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1. Independent and interactive effects of plant genotype and environment on plant traits and insect herbivore performance: A meta‐analysis with Salicaceae

   https://doi.org/10.1111/1365-2435.13249  

   Barker, Hilary L. ; Holeski, Liza M. ; Lindroth, Richard L. ; Koricheva, ed., Julia ( December 2018 , Functional Ecology)

   Ecological research has increasingly highlighted the importance of intraspecific variation in shaping the structure and function of communities and ecosystems. Indeed, the effects of intraspecific variation can match or exceed those of interspecific variation. Previous reviews of intraspecific variation in plant traits across heterogeneous environments have focused primarily onmeanphenotypic effects. We propose that a richer and fuller understanding of the ecological causes and consequences of intraspecific variation would be provided by partitioning traitvarianceinto its subcomponents (genetic, environment, genotype by environment interaction).

   We used a meta‐analysis of 352 sets of genetic, environment and genotype by environment (G×E) variation estimates from 72 studies of Salicaceae to compare these sources of variation across plant traits (growth, foliar nitrogen, defence compounds), insect herbivore performance metrics (e.g., survival, growth, fecundity) and environmental conditions (e.g., soil nutrients, water, defoliation).

   Our findings revealed that variation in levels of defence compounds (both condensed tannins and salicinoids) and insect herbivore performance were primarily genetically determined, while variation in plant growth and foliar nitrogen was more environmentally determined.

   Plasticity in plant growth, foliar nitrogen levels and insect herbivore performance varied substantially across different sites (year × location), and nutrient, water and carbon dioxide environments. Plasticity was lowest for chemical defence traits and all traits in contrasting ozone and defoliation environments.

   Our quantitative review also revealed several gaps in the literature, including a need for surveying more mature plants, a wider variety of insect herbivore species (e.g., leaf‐galling insects, specialist insects) and underrepresented environmental treatments (e.g., competition, defoliation, disease, light and water availability).

   Findings from this analysis highlight the importance of, and patterns within, intraspecific variation with respect to shaping the evolvability and plasticity of traits and governing the interactions of plants and insects.

   Aplain language summaryis available for this article.

    

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2. How does genome size affect the evolution of pollen tube growth rate, a haploid performance trait?

   https://doi.org/10.1002/ajb2.1326  

   Reese, John B. ; Williams, Joseph H. ( July 2019 , American Journal of Botany)

   Male gametophytes of most seed plants deliver sperm to eggs via a pollen tube. Pollen tube growth rates (PTGRs) of angiosperms are exceptionally rapid, a pattern attributed to more effective haploid selection under stronger pollen competition. Paradoxically, whole genome duplication (WGD) has been common in angiosperms but rare in gymnosperms. Pollen tube polyploidy should initially acceleratePTGRbecause increased heterozygosity and gene dosage should increase metabolic rates. However, polyploidy should also independently increase tube cell size, causing more work which should decelerate growth. We asked how genome size changes have affected the evolution of seed plantPTGRs.

   We assembled a phylogenetic tree of 451 species with knownPTGRs. We then used comparative phylogenetic methods to detect effects of neo‐polyploidy (within‐genus origins),DNAcontent, andWGDhistory onPTGR, and correlated evolution ofPTGRandDNAcontent.

   Gymnosperms had significantly higherDNAcontent and slowerPTGRoptima than angiosperms, and theirPTGRandDNAcontent were negatively correlated. For angiosperms, 89% of model weight favored Ornstein‐Uhlenbeck models with a fasterPTGRoptimum for neo‐polyploids, whereasPTGRandDNAcontent were not correlated. For within‐genus and intraspecific‐cytotype pairs,PTGRs of neo‐polyploids < paleo‐polyploids.

   Genome size increases should negatively affectPTGRwhen genetic consequences ofWGDs are minimized, as found in intra‐specific autopolyploids (low heterosis) and gymnosperms (fewWGDs). But in angiosperms, the higherPTGRoptimum of neo‐polyploids and non‐negativePTGR‐DNAcontent correlation suggest that recurrentWGDs have caused substantialPTGRevolution in a non‐haploid state.

    

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3. Neighborhood diversity simultaneously increased and decreased susceptibility to contrasting herbivores in an early stage forest diversity experiment

   https://doi.org/10.1111/1365-2745.13097  

   Grossman, Jake J. ; Cavender‐Bares, Jeannine ; Reich, Peter B. ; Montgomery, Rebecca A. ; Hobbie, Sarah E. ; Shefferson, ed., Richard ( December 2018 , Journal of Ecology)

   Forest structure and diversity can regulate tree vulnerability to damage by insects and pathogens. Past work suggests that trees with diverse neighbours should experience less leaf herbivory and less damage from specialist herbivores and diseases, and that the effect of neighbourhood diversity should be strongest at small spatial scales.

   In an early stage temperate tree diversity experiment, we monitored damage from leaf removing herbivores, specialist (gallers and leaf miners) herbivores, and two specialist fungal diseases (maple leaf anthracnose and cedar apple gall rust) over 3 years. The experimental design included treatments that varied independently in phylogenetic and functional diversity and we made our analyses across four spatial scales (1–16 m²).

   Neighborhood diversity simultaneously increased leaf removal for some species, decreased it for others, and had no effect on yet others. Height apparency—the difference between a focal plant’s height and its neighbours’—was the best single predictor of leaf removal across species and spatial scales, but the strength and direction of its effect were also species‐specific.

   Specialist pathogens and fungal foliar diseases showed signs of associational resistance and susceptibility. Oaks (Quercusspp.) were more resistant to leaf miners and maples were more resistant to anthracnose when surrounded by diverse neighbours (associational resistance). In contrast, birches (Betula papyrifera) were more susceptible to leaf miners and eastern red cedars (Juniperus virginiana) were more susceptible to cedar apple gall rust (Gymnosporangium juniperi‐virginianae) infection in diverse environments (associational susceptibility).

   Herbivore and pathogen damage was better predicted by community structure and diversity at small spatial scales (1 and 4 m²) than large scales (9 and 16 m²), suggesting a characteristic spatial scale for these biodiversity‐ecosystem functioning effects.

   Synthesis.Humans control forest diversity through selective harvesting and planting in natural stands and plantations. Our experimental demonstration of the role of local community structure and diversity in suppressing some forms of pest and pathogen damage to trees suggests that forest management can be most effective when diversity is considered at small spatial scales and the underlying biology of particular pests, pathogens, and hosts is taken into account. Pictured here: the “galls” formed by cedar apple gall rust (Gymnosporangium juniperae‐virginiae) on eastern red cedar (Juniperus virginiana) in early spring release wind‐dispersed teleospores. Junipers showed associational susceptibility: greater susceptibility to gall rust with more diverse neighbours.

    

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4. Fine‐scale environmental heterogeneity and spatial niche partitioning among spring‐flowering forest herbs

   https://doi.org/10.1002/ajb2.1593  

   Beck, Jared J. ; Givnish, Thomas J. ( January 2021 , American Journal of Botany)

   Environmental heterogeneity influences plant distributions and diversity at several spatial scales. In temperate forests, fine‐scale environmental variation may promote local coexistence among herbaceous species by allowing plants to spatially partition microsites within forest stands. Here we argue that shallow soils, low soil water‐holding capacity and fertility, and reduced light near tree boles should favor short, shallow‐rooted, evergreen species likeAnemone acutilobawith low moisture, nutrient, and light requirements. Farther from trees, richer, deeper soils should favor taller, deeper‐rooted herbs with greater moisture and nutrient demands, such asSanguinaria canadensisandTrillium flexipes.

   We tested these hypotheses by mapping the fine‐scale distributions ofAnemone,Sanguinaria, andTrilliumindividuals within a 50 × 50 m plot, comparing local species’ distributions with respect to soil depth and proximity to neighboring trees, and characterizing intraspecific and interspecific spatial associations.

   Local plant distributions were consistent with our predictions based on leaf height, physiology, and phenology.Anemonewas found in microsites on shallower soils and closer to trees than eitherSanguinariaorTrillium. In all three species, individual plants were spatially aggregated within 2 m, but spatially segregated from individuals of the other species beyond 2 m.

   Differential plant responses to fine‐scale environmental heterogeneity and observed spatial associations suggest that local species‐environment associations could facilitate coexistence. These findings illustrate how fine‐scale environmental heterogeneity coupled with phenological and physiological differences likely contribute to spatial niche partitioning among spring‐flowering forest herbs and maintain high local plant diversity within temperate forests.

    

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5. Extensive interlineage hybridization in the predominantly clonal Hydrilla verticillata

   https://doi.org/10.1002/ajb2.1392  

   Benoit, Lori K. ; Les, Donald H. ; King, Ursula M. ; Na, Hye Ryun ; Chen, Lei ; Tippery, Nicholas P. ( November 2019 , American Journal of Botany)

   The submersed aquatic plantHydrilla verticillata(“hydrilla”) is important ecologically and economically due to its aggressive growth in both indigenous and nonindigenous regions. Substantial morphological variation has been documented in hydrilla, including the existence of monoecious and dioecious “biotypes.” Whereas plastid sequence data have been used previously to explore intraspecific diversity, nuclear data have yet to be analyzed in a phylogenetic context. Molecular and morphological analyses were used to evaluate the genetic diversity and phylogenetic relationships of native and introduced populations.

   Nuclear (internal transcribed spacer—ITS; phytoene desaturase—PDS) and plastid (trnL‐F) sequence data were evaluated phylogenetically using likelihood and Bayesian methods. Leaf morphologies were compared among clades that were identified in phylogenetic analyses.

   Data from bothITSandPDSshow multiple instances of polymorphic sequences that could be traced to two or more lineages, including both invasive biotypes in the Americas. Leaf morphological data support the distinctness of lineages and provide a metric for distinguishing monoecious and dioecious biotypes in the United States.

   Nuclear molecular data indicate far greater genetic diversity than could be estimated using plastid markers. Substantially divergent copies of nuclear genes, found in multiple populations worldwide, likely result from interlineage hybridization. Invasive monoecious and dioecious hydrilla biotypes in the Americas are genetically distinct, with both biotypes resulting from admixture among Eurasian progenitors. Genetic similarity to populations in India and South Korea, respectively, implicates these as likely origins for the dioecious and monoecious biotypes.

    

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