Thee University of Florida has a wealth of information regarding trees.
You can learn more by visiting their site at:
http://hort.ifas.ufl.edu/woody/pruning/index.htm

The international society of arboriculture website has information for
the consumer tree care website information and how to check on an
arborists certification. You can learn more by visiting their site at:
www.isa-arbor.com

The International Society of Arboriculture (ISA) created
www.treesaregood.com to provide the general public with quality
arboriculture, or tree care, related information. You can learn more by
visiting the site at:
www.treesaregood.com

Colorado State University extension has listed a wealth of information
on their website regarding everything from plants to trees. For more
information visit their website at:
http://www.ext.colostate.edu/Pubs/garden/pubgard.html

Colorado Trees is a non-profit with the mission of leading in Colorado's
efforts to preserve, renew and enhance community forests. For more
information visit their website at:
http://www.coloradotrees.org/