Could it be "The Family Tree" by Sheri S Tepper? It's not been around for 20 years, published in 1998, but it is about nature (flora at least) fighting back against the concrete jungle. It's been a long time since I read it, but I think there was someone encased in a tree near the end.
http://www.harpercollins.com/books/F...=9780380791972