From 1973 to 1978, an interdisciplinary study of the pine and mixed conifer forests of the San Bernardino Mountains of southern California measured the effects of 30 years' exposure to photochemical oxidant air pollution on selected ecological systems. Average 24-hour ozone concentrations in the San Bernardino Mountains during the May through September period ranged from a background of 3-4 pphm up to a maxima of 10-12 pphm. Ponderosa pine was very ozone sensitive; foliar injury occurred at 24-hour concentrations of 5-6 pphm followed by, in decreasing order of sensitivity, Jeffrey pine, white fir, black oak, incense cedar, and sugar pine. Foliar injury and premature leaf fall caused decreased photosynthetic capacity, suppressed radial growth of stems (a negative exponential relationship), and reduced nutrient retention in the green biomass, all leading to weakened trees. Pines became more susceptible to root rot (Fomes annosus) and pine beetles (Dendroctonus brevicomis); mortality rates reached 2-3 percent in some years. Litter depth was greatest in stands receiving the most injury and associated defoliation, hindering pine seed establishment but encouraging oxidant-tolerant species in the understory.

Caption title. At head of title: Environmental research brief. Distributed to depository libraries in microfiche. "August 1982." Includes bibliographical references. "EPA/600-D-82-276."