Probably won't have much resolution for leaves. I've used ultrasonic transducers for a college class project almost 15 years ago. Ultrasound worked great for hard surfaces like walls, but softer materials like clothing reduced its range of detection. Snow covered trees, I think will have a similar signature that would be hard to detect except at close range. With a UAV, it could be done, but it'd have to be moving pretty slow I think. You'll probably have better luck with some sort of optical system.