As the name implies, tree sitting is a "form of protest which involves sitting in a tree. Until the demands of the tree sitter are met or the tree sitter is forcibly extracted, he or she will be supported by an extensive ground crew." Technically, tree sitting is illegal but it has become a popular form of symbolic protest in the name of standing up to clear cutting and the power of lumber companies.
Perhaps the best-known example of tree sitting came when Julia Butterfly Hill "sat" in a 180-foot tall California Redwood tree for 738 days. From December 10, 1997 to December 18, 1999, Hill lived in the 600-year-old tree she named "Luna" to prevent loggers of the Pacific Lumber Company from cutting it down. Eventually, the company agreed to preserve Luna and all trees within a three-acre buffer zone...and Hill ended her civil disobedience.
Currently, the longest running urban tree-sit continues in Berkeley, California