lepidoptera: Tortricidae) is a moth from Jamaica whose larvae bore into, and kill, the shoot tips of the invasive alien plant, Chromolaena odorata (L.) King and Robinson (Asteraceae). This study reports aspects of the biology of D. odorata, and also determined the host specificity (larval and adult no-choice trials) of the moth. Adults were short lived (ranging from 2 to 7 days), with females laying a mean of 15.4 eggs. Eggs took 9 days to hatch, larvae 20–23 days to develop and the pupal stage lasted 11–12 days, giving an overall lifecycle period of 41–45 days. Larval no-choice tests using 34 asteraceous test species indicated that only C. odorata could sustain complete development of D. odorata to adulthood, although there was slight initial boring 14 test species (plus chromolaena). Results from the adult nochoice trials, in which seven test-plant species were exposed to D. odorata, were consistent with those from larval trials, with larval damage, pupae and adults of D. odorata recorded from only C. odorata. This confirmed that only C. odorata is a suitable host for D. odorata in South Africa. Permission has subsequently been granted for the release of D. odorata in South Africa, thus making it the first shoot-tip attacking agent to be released against C. odorata. It is hoped that in the field, high levels of damage by the moth will reduce the height and therefore competitiveness of C. odorata, thereby contributing to the success of biological control of this plant.     

Postintroduction evolution contributes to the successful invasion of Chromolaena odorata

1. The evolution of increased competitive ability (EICA) hypothesis states that, when introduced in a novel habitat, invasive species may reallocate resources from costly quantitative defense mechanisms against enemies to dispersal and reproduction; meanwhile, the refinement of EICA suggests that concentrations of toxins used for qualitative defense against generalist herbivores may increase. Previous studies considered that only few genotypes were introduced to the new range, whereas most studies to test the EICA (or the refinement of EICA) hypotheses did not consider founder effects.
2. In this study, genetic and phenotypic data of Chromolaena odorata populations sampled across native and introduced ranges were combined to investigate the role of postintroduction evolution in the successful invasion of C. odorata.
3. Compared with native populations, the introduced populations exhibited lower levels of genetic diversity. Moreover, different founder effects events were interpreted as the main cause of the genetic structure observed in introduced ranges. Three Florida, two Trinidad, and two Puerto Rico populations may have been the sources of the invasive C. odorata in Asia.
4. When in free of competition conditions, C. odorata plants from introduced ranges perform better than those from native ranges at high nutrient supply but not at low nutrient level. The differences in performance due to competition were significantly greater for C. odorata plants from the native range than those from the introduced range at both nutrient levels. Moreover, the differences in performance by competition were significantly greater for putative source populations than for invasive populations.
5. Quantities of three types of secondary compounds in leaves of invasive C. odorata populations were significantly higher than those in putative source populations. These results provide more accurate evidence that the competitive ability of the introduced C. odorata is increased with postintroduction evolution.

     

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.