Yes, they are beautiful and natural. If you don't buy into a humanity/nature dichotomy then skyscrapers can be as well.
Skyscrapers can be beautiful. Personally, I really like seeing more plants incorporated into them. Roofs with lush gardens and scapes, plants growing along the sides.
Not really anything natural about a carefully manicured root system which has to be continuously maintained by people to grow a certain way or it goes "wild" and is unusable as a bridge or gets too heavy and collapses. However it is beautiful and certainly a great metaphor.
Nothing natural about a plant growing? I don't agree, irrespective of whether human intervention is required. It's a living organism that requires a dependency (humans) for longevity, but it is still very much alive and natural.