I know this advice is a little too late, but, I too live in the Sierras - frickin' trees everywhere. As I approach trees I tilt the camera down slightly - say15-20°. And I ask myself, "Can I see the 'tip-tops' of the trees?" If you can see the tips, then you are flying above them. If not, then prepare to call a tree service - as suggested above. Good luck - report back if you successfully recover the drone.