I SWEAR there was a thread on this, one of those rant threads, oneof those threads where people argue and crap. Over petty little things, too...
Anyway, why?
Ok, heres my opinion anyway.
The fact that you're calling it a forest in the first place already gives everyone who reads it a basic idea of what the whole thing looks like, an abundance of tall, verdant trees. Thats a forest. What you need to do in the description is portray the interesting parts.
So seeing as the whole thing has already been described with one word, it gets the reader more involved with the story and the character if you begin to describe it from his point of view, as they enter the forest. So describe the little area around him, and then we have the idea of what the forest is like, its characteristics. Then thats all we need - so long as you continue to describe parts in relevence to the plot throughout.
Just describe it, nobody can tell you how to describe, its writing, its an art, basically. You can't tell soemone how to put feel into their work, you can't tell them how they can personalise it, because its their work and only they know. Try it, and see what people think, then learn from your reviews. Don't ask how to describe an area.
