Air pollution increases forest susceptibility to wildfires: A case study in the San Bernardino Mountains in southern California

Abstract

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 factors have been exacerbating forest susceptibility to wildfires over the last century in southern California. Here we report on the significant role that air pollution has on increasing forest susceptibility to wildfires, as unfolded in the San Bernardino Mountains from 1999 to 2003. Air pollution, specifically ozone (O3), and wet and dry deposition of nitrogenous compounds from fossil fuel combustion, has significantly increased since industrialization of the region after WWII. Ozone and elevated nitrogen deposition cause specific changes in forest tree carbon, nitrogen, and water balance that enhance individual tree susceptibility to drought and bark beetle attack, and these changes contribute to whole ecosystem susceptibility to wildfire. For example, elevated O3 and N deposition increase leaf turnover rates and leaf and branch litter, and decrease decomposability of litter. Uncharacteristically, deep litter layers develop in mixed conifer forests affected by air pollutants. Elevated O3 and N deposition decrease the proportion of whole tree biomass in foliage and roots, the latter effect increasing tree susceptibility to drought and beetle attack. Because both foliar and root masses are compromised, carbohydrates are stored in the bole over winter. Elevated O3 increases drought stress by significantly reducing plant control of water loss. The resulting increase in canopy transpiration, combined with [O3 + N deposition]-induced decreases in root mass significantly increase tree susceptibility to drought stress, and when additionally combined with increased bole carbohydrates, perhaps all contribute to success of bark beetle attack. Phenomenological and experimental evidence is presented to support the role of these factors contributing to the susceptibility of forests to wildfire in southern California.