Trees and other urban vegetation could provide significant provisioning ecosystem services, or things humans can eat or otherwise use. This doesn’t surprise me, just like finding out the amount of theoretically usable material in our garbage is vast. The problem is coming up with a good business plan to collect and distribute it. I think it could be done, but not many of society’s creative thinkers are focused on this area.

http://www.sciencedirect.com/science/article/pii/S0169204617302566