One of the thing I find amazing in Southern California is how various the vegetation and climate can be within very short distances. One common thing, it is always very dry.
In this area more than anywhere else, it really looks like there is a different ecosystem, vegetation type on any single mountain slope, valley and canyon. The climate seems always to be hesitating between the mediterraneen type and desert arid type, and even a slight change in elevation, sun exposure, distance from the Pacific Ocean can result in dramatic changes.
Transition areas are always interesting, you can for instance see a cactus next to a pine.
On the picture, a pretty sharp transition between a forest-like vegetation and a desert. The difference between a north slope and a south slope of a mountain is pretty dramatic as well as the one between the summit of the mountain and the bottom of the valley. (This picture was taken in the San Jacinto Mountains)