Trees is a tie with a name borne of a lack of imagination, but it’s decorated with a multitude of little fir trees and Mulder wore it to look at a tree. Mulder only ever wore it in the one episode, Fresh Bones, which is criminal. Yes the episode was criminal but he fact that he never wore it again was also a crime. There’s a small possibility that the episode was filmed around Christmas time and this was a special Christmas tree tie, but sources would neither confirm nor deny this. You can’t get good imaginary sources these days.
The pattern really is too subtle for Mulder, but it does have a bit of a whimsical air, which is probably the real reason it disappeared. Scully has enough of a problem getting Mulder taken seriously, the last thing he needs is a whimsical tie, so after that assignment in North Carolina, Scully kindly offered to look after his dry cleaning so she could make sure that the Trees tie got ‘lost’. Somehow his underwear mysteriously got ‘lost’ as well, which he thought was odd because he didn’t usually get that dry cleaned. It does go to explain Mulder’s frequent noticeable lack of underwear. Not that I’d ever notice something like that.