A Guide to Dendrochronology
The study and dating of tree rings is called dendrochronology. Dendrochronologists are scientists who study tree rings to answer questions regarding the natural world and the place of humans in the world. There are many practical applications of the study of tree rings. Dendrochronology is considered an interdisciplinary science and its techniques and theory can be applied to various applications.
How Rings Are Counted
Dendrochronolgists do not simply count a tree's rings as it does not ensure accurate dating. There are actually many techniques used to date wood samples accurately. One such dating method is known as the skeleton plot. The skeleton plot is one of the most commonly used method for crossdating tree rings. Rings are studied by making a horizontal cross section cut through the trunk of a tree. The rings are visible due to changes in growth speed throughout the different seasons. Generally this means that one ring marks one year, though for accurate dating, tree rings must actually be studied further.
Dendrochronologists study timber core samples and measure the width of the growth rings. When scientists study samples from different sites within a particular area, they can build a comprehensive historical sequence. Dendrochronology techniques are more consistent in areas with marginal conditions. There are also certain types of trees that are more suitable for tree ring analysis than others. There are many obstacles that come with trying to date trees, such as the existence of insects that can inhabit trees and destroy the ring structure.
Tree Ring Formation
Trees feature two types of growth; primary and secondary. Primary growth takes place at the tips of stems and roots and results in the tree growing taller. Secondary growth occurs in the cork cambium and vascular cambium, resulting in the trunk's diameter increasing. Trees also contain xylem which is the vascular tissue of the tree through which most minerals and water are conducted. Then there is the phloem, or outer layer of the tree. The rings of a tree are the result of new growth occurring in the vascular cambium.
To figure out a tree's age, many factors need to be considered including temperature, age of the tree, precipitation, slope, and other growth factors. Some trees can produce false, or extra rings in a year. For scientists to develop a master tree ring dating system, all of the above factors need to be taken into account.
Dendrochronology can be useful in many applications. Dates from dendrochronology can be used as a check and calibration of radiocarbon dating. Climate can also be studied with tree rings as growth properties depend heavily on factors such as precipitation, temperature, and more. Dendrochronology is also important for art historians for their help in dating panel paintings, though there are limitations to what techniques can be used. In addition to art history, dendrochronology can be used in the dating of buildings and other wooden structures.
History of Tree Ring Dating
Tree rings were mentioned first by a Greek botanist named Theophrastus and later by Leonardo da Vinci. Then, in the mid 1700s, French investigators began examining the effect of growing conditions on the shape of a tree's rings. In the US, Alexander Catlin Twining was studying patterns in tree rings that could be used to learn about the past climate of different regions. Dendrochronology really began during the later half of the nineteenth century and is now used in many different applications.
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