The measurement of small-scale environmental heterogeneity using clonal transplants of Anthoxanthum odoratum and Danthonia spicata

Summary To assess the scale of micro-environmental heterogeneity perceived by two co-occurring grass species, Anthoxanthum odoratum and Danthonia spicata, cloned tillers of each species were planted into the natural habitat at a range of spacings (from 2 cm to more than 2 m apart) and measured for survival and fecundity over three years. A. odoratum responded to heterogeneity at a scale of 4–8 cm and at a scale of 2–8 m but not to intermediate scales. D. spicata did not respond significantly to heterogeneity. However one genotype infected with the systemic fungus Atkinsonella hypoxylon showed a large response to heterogeneity at both small and large spatial scales. The results showed that the scale and level of environmental heterogeneity as measured by its fitness impact depends on the species and genotype involved. The results indicate that small scale environmental heterogeneity could play a role in the maintenance of sexual reproduction in A. odoratum.     

A genetic analysis of the photosynthetic properties of populations of Danthonia spicata that have different growth responses to light level

Summary Three populations of the grass Danthonia spicata were observed to have different rates of biomass accumulation when grown in common environment treatments. The populations were native to adjacent sites of different successional age and different levels of shading. Twelve individuals from each population were clonally replicated and two replicates were grown in each of two light treatments, 100% and 22% of unshaded sunlight. Following growth in the treatments the populations all exhibited the same mean light-saturated photosynthetic rate of 11.7 mol m^-2s^-1. This rate is intermediate for published values of sun and shade species and for species from along a successional gradient. There was no difference in photosynthetic rate among treatments. There was significant genetic variation for lightsaturated photosynthetic rate within populations but no significant differences among populations. The populations had similar leaf water potential values of-1.12 MPa in all treatments. There were significant differences among treatments and genotypes for specific leaf weight which resulted in significant differences among treatments and no significant differences among genotypes in light-saturated photosynthetic rate expressed on a leaf weight basis. Lightsaturated photosynthetic rate had a high heritability and low plasticity. We postulate that photosynthetic rate is under strong selection and that the observed rates permit populations of D. spicata to grow in a wide range of habitat light levels.     

Isozyme evidence for host races of the fungus Atkinsonella hypoxylon (Clavicipitaceae) infecting the Danthonia (Poaceae) complex in the southern Appalachians

On granite outcrops around Highlands, North Carolina four sympatric Danthonia grass species are infected by the fungus Atkinsonella hypoxylon (Clavicipitaceae). Danthonia epilis and D. sericea (rare hosts) are infected only in this small region, although they range more widely, while D. compressa and D. spicata (widespread hosts) are infected throughout their ranges in eastern North America. To test the hypothesis that infection of the rare hosts has recently spread from the widespread hosts, the genetic structure of fungal populations on different Danthonia hosts was investigated by isozyme electrophoresis. Two hundred isolates were collected from 46 sites and analyzed for ten polymorphic isozyme loci to examine levels of variation, genetic differentiation, and gene flow among fungal populations infecting different hosts. Genetic identities (I) indicated that isolates from within each of the widespread and rare host species pairs were homogeneous (I > 0.99), but the pairs were highly differentiated (0.66 < I < 0.69). With one exception there was no overlap in fungal genotypes isolated from the widespread hosts (12 genotypes) vs. the rare hosts (seven genotypes). Two plants of D. compressa growing sympatrically with infected D. epilis at one site were infected with a genotype characteristic of the latter species. Estimates of gene flow suggest a high degree of host-mediated reproductive isolation in A. hypoxylon, indicating that the fungus forms distinct, long-standing host races or sibling species in the southern Appalachians.     

Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass

If males and females of a species differ in their effect on intraspecific competition then this can have significant ecological and evolutionary consequences because it can lead to size and mortality disparities between the sexes, and thus cause biased population sex ratios. If the degree of sexual dimorphism of competitive effect varies across environments then this variation can generate sex ratio variation within and between populations. In a California population of Distichlis spicata, a dioecious grass species exhibiting extreme within-population sex ratio variation (spatial segregation of the sexes), I evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types. The sex of seedlings was determined using a RAPD-PCR marker co-segregating with female phenotype. Distichlis spicata seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings in soil from microsites where the majority of D. spicata plants are female, and this sexual dimorphism of competitive effect was weaker or did not occur in other soil types. This study suggests that it is not just the higher costs of female versus male reproduction itself that cause spatial segregation of the sexes in D. spicata, but that differences in competitive abilities between the sexes—which occur as early as the seedling stage—can generate sex ratio variation.     

THE ECOLOGY AND GENETICS OF FITNESS IN CHLAMYDOMONAS III. GENOTYPE-BY-ENVIRONMENT INTERACTION WITHIN STRAINS

The fitness of genotypes created by crossing strains of Chlamydomonas reinhardtii was measured in axenic pure culture in a set of chemically defined environments. There was substantial and highly significant genotype-by-environment interaction, with genetic correlations between environments averaging only about +0.1 for both r and K. Higher-order interactions with combinations of environmental factors appeared to be no less important than simple interactions with single factors. The importance of genotype-by-environment interaction increased with the number of environmental factors manipulated. The linear reaction norms of genotypic score on environmental mean score varied substantially among genotypes and often intersected. There was also some evidence that nonallelic genetic interactions were present, and varied among environments. The genetic correlation of r with K also varied among environments, being significantly negative in some but not in others. These results are similar in all important respects to those previously obtained with different species, and suggest that genotype-by-environment interaction is important at all genetic scales. It is argued that they provide empirical support for a general theory of diversity, the “Tangled Bank,” based on the different response of genotypes to the range of conditions found in heterogeneous natural environments.     

Genetic interactions influence host preference and performance in a plant-insect system

Phytophagous insects generally feed on a restricted range of host plants, using a number of different sensory and behavioural mechanisms to locate and recognize their host plants. Phloem-feeding aphids have been shown to exhibit genetic variation for host preference of different plant species and genetic variation within a plant species can also have an effect on aphid preference and acceptance. It is known that genotypic interactions between barley genotypes and Sitobion avenae aphid genotypes influence aphid fitness, but it is unknown if these different aphid genotypes exhibit active host choice (preference) for the different barley genotypes. Active host choice by aphid genotypes for particular plant genotypes would lead to assortative association (non-random association) between the different aphid and plant genotypes. The performance of each aphid genotype on the plant genotypes also has the ability to enhance these interactions, especially if the aphid genotypes choose the plant genotype that also infers the greatest fitness. In this study, we demonstrate that different aphid genotypes exhibit differential preference and performance for different barley genotypes. Three out of four aphid genotypes exhibited preference for (or against) particular barley genotypes that were not concordant with differences in their reproductive rate on the specific barley genotype. This suggests active host choice of aphids is the primary mechanism for the observed pattern of non-random associations between aphid and barley genotypes. In a community context, such genetic associations between the aphids and barley can lead to population-level changes within the aphid species. These interactions may also have evolutionary effects on the surrounding interacting community, especially in ecosystems of limited species and genetic diversity.     

Predicting adaptive evolution under elevated atmospheric CO2 in the perennial grass Bromus erectus

Increasing concentrations of CO₂ in the atmosphere are likely to affect the ecological dynamics of plant populations and communities worldwide, yet little is known about potential evolutionary consequences of high CO₂. We employed a quantitative genetic framework to examine how the expression of genetic variation and covariation in fitness‐related traits, and thus, the evolutionary potential of a species, is influenced by CO₂. In two field experiments, genotypes of the dominant grassland perennial Bromus erectus were grown for several years in plots maintained at present‐day or at elevated CO₂ levels. Under noncompetitive conditions (experiment 1), elevated CO₂ had little impact on plant survival, growth, and reproduction. Under competitive conditions in plots with diverse plant communities (experiment 2), performance of B. erectus was reduced by elevated CO₂. This suggests that the effect of CO₂ was largely indirect, intensifying competitive interactions. Elevated CO₂ had significant effects on the expression of genetic variation in both the competitive and noncompetitive environment, but the effects were in opposite direction. Heritability of plant size was generally higher at elevated than at ambient CO₂ in the noncompetitive environment, but lower in the competitive environment. Selection analysis revealed a positive genotypic selection differential for plant size at ambient CO₂, indicating selection favoring genotypes with high growth rate. At elevated CO₂, the corresponding selection differential was nonsignificant and slightly negative. This suggests that elevated CO₂ is unlikely to stimulate the evolution of high biomass productivity in this species.     

Genotypic diversity enhances invasive ability of Spartina alterniflora

Although genetic diversity is very important for alien species, which have to cope with new environments, little is known about the role that genetic diversity plays in their invasive success. In this study, we set up a manipulation experiment including three levels of genotypic diversity to test whether genotypic diversity can enhance the invasive ability of alien species, in our case the invasive Spartina alterniflora in China, and to infer the underlying mechanisms. There was no significant relationship between genotypic diversity and parameters of performance in the first year; however, from the summer of the second year onwards, genotypic diversity enhanced four of the six parameters of performance. After two growing seasons, there were significant positive relationships between genotypic diversity and maximum spread distance, patch size, shoot number per patch, and aboveground biomass. Moreover, abundance of the native dominant species Scirpus mariqueter was marginally significantly decreased with genotypic diversity of S. alterniflora, suggesting that enhanced invasive ability of S. alterniflora may have depressed the growth of the native species. There was no significant difference in most measures of performance among six genotypes, but we observed a transgressive over performance in four measures in multiple‐genotype patches. At the end of the experiment, there were significant nonadditive effects of genotypic diversity according to Monte Carlo permutations, in six‐genotype, but not three‐genotype plots. Our results indicated that both additive and nonadditive effects played roles in the positive relationship between genetic diversity and invasion success, and nonadditive effects were stronger as duration increased.     

不同邻居物种的基因型多样性对冷蒿生长的影响

植物物种多样性与基因型多样性对群落的结构和功能具有重要的生态作用,近年来植物基因型多样性对植物间相互作用的影响已成为研究者关心的重要科学问题。实验选择退化草原优势种冷蒿(Artemisia frigida)为目标植物,稳定群落建群种羊草(Leymus chinensis)和群落伴生种洽草(Koeleria cristata)为邻居植物,来研究基因型多样性不同的邻居植物对冷蒿生长表现(株高、地上生物量、地下生物量和总生物量)的影响,并通过测量植物相对竞争强度及邻居植物性状变异来进一步探究邻居植物基因型多样性对目标植物影响的内在机制。结果表明:(1)邻居物种为羊草时,基因型多样性对冷蒿的生长表现影响显著,当邻居为6基因型时,冷蒿的株高、地上生物量以及总生物量显著低于单基因型和3基因型时的表现(P0.05),且相对竞争强度高于其他两种处理;而邻居物种为洽草时,基因型多样性对冷蒿所有观测指标以及相对竞争强度的影响均不显著(P0.05)。(2)利用主成分分析法来分析基因型多样性对自身性状变异的影响发现,邻居物种为羊草时,基因型多样性对性状变异响应显著,主要表现为3基因型时,羊草种群的株高、总生物量、地上生物量显著高于单基因型时的表现(P0.05);而邻居物种为洽草时,基因型多样性对性状变异影响不显著(P0.05)。(3)邻居物种为羊草时,羊草总生物量和比叶面积与冷蒿的地上生物量和总生物量呈显著负相关(P0.05);邻居物种为洽草时,洽草各性状与冷蒿性状间无显著相关性(P0.05)。实验结果揭示,基因型多样性对目标植物生长的效应受邻居植物种类的影响,稳定群落建群种羊草高基因型组合能显著抑制冷蒿的生长,这可能与羊草高基因型多样性种群性状变异大且对冷蒿有较高的相对竞争强度有关。所得结果为建群种基因型多样性影响种间相互作用提供了实验证据,为草原的合理利用和保护提供了理论指导。     

ASSORTATIVE MATING AND NATURAL SELECTION IN AN IRIS HYBRID ZONE

The phenology of different genotypes and the distribution of genetic variation among flowering plants and their progeny were examined to assess the levels of assortative mating and selection in a hybrid population of Iris. This study and a previous survey of RAPD nuclear markers and chloroplast markers indicate that the population consists of parental genotypes and recombinant hybrid genotypes that are similar to the parental species (I. fulva and I. brevicaulis), although lacking intermediate genotypes. Early in the season only I. fulva genotypes produced flowers, but as flowering in these plants decreased, the hybrid genotypes and I. brevicaulis genotypes began flowering, resulting in a 24-d period of coincidental flowering. The genotypic distribution of seeds produced during the period of flowering overlap contained a high frequency of intermediate genotypes that were not present in the adult generation. The degree of effective assortative mating was examined by comparing the observed progeny genotypic distributions with expected distributions from a mixed-mating model. The model included selfing and random outcrossing to the nearest plants that had pollen-bearing flowers on the day the recipient flower was receptive. The observed genotypic distribution of progeny from plants with I. brevicaulis chloroplast DNA (cpDNA) was not significantly different from the expected distribution. For I. fulva genotypes, however, there were higher than expected frequencies in the extreme genotypic classes, although intermediate genotypes were absent, indicating that these plants were preferentially mating with similar genotypes. Compared with the extreme genotypes, a larger proportion of the intermediate seed progeny produced were aborted, indicating that intermediate genotypes have lower viability. On the basis of the observed progeny genotypes and genetic disequilibria estimates for the adults and the progeny, there appears to be a pattern of effective asymmetrical mating in this population. This asymmetry is most likely due to pollen-style interactions that reduce the fertilization ability of genetically dissimilar pollen, or preferential abortion of genetically intermediate zygotes by I. fulva-like genotypes. The lack of any apparent discrimination by I. brevicaulis-like genotypes creates a directional exchange of nuclear genetic elements that will have implications for introgression and the evolution of hybrid genotypes.     

Population genetic analysis of Elliottiaracemosa (Ericaceae), a rare Georgia shrub

Genetic and genotypic diversity found within populations of threatened plant species can have important implications for their conservation and management. In this study we describe genetic and genotypic diversity found within 10 populations of the endemic shrub Elliottiaracemosa (Ericaceae), the Georgia plume. E. racemosa is a threatened species known from fewer than 50 locations, all within the state of Georgia, USA. Seedset is limited to nonexistent in some E. racemosa populations and sexual recruitment has not been documented. However, the species is known to spread vegetatively via root-sprouts. Twenty-one allozyme loci were resolved for E. racemosa, nine of which were polymorphic. Compared with other woody taxa, E. racemosa has low genetic (i.e. allelic) diversity within populations (H_ep = 0.063) and at the species level (H_es = 0.091). Most of the genetic variation (82%) was found within populations, and genetic identities between populations were high (mean I = 0.96). However, genotypic diversity (i.e. the number of multilocus genotypes) differed markedly among populations. Two of the 10 populations consisted almost entirely of single multilocus genotypes, whereas more than 20 multilocus genotypes (in samples of 48 stems) were detected at three sites. Sites in which few multilocus genotypes were detected have low seedset, suggesting that the lack of clonal diversity limits reproduction in some populations of this reportedly self-incompatible species.     

Genotypic diversity and environmental variability affect the invasibility of experimental plant populations

The world is experiencing increasing climatic variability, an ongoing loss of biodiversity and a growing spread of invasive species. Previous experimental studies demonstrated that the invasibility of plant populations is reduced with increasing resident genetic diversity and is promoted by environmental fluctuations, but their combined effect has so far not been considered. In a growth chamber experiment, we tested whether the genotypic diversity of experimental populations of Arabidopsis thaliana (1, 3 or 6 genotypes) and temperature fluctuations affect population invasion by Senecio vulgaris, and how these factors interact. We found that genotypic diversity tended to increase the invasion resistance of experimental plant populations in terms of the biomass ratio between the species, and that temperature fluctuations strongly favoured Arabidopsis (biomass: +49%) over Senecio (–28%). However, there were no interactions between environmental fluctuations and genotypic diversity. Nevertheless, the magnitude of net diversity effects and transgressive overyielding depended on temperature conditions, indicating that increased environmental variability can influence diversity mechanisms. Our study shows that, although genotypic diversity and environmental variability did not interact, these two factors independently affected the invasibility of plant populations.     

Genetic variation in tolerance of competition and neighbour suppression in Arabidopsis thaliana

Intraspecific competitive interactions can profoundly influence phenotypic evolution. However, prior studies have rarely evaluated the evolutionary potential of the two components of competitive ability, tolerance of competition and suppression of neighbours. Here, we grow a set of 20 Arabidopsis thaliana recombinant inbred lines in three competitive treatments (noncompetitive, intra‐genotypic competition and inter‐genotypic competition) to examine if there is genetic variation for the components of competitive ability and whether neighbour relatedness has an effect on fitness. We find evidence for genetic variation in tolerance of competition and neighbour suppression and that these two competitive strategies are correlated, such that genotypes that tolerate competition will also strongly suppress neighbours. We further observe that the effect of neighbour relatedness on fitness of target individuals depends on neighbour identity, i.e. whether target individuals perform better when competing against self vs. nonself individuals depends on the genotypic identity of the nonself neighbour. The results are particularly relevant to evolutionary responses under multi‐level selection.     

Stoichiometric differences in food quality: impacts on genetic diversity and the coexistence of aquatic herbivores in a <Emphasis Type="Italic">Daphnia</Emphasis> hybrid complex

The maintenance of genetic and species diversity in an assemblage of genotypes (clones) in the Daphnia pulex species complex (Cladocera: Anomopoda) in response to variation in the carbon:phosphorus ratio (quantity and quality) of the green alga, Scenedesmus acutus, was examined in a 90-day microcosm competition experiment. Results indicated that mixed assemblages of seven distinct genotypes (representing clonal lineages of D. pulex, D. pulicaria and interspecific hybrids) showed rapid loss of genetic diversity in all treatments (2 × 2 factorial design, high vs. low quantity, and high vs. low quality). However, the erosion of diversity (measured as the effective number of clones) was slowest under the poorest food conditions (i.e., low quantity, low quality) and by the conclusion of the experiment (90 days) had resulted in the (low, low) treatment having significantly greater genetic diversity than the other three treatments. In addition, significant genotype (clone) × (food) environment interactions were observed, with a different predominant species/clone found under low food quality versus high food quality (no significant differences were detected for the two food quantities). A clone of D. pulex displaced the other clones under low food quality conditions, while a clone of D. pulicaria displaced the other clones in the high food quality treatments. Subsequent life-history experiments were not sufficient to predict the outcome of competitive interactions among members of this clonal assemblage. Our results suggest that genetic diversity among herbivore species such as Daphnia may be impacted not only by differences in food quantity but also by those in food quality and could be important in the overall maintenance of genetic diversity in natural populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Intraspecific competitive interactions rapidly evolve via spontaneous mutations

Using a mechanistic spatially explicit trait-based neighborhood-model, we quantify the impact of mutations on intraspecific spatial interactions to better understand mechanisms underlying the maintenance of genetic variation and the potential effects of these evolved interactions on the population dynamics of Arabidopsis thaliana. We use 100 twenty-fifth generation mutation accumulation (MA) lines (genotypes) derived from one founder genotype to study mutational effects on neighbor responses in a field experiment. We created individual-based maps (15,000 individuals), including phenotypic variation, to quantify mutational effects within genotypes versus between genotypes on reproduction and survival. At small-scale (within 80 cm of the focal plant), survival is enhanced but seed-set is decreased when a genotype is surrounded by different genotypes. At large-scale (within 200 cm of the focal plant), seed set is facilitated by different genotypes while the same genotype has either no effect or negative effects. The direction of the interactions among MA lines suggests that at small scale these interactions may contribute to the maintenance of genetic variation and at large scale contribute to the survival of the population. This may suggest, that, mutations potentially have immediate effects on population and community dynamics by influencing the outcome of competitive and faciliatory interactions among conspecifics.

     

Recombinant hybrids retain heterozygosity at many loci: new insights into the genomics of reproductive isolation in Populus

The maintenance of species barriers in the face of gene flow is often thought to result from strong selection against intermediate genotypes, thereby preserving genetic differentiation. Most speciation genomic studies thus aim to identify exceptionally divergent loci between populations, but divergence will be affected by many processes other than reproductive isolation (RI) and speciation. Through genomic studies of recombinant hybrids sampled in the wild, genetic variation associated with RI can be observed in situ, because selection against incompatible genotypes will leave detectable patterns of variation in the hybrid genomes. To better understand the mechanisms directly involved in RI, we investigated three natural ‘replicate’ hybrid zones between two divergent Populus species via locus‐specific patterns of ancestry across recombinant hybrid genomes. As expected, genomic patterns in hybrids and their parental species were consistent with the presence of underdominant selection at several genomic regions. Surprisingly, many loci displayed greatly increased between‐species heterozygosity in recombinant hybrids despite striking genetic differentiation between the parental genomes, the opposite of what would be expected with selection against intermediate genotypes. Only a limited, reproducible set of genotypic combinations was present in hybrid genomes across localities. In the absence of clearly delimited ‘hybrid habitats’, our results suggest that complex epistatic interactions within genomes play an important role in advanced stages of RI between these ecologically divergent forest trees. This calls for more genomic studies that test for unusual patterns of genomic ancestry in hybridizing species.     

QUANTITATIVE VARIATION OF PROGENY FROM CHASMOGAMOUS AND CLEISTOGAMOUS FLOWERS IN THE GRASS DANTHONIA SPICATA

Progeny from chasmogamous (CH) and cleistogamous (CL) flowers of the grass Danthonia spicata were raised in their native habitat and in the greenhouse in order to determine how genetic variation was distributed among families and between CH and CL progeny within families. Twelve quantitative characters were measured on clones from individuals known to have arisen from either CH or CL flowers on a particular plant. Significant genetic variation existed for all characters measured. Most genetic variation was between families and two morphologically similar groups of families were identified. Relatively little genetic variation was found within families (approximately 5% of the total phenotypic variance). In field-raised plants, variance component analysis suggested that CL progeny were genetically more similar to each other than were CH progeny from the same plant. Levene's test of the average deviation of CH and CL progenies from their group means was nonsignificant but suggested there was a trend (0.05 < P < 0.10) for CH progeny to be more variable than CL progeny in the field but less variable in the greenhouse. The amount and distribution of genetic variation in the study population indicates that selective differentials would be larger among families than within families.     

羊草基因型多样性能增强种群对干扰的响应

研究了不同基因型多样性(1、3、6三种基因型组合)羊草种群的地上生物量、地下生物量、分蘖数、根茎芽数和根冠比5个指标对干扰强度(用不同留茬高度来模拟)的响应。结果表明:(1)基因型多样性和干扰强度对地上生物量、地下生物量、分蘖数和根茎芽数均有显著影响(P0.05),但两者的交互作用不显著(P0.05)。其中,多基因型组合(3、6基因型组合)羊草种群中这4个响应变量的值均显著高于单基因型羊草种群(P0.05);而干扰强度的增加显著降低了这4个响应变量的值(P0.05)。对于根冠比这一响应变量来说,仅干扰强度对其产生了显著地影响(P0.05)。(2)29个基因型多样性效应值中,有25个值大于0,其中12个表现为显著的基因型多样性正效应。依Loreau和Hector的方法将多样性净效应分解后发现,互补效应和选择效应共同主导12个响应指标的基因型多样性效应,而互补效应独自主导3个、选择效应独自主导5个响应指标的基因多样性效应;但对基因型多样性正效应起主要贡献的是互补效应。所得结果表明,基因型多样性能提高羊草种群的表现,能增强羊草种群对干扰的响应,不同基因型间的互补作用对这种正效应起主要贡献,这将为该物种种质资源保护和合理利用提供理论指导。     

Leaf litter decomposition differs among genotypes in a local <Emphasis Type="Italic">Betula pendula</Emphasis> population

Ecosystem processes, such as plant litter decomposition, are known to be partly genetically determined, but the magnitude of genetic variation within local populations is still poorly known. We used micropropagated field-grown saplings of 19 Betula pendula genotypes, representing genetic variation in a natural birch population, to examine (1) whether genotype can explain variation in leaf litter decomposition within a local plant population, and (2) whether genotypic variation in litter decomposition is associated with genotypic variation in other plant attributes. We found that a local B. pendula population can have substantial genotypic variation in leaf litter mass loss at the early stages of the decomposition process and that this variation can be associated with genotypic variation in herbivore resistance and leaf concentrations of soluble proteins and total nitrogen (N). Our results are among the first to show that fundamental ecosystem processes can be significantly affected by truly intraspecific genetic variation of a plant species.     

Seeking a sound index of competitive intensity: Application to the study of biomass production under elevated CO2 along a nitrogen gradient

Abstract The aim of this paper is to evaluate (i) the relevance of currently proposed measures of competitive intensity to elevated CO₂ studies by means of an example analysis, hypothesizing that competitive intensity is increased under elevated CO₂; and (ii) an alternative method for predicting species performance in mixtures from monocultures. Relative competition intensity (RCI), relative physiological performance and normalized ecological performance were used to characterize the competitive ability of two grasses (Danthonia richardsonii Cashmore, Phalaris aquatica L.) and two legumes (Lotus pedunculatus Cav., Trifolium repens L.) grown in monocultures and mixtures of the four species along a N gradient under conditions of ambient and elevated CO₂. Relative competition intensity could not be used to predict competitive outcomes in mixtures under conditions of elevated CO₂ because it failed to account for changes in the size of interspecific differences along the N gradient and between CO₂ concentrations. Relative physiological performance and relative ecological performance were more useful for investigating biomass production in mixtures and to predict species performance in mixtures from their performance in monocultures. Both indices of relative performance showed an increase in competitive intensity under elevated CO₂ conditions. They also showed a decrease in competitive intensity with increasing N supply over most of the range of N supply, but a reversal of that trend at high levels of N supply. The merits and utility of these relative performance indices for elevated CO₂ are discussed.