I believe your signal will be cut in half for about every 20 feet of trees it will pass through. I have seen estimates of .15db loss per foot, so .15 x 20 ft= 3db (50% loss) I have a similar situation, a friend 1.5 miles away, and there are too many trees to get a good link going between us.

here is a site with a wireless link analysis tool

http://www.gbonline.com/~multiplx/wireless/wireless.main.cgi