One of Peter Kahn's recent graduates, Jolina Ruckert, was quoted extensively by the Washington Post on the psychological benefits of exposure to nature.
You can read about it here: http://www.washingtonpost.com/business/on-leadership/the-business-of-e%20%3E%20scapism-means-that-roughing-it-in-the-outdoors-is-now-optional/2014/09/05/ae5db78c-3396-11e4-a723-fa3895a25d02_story.html