As I stumped by lucky chance on this thread, so I decided to throw my 2cents into this discussion.
Actually walking by now far and wide accross several simulations , I must agree with WB, that trees in Gunship! are, despite of some age, one of the kind . Unfortunaly, any attempt to change them may turn out to be QUITE of the task if you want maintain desired perception of uniformity of vast numbers of trees. Belief that by introducing some variety into trebun.pcx you would enhance overal look of trees may lead you to pretty undesirable result of seeing the same pattern repeated. Tiling system is quite paradoxical, but at the same time very intriguing noneless.
There is fng2k's palm replacement texture which is very well fitting into this scheme. Not much of a help, I know, but still....