Volume 28, Number 3, February 2016
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How can we tell how old a tree is?
Liz Sheffield
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:
www.youtube.com/watch?v=xmZO7aRgcW4
Radiocarbon dating
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
Philip Allan Publishers © 2016
www.hoddereducation.co.uk/biologicalsciencesreview
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 www.radiocarbon.com/about-carbon-dating.htm
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:
https://c14.arch.ox.ac.uk/embed.php?File=calibration.html
Dendrochronology
(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: www.youtube.com/watch?v=oRYsAn5QLYo. The
current record holder as the oldest living tree is a pine, Pinus longaeva, 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: www.rmtrr.org/oldlist.htm
Philip Allan Publishers © 2016
www.hoddereducation.co.uk/biologicalsciencesreview