The International Society of Arboriculture is a worldwide professional organization dedicated to fostering a greater appreciation for trees and to promoting research, technology, and the professional practice of arboriculture.
http://www.isa-arbor.com

National Arbor Day Foundation is an organization that helps people plant and care for trees and encourages the celebration of Arbor Day to advance global environmental stewardship for the benefit of this and future generations.
http://www.arborday.org

Street Tree Seminar Inc. is an organization comprised of urban forestry professionals who are dedicated to the development, health, care and management of street trees in the Southern California area.
http://www.streettreeseminar.com

The vision of West Coast Arborists, Inc.'s, the City's Urban Forestry Manager, is to deliver the best Urban Forestry Management Program in California. We achieve our vision by making Total Quality our way of doing business. Total Quality is assured by striving for 100% customer satisfaction, by leading the industry in state of the art urban tree care services, and by performing superior and safe operations on a 24 hour/7 day basis.
http://www.wcainc.com