We always cut off the base of the Christmas tree before placing it in water.  Experts can interpret the tree rings and judge the weather and conditions the tree faced each year with the light wood rings indicating the spring growth and the dark rings the summer growth.  Wide rings show a good growing season with sufficient rainfall whereas narrow rings mean that the tree had fewer nutrients and less sunlight.  A rough estimate of our 2 metre tree is about eleven years according to the rings.  (The dark marks are probably oil from the saw)