Walden Pond isn’t just the site of Henry David Thoreau’s two-year stint in which he documented a more simple, natural life, it is now the subject of a climate change study that shows how leaf-out times of trees and shrubs have changed since the 1850s. As a result of Thoreau’s observations, researchers at Boston University have revealed that the leaf-out times of trees and shrubs at Walden Pond are an average of 18 days earlier than observed by Thoreau in the mid 1800s.