Invaded grassland communities have altered stability‐maintenance mechanisms but equal stability compared to native communities

Theory predicts that stability should increase with diversity via several mechanisms. We tested predictions in a 5‐year experiment that compared low‐diversity exotic to high‐diversity native plant mixtures under two irrigation treatments. The study included both wet and dry years. Variation in biomass across years (CV) was 50% lower in mixtures than monocultures of both native and exotic species. Growth among species was more asynchronous and overyielding values were greater during and after a drought in native than exotic mixtures. Mean‐variance slopes indicated strong portfolio effects in both community types, but the intercept was higher for exotics than for natives, suggesting that exotics were inherently more variable than native species. However, this failed to result in higher CV's in exotic communities because species that heavily dominated plots tended to have lower than expected variance. Results indicate that diversity‐stability mechanisms are altered in invaded systems compared to native ones they replaced.     

Effects of plant species richness on stand structure and productivity

Aims Aboveground biomass production commonly increases with species richness in plant biodiversity experiments. Little is known about the direct mechanisms that cause this result. We tested if by occupying different heights and depths above and below ground, and by optimizing the vertical distribution of leaf nitrogen, species in mixtures can contribute to increased resource uptake and, thus, increased productivity of the community in comparison with monocultures.Methods We grew 24 grassland plant species, grouped into four nonoverlapping species pools, in monoculture and 3- and 6-species mixture in spatially heterogeneous and uniform soil nutrient conditions. Layered harvests of above- and belowground biomass, as well as leaf nitrogen and light measurements, were taken to assess vertical canopy and root space structure.Important findings The distribution of leaf mass was shifted toward greater heights and light absorption was correspondingly enhanced in mixtures. However, only some mixtures had leaf nitrogen concentration profiles predicted to optimize whole-community carbon gain, whereas in other mixtures species seemed to behave more 'selfish'. Nevertheless, even in these communities, biomass production increased with species richness. The distribution of root biomass below ground did not change from monocultures to three- and six-species mixtures and there was also no indication that mixtures were better than monocultures at extracting heterogeneously as compared to homogeneously distributed soil resources. We conclude that positive biodiversity effect on aboveground biomass production cannot easily be explained by a single or few common mechanisms of differential space use. Rather, it seems that mechanisms vary with the particular set of species combined in a community.     

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.     

More plant biomass results in more offspring production in annuals,or does it?

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi‐species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size‐related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between‐species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation.     

Interspecific interactions and biomass allocation among grassland plant species

Although a handful of studies have shown how interspecific interactions may influence plant shoot to root ratios, the issue of how these interactions influence biomass partitioning among coexisting plant species remains largely unexplored. In this study, we determined whether a given plant species could induce other plant species to allocate relative biomass to each of four zones (aboveground, and three soil depth layers) in a different manner to what they would otherwise, and whether this may influence the nature of competitive or facilitative interactions amongst coexisting plant species. We used a glasshouse study in which mixtures and monocultures of ten grassland plant species were grown in cylindrical pots to determine the effects of plant species mixtures versus monocultures on the production of shoots and of roots of other species for each of three soil depths. Across all experiments, stimulation of production in mixtures was far less common than suppression of production. Different plant species shifted their allocation to shoots or roots at different depths, suggesting that interspecific interactions can either: (1) increase the ratio of deep to shallow roots, perhaps because competition reduces root growth in the uppermost part of the soil profile; or (2) decrease this ratio by reducing plant vigour to such an extent that the plant cannot produce roots that can reach deep enough to exploit resources at lower depths. Further, these results suggest that there are instances in which competition may have the potential to enforce resource partitioning between coexisting plant species by inducing different species to root at different depths to each other.     

Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities

In many systems, native communities are being replaced by novel exotic-dominated ones. We experimentally compared species diversity decline between nine-species grassland communities under field conditions to test whether diversity maintenance mechanisms differed between communities containing all exotic or all native species using a pool of 40 species. Aboveground biomass was greater in exotic than native plots, and this difference was larger in mixtures than in monocultures. Species diversity declined more in exotic than native communities and declines were explained by different mechanisms. In exotic communities, overyielding species had high biomass in monoculture and diversity declined linearly as this selection effect increased. In native communities, however, overyielding species had low biomass in monoculture and there was no relationship between the selection effect and diversity decline. This suggests that, for this system, yielding behaviour is fundamentally different between presumably co-evolved natives and coevolutionarily naive exotic species, and that native-exotic status is important to consider.     

Interspecific competitive interactions between pairs of fungal species in natural substrates

The role of interspecific competition in fungal communities in natural substrates is poorly understood because fungi do not form easily definable populations. A new approach to investigating fungal competition, using natural substrates containing a range of known biomass concentrations of each of two species, is described. Relative competitive success of each species is assessed over time in terms of propagule production and substrate colonisation by each species. In an agricultural soil Mucor hiemalis usually out-competed Trichoderma harzianum. After 27 days, the success of both species in the mixtures was independent of the initial biomass concentration of either species, although the success of T. harzianum in these mixtures was substantially inhibited relative to the T. harzianum monocultures. In a forest soil, T. polysporum maintained a competitive advantage over M. hiemalis, and induced M. hiemalis to produce propagules rather than mycelia. Coexistence of both species always occurred in both experiments, and in the forest soil experiment the two-species mixtures all contained a higher total microbial biomass than the monocultures of either species by day 47, suggesting some niche differentiation.     

Are invasive plant species better competitors than native plant species? – evidence from pair‐wise experiments

Invasive plants often appear to be more competitive than native species, but there have been few tests of this hypothesis. We reviewed published pair-wise experiments between invading and native plant species. Although the designs that have been used allow only limited inferences, the available data suggest that the effect of invasive species on native species is usually stronger than vice versa. Furthermore, mixtures of invasive and native species are generally less productive than monocultures of the native species, but not less than monocultures of the invasive species. However, the selection of invaders and natives for study has not been random, and the data could be biased towards highly competitive invaders and natives that are weaker than average competitors. We attempt to clarify confusion surrounding the concept of competitive superiority in the context of plant invasions, and we discuss the limitations of the methods that have been used to investigate competition between invasive and native species. To rigorously test the generality of the hypothesis that invaders are better competitors than natives we need to compare the effects of closely related native and invasive species on each other. We suggest that the influence of an invading species on total plant community biomass is an important clue in understanding the role of competition in a plant invasion. The role of competition in the establishment and naturalization stages of the invasion process may be very different from its role in the "outbreak" stage.     

Plant biomass production and soil nitrogen in mixtures and monocultures of old field Mediterranean annuals

We examine the relationship between plant diversity and ecosystem properties in a Mediterranean grassland. Five legumes, three grasses and two forb species are grown in monocultures and compared with mixtures that include these ten species. Trifolium angustifolium L. (a legume), Lolium rigidum Gaudin (a grass), and Centaurea solstitialis L. (a forb), are replicated in monocultures. Plant cover, root length and biomass, and concentrations of soil nitrate and ammonium are measured in all plots in March and May. Aboveground biomass is measured at a final harvest in late May to early June. Root biomass is significantly higher in the species mixtures than the average of the monocultures. Plant cover and root length are marginally significantly higher (0.05 < P ≤ 0.1) in the mixtures compared to the average of the monocultures. Soil inorganic nitrogen concentrations and aboveground biomass do not significantly differ between the average of the monocultures and the mixtures. Aboveground biomass in T. angustifolium monocultures is significantly higher than in the mixtures, and on average the legume monocultures do not differ significantly from the mixtures. Root length and biomass in L. rigidum monocultures are higher than in the mixtures in March. Nitrate concentrations (which are negatively correlated with root length and biomass) are the lowest in C. solstitialis in May. Thus, we have evidence that some of the measures of ecosystem performance decline in the average of the monocultures when compared with the mixtures, but mixtures never outperform or do more poorly than the best performing monocultures.     

Rapid biodiversity declines in both ungrazed and intensely grazed exotic grasslands

Exotic-dominated ecosystems with low diversity are becoming increasingly common. It remains unclear, though, whether differences between native and exotic species (driver model), or changes in disturbances or resources (passenger model), allow exotics to become competitive dominants. In our field experiment, plant species origin (native or exotic), cattle grazing (ungrazed or intensely grazed once), and species composition treatments were fully crossed and randomly assigned to four-species mixtures and monocultures of grassland plants. We found that biodiversity declined more rapidly in exotic than in native species mixtures, regardless of our grazing disturbance treatment. Early declines in species evenness (i.e., increases in dominance) led to subsequent declines in species richness (i.e., local extinctions) in exotic mixtures. Specifically, Simpson’s diversity was 29% lower after 1 year, and species richness was 15% lower after 3 years, in exotic than in native mixtures. These rapid biodiversity declines in exotic mixtures were partly explained by decreased complementarity (i.e., niche partitioning and facilitation), presumably because exotic species lack the coevolutionary history that can lead to complementarity and coexistence in native communities. Thus, our results suggest that exotic species can drive biodiversity declines in the presence or absence of a grazing disturbance, partly because exotic species interactions differ from native species interactions. This implies that restoring plant biodiversity in grasslands may require removal of exotic species, in addition to disturbance management.     

Departure from naturalized to invasive stage: a disturbance-induced mechanism and associated interacting factors

Aims Within a habitat of multiple plant species, increased resource availabilities and altered species abundances following disturbances create opportunities for exotic species to successfully establish and subsequently naturalize into its non-native environment. Such post-disturbance changes in abiotic and biotic environments may also promote a naturalized exotic species (or invading species) to become invasive through rapid colonization of the habitat sites by reducing the extent and size of resident plant species. By combining species life history traits with that of the disturbance-induced changes in habitat characteristics, we aimed to determine those interacting factors and associated mechanism allowing an exotic invasion to start off.Methods We used a modified version of the classic competition–colonization (CC) model which was formulated first by Hastings (1980) and studied later by Tilman (1994) to explain spatial coexistence of multiple species. Within this model framework, recruitment-limited spatial competition has explicitly been linked with interspecific resource competition without altering the basic assumptions and structure of the original CC model.Important findings The model results showed that at a constant rate of resource supply, invading species can stably coexist with native species via trade-offs between species competitive ability and colonizing ability. On the other hand, the model predicted that with a fluctuating resource condition, invading species can successfully invade a habitat following continuous reductions in the size and extent of native species. Whether or not invading species holds competitive superiority over the native species for limiting resource, we showed that there exists a range of variation in available resource that allows an exotic invasion to start off in post-disturbance habitat. The associated disturbance-induced mechanism promoting invading species to become invasive has been identified. It states that occurrences of disturbances such as fire or clear-cutting influence variation in resource availability, and in addition open up many vacant microsites; given these disturbance-induced changes, invading species with a higher rate of propagule production and with a higher survival rate of adults particularly in low-resource condition recruits microsites at faster rate relative to native competitor species, and with a given range of variation in resource availabilities, it maintains continued expansions following reductions in size and extent of native species. Moreover, we identified those interacting factors and their specific roles that drive this mechanism. These factors include propagule supply, variable resource level and vacant microsite availability. Increased availability of vacant microsites following disturbances creates an opportunity for rapid colonization. Given this opportunity, higher number of propagules supplied by the invading species enhances the rate of colonization success, whereas the resource variation within a range of given thresholds maintains enhanced colonization rate of the invading species while it depresses native competitor species. Owing to the each factor's invasion regulatory ability, controlling one or all of them may have strong negative impact on the occurrence of exotic invasion.     

Competition along a nutrient gradient: A case study with Daucus carota and Chenopodium album

A binary competition experiment between carrot (Daucus carota L.) and Chenopodium album L. was conducted in a greenhouse at seven population densities and eight nutrient concentrations to investigate the effects of a nutrient gradient on plant competition in both monocultures and mixtures. The patterns of carrot biomass allocation (measured as root : shoot ratio) in monocultures and mixtures with C. album were affected by both nutrient availability and population density. Chenopodium album had a broader response to nutrient concentrations than carrot. The maximum yield of carrot in both monocultures and mixtures occurred at fourfold the standard concentration of nutrients, while C. album in both monocultures and mixtures had the maximum yield at 16-fold the standard nutrient concentration. The yield–density relationship of carrot tended to be increasing or asymptotic at lower nutrient concentrations but parabolic at higher concentrations, whereas that of C. album was little affected by nutrient availability. Nutrient availability had a profound influence on the competitive relationships between the two species: at both low and high nutrient concentrations, C. album tended to be more competitive than carrot, while at intermediate levels of nutrients, carrot was more competitive than C. album. Our results suggest that in relation to competitive performance, the weed has a greater ability to adapt itself to varying environments than does the crop. Additionally, the relative merits of the quantitative measures of competitive ability are briefly discussed.     

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.     

植物竞争对3种移植树苗生长的影响

在植物幼苗生长过程中,总是受到包括地下根竞争在内的各种竞争影响。植物间的竞争主要同有效光辐射、水分和各种营养相关。当外来植物侵入森林群落时,可能受到群落中其它植物竞争的影响。该文通过移植尾叶桉(Eucalyptus urophylla)、大叶相思(Acacia auriculaeformis)两种外来种和本地种荷木(Schima superba)幼苗,挖沟排除根竞争和砍树创造林窗来排除地上竞争的野外实验,研究植物竞争对幼苗生长的影响。有根竞争时,荷木、尾叶桉和大叶相思幼苗的生物量和净初级生产力均小于没有根竞争时,可见地下根竞争对3种幼苗生长有抑制性影响。尾叶桉、大叶相思和荷木3种幼苗受到的总竞争强度分别是0.357 9、0.338 3和0.198 9,受到的地下根竞争强度则分别是0.104 3、0.053 04和0.118 8,受到的地上竞争强度则分别是0.285 1、0.277 0和0.090 85。尾叶桉和大叶相思两种幼苗受到的总竞争强度间的差异不显著,但都显著大于荷木;3种幼苗受到的地上竞争强度间的差异同总竞争强度情况相似;尾叶桉和荷木两种幼苗受到的地下根竞争强度间的差异不显著,但都显著大于大叶相思。地上竞争对阳性树种尾叶桉和大叶相思两种幼苗的生长影响大,而地下根竞争则对耐阴性强的荷木幼苗影响大。尾叶桉和大叶相思两种外来种幼苗受到的总竞争强度均大于本地种荷木幼苗,这反映了这两种外来树种侵入次生林这样的群落受到竞争影响大。     

紫外线B辐射对几种植物种间竞争的影响

对大田条件下增强的紫外线B(UV-B 280～315nm,约相当于15％臭氧层衰减)对小麦和野燕麦等4个种对的竞争性平衡的影响进行了研究．结果表明,对照和UV-B处理时小麦和野燕麦的密度制约死亡规律没有显著差异,相对较大的竞争压力加强了UV-B对这两个物种生物量降低的效应．UV-B辐射处理后,按单株生物量和地上部生物量。UV-B增强了小麦对野燕麦的竞争优势,但是以单株籽粒数及籽粒重为依据的k1-2值在紫外辐射处理后却下降．竞争性平衡的改变伴随着两者总生物量的显著下降,特别是在较高的密度条件下．紫外辐射对其它3个种对的竞争性平衡有着不同程度与方向上的影响．一般情况下UV-B使竞争性平衡向有利于单子叶植物的方向发展．这一结果暗示,竞争胁迫,特别是种间竞争对正确评估UV-B辐射增强对农田生态系统的影响是至关重要的．     

两种入侵植物与三种本地植物根系特征的比较研究

以菊科2种入侵植物飞机草(Chromolaena odorata (L.)R. M. King & H. Rob)和紫茎泽兰( Eupatorium adenophorum Speng)以及生活型相似的3种本地植物异叶泽兰(Eupatorium heterophyllum DC.)、佩兰(Eupa-torium for...     

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

Previous research has found that plant diversity declines more quickly in exotic than native grassland plots, which offers a model system for testing whether diversity decline is associated with specific plant traits. In a common garden experiment in the Southern Great Plains in central Texas, USA, we studied monocultures and 9-species mixtures of either all exotic or all native grassland species. A total of 36 native and exotic species were paired by phylogeny and functional group. We used community-level measures (relative abundance in mixture) and whole-plant (height, aboveground biomass, and light capture) and leaf-level traits (area, specific leaf area, and C:N ratio) to determine whether trait differences explained native-exotic differences in functional group diversity. Increases in species’ relative abundance in mixture were correlated with high biomass, height, and light capture in both native and exotic communities. However, increasing exotic species were all C₄ grasses, whereas, increasing native species included forb, C₃ grass and C₄ grass species. Exotic C₄ grasses had traits associated with relatively high resource capture: greater leaf area, specific leaf area, height, biomass, and light capture, but similar leaf C:N ratios compared to native C₄ grasses. Leaf C:N was consistently higher for native than exotic C₃ species, implying that resource use efficiency was greater in natives than exotics. Our results suggest that functional diversity will differ between grasslands restored to native assemblages and those dominated by novel collections of exotic species, and that simple plant traits can help to explain diversity decline.     

Allelopathy as a mechanism for the invasion of <Emphasis Type="Italic">Typha angustifolia</Emphasis>

The direct competitive effects of exotic plants on natives are among the leading causes of plant extinctions worldwide. Allelopathy, one type of direct plant competition, has received relatively little research, particularly in aquatic and wetland systems, even though allelopathy can be a potent mechanism through which plant communities are structured. Typha angustifolia (narrow-leaved cattail) is an invasive exotic plant in North America that often forms monocultures in disturbed wetlands and is more invasive than native members of its genus. We tested whether T. angustifolia was allelopathic and whether it produced different biochemicals than a native congener by growing it with the native bulrush Bolboschoenus fluviatilis (river bulrush) in soil with and without activated carbon and by qualitatively and quantitatively comparing soluble phenolics produced in the roots of T. angustifolia and the native Typha latifolia (broad-leaved cattail). T. angustifolia had a strong allelopathic effect on B. fluviatilis, reducing the longest leaf length and root, shoot, and total biomass of B. fluviatilis. When the allelopathy of T. angustifolia was ameliorated by activated carbon, however, longest leaf length, ramet number, root biomass, shoot biomass, and total biomass of T. angustifolia were greatly reduced due to resource competition with B. fluviatilis. Furthermore, T. angustifolia produced different, but not more, soluble phenolics than T. latifolia suggesting that the identity of the phenolics is different between the two species rather than the concentrations. The allelopathic effects of T. angustifolia on a North American native wetland plant and its production of root biochemicals that appear to differ from those produced by a native congener are consistent with the possibility that T. angustifolia may use a novel allelochemical in its invasion of North American wetlands.     

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.     

Light partitioning in experimental grass communities

Through complementary use of canopy space in mixtures, aboveground niche separation has the potential to promote species coexistence and increase productivity of mixtures as compared to monocultures. We set up an experiment with five perennial grass species which differed in height and their ability to compete for light to test whether plants partition light under conditions where it is a limiting resource, and if this resource partitioning leads to increased biomass production in mixtures (using relative yield-based methods). Further, we present the first application of a new model of light competition in plant communities. We show that under conditions where biomass production was high and light a limiting resource, only a minority of mixtures outperformed monocultures and overyielding was slight. The observed overyielding could not be explained by species differences in canopy structure and height in monoculture and was also not related to changes in the canopy traits of species when grown in mixture rather than monoculture. However, where overyielding occurred, it was associated with higher biomass density and light interception. In the new model of competition for light, greater light use complementarity was related to increased total energy absorption. Future work should address whether greater canopy space-filling is a cause or consequence of overyielding.