The potential impact of oxidant air pollution on forest trees has been a major international focus of environmental concern and research. As more scientific data have become available, there has been an increasing realization of the complexity of interactions involved in understanding pollutant impacts at the levels of the individual forest tree, forest stand, and ecosystem. Pollutant effects from ozone, sulfur dioxide, acid deposition and nitrate deposition all interact and are further complicated by cycles of drought, pathogen outbreaks, fire history and other variables.

No better case study for understanding the complexity of environmental impacts of oxidant air pollution exists than that of the San Bernardino Mountains in Southern California. Lying downwind from the greater Los Angeles airshed, the montane forests of this range have had a long history of chronic exposure to high levels of ozone, particulate nitrate, and other pollutants. Observations of foliar necroses on native pines in the 1950s led to research that identified ozone as the causal agent. Moreover, early studies documented that pines exhibiting extensive foliar damage and associated reduction in needle longevity were more likely than healthy trees to be weakened and vulnerable to bark beetle attack and death.

The significance of these findings on forest health clearly established the importance of a better understanding of air pollutant impacts on montane forests of the San Bernardino Mountains and provided the impetus for the development of a large multidisciplinary study. This program of research, led by a team of scientists from the USDA Forest Service and the University of California, was initiated in 1974 and continues today. This volume presents an overview of 3 decades of research on oxidant air pollution impacts and nitrate deposition in these montane forests.

The volume is comprised of 18 chapters involving contributions from 35 authors and coauthors. Approximately half are current or former staff of the USDA Forest Service Fire Laboratory in Riverside, California. Chapters are grouped into four sections. Section 1, on physical geography, includes chapters on geomorphology and soils, climatology, and vegetation and fire dynamics. Section 2, on the effects of pollutants on vegetation and soils, groups 10 chapters covering a range of topics including the physiological ecology of pollutant effects, pollutant effects on conifer growth, wet- and dryfall inputs of pollutants, nitric acid impacts, and nitrogen deposition. The four chapters in Section 3 discuss interactions of pollutants with climate and biological factors and the associated influence on forest health and human use of forest habitats. Finally, Section 4 provides syntheses and conclusions for the volume in 2 chapters, one on ecological risk and policy implications of air pollutants and the other on forest management and research priorities under conditions of chronic air pollution.

The broad significance of the studies reported in this volume lies both with the long record of environmental studies and the multidisciplinary focus and intensity of this research. Long-term and detailed records of meteorological data and pollutant monitoring have allowed a better understanding of the complex interactions of pollutant concentrations and dynamics with climatic variables such as strength of the atmospheric temperature inversion, velocity of onshore winds, and differences in pressure gradients between ocean and interior deserts. While ozone is the primary pollutant directly responsible for foliar necroses and reduced needle longevity in pines, oxides of nitrogen in both gaseous and particular form are also shown to be highly significant. Nitrate particulates are the most important factor in reducing atmospheric visibility, and nitrate deposition on the forest canopy and soil have profound long-term impacts on nitrogen cycling and potential nitrogen saturation in these forest systems.

The major focus of this volume lies with the impacts of chronic ozone pollution on the needle condition, physiology, growth, and general longevity of dominant forest conifers, particularly Pinus ponderosa. However, it is useful to have additional contributions documenting the impacts of air pollution in the massive historical decline of lichen biomass and diversity in the San Bernardino Mountains and the evidence of widespread oxidant pollutant damage to understorey vegetation. A significant observation is that a number of annual species continue to show severe necroses from oxidant pollutants after more than 4 decades of chronic exposure of their populations, suggesting little selection toward ozone-resistant genotypes.

What has been the net impact of air pollution on forest structure and ecosystem function in the montane forests of the San Bernardino Mountains? Oxidant pollutant impacts to conifers reduce photosynthetic productivity and limit the supply of carbohydrates available for growth. Both decreased crown health from needle loss and decreased radial growth predispose trees to increased levels of bark beetle infestation and death. A reduction of cone and seed production in Pinus ponderosa has also been noted under conditions of chronic pollutant stress, reducing the regeneration capacity of these trees. Projections of forest succession models for the San Bernardino Mountains, using all of these data on pollution stress, indicate a likely gradual shift in dominance away from Pinus ponderosa toward Abies concolor and Calocedrus decurrens, which are more tolerant of oxidant pollutants. Moreover, field measurements have shown that increased dry and wet deposition of nitrates has augmented levels of soil nitrogen significantly. Simulation studies suggest that continued levels of deposition of this magnitude could lead to nitrogen saturation of soils and problems of nitrate pollution of ground water.

The breadth of topics covered in this volume should make it of interest to a broad range of scientists and resource managers with interests in issues relating to air pollution impacts on forests. The editors have accomplished a valuable task in pulling together these diverse studies into a comprehensive look at ozone and nitrate pollution impacts in Southern California's mountains.