Fire researchers at the National Institute of Standards and Technology (NIST) remind us that proper care of a cut Christmas tree is important to retaining high moisture content in the tree's needles to prevent accidental ignition and rapid flame spread.
And if you need any convincing, they have posted a series of cautionary videos, along with background information, on an updated Website, Fire Safety for the Holidays.
The most telling is a side-by-side comparison showing what happens to a properly maintained tree and a dry tree after they are exposed to a source of ignition. Within 15 seconds, the dry tree is engulfed in flames; after 90 seconds, all the needles have burned away and only the tree trunk and portions of some of the larger branches are intact. In contrast, needles of the properly maintained (wet) tree ignite and burn locally, but the flame does not spread.
The bottom line: A tree with dry needles can readily ignite and generate heat release rates that are capable of causing catastrophic fires in residential rooms. Videos can be downloaded from the site. Watch the side-by-side video on NIST's YouTube channel: http://youtu.be/AZk4vIXCnc8. (2017 NOTE: An updated version of the side-by-side video is now available on NIST's YouTube channel.)