You cannot disable trees. Its possible to use a different model for them though.. the most common used is the tree stump mod. Looks about this:
Search here for "tree hack" and you can find a link to the patch, I think it was in one of the guild forums.
Direct link, you need winrar to open it. Log all your accounts out, extract directly into your UO file. It will ask you replace all, say yes all. Then log back in. Makes you wonder how you ever played this game without it. http://download1697.mediafire.com/redy7f7pqtwg/aoglpz9d5j9pcls/UO-ArtFilesMUL_12-05-2013.rar Side note: This is not a "hack" you are only altering tree files and static deco files. So don't be surprised if you don't see hay on the ground anymore, enjoy.
side note, you may feel mod remorse down the road. I have two UO files, regular UO and one I named UOMOD. If I have some serious shtuff to do, I load the modded version... when I'm doing some RP, unmodded, dark nights etc. G'luck... Canute
Tree stump mod is bullshit. Somewhere ages ago I found a mod that makes trees felucca style, no leaves, much better.