Via the Sunset Scavenger RecologySF.com newsletter:
If you're wondering what happens to your tree once Sunset Scavenger picks it up: it's fed to a wood chipper. The wood chips are then trucked to Tracy, burned to power turbines and create electricity – and then sold back to PG&E to power that iPad you got as a gift this year. In 2009 San Francisco recycled nearly 500 tons (!) of holiday trees, enough to power 20,000 houses for a month.
Christmas tree collection will take place January 3-8 and January 10-15. Please place clean, unflocked trees next to your carts when you take them to the curb for your regularly scheduled pickup. Be sure to remove all tinsel, decorations, plastic bags, stands, and lights. If your tree is over 6 feet, please cut it in half.