@ ProfGremlin, thanks for sharing this technique. I may play with it a bit myself.

@ Master TMO, thank you as well for sharing your technique. The end result looks wonderful. GIMP doesn't have some of the filters you mentioned; neither does it come with layer styles. However, there is a plugin for GIMP that emulates some of the capacities of PS's layer styles. I am in the process of experimenting with other ways to make photo-realistic looking trees, and I'll definitely see if I can reproduce your method in GIMP.

Cheers,
-Arsheesh