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

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

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

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 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