To evaluate plant and herbivore responses to nitrogen we conducted a fertilization study at a low and high pollution site in the mixed conifer forests surrounding Los Angeles, California. Contrary to expectations, discriminant function analysis of oak herbivore communities showed significant response to N fertilization when atmospheric deposition was high, but not when atmospheric deposition was low. We hypothesize that longer-term fertilization treatments are needed at the low pollution site before foliar N nutrition increases sufficiently to affect herbivore communities. At the high pollution site, fertilization was also associated with increased catkin production and higher densities of a byturid beetle that feeds on the catkins of oak. Leaf nitrogen and nitrate were significantly higher at the high pollution site compared to the low pollution site. Foliar nitrate concentrations were positively correlated with abundance of sucking insects, leafrollers and plutellids in all three years of the study.

Understanding and controlling air pollution in highly populated areas is very important, although interpreting the levels of gaseous pollutants and airborne particulate matter is complicated by dominant natural and anthropogenic emissions, micro-meteorological processes, and chemical reactions which take place directly in the atmosphere. For this reason, it is very difficult to relate the characteristics of air pollution to one or more specific emission sources. The aim of this paper is to detect associations among elements and organic compounds emitted from specific sources by means of chemical analyses, statistical processing of data, seasonal evolution study, and geochemical considerations to trace their origin. A detailed characterization of air quality during the period September 2000-September 2001 was carried out in three locations of the Venice region: A heavy traffic urban site, a public park, and the island centre of the city of Venice. Twenty-eight inorganic elements, four polycyclic aromatic hydrocarbons, CO and benzene were quantified and processed by a statistical procedure based on factor analysis considering variations on a seasonal basis. Results show the presence of associations between elements and compounds with the same behaviour in all sampling points. This indicates that several pollutants originate from a common source, and are then “diluted” throughout the study area, maintaining the imprint of their origin. Pt, polycyclic aromatic hydrocarbons, CO and benzene originating from the exhaust gas of vehicles are all linked in the traffic factor, whereas Cd is associated with Se, having a common source in industrial processes.