An Italian architect named Stefano Boeri has designed and is building high-rise apartments with forests growing up the sides. Pretty cool concept...I wonder how well it's going to work long-term, but a really great answer to some of the issues of urbanism.
I like the idea, but what about the forest floor? It seems like we are dealing with ideals verses reality. A forest ecology needs no more inputs than just sun and rain, will this system stand up to that? I would assume not, each floor will have to maintain its own ecology and that seems very unlikely considering no non flying animals will have access. So it's cool, but what are we going to have to license building maintenance workers as horticulturalists or ecologists? This is cool and I like it, but realistically this system will consume resources like water, attention, and maintenance that a forest by it's very nature does not need. So I would call bs on this being "forests" at all.
Personally I would have designed the 'forest' to be a large spiral that loops around the building, and include 'park land' inset areas or balconies that are a little larger. Not only would it offer more habitat movement for small animals, but it would also mean you could build a 'park path' option for building tenants to use to walk up to their floor instead of relying on the building's elevator.
With careful selection of plant life it wouldn't be that hard to have a far greener city core with several structures built in this concept. It wouldn't be like a natural deep forest region, but it would offer more space for growing things and small animal habitat.
My gut, too, is that it's seems a bit too tricky. I suppose I should have put 'forest' in quotes originally, because I don't think it's meant to be a literal forest. But it is a neat way (in theory, at this point) to help mitigate pollution and create habitat for birds and insects in an otherwise inhospitable place. I think the idea behind the idea, that cites need to be designed in a way that allows nature in, is a good one.
It's actually, I believe, a huge hydroponic system. The trees have been specially grown to be able to survive like that, and I would guess that's how they avoid the tree roots ripping apart the buildings as they grow.
Of course, a better way to invite nature into the urban environment is to design cities better to begin with. But maybe more difficult with a city already developed, so this is one idea of a retrofit. I wonder if other plant life might not work better, like big hanging plants that they could then "chop and drop", removing plant matter (and thus sequestered carbon and pollution) to maybe a big compost pile on the edge of the city. Could be collected along with food scraps, to return nutrients to the place where the food for the city was being produced.