Some of the trees are not a 100% own design of mine, but have been heavily influenced by great models of two other top-LegoŽ-builders:

Take a look at John Gerlach's tree here:
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=35575

And have a look at these trees of Heiner Berg:
http://home.t-online.de/home/HeinerBerg/lokse.htm