Not really a universal index, but interesting nevertheless: MIT Treepedia has indexed some cities based on tree canopies from Google street view. Some (to me)

Not really a universal index, but interesting nevertheless: MIT Treepedia has indexed some cities based on tree canopies from Google street view. Some (to me) surprising finds, like Paris (8.8%) vs NY (13.5%). https://senseable.mit.edu/treepedia https://senseable.mit.edu/treepedia