Vertical climbing tank/rover
Don't forget that you will be needing (I'm no physicist either, but I'll venture a guess) 3 of your 5lbs of thrust to hold it against the tree. And the extra friction involved will undoubtedly increase its weight in the form of drag.
If it weighs 5lbs, I'm thinking you'll need at least 8lbs of thrust just to keep it glued to the tree without rising. so 10 -15lbs to make it advance. (and don't forget to add the weight of the engine at 10 - 15- lbs of thrust, you're talking about a big engine!)