3 literature references:

Author: Farjon, A.
Year: 2010
Title: A Handbook of the World's Conifers
Source: E.J. Brill, Leiden
Author: Eckenwalder, J.
Year: 2009
Title: Conifers of the world
Source: Timber Press, Portland, Oregon
Author: Little, E.L.
Year: 1979
Title: Checklist of United States trees (native and naturalized)
Source: Forest Service, USDA, Washington, D.C.