No look at tree art would be complete without profiling the practice of Pooktre. The hobby of training/shaping living trees and other woody plants into living art has been around for centuries and is known as Pooktre (aka arborsculpture.) It continues today by skilled arborists and enthusiastic green thumbs around the globe.
Here's a great link to the history, a world tour of the art
and a how-to to growing your own living chair and fence.
Living fiscus trees... amazing!
The following (also pics #2 & 3) were shaped by Axel Erlandson, one of the most famous Pooktre artists of the 20th century. At one point, in the late 1940's, he had over 55 shaped trees on his grounds in an attraction he called "Tree Circus" The above link is a great look at his life and contribution to the art form.