Description:
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 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).
