Welcome to the neighbourhood: interspecific genotype by genotype interactions in Solidago influence above- and belowground biomass and associated communities.

Intra- and interspecific plant-plant interactions are fundamental to patterns of community assembly and to the mixture effects observed in biodiversity studies. Although much research has been conducted at the species level, very little is understood about how genetic variation within and among interacting species may drive these processes. Using clones of both Solidago altissima and Solidago gigantea, we found that genotypic variation in a plant's neighbours affected both above- and belowground plant traits, and that genotype by genotype interactions between neighbouring plants impacted associated pollinator communities. The traits for which focal plant genotypic variation explained the most variation varied by plant species, whereas neighbour genotypic variation explained the most variation in coarse root biomass. Our results provide new insight into genotypic and species diversity effects in plant-neighbour interactions, the extended consequences of diversity effects, and the potential for evolution in response to competitive or to facilitative plant-neighbour interactions.

Differences in spatial distribution, morphology, and communities of herbivorous insects among three cytotypes of Solidago altissima (Asteraceae).

PREMISE OF THE STUDY: Polyploidy in plants can result in genetic isolation, ecological differences among cytotypes, and, ultimately, speciation. Cytotypes should be sympatric only if they are segregated in an ecological niche or through prezygotic isolation. We tested whether sympatric diploid, tetraploid, and hexaploid ramets of Solidago altissima L. (Asteraceae) differ in their ecological niche. METHODS: We measured how cytotypes were distributed within habitats, their morphology, and the composition of their communities of herbivorous insects at 10 natural field sites. We also conducted a common garden experiment to confirm whether observed differences in morphology or communities of herbivores were due to cytotype or environmental effects. KEY RESULTS: Diploid ramets often grew in open areas, relatively far from woody plants, and were associated with a high species richness of herbaceous plants, especially grasses. Hexaploids often grew in heavy shading under woody plants where grasses were scarce. Finally, tetraploids usually grew in transition areas between diploids and hexaploids. Hexaploid ramets also were taller than ramets of the other cytotypes and had larger leaves. Two species of insects, the leaf-galling fly Asteromyia carbonifera and the phloem-tapping aphid Uroleucon nigrotuberculatum, were more abundant on hexaploid ramets than on ramets of other cytotypes in the field. When grown in a common garden, however, cytotypes were similar in morphology and communities of herbivores. CONCLUSIONS: We conclude that cytotypes of S. altissima differ in their spatial distribution within habitats and that spatial variation in environmental factors influence plant morphology and communities of herbivorous insects.

Divergence of Eurosta solidaginis in response to host plant variation and natural enemies.

We tested the hypothesis that forest and prairie populations of the gall-inducing fly, Eurosta solidaginis, have diverged in response to variation in selection by its host plant Solidago altissima, and its natural enemies. A reciprocal cross infection design experiment demonstrated that fly populations from the prairie and forest biomes had higher survival on local biome plants compared to foreign biome host plants. Flies from each biome also had an oviposition preference for their local plants. Each fly population induced galls of the size and shape found in their local biome on host plants from both biomes indicating a genetic basis to the differences in gall morphology. Solidago altissima from the prairie and forest biomes retained significant morphological differences in the common garden indicating that they are genetically differentiated, possibly at the subspecies level. The populations are partially reproductively isolated as a result of a combination of prezygotic isolation due to host-associated assortative mating, and postzygotic isolation due to low hybrid survival. We conclude that E. solidaginis is undergoing diversifying selection to adapt to differences between prairie and forest habitats.

Effects of metal lead on growth and mycorrhizae of an invasive plant species (Solidago canadensis L.).

It is less known whether and how soil metal lead (Pb) impacts the invasion of exotic plants. A greenhouse experiment was conducted to estimate the effects of lead on the growth and mycorrhizae of an invasive species (Solidago canadensis L.) in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that allowed arbuscular mycorrhizal (AM) fungal hyphae rather than plant roots to grow into the TEST compartments. Three Pb levels (control, 300, and 600 mg/kg soil) were used in this study to simulate ambient soil and two pollution sites where S. canadensis grows. Mycorrhizal inoculum comprised five indigenous arbuscular mycorrhizal fungal species (Glomus mosseae, Glomus versiform, Glomus diaphanum, Glomus geosporum, and Glomus etunicatum). The 15N isotope tracer was used to quantify the mycorrhizally mediated nitrogen acquisition of plants. The results showed that S. canadensis was highly dependent on mycorrhizae. The Pb additions significantly decreased biomass and arbuscular mycorrhizal colonization (root length colonized, RLC%) but did not affect spore numbers, N (including total N and 15N) and P uptake. The facilitating efficiency of mycorrhizae on nutrient acquisition was promoted by Pb treatments. The Pb was mostly sequestered in belowground of plant (root and rhizome). The results suggest that the high efficiency of mycorrhizae on nutrient uptake might give S. canadensis a great advantage over native species in Pb polluted soils.

Intraspecific diversity and dominant genotypes resist plant invasions.

Numerous studies have asked whether communities with many species deter invasions more so than do species-poor communities or whether dominant species deter invasion by colonizing species. However, little is known about whether high intraspecific diversity can deter biological invasions or whether particular genotypes might deter invasions. In this study, we present experimental evidence that intraspecific diversity and particular genotypes of tall goldenrod, Solidago altissima, can act as a barrier to colonization by new species. We found that biomass of colonizing species was negatively correlated with genotypic diversity, and particular genotypes affected the richness, cover, and biomass of colonizing species. Stem density of S. altissima increased with genotypic diversity and varied among genotypes, suggesting that stem density is a key mechanism in limiting colonization dynamics in this system. Our results indicate that the loss of intraspecific diversity within a dominant plant species can increase susceptibility to plant invasions.