This editorial in today's LA Times deserves a full quote, but in the interest of fair use, I'll just give you the gist of it below.
More frequent and intense brush fires are threatening Southern California's native landscape.
April 22, 2009
Spring is in its early stages, which means the wilderness parks are still abloom with red paintbrush and blue-eyed grass. In Chino Hills State Park, though, the palette is markedly different: pale yellow mustard, purple thistle and the white-and-lavender flowers of wild radish, none of them native plants. It's a pretty scene right now, but one that tells a story about a worsening cycle of wildfire that threatens to transform the Southern California landscape.
Ravaged by two brush fires in November, the park is now firmly in the grip of opportunistic invaders. Aside from a lone poppy hidden amid the radish, there wasn't a native plant to be found on a recent hike along a mile and a half of Aliso Canyon.
The 2008 fires came too soon after other wildfires. They burned with particular intensity and blackened 95% of the park. The combination provided a perfect opening for weeds such as mustard to choke out native flora.
Noxious plants make an inferior habitat for wildlife; they also turn brown and dry faster than native plants, lengthening the fire season. As global warming accelerates the fire cycle, Chino Hills could be an early example of sweeping, long-term change in the open spaces that Southern Californians cherish. ...