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Terpenoids are secondary metabolites produced in most plant tissues and are often considered toxic or repellent to plant enemies. Previous work has typically reported on intra-specific variation in terpene profiles, but the effects of plant sex, an important axis of genetic variation, have been less studied for chemical defences in general, and terpenes in particular. In a prior study, we found strong genetic variation (but not sexual dimorphism) in terpene amounts in leaves of the dioecious shrub Baccharis salicifolia. Here we build on these findings and provide a more in-depth analysis of terpene chemistry on these same plants from an experiment consisting of a common garden with male (N = 19) and female (N = 20) genotypes sourced from a single population. Our goal in the present study was to investigate quantitative and qualitative differences in terpene profiles associated with plant sex and genotypic variation. For this, we quantified leaf mono- and sesquiterpene amount, richness, and diversity (quantitative profile), as well as the composition of compounds (qualitative profile). We found no evidence of sexual dimorphism in monoterpene or sesquiterpene profiles. We did, however, find significant genotypic variation in amount, diversity, and composition of monoterpenes, but no effects on sesquiterpenes. These findings indicated that genotypic variation in terpene profiles largely surpassed variation due to sexual dimorphism for the studied population of this species.

The evolutionary processes that underlie variation in plant genome size have been much debated. Abiotic factors are thought to have played an important role, with negative and positive correlations between genome size and seasonal or stressful climatic conditions being reported in several systems. In turn, variation in genome size may influence plant traits which affect interactions with other organisms, such as herbivores. The mechanisms underlying evolutionary linkages between plant genome size and biotic and abiotic factors nonetheless remain poorly understod. To address this gap, we conducted phylogenetically controlled analyses testing for associations between genome size, climatic variables, plant traits (defenses and nutrients), and herbivory across 29 oak (Quercus) species. Genome size is significantly associated with both temperature and precipitation seasonality, whereby oak species growing in climates with lower and less variable temperatures but more variable rainfall had larger genomes. In addition, we found a negative association between genome size and leaf nutrient concentration (found to be the main predictor of herbivory), which in turn led to an indirect effect on herbivory. A follow‐up test suggested that the association between genome size and leaf nutrients influencing herbivory was mediated by variation in plant growth, whereby species with larger genomes have slower growth rates, which in turn are correlated with lower nutrients. Collectively, these findings reveal novel associations between plant genome size and biotic and abiotic factors that may influence life history evolution and ecological dynamics in this widespread tree genus. [ABSTRACT FROM AUTHOR]

Intraspecific plant trait variation can have cascading effects on plant-associated biotic communities. Sexual dimorphism is an important axis of genetic variation in dioecious plants, but the strength of such effects and the underlying mechanisms relative to genetic variation are unknown. We established a common garden with 39 genotypes of Baccharis salicifolia sampled from a single population that included male and female genotypes and measured plant traits and quantified associated arthropod communities. Genetic variation sensu lato (genotypic variation) had strong effects on most plant traits (flower number, relative growth rate, specific leaf area, percent water content, carbon-nitrogen ratio, monoterpene but not sesquiterpene concentrations) and on herbivore and predator density, and on arthropod community composition (relative abundance of 14 orders). In contrast, sexual dimorphism had weaker effects on only a few plant traits (flower number and relative growth rate), on predator density, and on arthropod community composition, but had no effect on herbivore density. Variation in flower number drove genetic variation sensu lato and sex dimorphism in predator density and arthropod community composition. There was unique genetic variation sensu lato in herbivore density (positively) associated with monoterpene concentration and in arthropod community composition associated with specific leaf area and carbon-nitrogen ratio. There was unique sexual dimorphism in arthropod community composition associated with plant relative growth rate. Together, these results demonstrate that genetic variation sensu lato and sexual dimorphism can shape plant-associated arthropod communities via both parallel and unique mechanisms, with greater overall effects of the former.

The abiotic environment exerts strong effects on plant‐associated microbes, shaping their interactions with plants and resulting ecosystem processes. However, these abiotic effects on plant–microbe interactions are often highly specific and contingent on the abiotic driver or microbial group, requiring synthesis work describing general patterns and from this generate hypotheses and guide mechanistic work. To address this, we conducted a meta‐analysis of the effects of climate change‐related abiotic factors, namely warming, drought, and eCO2, on plant‐associated microbes distinguishing by microbial taxonomic or biological group (bacteria, fungi or virus) and the plant part where microbes are found or associated with (phyllosphere or rhizosphere). We found abiotic driver‐specific patterns, whereby drought significantly reduced microbial abundance, whereas warming and eCO2 had no significant effects. In addition, these abiotic effects were contingent on the microbial taxonomic group, with fungi being negatively affected by drought but positively affected by warming (eCO2 enrichment had no effect), whereas bacteria and viruses were not significantly affected by any factor. Likewise, rhizospheric microbes were negatively affected by drought but positively affected by warming (eCO2 enrichment had no effect), whereas phyllospheric microbes were not significantly affected by any factor. Collectively, these findings point to important implications for global change research by highlighting contrasting effects of climate change‐related abiotic drivers on plant‐associated microbes and the contingency of such effects on microbe life histories and the nature of their interactions with plants. [ABSTRACT FROM AUTHOR]

A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underpinning such effects. However, there are currently a number of key gaps in knowledge that have hindered the development of a predictive framework of plant diversity effects on consumers. For instance, we still know very little about how the magnitude of plant trait variation (e.g. intra-specific vs. inter-specific), as well as the identity and combined effects of plant, herbivore and natural enemy traits, mediate plant diversity effects on consumers. Moreover, the fine-scale mechanisms (e.g. changes in consumer behaviour or recruitment responses) underlying such diversity effects in many cases remain elusive or have been overlooked. In addition, most studies of plant diversity effects on associated consumers have been developed under a static, unidirectional (bottom-up) framework of effects on herbivores and predators without taking into account the potential for dynamic feedbacks across trophic levels. Here we seek to address these key gaps in knowledge as well as to capitalize on recent advances and emerging frameworks in plant biodiversity research. In doing so, we provide new insights as well as recommendations which will stimulate new research and advance this field of study.

The effects of tree diversity on herbivore-enemy interactions have received relatively little attention and even fewer studies have compared the relative influence of tree intra- versus interspecific diversity on such dynamics. We evaluated the effects of mahogany (Swietenia macrophylla) genotypic diversity and tree species diversity on parasitoid attack and species richness associated with Phyllocnistis meliacella, a specialist herbivore on mahogany, in a forest diversity experiment consisting of 74 plots (21×21m2; 64plants/plot). We sampled 34 of such plots classified as: mahogany monocultures of one maternal family (i.e. genotype), mahogany monocultures of four families and polycultures of four species (including mahogany). We surveyed leafminer abundance and collected mined leaves to estimate parasitism and parasitoid species richness. Leafminer abundance was not influenced by either type of diversity. Similarly, there were no effects of genotypic diversity or species diversity on parasitism or parasitoid species richness. Plant diversity effects on parasitoids were probably absent because the species attacking P. meliacella are dietary generalists that likely recruited to multiple host species (in addition to P. meliacella) and their responses to diversity could have cancelled each other out. Future work should explicitly investigate how predator and parasitoid traits mediate the effects of plant diversity on tritrophic interactions.

© 2015 British Ecological Society. Mechanisms by which climatic factors drive reproductive investment and phenology in masting species are not completely understood. Climatic conditions may act as a proximate cue, stimulating the onset of reproduction and indirectly increasing fitness through benefits associated with synchronous reproduction among individuals. Alternatively, climatic conditions may directly influence individual-level allocation to reproduction and reproductive success through effects occurring independently of synchronous reproduction. We previously showed that masting in a ponderosa pine (Pinus ponderosa) population was strongly influenced by spring mean temperature 2 years before seed cone maturation (Ti-2). However, recent work shows that the difference in temperature between previous growing seasons (ΔT) is more predictive of reproductive investment in long-lived tree species. Here, we compared four candidate models that predict seed cone production in P. ponderosa based upon different climatic factors (including Ti-2 and ΔT models). After determining the best climatic predictor, we tested for a potential mechanism by which climate might directly influence seed cone production independent of benefits via synchrony, namely effects of temperature on trade-offs between current and past reproduction (determined by underlying resource availability). We found that Ti-2 (rather than ΔT) was the best predictor of seed cone production. We further show that this same climatic factor exerts a direct fitness benefit to individuals by reducing the strength of trade-offs between current and past reproductive efforts. Synthesis. We demonstrate that a single climatic factor provides fitness benefits to individuals directly, by weakening reproductive trade-offs, and indirectly through the benefits associated with synchrony and masting. This suggests a mechanism for the origin and maintenance of masting: individuals initially respond to climatic cues that directly enhance reproduction (e.g. lower reproductive costs through weakened trade-offs) and this dynamic, expressed across multiple individuals, reinforces these benefits through the economies of scale associated with synchrony and masting. We propose a new mechanism for the origin and maintenance of masting: individuals initially respond to climatic cues that directly enhance reproduction (e.g. lower reproductive costs through weakened trade-offs) and this dynamic, expressed across multiple individuals, reinforces these benefits through the economies of scale associated with synchrony and masting.

Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant anti-herbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses.