It depends on your location, the satellite(s) you're trying to receive, and the height of the tree(s) in question.
I have a 20' tree that is about 15' or so in front of my 10' and 4' dishes, but since my lattitude is around 30 degrees and the tree is due south it doesn't really interfere with my reception. The 10' dish is about 8 feet off the ground and my 4' is about 6 feet off the ground.
You can try using dish pointer...it has a tool that allows you to judge whether an obsticle's height will pose a problem.