[Readers, please note that I am building this page bit by bit for western states. Thanks for your understanding and patience. Last update, 8.1.2011.]
Lady Bird Johnson Native Plant Database. Centered near Austin, Texas, the Lady Bird Johnson Wildlife Center has a database that includes trees in all states.
The Texas Forest Service has a great guide and identification analysis: Texas Forest Service Tree Guide.
Gardening from the Ground Up: Trees and Shrubs Native to New Mexico. An abbreviated source without photographs, but does provide common and scientific names of trees with a brief description. Some links with other databases.
New Mexico Envirothon Tree Identification Forestry Key. A very good .pdf file. This file has a study guide for the botanical attributes of conifers and other New Mexico trees, color photographs, scientific names of trees and a short reference section. A short identification guide that is quite scientific.
GENERAL SOURCES NORTH AMERICA
Rutgers Landscape and Nursery Services, New Jersey. Although outside the western states, this site contains several categories of trees with accompanying photographs. This website is NOT affiliated with Rutgers University, but is a commercial website. It does, however, offer some data on trees and shrubs.
Documents, ebooks and guides for trees throughout North America. This is a collection of documents of all types that may be downloaded. Managed by Netopdf.info. Extensive anthology, mostly by states.