The trees that shade, cool and feed people from Ventura County to the Mexican border are dying so fast that within a few years it’s possible the region will look, feel, sound and smell much less pleasant than it does now.“We’re witnessing a transition to a post-oasis landscape in Southern California,” says Greg McPherson, a supervisory research forester with the U.S. Forest Service who has been studying what he and others call an unprecedented die-off of the trees greening Southern California’s parks, campuses and yards. Botanists in recent years have documented insect and disease infestations as they’ve hop-scotched about the...