The Effects of Nitrogen Deposition, Ambient Ozone, and Climate Change on Forests in the Western U.S

Nitrogen (N) deposition in the western United States is most severe near major urban areas or downwind of agricultural regions, particularly in areas where confined animal feeding operations such as dairies or feedlots are located. Nitrogen saturated ecosystems are predominantly found in hotspots located within 60 km of urban or agricultural emissions source areas, where N deposition inputs are 20 kg ha-1 yr-1 or greater. Nitrogen deposition gradients are steep with rapidly decreasing deposition with increasing distance from the source area. More subtle ecological effects of N deposition, such as fertilization effects and changes in sensitive biotic communities (for example, lichens and diatoms) occur over a much wider area than the severely affected hotspots. Effects on these sensitive ecosystem components are observed with N deposition levels as low as 3 to 8 kg ha-1 yr-1. Visual ozone injury in the West is most severe in pine trees in forests in southern California and in the southern Sierra Nevada in central California. Recent ozone exposure data from passive monitoring networks demonstrate that elevated ozone exposures can occur as far as 250 km from emissions source areas. The geographic scope of the areas affected by ozone has increased in the past 30 years as human populations and urban zones have increased in size, and this trend is expected to continue. The combined effects of ozone and N deposition result in profound changes in plant physiological function, nutrient cycling, C storage, fuel accumulation, and susceptibility to insect attack. Predicting future ecosystem condition under scenarios of increasing CO2 and temperature and altered precipitation patterns presents a complex research problem, particularly for areas also exposed to ozone and N deposition. Research approaches including controlled studies, manipulative field experiments, simulation modeling, and a consideration of disturbances such as pests, introduced species, fire, and drought are needed. Combined biogeography-biogeochemistry simulation models (also known as dynamic general vegetation models) incorporating all of these interacting factors will be needed to advance our understanding of these complex interactions.