Common Garden Study Research Articles

Floral and mating system divergence in secondary sympatry: testing an alternative hypothesis to reinforcement in Clarkia.

Reproductive character displacement (RCD) is often an important signature of reinforcement when partially cross-compatible taxa meet in secondary sympatry. In this study, floral evolution is examined during the Holocene range expansion of Clarkia xantiana subsp. parviflora from eastern Pleistocene refugia to a western zone of sympatry with its sister taxon, subsp. xantiana. Floral divergence between the two taxa is greater in sympatry than allopatry. The goal was to test an alternative hypothesis to reinforcement - that floral divergence of sympatric genotypes is simply a by-product of adaptation to pollination environments that differ between the allopatric and sympatric portions of the subspecies' range. Floral trait data from two common garden studies were used to examine floral divergence between sympatric and allopatric regions and among phylogeographically defined lineages. In natural populations of C. x. parviflora, the magnitude of pollen limitation and reproductive assurance were quantified across its west-to-east range. Potted sympatric and allopatric genotypes were also reciprocally translocated between geographical regions to distinguish between the effects of floral phenotype versus contrasting pollinator environments on reproductive ecology. Sympatric populations are considerably smaller flowered with reduced herkogamy. Pollen limitation and the reproductive assurance value of selfing are greater in sympatric than in allopatric populations. Most significantly, reciprocal translocation experiments showed these differences in reproductive ecology cannot be attributed to contrasting pollinator environments between the sympatric and allopatric regions, but instead reflect the effects of flower size on pollinator attraction. Floral evolution occurred during the westward range expansion of parviflora, particularly in the zone of sympatry with xantiana. No evidence was found that strongly reduced flower size in sympatric parviflora (and RCD between parviflora and xantiana) is due to adaptation to limited pollinator availability. Rather, floral divergence appears to have been driven by other factors, such as interactions with congenerics in secondary sympatry.

Mechanisms underlying plant sexual dimorphism in multi‐trophic arthropod communities

A growing body of research documents the importance of plant genetic effects on arthropod community structure. However, the mechanisms underlying these effects are often unclear. Additionally, plant genetic effects have largely been quantified in common gardens, thus inflating the estimates of their importance by minimizing levels of natural variation. Using Valeriana edulis, a dioecious plant with genetically based sex determination, we conducted surveys and experiments on wild-grown individuals to document field patterns of arthropod association between the sexes and the mechanisms underlying these plant genetic effects. Three years of surveys revealed strong and consistent sex-biased arthropod association in wild-grown plants: female plants supported 4-fold, 1.5-fold, and 4-fold higher densities of aphids, aphid predators, and aphid-tending ants, respectively, compared to males. There was mixed evidence that the female bias for aphids was due to higher plant quality, while we found no difference between plant sexes in aphid preference or the top-down effects of predators and tending ants. Female bias for ants was due to both the greater attractiveness of female plants (direct effect mediated by floral nectar) and an independent, weaker effect of higher aphid abundance on females (density-mediated indirect effect). Conversely, the female bias for predators was driven solely by the greater attractiveness of female plants. We did not find interaction modification, i.e., ant-aphid and predator-aphid interactions were equivalent between plant sexes. Plant sex explained 0.24%, 2.28%, and 4.42% of the variance in aphids, predators, and ants, respectively, values comparable to but slightly weaker than those previously reported from common-garden studies. In contrast to the prediction of diminished plant genetic effects with increasing trophic level, we show how weak indirect effects on predators and parasitoids (via herbivores) can be complemented by strong direct effects via common plant traits (floral resources). In summary, we document direct and indirect effects of genetically based sex on a multi-trophic arthropod community that were expressed in wild-grown plants across multiple years.

Convergent effects of elevation on functional leaf traits within and among species

Summary Spatial variation in filters imposed by the abiotic environment causes variation in functional traits within and among plant species. This is abundantly clear for plant species along elevational gradients, where parallel abiotic selection pressures give rise to predictable variation in leaf phenotypes among ecosystems. Understanding the factors responsible for such patterns may provide insight into the current and future drivers of biodiversity, local community structure and ecosystem function. In order to explore patterns in trait variation along elevational gradients, we conducted a meta‐analysis of published observational studies that measured three key leaf functional traits that are associated with axes of variation in both resource competition and stress tolerance: leaf mass:area ratio (LMA), leaf nitrogen content per unit mass (Nmass) and N content per unit area (Narea). To examine whether there may be evidence for a genetic basis underlying the trait variation, we conducted a review of published results from common garden experiments that measured the same leaf traits. Within studies, LMA and Narea tended to decrease with mean annual temperature (MAT) along elevational gradients, while Nmass did not vary systematically with MAT. Correlations among pairs of traits varied significantly with MAT: LMA was most strongly correlated with Nmass and Narea at high‐elevation sites with relatively lower MAT. The strengths of the relationships were equal or greater within species relative to the relationships among species, suggesting parallel evolutionary dynamics along elevational gradients among disparate biomes. Evidence from common garden studies further suggests that there is an underlying genetic basis to the functional trait variation that we documented along elevational gradients. Taken together, these results indicate that environmental filtering both selects locally adapted genotypes within plant species and constrains species to elevational ranges based on their ranges of potential leaf trait values. If individual phenotypes are filtered from populations in the same way that species are filtered from regional species pools, changing climate may affect both the species and functional trait composition of plant communities.

Transgenerational effects of herbivory in a group of long‐lived tree species: maternal damage reduces offspring allocation to resistance traits, but not growth

Summary Numerous studies have explored plant strategies of resource allocation to growth and/or resistance traits within a single generation. In contrast, exceedingly little is known about whether such patterns hold across generations; that is, in seedlings of plants that experienced maternal herbivory. In a common garden study with clonally replicated genotypes of three cottonwood taxa (Populus angustifolia, Populus fremontii and their F1 hybrids), we examined transgenerational response to maternal herbivory in terms of half‐sibling seedling offspring (i) germination and growth and (ii) constitutive vs. transgenerational plastic allocation to resistance (measured as both phytochemical content and concentration). Two major results emerged. First, we found that taxa (and often genotypes within a taxon) significantly differed in their constitutive allocation to both growth and resistance. Fremont (P. fremontii) seedlings grew up to seven times more rapidly than did narrowleaf (P. angustifolia) seedlings and had higher or similar content of two key phytochemical resistance traits. Overall, this led to a dilution effect in Fremont relative to narrowleaf, whereby concentrations of two key phytochemical resistance traits were more than 50% lower. Secondly, maternal herbivory by cottonwood leaf beetle larvae on foliage adjacent to developing seeds did not significantly alter offspring growth, but did decrease offspring phytochemical content by 10–55% relative to offspring of maternal control (undamaged) trees. As a result, concentrations of offspring phytochemical resistance traits were reduced by 10–18% in seedlings with maternal herbivory, relative to maternal control seedlings, across all three taxa. These patterns suggest an allocational trade‐off, whereby maternal damage results in maintenance of offspring seed size and growth traits at the expense of phytochemical defences in the next generation. Synthesis: This is the first instance in which transgenerational effects of herbivory on growth and defence traits have been described in long‐lived, woody plant species. Populus differs substantially from herbaceous plant species or short‐lived animals in which transgenerational plasticity of resistance has been examined, in terms of life history (time from germination or hatching to reproductive maturity) and/or in the lag time between generations. These differences may influence the ecological and evolutionary relevance of transgenerational plasticity in defence.

Evolution of Competitive Ability within Lonicera japonica’s Invaded Range

Factors influencing invasive taxa may change during the course of an invasion. For example, intraspecific competition is predicted to be more important in areas with older stands of dense monospecific invaders than at the margins of an invaded range. We evaluated evolution in response to predicted changes in competition by comparing the intraspecific competitive ability of North American populations of Japanese honeysuckle (Lonicera japonica) from areas established 100–150 yr ago, which make up the core of the invaded range, to populations from the northern range margin, established in the past 50 yr. Plants from populations in the core and margin regions of the invaded range were grown in two common garden studies. The first had three levels of intraspecific competition, and vines differed in their access to a lattice for climbing. The second asked whether plants from older core or newer margin populations were better competitors. Intraspecific competition decreased the size of plants from both core and margin regions. Contrary to predictions, there was no increase in climbing shoot frequency or size in plants from denser core populations, although these did increase in the presence of a lattice structure. Also contrary to predictions, plants from the margin of the invaded range were larger and stronger competitors than those from the core, regardless of competition level. Enhanced intraspecific competitive ability of plants from low-density margin populations is likely driven by selection for more rapid growth rate in the north, rather than direct selection on competitive ability. Results counter to prediction highlight the importance of considering the broader selective context to understand evolution. Furthermore, this differentiation across the invaded range is likely to enhance L. japonica’s invasive potential.

Driftsel: an R package for detecting signals of natural selection in quantitative traits

Approaches and tools to differentiate between natural selection and genetic drift as causes of population differentiation are of frequent demand in evolutionary biology. Based on the approach of Ovaskainen et al. (2011), we have developed an R package (DRIFTSEL) that can be used to differentiate between stabilizing selection, diversifying selection and random genetic drift as causes of population differentiation in quantitative traits when neutral marker and quantitative genetic data are available. Apart from illustrating the use of this method and the interpretation of results using simulated data, we apply the package on data from three-spined sticklebacks (Gasterosteus aculeatus) to highlight its virtues. DRIFTSEL can also be used to perform usual quantitative genetic analyses in common-garden study designs.

Ecological variation in leaf biomechanics and its scaling with tissue structure across three mediterranean‐climate plant communities

Summary The mechanical resistance of leaves has key ecological implications but its basis has not been well understood, particularly at the tissue scale. We tested the hypotheses that leaf mechanical resistance should be a function of tissue density, increasing from the lamina to the midrib, and higher in drought‐tolerant than drought‐avoiding species. In a common garden study, we quantified nine leaf biomechanical traits, including measurements of material and structural resistance, and in addition 17 morphological traits, at the tissue and whole‐leaf scales, for 21 species from three semi‐arid communities of California, USA. The mechanical properties of leaves depended strongly on tissue density. Material resistance was significantly greater in the midrib than in the leaf lamina, and tissue resistances were significantly correlated among tissues, lower in deciduous coastal sage species and higher in evergreen drought‐tolerant chaparral species. The proportion of the biomass invested in the midrib was lower in species bearing midribs and laminas of high material resistance. Our results support the hypothesis of a hierarchical partitioning of leaf mechanical resistance among leaf tissues reflecting the investment of dry mass. Also, our data indicated a mechanical compensation in leaf design, where leaves with high material resistance and density deploy a relatively minor proportion of support tissue in the midrib. Finally, our results establish a quantitative basis for differences among communities in leaf biomechanics. Our results supported the classical characterization of the mediterranean‐climate flora of California according to the dramatic increase in the mean leaf mechanical resistance from species of coastal sage to chaparral, with diverse leaf types in the Mojave Desert species.

The Ghost of Outcrossing Past in Downy Brome, an Inbreeding Annual Grass

We investigated the frequency of outcrossing in downy brome (Bromus tectorum L.), a cleistogamous weedy annual grass, in both common garden and wild populations, using microsatellite and single nucleotide polymorphic (SNP) markers. In the common garden study, 25 lines with strongly contrasting genotypes were planted in close proximity. We fingerprinted 10 seed progeny from 8 individuals of each line and detected 15 first-generation heterozygotes for a t-value (corrected for cryptic crosses) of 0.0082. Different genotypes were significantly overrepresented as maternal versus paternal parents of heterozygotes, suggesting gender-function-dependent genetic control of outcrossing rates. In 4 wild populations (>300 individuals each), expected heterozygosity ranged from 0.149 to 0.336, whereas t-values ranged from 0.0027 to 0.0133, indicating high levels of both genetic diversity and inbreeding. Up to a third of the individuals in each population belonged to groups with identical or nearly identical SNP genotypes, whereas many of the remaining individuals were members of loose clusters of apparently related plants that probably represent descendants from past outcrossing events. Strict inbreeding in some lineages within a population with occasional outcrossing in others may be related to positive selection on adaptive syndromes associated with specific inbreeding lineages, or possibly to among-lineage differences in genetic regulation of outcrossing.

Inter-island but not intra-island divergence among populations of sea oats, Uniola paniculata L. (Poaceae)

Understanding the underlying causes of phenotypic trait variation among populations is important for informing conservation decisions. This knowledge can be used to determine whether locality matters when sourcing populations for habitat restoration. Uniola paniculata is a federally protected coastal dune grass native to the southeastern Atlantic and the Gulf coasts of the USA that is often used to stabilize restored dune habitats. This study uses neutral genetic markers (allozymes) and a greenhouse common garden study to determine the relative contributions of neutral evolutionary processes and natural selection to patterns of phenotypic variation among natural populations of U. paniculata. Seeds were sourced from foredune and backdune populations spanning shoreline-to-landward environmental gradients on each of four Georgia barrier islands. Based on previous work, we expected to find evidence of divergent selection among populations located on the shoreline-to-landward environmental gradient. However, differences among islands, rather than intra-island habitat differences, drive divergent selection on aboveground and total biomass. The lack of evidence for divergent selection across the shoreline-to-landward gradient suggests that previously documented intra-island trait variation is likely due to phenotypic plasticity. Our findings have implications for conservation and restoration efforts involving U. paniculata, as there is evidence for divergent selection among populations located on neighboring islands.

Land use causes genetic differentiation of life‐history traits in Bromus hordeaceus

There is increasing evidence that species can evolve rapidly in response to environmental change. However, although land use is one of the key drivers of current environmental change, studies of its evolutionary consequences are still fairly scarce, in particular studies that examine land-use effects across large numbers of populations, and discriminate between different aspects of land use. Here, we investigated genetic differentiation in relation to land use in the annual grass Bromus hordeaceus. A common garden study with offspring from 51 populations from three regions and a broad range of land-use types and intensities showed that there was indeed systematic population differentiation of ecologically important plant traits in relation to land use, in particular due to increasing mowing and grazing intensities. We also found strong land-use-related genetic differentiation in plant phenology, where the onset of flowering consistently shifted away from the typical time of management. In addition, increased grazing intensity significantly increased the genetic variability within populations. Our study suggests that land use can cause considerable genetic differentiation among plant populations, and that the timing of land use may select for phenological escape strategies, particularly in monocarpic plant species.

Effects of litter on seedling establishment in natural and semi‐natural grasslands: a meta‐analysis

SummaryPlant litter is a key component in terrestrial ecosystems. It plays a major role in nutrient cycles and community organization. Land use and climate change may change the accumulation of litter in herbaceous ecosystems and affect plant community dynamics. Additionally, the transfer of seeds containing plant material (i.e. litter) is a widespread technique in grassland restoration.Ecosystem responses to litter represent the outcome of interactions, whose sign and strength will depend on many variables (e.g. litter amount, seed size). A previous meta‐analysis (from 1999) reported that litter had an overall negative effect on seed germination and seedling establishment in different ecosystems. However, recent studies indicated that this might not be the case in grassland ecosystems.We used 914 data from 46 independent studies to analyse the effects of litter on seedling (i) emergence, (ii) survival and (iii) biomass, employing meta‐analytical techniques. Each data set was stratified according to methodology, grassland type, irrigation conditions, litter amount and seed size.We found an overall neutral effect of litter presence on seedling emergence and survival and a positive effect on seedling biomass. However, whereas for field experiments the response remained neutral, it was positive for common garden studies. In glasshouse experiments, litter effects were negative for emergence and positive for biomass.Litter may have a positive effect on seedling recruitment in dry grasslands or under water‐limited conditions, or in the presence of low to medium litter amounts (< 500 g m−2). However, high litter amounts (> 500 g m−2) will inhibit seedling recruitment. Large seeds showed a more positive response to litter presence with respect to seedling emergence and survival, but not concerning biomass.Synthesis. Under dry conditions (e.g. dry grasslands or dry periods) or with low to medium litter amounts, litter presence has a positive effect on seedling establishment. However, climate and land use change may promote litter accumulation and reduce seedling establishment, affecting grasslands composition and ecosystem functions.

Integrating trait‐ and niche‐based approaches to assess contemporary evolution in alien plant species

Summary Two primary lines of indirect evidence for contemporary evolution in alien species are based on differences between native and introduced ranges in one or more functional traits or a shift in environmental niche. Although the integration of trait and environmental niche perspectives is increasingly recognised as a key to understanding the role of life‐history evolution in range‐shifting populations, there has been no attempt to bring together these perspectives on the contemporary evolution of alien plant species. We develop a set of scenarios that contrast trait–environment relationships observed in the field for alien species in the native and introduced range, and explore how they might be shaped by contemporary evolution. In each case, the limitations of uniquely trait or environmental niche perspectives are highlighted. The scenarios are examined in relation to long‐term trends in covariation between temperature and first flowering date of European plant species introduced into the USA. Support for four of the scenarios is found. Field studies examining how species traits respond to environmental variation along natural gradients cannot by themselves distinguish relationships arising from genetic variation, phenotypic plasticity or genetic variation for such plasticity. This is best assessed by reciprocal transplant experiments. However, trait–environment relationships provide a basis for better targeted common garden studies that are more hypothesis driven and that pinpoint the traits of interest, ascertain the appropriate selection gradients and the range over which they need to be observed, as well as identify candidate species for further study. Synthesis. The need to improve species distribution models through a better understanding of underlying ecological and evolutionary processes makes assessments of trait–environment relationships, in both the native and introduced ranges, significant. They are of paramount importance when explaining the differential success of alien plants in novel environments as well as when predicting the potential for future range shifts following introduction. The current paucity of such comparisons represents a significant gap in our understanding of biological invasions.

Growth pattern and survival in populations of Poecilia vivipara (Teleostei; Poeciliidae) inhabiting an environmental gradient: a common garden study

We performed a common garden experiment to assess the existence of genetic differences on growth and body size between two populations of Poecilia vivipara inhabiting extremes of an environmental gradient caused by water salinity in lagoons of Northern Rio de Janeiro State, Brazil: the Campelo lagoon (freshwater) and Acu lagoon (brackish/saltwater). The two populations show extreme differences in average phenotypes for body size, shape and life history (freshwater populations with smaller body size, lower fecundity and larger reproductive allotment). Pregnant females were brought to the lab and the offspring from both groups were kept in a common recirculating system with freshwater. Standard length and survival were measured weekly over a period of 200 days and growth models were fitted and selected with information criteria. The offspring originally from the brackish water lagoon presented larger asymptotic length, higher maximum growth rate but lower survival than the offspring originally from the freshwater lagoon. Potential confounding variables such as density differences due to mortality and maternal effects (offspring size) were included as covariates in comparisons of growth rates between groups. The results are consistent with phenotypic differences among populations having some genetic basis, and with the existence of a trade-off between growth and maintenance due to the high growth/low survival observed in the group that changed from salt to freshwater. Comparisons of captive and natural populations suggest that the influence of environmental factors, such as salinity, food availability, fish density and predation should also be considered relevant to explain phenotypic variation in this system.

Response of different white fir geographic provenances to Trichosporium symbioticum inoculation in California

We inoculated the fir engraver ( Scolytus ventralis LeConte) associated fungus Trichosporium symbioticum Wright onto 56 white fir ( Abies concolor (Gordon & Glend.) Lindl. ex Hildebr.) trees planted in a common garden study near Camino, California, that represented five geographic provenances of this species. The objective was to determine if there is a differential lesion length response of white fir provenances with respect to provenance. We found a significant (P < 0.019) difference between the provenances from Arizona and those of eastern Nevada origins 28 days after inoculation. There was a significant interaction between the two T. symbioticum isolates and season of inoculation. Fall inoculations tended to have smaller lesions than those in the spring but this varied by isolate in that the one from eastern Nevada tended to produce longer lesions in the spring (P = 0.0001) whereas the isolate from the Camino plantation did not differ between spring and fall (P = 1.000). There is evidence for genetic variability relative to white fir provenance lesion length in response to T. symbioticum inoculation, and in future studies, isolate variability should also be taken into account.

Boldness behavior and stress physiology in a novel urban environment suggest rapid correlated evolutionary adaptation

Novel or changing environments expose animals to diverse stressors that likely require coordinated hormonal and behavioral adaptations. Predicted adaptations to urban environments include attenuated physiological responses to stressors and bolder exploratory behaviors, but few studies to date have evaluated the impact of urban life on codivergence of these hormonal and behavioral traits in natural systems. Here, we demonstrate rapid adaptive shifts in both stress physiology and correlated boldness behaviors in a songbird, the dark-eyed junco, following its colonization of a novel urban environment. We compared elevation in corticosterone (CORT) in response to handling and flight initiation distances in birds from a recently established urban population in San Diego, California to birds from a nearby wildland population in the species' ancestral montane breeding range. We also measured CORT and exploratory behavior in birds raised from early life in a captive common garden study. We found persistent population differences for both reduced CORT responses and bolder exploratory behavior in birds from the colonist population, as well as significant negative covariation between maximum CORT and exploratory behavior. Although early developmental effects cannot be ruled out, these results suggest contemporary adaptive evolution of correlated hormonal and behavioral traits associated with colonization of an urban habitat.

No consistent association between changes in genetic diversity and adaptive responses of Australian acacias in novel ranges

Common garden studies comparing trait differences of exotic species between native and introduced ranges rarely incorporate an analysis of genetic variation, but simply infer that trait shifts between ranges are genetically determined. We compared four growth-related traits (total biomass, relative growth rate RGR, specific leaf area SLA, and root to shoot ratio R:S) of five invasive Fabaceae species (Acacia cyclops, A. longifolia, A. melanoxylon, A. saligna, Paraserianthes lophantha), grown in a common garden experiment using seeds from introduced and native ranges across Australia. Chloroplast microsatellite loci were used to compare genetic diversity of native and introduced populations to determine standing genetic diversity and infer introduction history. We asked whether shifts in traits associated with faster growth due to enemy release in the introduced range were associated with levels of genetic diversity associated with introduction history. We found differences in traits between ranges, although these traits varied among the species. Compared to native-range populations, introduced-range Acacia longifolia had greater biomass and larger SLA; A. cyclops had greater RGR; and A. melanoxylon displayed lower R:S. Genetic diversity in the introduced range was lower for one of those species, A. longifolia, and two others that did not show differences in traits, A. saligna and P. lophantha. Diversity was higher in the introduced range for A. melanoxylon and did not differ among ranges for A. cyclops. These patterns of genetic diversity suggest that a genetic bottleneck may have occurred following the introduction of A. longifolia, A. saligna and P. lophantha. In contrast greater or comparable genetic diversity in the introduced range for A. melanoxylon and A. cyclops suggests introductions from multiple sources. This study has shown that a reduction in genetic diversity in the introduced range is not necessarily associated with a reduced capacity for adaptive responses or invasion potential in the novel range.

A Common-Garden Study of Resource-Island Effects on a Native and an Exotic, Annual Grass After Fire

Plant-soil variation related to perennial-plant resource islands (coppices) interspersed with relatively bare interspaces is a major source of heterogeneity in desert rangelands. Our objective was to determine how native and exotic grasses vary on coppice mounds and interspaces (microsites) in unburned and burned sites and underlying factors that contribute to the variation in sagebrush-steppe rangelands of the Idaho National Lab, where interspaces typically have abiotic crusts. We asked how the exotic cheatgrass (Bromus tectorum L.) and native bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love) were distributed among the microsites and measured their abundances in three replicate wildfires and nearby unburned areas. We conducted a common-garden study in which soil cores from each burned microsite type were planted with seed of either species to determine microsite effects on establishment and growth of native and exotic grasses. We assessed soil physical properties in the common-garden study to determine the intrinsic properties of each microsite surface and the retention of microsite soil differences following transfer of soils to the garden, to plant growth, and to wetting/drying cycles. In the field study, only bluebunch wheatgrass density was greater on coppice mounds than interspaces, in both unburned and burned areas. In the common-garden experiment, there were microsite differences in soil physical properties, particularly in crust hardness and its relationship to moisture, but soil properties were unaffected by plant growth. Also in the experiment, both species had equal densities yet greater dry mass production on coppice-mound soils compared to interspace soils, suggesting microsite differences in growth but not establishment (likely related to crust weakening resulting from watering). Coppice-interspace patterning and specifically native-herb recovery on coppices is likely important for postfire resistance of this rangeland to cheatgrass.

Morphological adaptation of a palatable plant to long-term grazing can shift interactions with an unpalatable plant from facilitative to competitive

Unpalatable plants can protect palatable neighbor plants from grazing pressure, but morphological evolution of a palatable species might change its interactions with unpalatable plants. We predicted that when a palatable species has locally adapted to grazing by expressing a dwarf phenotype that reduces grazer accessibility, the dwarf plants experience relatively more competitive effects than facilitative effects from large, well-defended, unpalatable species. We used a transplant experiment, in which both dwarf and large ecotypes of a palatable annual species, Persicaria longiseta, were transplanted outside and inside the canopy of an unpalatable nettle, Urtica thunbergiana, in a long-term deer grazing habitat of Nara Park, Japan. The dwarf ecotype of Persicaria has adapted to the grazing environments of the park by exhibiting inherently short shoots and small leaves, whereas the large ecotype is found in habitats with no grazing history. A previous common-garden study suggested that the phenotypic differences were genetically based and that phenotypic plasticity contributed little to the morphological difference. The large-phenotype of Persicaria experienced significantly increased morphological size, survival, and reproductive output under the Urtica canopy compared to outside the canopy, whereas these traits of the dwarf phenotype were reduced under the Urtica canopy compared to outside. These results indicate that the net effects of Urtica on Persicaria were positive for the large ecotype and negative for the dwarf ecotype. Thus, the morphological adaptation of a palatable species to avoid grazing altered its interactions with a large, well-defended neighbor.

Hybridization and invasion: an experimental test with diffuse knapweed (Centaurea diffusa Lam.).

A number of studies have suggested a link between hybridization and invasion. In this study, we experimentally test the potential for hybridization to influence invasion through a greenhouse common garden study. Diffuse knapweed (DK) (Centaurea diffusa Lam.) was introduced to North America with admixture from spotted knapweed (SK) (Centaurea stoebe subsp. stoebe L.). Comparisons between North American DK (including hybrid phenotypes) and native (European) DK in a common garden did not reveal enhanced performance or increased phenotypic variance, suggesting that pre-introduction hybridization or, more generally, post-introduction evolutionary change has not significantly contributed to the invasion of DK. In contrast, early generation hybrids [artificially created Backcross 1 (BC1) plants] exhibited increased variance for eight of the examined traits, and greater leaf and reproductive shoot production when compared to North American DK. Individual BC1 lines differed for several traits, suggesting the importance of the cross for drawing conclusions from such comparisons. When compared to the parental species (DK and SK), the BC1 plants were not transgressive for any of the measured traits. Overall, these findings suggest that if diploid SK is introduced to North America, interspecific hybridization has the potential to result in even more aggressive invaders.

Gone with the wind: understanding evolutionary transitions between wind and animal pollination in the angiosperms

Gone with the wind: understanding evolutionary transitions between wind and animal pollination in the angiosperms