I needed a snow covered pine and I remembered the Mundi collection by Nobiax (aka yughues at OGA) so I converted it to work in RE Here it is:
The files in the attachment are set up with a snow and no snow version. I had problems with converting the transperancy of the original tga's into pngs, hence the no snow skin is pretty crappy. There was also a half snow tga in the original download - http://opengameart.org/content/freebies-mundo-commissions as well as some other good models.
I have them in the RE vegetation folder, so to use them in your map you would add:
mmodel "vegetation/snowpine/snow"
mmodel "vegetation/snowpine/none"
Addendum: I had the name wrong (nobias) initially, which I have corrected to Nobiax. He is a very good artist and released heaps through OGA. I have converted a few other trees which I will post as I come across them.