Ideas Wise
I reckon the idea of a green enviromental forum is FANTASTIC. It's about time enviromentalists and people who actually care for the enviroment voice out opinions and discuss issues as a group. Who knows in the near future the forum members might even help in a greater movement towards this global issue.
I think marketing the forum to some australians might be good as well, there's heaps of enviromentalists here where I am. So that might direct more traffic to your forum as well.
Graphics and Layout Wise
I suppose a more greenish layout might do the trick, something fresher and more leafy
The logo can be better, something more FRESH and JUICY and add a header behind the logo, that would totally make the forum shine.
A favicon could be added once there's heaps of traffic.
Cheers