http://home.arcor.de/Matthiasth/b�ume2.JPG
31 sec render time
31.5 MB blend file 
I see why it took you 31 MB of blend ! lots of vertices from the distance !
Not bad at all.
However, in my own experience, you’ll get better results by cheating a little… using complex textures but simpler models for the leaves and things like that.
Good luck and keep it up !
good work, but as said above a few more textures can make a pic 10 times better
oh btw you should do a google search for things like tree textures etc, that will give the trees a very real edge, but once again well done.
Um, the trees look very similar… A bit of variance would help. Not all trees stand perfectly straight up and down either…
You may get good results by faking some of the trees in the distance, use tree images mapped onto planes. These will look OK in the distance, and will save your vertices for the trees in the foreground!
Do you have access to a flat bed scanner? If so, try scanning some leaves in, or some twigs, or even some bits of bark from dead branches. Use these for making texture maps…
regards
brian