Introduction

Diagram of factors

Many factors combine to increase forest susceptibility to wildfire in southern California, and most of these were set in motion decades ago. These factors include a rapid increase in human population and resource use; a shift from timber production to recreational forest use; fire suppression with subsequent forest densification; periodic, extreme drought; and bark beetle outbreaks. The contribution of air pollution to forest susceptibility to wildfire has not been studied extensively. In this paper, we will link air pollution to increasing forest densification, litter build up, drought stress, tree susceptibility to successful bark beetle attack, tree mortality, and increased forest susceptibility to wildfire (Figure on the right). A case study will be presented for the San Bernardino Mountain Range in the Transverse Range north and east of Los Angeles, California. We will focus on pollutant effects on ponderosa pine, which dominates the mixed conifer forest in the western part of the range.

In the late 19^th century, gold and other valuable minerals were discovered in the San Bernardino Mountains, and the population rapidly increased (Minnich 1988). The forest was logged for buildings, mine shaft support, and for fuel. In 1899, a severe drought occurred, water was limiting, and a premium was placed on reservoir development (Lake Gregory, Arrowhead, Big Bear). As the reservoirs were established, they became magnets for recreation use in the 1920s. With the shift from resource utilization to recreation, incursions of fire from the chaparral into the forest were suppressed, and forest density increased through the 1940s. In the 1950s, the Forest Service made an attempt to thin the forests, but, for aesthetic reasons, the mountain communities strongly opposed both branch trimming and stand thinning. As a consequence, the forest continued to increase in density, and trees grew increasingly closer to structures. In the 1980s, the community councils drew up “Forest Plans” that included branch trimming and thinning of trees within 30 m of valued structures (Asher and Forrest 1982). However, these recommendations were not followed or enforced. The region was, and is, highly susceptible to wildfire.

Next: Effects of Periodic Drought

Click to view citations... Literature Cited

• Asher, J.E.; Forrest, D.W. 1982. Lake Arrowhead communities forest management plan’s “State of the Forest” report. Lake Arrowhead, CA: Lake Arrowhead Property Owners Association.
• Minnich, R.A. 1988. The biogeography of fire in the San Bernardino Mountains of California: A historical survey. University of California Publication in Geography 28. 128 p.