Actually, just ticking the collide box usually sucks, cause the two planes forming trees are much bigger than the texture.. So you might collide much unexpended.. What I suggest doing is not making tree collide, but making a tire pillar on the trees place, and making it collidable, and then turn off rendering so that it appears invisible in game. So now you have the tree, and the tire collision posing instead of the tree one.. It should work quite nice. I hope I was understandable enough.. xD