It depends on how you get your tree. If you go out and chop a tree down in the forest, this is not the best way. You can disrupt local ecosystems, especially when many people choose to do it this way. Christmas tree farms, however, are extremely sustainable as they plant 1-3 seedlings for every tree they harvest. Artificial trees should be avoided as they are made of non-biodegradable plastic. Due to their size, you want to avoid buying this much plastic as much as possible. If you do want an artificial tree, try to find a used one to give it a second life.