Note: This story is part of a special HCN magazine issue devoted to travel in the West. San Francisco, California Let's say you've freed up a couple days and more than a couple bucks to visit San Francisco. Unlike the hordes of tourists who visit this city each year, you'd rather not spend your entire visit doing predictable activities like slurping on crab legs at Fisherman's Wharf and photographing the heck out of the Golden Gate Bridge. You're all for urban culture, good eats and fancy shoes, and maybe even an evening at the theater, but your deeper allegiance lies with ferns and boulders and watersheds, the lineaments of the land and the elemental realities of place. So explore the city's trees. You can enjoy this kind of special tourism in many cities, but San Francisco might be the best place for it. The trees here are unlike other