Cutting down trees (especially over a limit that you've agreed on) pisses off elves.
This is good.
Elves that are in a good mood only bring along cloth and other such things to trade because they think that these things are most valuable. Sometimes they bring animals but usually normal ones (Dogs, cats, cows, goats).
Elves that are getting more annoyed tend to bring the real cool stuff, Giant Eagles, Bears, Cougars, and other Giant or otherwise cool animals.
Elves that are really pissed off go to war with you, and since they only use wooden weapons, they will loose horribly, as long as you have a nice set of traps ready, or a competent military and have them armed and armored, even in copper, they will own the elves. Sometimes almost never one gets a lucky shot with a bow and a dwarf will need some health care, but they'll be ok in the end.
You may not be aware, but it's normal for many members of the forum (including me) to just take everything the elves bring until they get mad enough to bring the cool stuff. Yeah you'll need to make a few copper weapons and sets of armor, but you don't even need to train your dorfs to fight, the first time the elves went to war with me I just had my woodcutter and 3 friends go and pickup the gear and they owned the elves, they had some bruises but they were fine.
Two last things:
1. Traps (like I said up there^) are pretty good for taking out elves, and cage traps are a good idea if you want to capture the elves, my current fort has 24 captured elves that I use to fight in my arena.
2. I apologize if you already know all this stuff, but I figured, I may as well share.