Two other things about trees: * They hold tons and tons of water and are generally massive (if you've never given a hardwood tree a good pruning, the volume an

Two other things about trees: * They hold tons and tons of water and are generally massive (if you've never given a hardwood tree a good pruning, the volume and mass are surprising). A large, growing tree and its root system would add very significant load to the structure. * They blow over sometimes. Probably frequently, on an exposed, elevated rooftop with limited soil depth (shallow roots, fairly easy to saturate). 20 tons of tree flying off a tower during a storm doesn't sound like fun.