WOW! That's some good old Yankee ingenuity, to be sure. Since you asked for suggestions on preserving the wood, I would recommend a polyurethane finish - the kind that is used to coat furniture after the stain has been applied. You can find it at any hardware store. The advantage to making trees out of real twigs is that they LOOK real. I applaud your dedication to the hobby. Can we see pics of the trees in your layout when they are done?