The video clip above from the Building and Fire Research Laboratory of the National Institute of Standards and Technology illustrates what happens when fire touches a dry tree. Within three seconds of ignition, the dry Scotch pine is completely ablaze. At five seconds, the fire extends up the tree and black smoke with searing gases streaks across the ceiling. Fresh air near the floor feeds the fire. The sofa, coffee table and the carpet ignite prior to any flame contact.
Wet trees tell a different story.
For comparative purposes, the NIST fire safety engineers selected a green Scotch pine, had it cut in their presence, had an additional two inches cut from the trunk's bottom, and placed the tree in a stand with at least a 7.6 liter water capacity. The researchers maintained the Scotch pine's water on a daily basis. A single match could not ignite the tree. A second attempt in which an electric current ignited an entire matchbook failed to fire the tree. Finally they applied an open flame to the tree using a propane torch.