The following sites are the research showing that properly cared for real trees do not burn. 

          http://www.fpl.fs.fed.us/documnts/pdf1997/white97b.pdf 
http://www.redlionpa.org/Christma.htm
http://fire.nist.gov/bfrlpubs/fire00/PDF/f00147.pdf
http://www.mcta.org/firesafety.htm

This is the web site of the National Christmas Tree Association.  All kinds of info on Christmas Trees.

http://www.realchristmastrees.org

Info about many of the Christmas tree farms in South Dakota. 

http://www.christmas-tree.com/real/sd//real/sd/

Detailed description of the various Christmas tree species.

http://www.urbanext.uiuc.edu/trees/treetypes.html