Insect herbivore communities and air pollution: Focusing on the effects of nitrogen and ozone on herbivores of bracken fern, California black oak, and ponderosa pine in dry temperate forests

Nitrogen emissions from urban centers and industrial centers have led to high nitrogen deposition in previously nitrogen-limited systems. I examined changes in arthropod herbivore communities along the ambient pollution gradient in the San Bernardino Mountains which are heavily impacted by both nitrogen deposition and ozone. I also examined how herbivore communities and their host plants responded to nitrogen addition treatments at a high and low pollution site. Oak herbivore communities at high pollution sites were dominated by chewing insects. If air pollution exposure mediated a shift from herbivore communities dominated by fluid-feeding insects to communities dominated by chewing insects, it could impact nutrient cycling in these forests. Changes in pine communities where not strongly associated with the air pollution gradient. Fern herbivores showed the most fluctuation in herbivore community comparisons.; Oak herbivore communities showed no response to nitrogen fertilization when atmospheric deposition was low, but did show changes when atmospheric deposition was high. At the high pollution site, fertilization was associated with increased catkin production and higher densities catkin-feeding beetle larvae. Pine herbivores responded more strongly to nitrogen fertilization at a low pollution site compared to a high pollution site. Bracken fern herbivore responses to nitrogen addition treatments were variable. However, fern foliage showed the largest differences in % total nitrogen and nitrate concentrations between high and low pollution sites. None of the plant species examined showed a consistent effect of fertilization on total foliar nitrogen at the low pollution site.; Bark beetle activity and associated tree mortality increased in response to nitrogen fertilization of pines at the low pollution site, suggesting that increased nitrogen availability through atmospheric deposition could play a role in increased tree susceptibility to bark beetles at the high pollution site. In contrast, while sawflies on bracken fern and lepidopterans on oak were responding to differences in plant nitrogen between high and low pollution sites, fertilization at the low pollution site did not affect these herbivores. Other factors that may influence differences in plant nitrogen between sites include differences in ozone exposure, the role of nitrogen deposition to foliar surfaces and differences in precipitation.