Air Pollution and Vegetation Change in Southern California Coastal Sage Scrub: A Comparison with Chaparral and Coniferous Forest 1

The coastal sage scrub (CSS) vegetation of southern California is rapidly converting to annual grasslands, perhaps in part because of air pollution. By contrast, chaparral and coniferous forest are subject to equally high levels of air pollution but are relatively stable. A comparative analysis of ozone and nitrogen deposition on plants of CSS, exotic annual grassland, chaparral, and coniferous forest shows these vegetation types have different susceptibilities to each pollutant. Historically high concentrations of ozone in the local mountains weakened pines, contributing to tree mortality. Native shrub seedlings had decreased growth in chambers with current-day levels of 150 ppb ozone. Under natural field conditions the shrubs may escape ozone injury by being physiologically active early in the season when ozone concentrations are below phytotoxic levels. Summer-active pines are more susceptible to ozone than summer-deciduous CSS shrubs and senescent annual grasses. Nitrogen deposition has different impacts from ozone because N accumulates on leaf and soil surfaces during the summer. Conifers are more susceptible to leaf-deposited nitric acid because they are physiologically active in summer, while chaparral may be less so because of thick cuticles and reduced summertime activity. Deciduous CSS and senescent grasses are less susceptible to direct leaf damage. However, N becomes available for root uptake after the first fall rains. Soil accumulations up to 87 µg/g extractable N have been measured in surface soil of CSS shrubland, levels that have caused mortality in the greenhouse. Grasses may escape the deleterious effects of high soil N levels because of their annual habit. Coniferous forest may have a higher threshold for N damage because of high stand biomass, high N immobilization in soil organic matter, and watershed N runoff. The resistance of chaparral to high N is less well understood, but may be due to higher biomass and slower growth rates than CSS and also high leachate losses of N.