Least-square means (± S.E.) of density (# insects per kg of dry shoot biomass) of the generalist aphid <i>Aphis gossypii</i> (A), the specialist aphid <i>Uroleucon macolai</i> (B), ants (<i>Linepithema humile</i>; C), <i>A</i>. <i>gossypii</i> parasitoids (Braconidae; D), and mycorrhizal percent root colonization (E) for growth rate monocultures (“mono”) and polycultures (“poly”) of fast- and slow-growing plant genotypes, and plant sex monocultures and polycultures of male and female genotypes of the shrub <i>Baccharis salicifolia</i> Least-square means account for the effects of plot.

<p>* = Significant (<i>P</i> < 0.05). The mean of sex and growth rate monocultures is the mean of male and female monocultures or the mean of slow- and fast-growing monocultures, respectively.</p

Least-square means (± S.E.) for total biomass (A), shoot biomass (B), and root biomass (C; g, dry weight in all cases) for growth rate monocultures (“mono”) and polycultures (“poly”) of fast- and slow-growing genotypes, and plant sex monocultures and polycultures of male and female genotypes of the shrub <i>Baccharis salicifolia</i>.

<p>Least-square means account for the effect of plot to control for effects of spatial heterogeneity. * = significant (<i>P</i> < 0.05); ms = marginally significant (0.05 < <i>P</i> < 0.10). The mean of sex and growth rate monocultures is the mean of male and female monocultures or of slow- and fast-growing monocultures, respectively.</p

Least-square means (± S.E.) of abundance of the generalist aphid <i>Aphis gossypii</i> (A), the specialist aphid <i>Uroleucon macolai</i> (B), ants (<i>Linepithema humile</i>; C), <i>A</i>. <i>gossypii</i> parasitoids (Braconidae; D), and mycorrhizae (percent root colonization × dry root biomass; E) for growth rate monocultures (“mono”) and polycultures (“poly”) of fast- and slow-growing plant genotypes, and plant sex monocultures and polycultures of male and female genotypes of the shrub <i>Baccharis salicifolia</i>.

<p>Least-square means account for the effect of plot. * = Significant (<i>P</i> < 0.05). The mean of sex and growth rate monocultures is the mean of male and female monocultures or the mean of slow- and fast-growing monocultures, respectively.</p

Traits underlying community consequences of plant intra-specific diversity

<div><p>A plant’s performance and interactions with other trophic levels are recorgnized to be contingent upon plant diversity and underlying associational dynamics, but far less is known about the plant traits driving such phenomena. We manipulated diversity in plant traits using pairs of plant and a substitutive design to elucidate the mechanisms underlying diversity effects operating at a fine spatial scale. Specifically, we measured the effects of diversity in sex (sexual monocultures vs. male and female genotypes together) and growth rate (growth rate monocultures vs. fast- and slow-growing genotypes together) on growth of the shrub <i>Baccharis salicifolia</i> and on above- and belowground consumers associated with this plant. We compared effects on associate abundance (# associates per plant) vs. density (# associates per kg plant biomass) to elucidate the mechanisms underlying diversity effects; effects on abundance but not density suggest diversity effects are mediated by resource abundance (i.e. plant biomass) alone, whereas effects on density suggest diversity effects are mediated by plant-based heterogeneity or quality. Sexual diversity increased root growth but reduced the density (but not abundance) of the dietary generalist aphid <i>Aphis gossypii</i> and its associated aphid-tending ants, suggesting sex mixtures were of lower quality to this herbivore (e.g. via reduced plant quality), and that this effect indirectly influenced ants. Sexual diversity had no effect on the abundance or density of parasitoids attacking <i>A</i>. <i>gossypii</i>, the dietary specialist aphid <i>Uroleucon macolai</i>, or mycorrhizae. In contrast, growth rate diversity did not influence plant growth or any associates except for the dietary specialist aphid <i>U</i>. <i>macolai</i>, which increased in both abundance and density at high diversity, suggesting growth rate mixtures were of higher quality to this herbivore. These results highlight that plant associational and diversity effects on consumers are contingent upon the source of plant trait variation, and that the nature of such dynamics may vary both within and among trophic levels.</p></div

Effects of plant diversity on insect abundance and density.

<p>Effects of plant diversity on insect abundance and density.</p

A test for clinal variation in <i>Artemisia californica</i> and associated arthropod responses to nitrogen addition

<div><p>The response of plant traits to global change is of fundamental importance to understanding anthropogenic impacts on natural systems. Nevertheless, little is known about plant genetic variation in such responses or the indirect effect of environmental change on higher trophic levels. In a three-year common garden experiment, we grew the shrub <i>Artemisia californica</i> from five populations sourced along a 700 km latitudinal gradient under ambient and nitrogen (N) addition (20 kg N ha^-1) and measured plant traits and associated arthropods. N addition increased plant biomass to a similar extent among all populations. In contrast, N addition effects on most other plant traits varied among plant populations; N addition reduced specific leaf area and leaf percent N and increased carbon to nitrogen ratios in the two northern populations, but had the opposite or no effect on the three southern populations. N addition increased arthropod abundance to a similar extent among all populations in parallel with an increase in plant biomass, suggesting that N addition did not alter plant resistance to herbivores. N addition had no effect on arthropod diversity, richness, or evenness. In summary, genetic variation among <i>A</i>. <i>californica</i> populations mediated leaf-trait responses to N addition, but positive direct effects of N addition on plant biomass and indirect effects on arthropod abundance were consistent among all populations.</p></div

Plant population and nitrogen addition effects on plant traits.

<p>Main N addition effect (right side of each plot; squares) and interactive population x N addition effect (left side of each plot; circles) on a) specific leaf area (cm²g^-1 dry weight), b) percent N, c) carbon to nitrogen ratio, and d) percent water content ((wet weight-dry weight)/wet weight). Letters represent populations: SD = San Diego, SM = Santa Monica, CAM = Cambria, SC = Santa Cruz, and GG = Golden Gate National Recreation Area. Numbers below letters represent population latitude. Error bars represent ±1SE. Treatments (P = plant population, N = nitrogen addition, and P x N = plant population x nitrogen addition) are listed at the top of each panel with (*) designating statistical significance.</p

Nitrogen addition and population effects on plants and arthropods.

<p>Nitrogen addition and population effects on plants and arthropods.</p

Plant population and nitrogen addition effects on plant biomass and arthropods.

<p>Main N addition effect (right side of each plot; squares) and interactive population and N addition effect (left side of each plot; circles) on a) estimated dry plant biomass (g) and b) arthropod abundance (individuals per plant). Letters represent populations: SD = San Diego, SM = Santa Monica, CAM = Cambria, SC = Santa Cruz, and GG = Golden Gate National Recreation Area. Numbers below letters represent population latitude. Error bars represent ±1SE. Treatments (P = plant population, N = nitrogen addition, and P x N = plant population x nitrogen addition) are listed at the top of each panel with (*) designating statistical significance.</p

Estimated total and dry N deposition (kg N ha^-1 yr^-1) in 2009.

<p>Estimated total and dry N deposition (kg N ha^-1 yr^-1) in 2009.</p