Volume 28, Number 3, February 2016
How can we tell how old a tree is?
There are two methods, both based on the fact that tree trunks expand by laying down new growth on top of the old growth. So the innermost tissues of the trunk are the oldest, the outermost layers are the most recent (see the following diagram).
This means that the wood across the diameter of a tree trunk has been laid down throughout the life of the plant. Trees do not grow at a constant rate. When conditions are favourable (e.g. in the spring and summer) they grow quickly — creating big cells — ‘earlywood’. During the autumn and winter in temperate zones growth slows then stops — creating smaller cells — latewood. When growth starts up again in spring, this creates a distinct line in the wood. These lines mark between growth rings, and the width of the ring provides a good indication of what sort of conditions the tree experienced. In a good year the width is greater than in years when conditions are harsh —for example, when there has been a drought, or low temperatures. The movie in the following link shows how this information can be used to help water suppliers plan for periods of drought:
The wood in a tree trunk locks in place the carbon that was in the air as carbon dioxide when the leaves on the tree grabbed it for photosynthesis. So even if there are not clear rings, which it true of trees in the tropics, we can use our knowledge of the decay rate of radioactive isotopes to get a pretty accurate estimate of the age of a tree. Carbon comes in three forms. Radiocarbon — carbon 14 — is an isotope that is unstable and weakly radioactive (there are also two stable isotopes — carbon 12 and carbon 13). We know how long C14 takes to decay, so the amount of C14 left in a sample gives a pretty good indication of how old the sample is. See for a good explanation of how this works, and how the radioactivity is measured. The following site shows how tree growth rings can be used to provide a radiocarbon record:
(dendro- Greek for tree; chronology is the study of time)
Where tree trunks form growth rings we can get very accurate measurements of the age of particular specimens. Where there are both live and dead specimens, by making detailed measurements of the width of rings in different specimens we have generated records spanning back over 10,000 years. This kind of cross matching gives the most accurate measurements of the age of trees, and there is no need to cut a tree down to measure growth rings. Instead we can remove very slim cores from the trunk of living specimens, leaving the tree to seal the wound and carry on growing (see the resin canals in the diagram above, and read more in the centrespread of the current issue —‘High and mighty pines’). The following movie shows how a core is taken and explains how cores can provide a record of climatic conditions in times gone past: The current record holder as the oldest living tree is a pine, Pinuslongaeva, growing in the White Mountains of eastern California. See here for a list of the oldest known trees in the world, and how the information was obtained:
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