Physiological/ecological interactions between ozone and nitrogen deposition in forest ecosystems

Ozone (O-3) is considered as the main factor in the air pollution-related decline of forests in North America and Europe. Decline of ponderosa and Jeffrey pines in the San Bernardino Mountains of southern California and pronounced foliar injury of these species in the Sierra Nevada Mountains of central California are well-known examples of the O-3 effects. Other components of photochemical smog, especially nitrogenous (N) compounds, may also play an important role in the observed changes in forests and other ecosystems. Although direct effects of HNO3 Vapor and peroxyacetyl nitrate (PAN) have not been observed in field conditions, these pollutants have a potential for causing foliar injury of sensitive plants at elevated concentrations. In addition, long-term dry deposition of N gases and particles as well as wet deposition of NO3- and NH4+ can significantly change plant N status. Below critical load (threshold) values of N deposition, changes in phenology of plants, higher rates of physiological processes, changed chemical composition, altered biochemical processes and improved growth of plants may occur. At levels exceeding N critical load, nutrient deficiencies and other negative effects may lead to decreased growth of trees and changes in species composition. Possible biochemical, physiological, and ecological effects Of O-3 and N deposition singly and in combination, mainly based on results of studies performed in mixed conifer forest of California, are discussed.