Im willing to help do models and textures, just give me reference pictures and Ill get to work.
Certainly. I have some reference images on my comp some from Avatar itself and some from random artists/artworks I have found on the net.
However Deviantart is a good place to look for good reference images.
Here are a few i just found:
http://marcadamus.deviantart.com/art/Rainforest-Complexities-58690195
http://pmp.deviantart.com/art/Rainforest-Water-II-40371518
http://innocenteye.deviantart.com/art/Rainforest-37417788
http://nathie.deviantart.com/art/The-Old-Tree-75736813
http://inshield.deviantart.com/art/Boring-tree-9454030
EDIT: If you would like to make some trees that would be cool, I could then focus more on making the foliage, rocks and some fell tree's/logs. I would prefer some tree's to be rather large, think redwoods, and some trees to be more of an average size to add some diversity. As far as over all appearance something similar to the 3rd image linked above with moss and vines and such covering and wrapped around the trees. And also some more gnarly trees with interesting bends and twists. I have one tree similar to what i have described that I made a bit ago that can possibly give you an idea of what I mean for the more gnarly type of tree's.
I can cobble a few unique creatures to flit about/graze/lurk to add to the atmosphere (should there be a need ).
That would be cool, would add a bit more life to the place rather then just some boring foliage and junk.