Simply put, dendrochronology is the dating of past events (climatic
changes) through study of tree ring growth. Botanists, foresters and
archaeologists began using this technique during the early part of the
20th century. Discovered by A.E. Douglass from the University of Arizona,
who noted that the wide rings of certain species of trees were produced
during wet years and, inversely, narrow rings during dry seasons.
Each year a tree adds a layer of wood to its trunk and branches thus
creating theannual rings we see when viewing a cross section. New wood
grows from the cambium layer between the old wood and the bark. In the
spring, when moisture is plentiful, the tree devotes its energy to
producing new growth cells. These first new cells are large, but as the
summer progresses their size decreases until, in the fall, growth stops
and cells die, with no new growth appearing until the next spring. The
contrast between these smaller old cells and next year's larger new cells
is enough to establish a ring, thus making counting possible.
Principles of Dendrochronology
A principle basic to any study of the past is the principle of "uniformity
in the order of nature", first proposed by James Hutton in 1785. It is
commonly stated as:
The present is the key to the past.
When applying this principle to dendrochronology, it says that
the variations in conditions present today must have been present in the
past. This does not mean to say that the conditions are exactly the same
but that similar kinds of influences affected the similar kinds of
processes. Checked through a variety of methods independent of tree-ring
growth, this principle has proved to be valid.
The climatic changes or patterns in specific geographic areas can be
traced by the study of old living trees. Samples taken from trees of
unknown age can then be studied for matches with samples from trees with
known sequences of growth. Using this process, when the rings "match" or
are found to be overlapping in age, we are able to "see" even further back
n example of this occurred in the 1920's when expeditions led by Douglass
dated Pueblo Bonito, a prehistoric native American settlement in New
Mexico. By analysing the timbers used in its construction, they determined
its existence 800 years before Columbus.
A chronology (arrangement of events in time) can be made by comparing
different samples. Using a boring tool, a long slender core sample about
.423 centimetres in diameter is extracted.
Lets say the sample was taken from a standing 4,000 year-old (but long
dead) bristlecone. Its outer growth rings were compared with the inner
rings of a living tree. If a pattern of individual ring widths in the two
samples prove to be identical at some point, we can carry dating further
into the past. With this method of matching overlapping patterns found in
different wood samples, bristlecone chronologies have been established
almost 9,000 years into the past.
It is an oversimplification to say that dendrochronology is ring counting
based on rainfall and the physiology of trees. Many other factors are
considered. This is especially true with the old bristlecones, as their
growth can be affected by slope gradient, sun, wind, soil
properties, temperature and snow accumulation.
The more a tree's rate of growth has been limited by such environmental
factors, the more variation in ring to ring growth will be present. This
variation is referred to as sensitivity and the lack of ring variability
is called complacency. Trees showing sensitive rings are those affected by
conditions like slope gradient, poor soils, little moisture. Those showing
complacent rings have generally constant climatic conditions such as a
high water table, good soil, or protected locations.
A number of tree samples must be examined and cross dated from any given
site to avoid the possibility of all the collected data showing a missing
or extra ring. Further checking is done until no inconsistency appears.
Often several sample cores are taken from each tree examined. These must
be compared not only with samples from other trees at the same location
but also with those at other sites in the region. Additionally, the
average of all data provides the best estimate of climate averages. A
large portion of the effects of non climatic factors that occur in the
various site data is minimized by this averaging scheme.
A number of computer programs are used to calculate statistical data.
Yearly climatic changes can be seen by first removing, statistically, the
gradual changes associated with the age of the tree. The end result of all
the analytical work is the master chronology, an absolute form, which can
be understood and used by others.
Dendrochronology can help solve many environmental problems. Tree-ring
analysis can be used in a variety of ways to diagnose pollution effects in
its early stages, and to help adjust certain irrigation projects. The
bristlecone chronologies had raised questions regarding radiocarbon
dating methods and have been use to recalibrate the C-14
The bristlecone pine chronology in the White Mountains currently extends
back almost 9,000 years continuously. That's to 7,000 BC! Several pieces
of wood have been collected that will extend this date back even further.
The hope is to push the date back to at least 8,000 BC. This will be
important as the last Ice Age ended about 10,000 years ago, and to have a
record of this transition period would offer scientists a wealth of