As trees grow in age a layer of wood cells are produced each year that usually extends around
the entire perimeter of the tree. This annual ring of growth consists of thin-walled cells formed early in the
growing season (earlywood) and thicker-walled cells produced later in the growing season (latewood). From
the start of earlywood growth to the end of the latewood growth constitutes one annual tree ring. Because
new wood is also added upwards, the number of rings through cross sections of the trunk will decrease with height.
Thus to determine the age of the tree, a tree ring researcher (dendrochronologist) typically counts the number
of annual rings near the base or at "belt height". Ideally, to determine the age of a tree a researcher
will saw through the tree to get a cross section. To preserve living trees, researchers use an increment
borer, which is a hollow auger-like device that is twisted into the trunk. When the borer, about a centimeter
in diameter, reaches the center of the tree, the interior plug or core of wood is removed. The core is then
taken to the lab where tree wood and rings are analyzed. In practice, at least 2 samples are usually taken
from the same tree to reduce the amount of variability in the tree ring data.
It's a general rule that tree ring widths decrease as the tree ages. This fact is well
supported by the data, especially in the first 100 years of growth. Overall we see annual fluctuations in tree
growth most likely due to seasonal differences in climate variables such as temperature and precipitation.
What's really interesting is the periodic fluctuations beginning around 1650 and continuing through 3-4 cycles
into the mid 1800's. Are these an indication that there were 50-year fluctuations in climate in the Pacific
Northwest during this period? If so, what climate variables were limiting factors during periods of slow growth?
An excellent exercise for the student is to determine the best fitting curve through the time series or analyze
the differences in variation between the two samples.
The study that uses tree rings to reconstruct
the past climate of an area is called dendroclimatology, a subfield
of dendrochronology -- the science of using tree ring dating to analyze
patterns in physical and social sciences. Other subfields of
dendrochronology include dendroarchaelogy (using tree rings to date
wooden artifacts), dendrohydrology (using tree rings to study changes
in river flow, surface runoff, and lake levels), and dendropyrochronology
(using uses tree rings to study past and present changes in wildfires).
An excellent source on dendrochronology is Henri D. Grissino-Mayer's
Ultimate Tree Ring Web Pages.
Source of the data: Earle, C.J., Brubaker, L.B., Segura, G., International Tree Ring Data Base,
NOAA/NGDC Paleoclimatology Program, Boulder, Colorado, USA. http://www.ngdc.noaa.gov/paleo/ftp-treering.html