Yacht(sailboat) racing term referring to the visual speed indicator of looking at a competitors sail edges and the trees on the land behind. If trees are appearing in front of the other vessel, then you are going faster. If the trees are appearing behind the other vessel, then it is going faster.
Related term: A Real Tree Maker - meaning a fast boat.
Making Trees/Tree Maker: Hey, I just spent three days fairing the boats hull. It's going to be a Real Tree Maker now.