I've been doing some work on trees for Targetware and have two diffierent approaches, one is a detailed molel, with 4000 polys, too much for most peoples systems, the other a hybrid flat model with 8-110 polys depending on shape.
Approch One, high poly trees:
http://www.mossie.me.uk/ww1/models/m...ery/image2.jpg
Approach Two, max 110 polys. The second way allows some 10,000 trees before fps settings drop to low levels.
http://www.mossie.me.uk/ww1/models/m...cenery/004.jpg
http://www.mossie.me.uk/ww1/models/m...cenery/005.jpg
http://www.mossie.me.uk/ww1/models/m...cenery/006.jpg
I've continued the development to cover a wide range of tree types, and building more to suit specific mod locations.