The life-span of trees is determined by growth rings. These can be seen if the tree is cut down or in cores taken from the edge to the center of the tree. Correct determination is only possible for trees which make growth rings, generally those which occur in seasonal climates. Trees in uniform non-seasonal tropical climates are always growing and do not have distinct growth rings. It is also only possible for trees which are solid to the center of the tree; many very old trees become hollow as the dead heartwood decays away. For some of these species, age estimates have been made on the basis of extrapolating current growth rates, but the results are usually little better than guesses or speculation. White proposed a method of estimating the age of large and veteran trees in the United Kingdom by correlation between a tree's stem diameter, growth character and age.