I received this question sometime ago, and I thought it was very insightful: “When a Christmas (Fraser fir) tree is cut down, is it still alive and why or why not? Also if it is still alive, how long can it live separated from the root ball?”
A cut Christmas tree is still alive after it has been severed from its roots. The tree has a finite amount of water in its tissues that is lost over time via transpiration from the needles. Christmas trees have a variable post-harvest shelf-life, which depends on the species. Eastern red cedar and Atlantic white cedar dry out very quickly, but Fraser fir and noble fir have a much slower drying time.
When a cut tree is rehydrated, it will absorb about 1 qt of water per day per inch of stem diameter. Water enters the tree from the cut end and is lost from the needles. To facilitate rehydration remove a 1/2 to 1 inch section from the base before putting the tree in a water-filled stand. The cut should be perpendicular to the trunk. Researchers have found that plain, unadulterated water is best. L. E. Hinesley and S. M. Blankenship, reported in HortTechnology (Oct/Dec 1991, 90-91; Effect of water additives on Fraser fir needle retention,) that water alone was better than the commercial floral and tree preservatives they evaluated as well as bleach (sodium hypochlorite), aspirin, sugar, and 7-Up®.