Basic Dendrochronology

William Cardwell, 11/24/2004

ES767 Quaternary Geology

In the scientist or historian’s efforts to determine dates of past events, many methods exist to interpret the available data. Age determination is an important part of research, but most methods yield only a range of ages. For example radiocarbon dating is an accurate and widely accepted means of age determination; however, the information obtained does not specify an exact year. While having a limited useful range, approximately 11,000 years, dendrochronology is a useful tool for determining a precise age down to the year, and on some occasions the season of that year. While it may seem a simple matter to count tree rings, dendrochronology is a science and as such is regulated by a set of guidelines or principles. Many of the principles are not exclusive to dendrochronology and are used for other disciplines, while others are specifically used for tree ring research. These principles are:

The Uniformitarian Principle
The Principle of Limiting Factors
The Principle of Aggregate Tree Growth
The Principle of Ecological Amplitude
The Principle of Site Selection
The Principle of Crossdating
The Principle of Replication

The Uniformitarian Principle

In 1785 James Hutton stated that, “the present is the key to the past.” This statement basically means that the same processes that link biological processes to environmental conditions today also did so in the past. While Hutton was talking about geological processes, not tree growth, this statement still holds true. There is nothing to suggest that anything has changed that would alter the impact that environmental conditions have on the growth of plants and trees.

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The Principle of Limiting Factors

This principle, as it relates to dendrochronology, states that the rate of plant growth is limited by the environmental resource which is in least supply. For example, moisture is often a limited resource, especially in arid and semiarid regions, while temperature may be a limiting factor in higher altitude settings. In addition, limiting factors may be compounded, such as in an area which receives little rainfall and also has well drained sandy soil. The factor of little rainfall has an impact, but since little water is retained in the soil the effects are multiplied in limiting the plants growth.

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The Principle of Aggregate Tree Growth

This principle states that any individual tree growth series can be broken down into a collective of environmental factors that affected the growth of the tree over time. For example, the growth of a tree ring in one year is a result of an amassed collection of factors, including: 1) The growth of the tree as it relates to the normal growth process among others of the same species. 2) Climatic conditions which existed in the forest during the year, including: -amount of rainfall -temperature -storms, resulting in damage to trees 3) Factors from outside the forest which affect growth, including: -insect outbreak -volcanic eruption -intervention by man 4) Random error: Factors other than the three above, which may affect tree growth. In order to minimize the affects caused by these environmental factors, trees are chosen from areas that have had the least exposure to any processes which may have impacted normal tree growth.

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The Principle of Ecological Amplitude

This principle states that a tree species “may grow and reproduce over a certain range of habitats, referred to as it ecological amplitude” (Fritts, 1976). An example of this is the Ponderosa pine, which has a large ecological amplitude due to its being distributed throughout North America. On the other hand, giant sequoia trees have a small ecological amplitude due to their being limited to the western slopes of the Sierra Nevada of California. This principle is valuable to dendrochronology because the most useful trees are often found at the margins of their habitat range.

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The Principle of Site Selection

This principle states that sites that will yield data useful to dendrochronology can be chosen on the basis that trees will produce rings that will reflect the environmental condition being studied. For example, if past drought conditions were to be studied, samples of trees growing in areas that are known to have experienced drought conditions would be sampled.

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The Principle of Crossdating

This principle states that the exact year a tree ring was formed may be determined by matching certain characteristics, such as width or density, of the rings to that of several other tree ring series. For example the date of the construction of a building or even a door may be determined by matching tree ring patterns of the structure with that of a living tree. Crossdating is the most fundamental principle of dendrochronology.

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The Principle of Replication

This principle states that multiple samples taken from a single tree and sampling multiple trees in a single area results more accurate data gathering. In this manner, events that may be exclusive to only a single tree may be filtered out.

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The concept behind dendrochronology is a simple one. As a tree grows, it adds a new ring of wood under the bark for every year. But it’s not as simple as just counting rings. For each year, two rings are formed, a light colored one and a dark brown one. The light colored ring is formed during the early part of the year, and the darker colored ring is formed in the later part of the year. In the early part of the year, when rain is abundant, the tree may grow and devote its energy to the production of large new cells forming the light colored rings. As the year progresses, growth slows producing smaller cells and some cells die forming the dark colored rings. By looking at the width of the rings, it may be determined if the year was favorable or unfavorable for growth. A narrow ring means unfavorable conditions, and a wide rings means favorable conditions. Over a long period, there will be a pattern of ring sizes reflecting the conditions of the trees environment. These patterns may be then compared and aligned, using the above mentioned principles, with trees within the same geographic area creating a record of environmental conditions spanning hundreds or thousands of years.

Collection Method

Samples for the study of dendrochronology are generally small cores taken from living trees. These cores are similar in appearance to dowel rods. The process of extracting a core from a tree is fairly simple. A tool called an increment borer (fig. 1), which is a hollow tube with a threaded end, is screwed into the trunk of the desired tree (fig. 2). As the borer threads into the tree, its sharp edge cuts a cylindrical portion of wood, which slides into the hollow shaft of the borer. When the desired depth is reached, an extractor tool is inserted into the hollow borer and the small core of wood is removed (fig. 3). Due to the fragile nature of the cores, special care is taken. After the core is removed from the coring tool, it is stored in a drinking straw to protect it until it may be prepared for anlysis (Fig. 4).

(Figure 1) Dismantled increment borer showing all parts

(Figure 2) Coring tool being used to cut a core

(Figure 3) Core being removed from increment borer

(Figure 4) Core being placed into a drinking straw for protection

Preparation Method

Once the desired cores are collected, they are taken to the lab for preparation. First the small cores are glued on specially prepared wood blocks. These blocks will have a narrow groove cut into them to receive the core. Once the core has been glued to the block it is then sanded with progressively finer and finer sand paper to reveal the internal structure of the wood (fig. )

ponderosa core sample that has been sanded and is ready for analysis

Analysis

The next step of the process is to analyze the core. While there are many methods of analysis, the most fundamental is cross dating. Cross dating simply overlays the ring patterns of one tree onto those of another tree, in an attempt to trace the progression of the growing conditions and attach dates to the rings. The image below demonstrates crossdating using three cores from three different trees to determine continuous growing conditions from 600 to 1500 years.

Illustration of crossdating of three cores

References:

Grissino-Mayer, Henri D., Ultimate Tree-Ring Web Pages, World Wide Web homepage URL: http://web.utk.edu/~grissino/default.html. Retrieved 11/09/2004.

Oxford Dendrochronology Laboratory, World Wide Web homepage URL: http://www.dendrochronology.com/ Retreived 11/09/2004.

Fritts, Harold C.,1976. Tree Rings and Climate. New York, NY: Academic Press. 567 pp.

McCarthy, Brian C., Introduction to Dendrochronology, Ohio University, World Wide Web homepage URL: http://www.plantbio.ohiou.edu/epb/instruct/ecology/dendro.htm. Retreived 11/10/2004

Figures 1, 2, 3, and 4 were obtained from Introduction to Dendrochronology,URL: http://www.plantbio.ohiou.edu/epb/instruct/ecology/dendro.htm. and used with the permission of Brian C. McCarthy and Darrin Rubino.