Some info about fire and the San Gabriel Mts. plant life:

Fire is the primary regenerative force in the chaparral plant habitat. In fact, many plant species require fire to reproduce. Historically, the fire cycle in the mountain ranges of Southern California has been approximately 40-60 years. However, with increased urbanization, human ignitions have shortened this cycle to 30-40 years. As a result, particularly in the San Gabriel foothills, chaparral shrubs are less able to re-colonize amidst frequent seedling-killing fires and must also face tough competition from non-native grasses. This has led to a habitat conversion from shrubs to grasses.

(link to pdf: 74.125.155.132)