I had the opportunity to visit Nagold this summer in Germany, where an architect had a project called the tree cube. http://www.ferdinandludwig.com/plane-tree-cube-nagold/articles/plane-tree-cube-nagold.html
This project is by Ferdinand Ludwig, who is in the Research Group Baubotanik at the university of Stuttgart, a group that consists of just himself.
This tree cube takes conception from the idea of parasites that grow around trees, and when the trees actually die, the parasites actually become sturdy enough that they form a “tree” themselves.
So the tree cube is initially supported by steel members, which will be removed as the parasitic plants grows more and becomes structural.
one can see the steel members on the inside, each one marked with a year, which is the year that the steel can be removed. Eventually they will all be removed, and the tree cube is supported by the plants alone.
The plants are rooted in these planters arrayed on the outside of the cube.
The one at Nagold is still in baby form, here are pictures of a footbridge, where the trees have grown big enough to support the weight of the bridge and people.