COS 68-6
Variation in the response of Artemisia californica populations to nitrogen addition and impacts on associated arthropods
Geographic variation in ecologically important plant traits and plant responses to environmental change can influence the strength of plant-arthropod interactions across a species range. Atmospheric nitrogen (N) deposition, one example of environmental change, has increased dramatically since the industrial revolution and represents an important source of N in coastal sage scrub habitats of Southern California. N-deposition can cause changes in plant nutritional quality and tissue chemistry that are likely to affect the performance of plant-associated arthropods. We examined population variation in plant functional traits and growth for the shrub Artemisia californica, and the performance of the specialist aphid, Obtusicauda cowenii, along a 700 km geographic gradient in coastal California. Plants cloned from five populations along this gradient were grown for three years in ambient and nitrogen addition treatments (+ 20 kg N/Ha). During one growing season, we placed 5 aphids contained in a fine mesh bag on one branch of each experimental plant. Aphid population growth (performance) was recorded weekly for five weeks to determine the influence of plant trait variation and nitrogen addition on this specialist herbivore. Additionally, we collected whole arthropod communities from experimental plants to assess the likely consequences of plant responses to nitrogen addition for plant-arthropod interactions.
Results/Conclusions
Plant growth differed among the five source populations and increased in the nitrogen addition treatment. Nitrogen addition increased leaf percent water content but did not influence specific leaf area; neither of these traits differed among source populations. Source populations differed in their response to nitrogen addition in terms of C:N ratio, with two populations increasing C:N, two populations decreasing C:N, and one population showing no change in C:N with nitrogen addition. Aphid population growth rate did not differ among plant source populations or treatments. However, aphid population survival (i.e. the number of populations that did not go extinct during the course of the experiment) was greater on plants in the nitrogen addition treatment. We identified over >2800 individual arthropods belonging to 125 morphospecies in 11 Orders. Nitrogen addition increased arthropod densities by almost 50% across all populations. In conclusion, nitrogen addition significantly impacted plant functional traits and growth, the survival of a specialist herbivore, and whole community arthropod densities on the foundation species A. californica. The inconsistent response among A. californica populations to N addition indicates that responses to this important environmental change is likely to have differential affects on plant associates across this species range.