Robert S. Thompson, Katherine H. Anderson and Patrick J. Bartlein. 1999. Atlas of Relations Between Climatic Parameters and Distributions of Important Trees and Shrubs in North America. U.S. Geological Survey Professional Paper 1650 A&B. URL=http://greenwood.cr.usgs.gov/pub/ppapers/p1650-a/pages/conifers.html, accessed 2007.11.11. Maps and ArcView shapefiles can also be downloaded at http://esp.cr.usgs.gov/data/atlas/little/, accessed 2007.11.11.