Interspecific Trait Differences Research Articles

A phylogenomic perspective on interspecific competition.

Evolutionary processes may have substantial impacts on community assembly, but evidence for phylogenetic relatedness as a determinant of interspecific interaction strength remains mixed. In this perspective, we consider a possible role for discordance between gene trees and species trees in the interpretation of phylogenetic signal in studies of community ecology. Modern genomic data show that the evolutionary histories of many taxa are better described by a patchwork of histories that vary along the genome rather than a single species tree. If a subset of genomic loci harbour trait-related genetic variation, then the phylogeny at these loci may be more informative of interspecific trait differences than the genome background. We develop a simple method to detect loci harbouring phylogenetic signal and demonstrate its application through a proof-of-principle analysis of Penicillium genomes and pairwise interaction strength. Our results show that phylogenetic signal that may be masked genome-wide could be detectable using phylogenomic techniques and may provide a window into the genetic basis for interspecific interactions.

Plant functional traits affect competitive vigor of pasture grasses during drought and following recovery

AbstractGrassland biomass production is strongly linked with the timing and intensity of precipitation events. While the direct effects of precipitation patterns on grasses are well‐studied, less is known regarding plant–plant interactions during different phases of drought (i.e., dry down vs. recovery). Here, we examined how the intensity and timing of drought affected biomass production, traits related to growth rate and competitive vigor (specific leaf area [SLA], leaf dry matter content [LDMC], and height [HT]), and competitive effects in three common pasture grasses. Each species was grown alone (one individual per 45‐L planter) or with competition (one individual of each species per 45‐L planter) under three different drought types: (1) “short‐term” drought where water was withheld until the first species reached stomatal closure; (2) “prolonged” drought where water was withheld until all three species reached stomatal closure; and (3) “repeated” short‐term drought where water was withheld until the first species reached stomatal closure, plants were rewatered to capacity, and then, the drought was repeated. In all three drought types, replicates were assessed for biomass and traits pre‐ and post‐rewatering to represent the “resistance” and “recovery” phases of drought, respectively. Overall, we found (1) competitive interactions during phases of drought were primarily mediated by plant HT and LDMC, not SLA; (2) the severity and frequency of drought were key factors in describing plant–plant interactions during phases of drought; and (3) interspecific differences in traits and trait responses to drought phases were key in predicting plant–plant competition. Such shifts in competition interactions associated with interspecific trait responses to drought and during recovery from drought are likely to have significant effects on the productivity and composition of multispecies, grass‐dominated plant communities.

Importance and benefit of incorporating the responses of species mean trait values in trait-based community assembly

Species mean trait values are the common currency of current trait-based community ecology, in which species are approximated by a set of constant mean trait values and are assumed to be passively selected by abiotic and biotic filters according to these values. However, species mean trait values can respond to both species interactions and the environmental context. Do such responses in traits subsequently alter the effect of filters on communities? Can we benefit by incorporating the responses of species mean trait values in trait-based community assembly? We tested the responses of mean values of four key functional traits to reduced light and interspecific competition in a two-year common garden experiment, using 1,584 seedlings of 11 tree species. We quantified the changes in interspecific trait differences, community-weighted means (CWM), and functional diversity (FD) and compared these to their corresponding changes under the assumption of constant mean trait values. Species mean values of most examined traits underwent substantial changes (mean = 18 ± 3%) to shading and interspecific competition treatments, but the direction and extent of these responses varied among traits and species, resulting in significant changes in interspecific trait differences, CWM and FD. Considering these trait responses largely improved our ability to identify these effects of low light and interspecific competition on CWM and FD. Further, we showed that strong abiotic filters could promote trait divergence if trait responses were considered, but that this pattern would be obscured if species mean trait values were considered as constant. Consequently, incorporating the responses of species mean trait values, e.g. using multiple mean values for a trait of a species in a heterogeneous environment, into the current trait-based community ecology offers us a more accurate, sensitive, and time-saving way to identify the underlying ecological mechanisms.

Substrate quality regulates density loss, cellulose degradation and nitrogen dynamics in downed woody debris in a boreal forest

Quantifying forest carbon (C) and nitrogen (N) turnover rates requires better understanding of the mechanisms of decomposition of coarse woody debris (CWD). We examined the dynamics of bulk density, C, N, cellulose and lignin in downed logs, taking into account initial interspecific trait differences in bark and wood, in a 66-year-long decomposition chronosequence in an old-growth mixed boreal forest.Wood and bark density decreased with increasing decay class and time since tree death. The C concentration per mass did not change or slightly decreased. Nitrogen increased in wood faster than in bark. In aspen wood, it increased two times more intensively than in wood of conifers. The C/N ratio and the cellulose content decreased, whereas the lignin content increased, decreased or remained stable in wood depending on tree species and calculation basis. Spruce wood was characterized by the greatest cellulose degradation rates. The N availability positively influenced wood bulk density loss and cellulose degradation in wood of all tree species, as well as the C loss in conifer bark.The total C amount in bark and wood of individual downed logs decreased. The total N amount decreased in bark and increased in wood with different rates depending on tree species identity.The results indicate the importance of accounting for the CWD tissue tree species specific traits in decomposition studies. In old-growth boreal forests, downed logs contribute significantly to long-term C and N sequestration.

Intraspecific variability of specific leaf area fosters the persistence of understorey specialists across a light availability gradient.

Forest understorey plants are sensitive to light availability, and different species groups can respond differently to changing light conditions. A plant trait tightly linked to light capture is specific leaf area (SLA). Studies considering the relative role of within- and among-species SLA variation across different species groups (e.g. specialists and generalists) are rarely implemented in temperate forest understories varying in their maturity. We examined community-level SLA patterns of beech forest understories along a light availability gradient, and for habitat specialists and generalists separately. We then disentangled and quantified the contribution of intraspecific trait variability and interspecific trait differences in shaping SLA patterns. We revealed that the increase in community-level SLA with decreasing light availability was primarily driven by beech forest specialists (and, to a lesser extent, by forest generalists), and this pattern was mainly determined by specialists' high intraspecific variability. Community-level SLA was therefore formed by different responses at different organizational levels, i.e. within and among species, and for separate species groups. This study provides insights into factors shaping the shade tolerance strategy in beech forest understorey plants; specialists persistence under putative less favourable conditions (i.e. high irradiation) may be fostered by their ability to adjust their light capture strategies intraspecifically.

Intestinal microbes: an axis of functional diversity among large marine consumers.

Microbes are ubiquitous throughout the world's oceans, yet the manner and extent of their influence on the ecology and evolution of large, mobile fauna remains poorly understood. Here, we establish the intestinal microbiome as a hidden, and potentially important, ‘functional trait’ of tropical herbivorous fishes—a group of large consumers critical to coral reef resilience. Using field observations, we demonstrate that five common Caribbean fish species display marked differences in where they feed and what they feed on. However, in addition to space use and feeding behaviour—two commonly measured functional traits—we find that interspecific trait differences are even more pronounced when considering the herbivore intestinal microbiome. Microbiome composition was highly species specific. Phylogenetic comparison of the dominant microbiome members to all known microbial taxa suggest that microbiomes are comprised of putative environmental generalists, animal-associates and fish specialists (resident symbionts), the latter of which mapped onto host phylogeny. These putative symbionts are most similar to—among all known microbes—those that occupy the intestines of ecologically and evolutionarily related herbivorous fishes in more distant ocean basins. Our findings therefore suggest that the intestinal microbiome may be an important functional trait among these large-bodied consumers.

Leaf functional traits at home and abroad: A community perspective of sycamore maple invasion

Understanding the mechanisms behind the successful invasion of alien plant species remains a significant research challenge in invasion ecology. There are surprisingly few cross-continental comparative studies which investigated interspecific trait differences between native and invaded ranges. Here, we compare leaf functional traits of sycamore (Acer pseudoplatanus L.), a deciduous tree species native to Europe and invasive in New Zealand (NZ), to co-occurring resident woody species in both the native and invaded ranges. We analyze a suite of 14 physiological traits on individuals growing either in shade or in sun light conditions. Despite similar environmental conditions, the leaf economics spectrum of forest communities is narrower in NZ than in France. Sycamore extends the leaf economics spectrum of the recipient community towards greater photosynthetic efficiency in the invaded range at a relatively low construction cost. This feature increases the competitive advantage of sycamore over native species. Biogeographic history likely has resulted in a native insular temperate forest community in NZ with unoccupied trait space along the leaf economics spectrum, making these communities susceptible to invasion by ‘pre-adapted’ alien species, according to the Evolutionary Imbalance Hypothesis. Sycamore may thus successfully invade NZ evergreen forests simply because it fills a functional gap at the fast return end of the leaf economics spectrum, corresponding to a formerly empty niche. As a consequence, forest management can hardly extirpate the invasive alien tree.

Trait hierarchies and intraspecific variability drive competitive interactions in Mediterranean annual plants

Abstract Both intra‐ and interspecific differences in traits may modulate interactions between plants. Two mechanisms are hypothesized to regulate these effects: competitive hierarchies and trait dissimilarities, but it is unclear how the prevalence of each might depend on environmental conditions and on intra and interspecific differences. We sowed six replicates of all possible pairwise combinations across eight annual species (including conspecific competition and individuals without competitors), in pots under two levels of fertilization. We measured above‐ and below‐ground traits and total biomass of the focal individuals. We expressed competition as the decrease in biomass of focal individuals relative to biomass without competition, and modelled competition using directional (hierarchy) or non‐directional (dissimilarity) trait differences, with or without intraspecific variability (ITV). We found evidence of different trait hierarchies operating under different fertilization conditions but little support for trait dissimilarities. The negative effect of competition on focal plants increased with the height of the competitors in both of fertilization levels. Further, in unfertilized conditions, plants with lower specific leaf area (SLA) and larger root systems experienced less competition. Including ITV in trait hierarchies substantially improved our ability to predict the intensity of competition. This was partly due to ITV reducing competitive hierarchies. Synthesis. Our results underscore the importance of traits in driving interactions among plants. Competitive relationships between species depend on complex interactions between trait intra and interspecific differences and resource availability. ITV appears to be a mechanism capable of reducing trait hierarchies, and hence the intensity of competition between coexisting plants.

Intraspecific Variation in Worker Body Size Makes North American Bumble Bees (Bombus spp.) Less Susceptible to Decline.

Population declines have been documented in approximately one-third of bumble bee species. Certain drivers of these declines are known; however, less is known about the interspecific trait differences that make certain species more susceptible to decline. Two traits that have implications for responding to rapidly changed environments may be particularly consequential for bumble bee populations: intraspecific body size variation and brain size. Bumble bee body size is highly variable and is likely adaptive at the colony level, and brain size correlates with cognitive traits (e.g., behavioral plasticity) in many groups. Trait variation and plasticity may buffer species against negative effects of rapidly changed environments. Using phylogenetically controlled analyses of 31 North American bumble bee species, we find that higher intraspecific body size variation is associated with species having increased their relative abundance over time. However, this variation does not significantly interact with tongue length, another trait thought to influence bees' decline susceptibility. Head size, a proxy for brain size, is not correlated with change in relative abundance. Our results support the hypothesis that variation in body size makes species less susceptible to decline in rapidly altered environments and suggests that this variation is important to the success of bumble bee populations.

Molecular diet analysis finds an insectivorous desert bat community dominated by resource sharing despite diverse echolocation and foraging strategies.

Interspecific differences in traits can alter the relative niche use of species within the same environment. Bats provide an excellent model to study niche use because they use a wide variety of behavioral, acoustic, and morphological traits that may lead to multi‐species, functional groups. Predatory bats have been classified by their foraging location (edge, clutter, open space), ability to use aerial hawking or substrate gleaning and echolocation call design and flexibility, all of which may dictate their prey use. For example, high frequency, broadband calls do not travel far but offer high object resolution while high intensity, low frequency calls travel further but provide lower resolution. Because these behaviors can be flexible, four behavioral categories have been proposed: (a) gleaning, (b) behaviorally flexible (gleaning and hawking), (c) clutter‐tolerant hawking, and (d) open space hawking. Many recent studies of diet in bats use molecular tools to identify prey but mainly focus on one or two species in isolation; few studies provide evidence for substantial differences in prey use despite the many behavioral, acoustic, and morphological differences. Here, we analyze the diet of 17 sympatric species in the Chihuahuan desert and test the hypothesis that peak echolocation frequency and behavioral categories are linked to differences in diet. We find no significant correlation between dietary richness and echolocation peak frequency though it spanned close to 100 kHz across species. Our data, however, suggest that bats which use both gleaning and hawking strategies have the broadest diets and are most differentiated from clutter‐tolerant aerial hawking species.

Functional groups differ in trait means, but not in trait plasticity to species richness in local grassland communities.

Despite growing interest in incorporating intraspecific variation of functional traits in community-level studies, it remains unclear whether species classified into functional groups based on interspecific trait differences are similar regarding their variation in trait expression in response to varying plant diversity and composition in local communities. In a large biodiversity experiment (Jena Experiment) designed on a trait-based a priori definition of functional groups (grasses, legumes, small herbs, tall herbs), we studied means, extent of variation (coefficient of variation across communities) and plasticity to increased plant diversity (slopes over a logarithmic species richness ranging from 1, 2, 4, 8 and 16 to 60 species) for nine functional traits. Species means and extent of variation in traits related to nitrogen (N) acquisition and N use differed among functional groups and were more similar in phylogenetically closely related species than expected by chance. Species in the same functional group showed a weak phylogenetic signal and varied widely in means and extent of variation in traits related to shoot architecture and to a smaller extent in leaf traits related to carbon acquisition. This indicated that functional groups were less distinguishable in light than in nitrogen acquisition strategies. The direction and degree of trait plasticity to increasing species richness did not show a phylogenetic signal and were not different among functional groups, but varied largely among species within functional groups. Correlation structures in trait means, extent of trait variation and trait plasticity revealed functional tradeoffs in the acquisition of nitrogen and light across species. While correlations between trait means and extent of trait variation varied from trait to trait (positive, negative or unrelated), trait means and trait plasticity were mostly unrelated. Our results suggest that the concept of functional groups is viable, but context-specific trait measurements are required to improve our understanding about the functional significance of intraspecific trait variation and interspecific trait differences in local plant communities.

Interspecific trait differences rather than intraspecific trait variation increase the extent and filling of community trait space with increasing plant diversity in experimental grasslands

Niche differentiation among species is supposed to be reflected in the distribution of trait values in communities. Yet, how species-specific trait expression responds to changing plant diversity and how these changes affect community-level trait distributions is less clear. We used monocultures and mixtures of a large biodiversity experiment (Jena Experiment) to evaluate this question. We applied additive partitioning of net biodiversity effects (NE) into complementarity (CE) and selection effects (SE) to community-weighted trait means (CWM). Between 60–100% of NEs on CWMs (i.e. their deviation from the abundance-weighted means of monoculture trait values) were explained by CEs indicating that trait plasticity of different species contributed to shifts in CWMs. Positive CEs for shoot-architectural traits, tissue elemental concentrations and specific leaf area (SLA) indicated on-average higher values of these traits in the mixtures, while negative SEs in these traits suggested that dominant species showed the opposite response to the mixture-environment. Trait plasticity in response to mixture growth conditions increased species-level trait deviation from CWMs in several traits related to structural stability and SLA, while species-level deviation in shoot length from CWMs decreased. Community-wide occupation of trait space (range and variance of trait values) increased with plant diversity. Trait plasticity in response to the mixture-environment reduced the occupied trait space in several size-related traits, while it increased trait space in SLA. Measures of trait-based niche differentiation did not change (kurtosis, i.e. “peakedness” of trait distribution) or decreased (standard deviation of nearest neighbour distances, SDNN) at increased species richness, indicating that plasticity did not reduce niche overlap. Our results suggest that fixed trait differences among species primarily explain the greater extent and denser filling of community trait space at increased plant diversity. Species-specific trait plasticity in response to increased plant diversity does not increase community-wide trait space or decrease niche overlap between species.

On the challenges of using field spectroscopy to measure the impact of soil type on leaf traits

Abstract. Understanding the causes of variation in functional plant traits is a central issue in ecology, particularly in the context of global change. Spectroscopy is increasingly used for rapid and non-destructive estimation of foliar traits, but few studies have evaluated its accuracy when assessing phenotypic variation in multiple traits. Working with 24 chemical and physical leaf traits of six European tree species growing on strongly contrasting soil types (i.e. deep alluvium versus nearby shallow chalk), we asked (i) whether variability in leaf traits is greater between tree species or soil type, and (ii) whether field spectroscopy is effective at predicting intraspecific variation in leaf traits as well as interspecific differences. Analysis of variance showed that interspecific differences in traits were generally much stronger than intraspecific differences related to soil type, accounting for 25 % versus 5 % of total trait variation, respectively. Structural traits, phenolic defences and pigments were barely affected by soil type. In contrast, foliar concentrations of rock-derived nutrients did vary: P and K concentrations were lower on chalk than alluvial soils, while Ca, Mg, B, Mn and Zn concentrations were all higher, consistent with the findings of previous ecological studies. Foliar traits were predicted from 400 to 2500 nm reflectance spectra collected by field spectroscopy using partial least square regression, a method that is commonly employed in chemometrics. Pigments were best modelled using reflectance data from the visible region (400–700 nm), while all other traits were best modelled using reflectance data from the shortwave infrared region (1100–2500 nm). Spectroscopy delivered accurate predictions of species-level variation in traits. However, it was ineffective at detecting intraspecific variation in rock-derived nutrients (with the notable exception of P). The explanation for this failure is that rock-derived elements do not have absorption features in the 400–2500 nm region, and their estimation is indirect, relying on elemental concentrations covarying with structural traits that do have absorption features in that spectral region (constellation effects). Since the structural traits did not vary with soil type, it was impossible for our regression models to predict intraspecific variation in rock-derived nutrients via constellation effects. This study demonstrates the value of spectroscopy for rapid, non-destructive estimation of foliar traits across species, but highlights problems with predicting intraspecific variation indirectly. We discuss the implications of these findings for mapping functional traits by airborne imaging spectroscopy.

Bumble bee species exhibit divergent responses to urbanisation in a Southern California landscape

1. Bees are ecologically important pollinators that are threatened by disease, habitat degradation, and habitat loss. Bee species that share ecological traits (e.g. body size, diet breadth, and sociality) may often respond similarly to environmental disturbance; however, few studies have examined the extent to which closely related, ecologically similar species exhibit divergent responses to the same forms of disturbance.2. In the present study, the responses of bumble bees (Apidae: genus Bombus) to urbanization are examined by combining field surveys with the quantification of local‐ and landscape‐level variables using Geographic Information Systems (GIS). During the spring and summer of 2012 and 2013, 57, bumble bees were surveyed in 1‐ha plots in San Diego County, California that spanned a gradient of urbanisation. Species‐level, occurrence (presence/absence) data and logistic regressions were then used to identify determinants of Bombus occurrence across this landscape.3. Approximately 1000 Bombus individuals belonging to three species were observed: B. californicus Smith, B. melanopygus Cresson, and B. vosnesenskii Radoszkowski. The occurrence of all Bombus species was negatively related to impermeable surface cover within study plots, but species responded uniquely to other plot‐scale variables and exhibited distinctive patterns of scale‐dependency with respect to impermeable surface cover surrounding plots.4. Urbanisation negatively affected all three focal bumble bee species in this study. Species‐specific responses to plot‐scale and landscape‐scale variables presumably reflect interspecific trait differences (e.g. body size, tongue length, and foraging behaviour). Unique responses to urbanisation caution against pooling species into functional groups based merely on taxonomic relationships or perceived ecological similarities.

The contribution of phenotypic plasticity to complementary light capture in plant mixtures.

Interspecific differences in functional traits are a key factor for explaining the positive diversity-productivity relationship in plant communities. However, the role of intraspecific variation attributable to phenotypic plasticity in diversity-productivity relationships has largely been overlooked. By taking a wheat (Triticum aestivum)-maize (Zea mays) intercrop as an elementary example of mixed vegetation, we show that plasticity in plant traits is an important factor contributing to complementary light capture in species mixtures. We conceptually separated net biodiversity effect into the effect attributable to interspecific trait differences and species distribution (community structure effect), and the effect attributable to phenotypic plasticity. Using a novel plant architectural modelling approach, whole-vegetation light capture was simulated for scenarios with and without plasticity based on empirical plant trait data. Light capture was 23% higher in the intercrop with plasticity than the expected value from monocultures, of which 36% was attributable to community structure and 64% was attributable to plasticity. For wheat, plasticity in tillering was the main reason for increased light capture, whereas for intercropped maize, plasticity induced a major reduction in light capture. The results illustrate the potential of plasticity for enhancing resource acquisition in mixed stands, and indicate the importance of plasticity in the performance of species-diverse plant communities.

Experimentally reduced root-microbe interactions reveal limited plasticity in functional root traits in Acer and Quercus.

Interactions between roots and soil microbes are critical components of below-ground ecology. It is essential to quantify the magnitude of root trait variation both among and within species, including variation due to plasticity. In addition to contextualizing the magnitude of plasticity relative to differences between species, studies of plasticity can ascertain if plasticity is predictable and whether an environmental factor elicits changes in traits that are functionally advantageous. To compare functional traits and trait plasticities in fine root tissues with natural and reduced levels of colonization by microbial symbionts, trimmed and surface-sterilized root segments of 2-year-old Acer rubrum and Quercus rubra seedlings were manipulated. Segments were then replanted into satellite pots filled with control or heat-treated soil, both originally derived from a natural forest. Mycorrhizal colonization was near zero in roots grown in heat-treated soil; roots grown in control soil matched the higher colonization levels observed in unmanipulated root samples collected from field locations. Between-treatment comparisons revealed negligible plasticity for root diameter, branching intensity and nitrogen concentration across both species. Roots from treated soils had decreased tissue density (approx. 10-20 %) and increased specific root length (approx. 10-30 %). In contrast, species differences were significant and greater than treatment effects in traits other than tissue density. Interspecific trait differences were also significant in field samples, which generally resembled greenhouse samples. The combination of experimental and field approaches was useful for contextualizing trait plasticity in comparison with inter- and intra-specific trait variation. Findings that root traits are largely species dependent, with the exception of root tissue density, are discussed in the context of current literature on root trait variation, interactions with symbionts and recent progress in standardization of methods for quantifying root traits.

Intraspecific trait variability mediates the response of subalpine grassland communities to extreme drought events

Summary Climate change is expected to increase the magnitude and the frequency of extreme climatic events such as droughts. Better understanding how plant communities will respond to these droughts is a major challenge. We expect the response to be a shift in functional trait values resulting from both species turnover and intraspecific trait variability, but little research has addressed the relative contribution of both components. We analysed the short‐term functional response of subalpine grassland communities to a simulated drought by focusing on four leaf traits (LDMC: leaf dry matter content, SLA: specific leaf area, LNC: leaf nitrogen concentration and LCC: leaf carbon concentration). After evaluating species turnover and intraspecific variability separately, we determined their relative contribution in the community functional response to drought, reflected by changes in community‐weighted mean traits. We found significant species turnover and intraspecific variability, as well as significant changes in community‐weighted mean for most of the traits. The relative contribution of intraspecific variability to the changes in community mean traits was more important (42–99%) than the relative contribution of species turnover (1–58%). Intraspecific variability either amplified (for LDMC, SLA and LCC) or dampened (for LNC) the community functional response mediated by species turnover. We demonstrated that the small contribution of species turnover to the changes in community mean LDMC and LCC was explained by a lack of covariation between species turnover and interspecific trait differences. Synthesis. These results highlight the need for a better consideration of intraspecific variability to understand and predict the effect of climate change on plant communities. While both species turnover and intraspecific variability can be expected following an extreme drought, we report new evidence that intraspecific variability can be a more important driver of the short‐term functional response of plant communities.

Plasticity influencing the light compensation point offsets the specialization for light niches across shrub species in a tropical forest understorey

Summary Shade tolerance can be defined as the light level at which plants can survive and possibly grow. This light level is referred to as the whole‐plant light compensation point (LCP). The LCP depends on multiple leaf and architectural traits. We are still uncertain how often interspecific trait differences allow species to specialize for separate light niches, as observed between shade‐tolerant species and light‐demanding species. Alternatively, trait plasticity may allow many species to grow in similar light conditions. We measured leaf and architectural traits of up to 1.5‐year‐old seedlings of 15 sympatric Psychotria shrub species grown at three light levels. We used a 3D plant model to estimate the impacts of leaf traits, architectural traits and plant size on the whole‐plant light compensation point (LCPplant). Plant growth rates were estimated from destructive harvests and allometric relationships. At lower light levels, plants of all species achieved a lower leaf light compensation point (LCPleaf). The light interception efficiency (LIE), an index of self‐shading, decreased with increasing plant size and was therefore lower in high‐light treatments where plants grew more rapidly. When corrected for size, LIE was lower in the low‐light treatment, possibly as a result of lower investments in woody support. Species did not show trade‐offs in growth under low‐ and high‐light conditions, because species with the greatest plasticity in LCPplant and underlying traits (LCPleaf and LIE) achieved the highest growth rates at lower light levels. Synthesis. The interspecific differences in LCPplant did not result in a growth or survival trade‐off between low‐ and high‐light conditions. Instead, these differences were more than offset by the greater plasticity in LCPplant in some species, which was driven by greater plasticity in both leaves and architecture. The most plastic species achieved the fastest growth at different light levels. The results show that plasticity largely neutralizes the separation of light niches amongst species in this forest understorey genus and imply that differential preferences of species for either gaps or forest understorey occur in later life phases or are driven by other stress factors than low light alone.

Plastic and constant developmental traits contribute to adaptive differences in co‐occurringPolygonumspecies

Adaptive differences among species are often thought to result from developmentally constant trait differences that enhance fitness in alternative environments. Species differences in patterns of individual phenotypic plasticity can also have ecological consequences. Indeed, functionally related constant and plastic traits may interact to determine the phenotype's adaptive value in particular conditions. We compared juvenile shade avoidance traits (height and its components, internode length and node number) across two field density treatments inPolygonumpersicariaandP. hydropiper, annual plant species that co‐occur in pastures comprised of a mosaic of plant densities. We used selection analyses to test trait contributions to fitness in alternative density treatments. Seedlings of both species expressed plasticity for internode elongation in response to density;P. persicariaplants increased internode length and consequently height significantly more in high density than did those ofP. hydropiper. As predicted by the shade avoidance hypothesis, increased height was adaptive for both species in high density stands, soP. persicariaplants had higher fitness in this environment. By contrast, node numbers were relatively constant across density treatments in both species:P. hydropiperseedlings consistently produced more nodes than did those ofP. persicaria. This constant trait difference contributed toP. hydropiper'sgreater relative fitness at low density, where more nodes and hence leaves enable plants to better exploit available light. Differences between species in these juvenile shade‐avoidance traits did not result from the evolutionary constraints of lack of heritable variation or costs of plasticity. We discuss how these interspecific trait differences may have been generated by divergent selective histories resulting from differences in herbivore resistance. These results illustrate how adaptive differences in both plastic and constantly expressed traits may jointly contribute to ecological distribution, including coexistence in patchy habitats.