I've noticed trend amongst some of my favorite artists -- fantasy cities in forests and trees. Of course, everyone has the childhood connection to tree houses. However, I see this trend as a need to connect back to nature. In many modern cities it seems that we lost our connection to the natural world and many of us yearn to get that connection back. Most of us have grown accustomed to the every day comforts and luxuries that modern cities bring us. These artist renderings are a way to merge those two worlds, modern day luxury and nature combined in harmony - cities growing in the trees.
Also check out some crazy - actual tree house and tree house concepts at Inhabitat. Especially the treehouses from Free Spirit Spheres, they remind me of the scene in the Oompa Loompa village in the newer Charlie and the Chocolate Factory. Also check out these renderings of the Green Towers, planned to be built in Seoul, South Korea.