Conclusions

The role of air pollutants in increasing tree susceptibility to drought, successful bark beetle attack, tree mortality, and the susceptibility of forests to wildfire have not been studied extensively. Air pollutants, specifically strong oxidants and nitrogen deposition, contribute to increased litter accumulation and increased tree susceptibility to drought stress. It is well known that drought-stressed trees are more susceptible to successful bark beetle attack. The combination of chronic drought in 1999-2001 and acute drought in 2002 resulted in a bark beetle epidemic in the western San Bernardino Mountains. We contend that the severity of tree mortality in the western San Bernardino Mountains was significantly exacerbated by the higher air pollutant deposition in this region.

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