Air Pollution Increases Forest Susceptibility to Wildfires in Southern California

N.E. Grulke, R.A. Minnich, T. Paine, A. Dunn, and D. Chavez

USDA Forest Service Pacific Southwest Research Station (1,5)
University of California Department of Geography (2) and Department of Entomology (3)
USDA Forest Service San Bernardino National Forest (4)

Many factors increase susceptibility of forests to wildfire in California. Among them are increases in human population, changes in land use, increased fire starts, 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. Air pollution, specifically ozone 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, bark beetle attack, and combined, and these changes contribute to whole ecosystem susceptibility to wildfire. For example, elevated O3 and N deposition increase leaf turnover rates, N content of leaf 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 mass is compromised, overwinter carbohydrates are stored in the bole, perhaps contributing to increased beetle fecundity. 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, 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.

Native Pests Session - Wednesday Afternoon

corresponding author:

Nancy E. Grulke
Pacific Southwest Research Station
4955 Canyon Crest Drive
Riverside, CA 92507
951-680-1556
ngrulke@fs.fed.us