But this is doable, if you are serious about it.
Ubiquiti wireless gear is the way to go right now, and there's lots of technical help on their forum and others. Their 900Mhz gear will handle SOME tree coverage, as will the 2.4Ghz. Their gear is so cheap that you can afford to make little house-to-house relays to get into hard to reach spots. Their wiki has a decent write-up of how to build a WISP with their gear.
There are lots of other gotchas in the biz, arranging tower sites (private landowners are good, but you'll need a solid contract), getting customers to actually pay you (at all, not just on time), each install is going to have to be paid for up front ($150-200) and you won't make any money off that customer for about 6-8 months, service truck & tools, insurance (wispinsurance.com) and lots more.
Go lurk on the Ubiquiti and Mikrotik forums for a few months, and you'll start getting a clear picture of what running a small ISP day-to-day is like.