Around here they get chipped and sold for mulching. Smells great!
My uncle grew trees for resale. We harvested over Thanksgiving weekend, making them the "freshest" out there unless you cut it yourself, we planted two trees for every one harvested, and the university he worked for ran a recycling program to pick up and chip the trees then distribute the mulch. Also, you usually do not harvest the trees until they are 7 yrs old minimum, and thats 7 years of oxygen production under it's belt!