Air pollution increases forest susceptibility to wildfires: A case study

Many factors increase susceptibility of forests to wildfire. Among them are increases in human population, changes in land use, fire suppression, and frequent droughts. These and other factors have been exacerbating forest susceptibility to wildfires over the past century in southern California. We report on the significant role that air pollution has had on increasing forest susceptibility to wildfires, based on a 1999–2003 case study in the San Bernardino Mountains. Air pollution, specifically ozone (O3) and wet and dry deposition of nitrogenous (N) compounds as a by-product of fossil fuel combus-tion, has significantly increased since urbanization and industrializa-tion of the region after 1945. Ozone and elevated N deposition cause specific changes in forest tree carbon (C), N, and water balance that enhance individual tree susceptibility to drought, bark beetle attack, and disease, and when combined, contribute to whole ecosystem susceptibility to wildfire. For example, elevated O3 and N deposition increase leaf turnover rates, leaf and branch litter, and decrease decomposability of litter, creating excessively deep litter layers in mixed-conifer forests affected by air pollutants. Elevated O3 and N deposition decrease the proportion of whole tree biomass in foliage and roots, thereby increasing tree susceptibility to drought and beetle attack. Because both foliar and root mass are compromised, carbohydrates are stored in the bole over winter. Elevated O3 increases drought stress by significantly reducing plant control of Corresponding author