One of the things I hear from clients who have moved to California from other regions of the country is, "there's no fall color here." And while I'll grant that the Bay Area is no New England, I have to say that this autumn the trees seem particuarly brilliant.
While lots of factors influence fall leaf color, what I've noticed this year is that [a] we had a fairly mild summer, so fewer leaves scorched and departed early; [b] the cold nights arrived with a vengeance right around the autumnal equinox, giving the trees not one but two cues (light and temperature) that it's time for dormancy; and [c] the dry summer has persisted into fall, stressing trees into withdrawing their green chlorophyll earlier and more quickly (thereby revealing the natural anthocyanins and carotene colors).
Some trees with legendary fall color include Acer freemanii (Red Maple), Ginkgo biloba (Maidenhair Tree), Quercus coccinea (Scarlet Oak), Acer griseum (Paperbark Maple), the native Acer circinatum (Vine Maple),
and of course the reliably fluorescent Pistacia chinensis (Chinese Pistache) and Liquidambar styraciflua (Sweet Gum). From a drive around my Palo Alto neighborood this morning, I've posted photos of these and more at my Flickr site.
To my eye, the colors peaked late last week, so as you're out and about over the next few days, I hope you'll make it a point to notice and appreciate the colors before they come back to earth. If you see something particularly stunning, feel free to send it in. And if you have favorite autumn plants of your own, please share!