The "recycling" of Christmas trees in Los Angeles County is actually kind of complicated. According to a story posted at ZevWeb, the 100,000 or so trees the city of Los Angeles picks up are processed and made available as free mulch to residents of the city, or sent off to farms in Kern and Ventura counties. The 25 tons worth of trees recycled in Santa Monica are used as mulch around that city. On Catalina Island, old Christmas trees are placed into gullies by school kids for erosion control.
Then there's the 300,000 or so trees picked up elsewhere in the county that are put to use in the actual landfills where they are, in theory, being recycled from. The trees are used to cover the day's delivery of regular trash to reduce odors and prevent the garbage from being blown around by the wind. These trees will end up buried in the landfill, but they still are technically recycled.
More at the website of Supervisor Zev Yaroslavsky.