By: K. Gajewski, R. Zalatan, N. Ayotte
On this page is some basic information about collecting tree cores. For information about the general field of dendrochronology, please refer to Stokes and Smiley (1968), Fritts (1976), and Cook and Kairiukstis (1990).
A good source of basic information is the tree-ring faq (frequently-asked-questions). Here you can also find a number of useful tips and solutions to problems:
The Laboratory of Tree-Ring Research is a great place to start learning about the field of dendrochronology: http://www.ltrr.arizona.edu/
(A) Field Analysis
Site selection depends on the question you are investigating. See the following reference: Fritts, H.C. 1976. Tree rings and climate. Academic Press.
The tree-ring borer and its use
The borer consists of three parts: the handle, the borer and the spoon. Unscrew the spoon and pull it out. Stick the borer into the square hole in the handle and click the snap shut. There is only one way it will fit. When the borer is assembled, simply screw it into the tree. Areas where annual rings may be distorted, such as near branches on uphill or downhill sides of the trunk, should be avoided. When you have reached the centre of the tree, insert the spoon carefully. Push straight in - do not bend it. Push it in as far as it will go, then unscrew the borer 1 turn. Remove the spoon, place the core in a straw and tape the ends with masking tape. Remove the borer from the tree. You may want to take more than one core to ensure the pith has been reached and for crossdating purposes. Once you have collected your cores, remove the taped ends to allow the cores to dry overnight or until needed.
The Stokes and Smiley (1968) and Fritts (1976) books provide information about core collection. You should collect the cores as close to the ground as possible, to get the maximum number of rings. Since growth of the tree begins at the root system, a core taken above the base may have a much younger estimated age than one taken directly at ground level. You need multiple cores of a tree to be able to crossdate the cores. Complete sections make core dating easier, although this is not always possible!
The tree-ring pages have a nice form that you can print out. At least, look at this to see the kinds of information you should be collecting about your site. http://tree.ltrr.arizona.edu/~grissino/supplies.htm#forms (look at the bottom of the page of this link)
Care of the borer
Every year, one or two of the borers get destroyed. There is no excuse for this. These are sturdy and simple instruments and can last for years if properly used and maintained.
(B) Laboratory Analysis
The basic dendrochronological study involves the development of a chronology derived from annual tree-ring increments.
After the cores are left to dry, they are glued vertically into a groove in a wooden block using water-soluble glue (carpenter's glue). Once the cores are dry (left overnight), they are ready to be sanded using four progressively finer grades of sandpaper (120, 220, 320 and 600). For increased enhancement of cells, use a 1500 grit sandpaper.
Dating the cores using crossdating
The first step is to use skeleton plots of the tree-ring sequences to identify rings with unusual (very large or small) growth. These plots are used to relate a group of cores to each other by pattern matching. Narrow rings are recorded as they occur on strips of graph paper with 2 mm divisions. The width of each ring is compared to that of the rings on either side of it; a long vertical line is drawn where a ring is considerably narrower than its neighbor, and a shorter line when it is only slightly narrower. The oldest cores are crossdated to all other cores in order to formulate a master chronology. When discrepancies are found, another researcher checks the cores to verify proper crossdating techniques.
Problems may arise when missing, locally absent or false rings are present. A ring is normally formed every growing season, but in some years of unfavorable climatic conditions, a tree may not develop an annual ring (missing ring). In other circumstances, a ring may be absent at some point on the tree, and therefore is only seen when a cross-section of the tree is taken (locally absent). False rings (or double rings) may also occur during a favorable growing season, whereby two rings are formed. For example, a tree will develop an annual ring in a normal growing season, but then favorable climatic conditions allow the tree to grow again and as a result, form a new ring. This second annual ring is considered the false ring. These missing, locally absent, and false rings can be detected by crossdating several trees from the same plot or region.
Radial growth analysis
To compute a chronology based on tree rings, the width of each ring must be measured. The type and accuracy of the measuring devices may vary. In our lab, annual growth rings are measured using Measure J2X and a Mini-scale and Mate System Encoder connected to an Acurite III digital counter with a precision of 0.001 mm. The accuracy of crossdating is verified using the computer program COFECHA (Grissino-Mayer et al. , 1992). COFECHA is used as a method of quality control on the measured ring-widths. Each measured ring-width series must be compiled into one notepad document and saved as an *.rwl file. Also, ensure that each ring-width series ends in 999. The following is an example of the *.rwl file that is run through COFECHA:
Ring widths are not only affected by climate but also change with tree age, height within the stem, site-specific characteristics and site productivity (Fritts, 1976). For the purpose of climate reconstructions, it is essential to determine the changes in ring widths associated with age or other factors and to remove them from the measurements. This correction is known as standardization, and the transformed values are referred to as ring-width indices (Fritts, 1976). The purpose of standardization is to remove an overall growth trend in tree-ring measurement series, and to remove part of the variance at very low frequencies approaching the length of the series (Grissino-Mayer et al. , 1992). There exists a large array of techniques that can be used for standardizing ring widths and creating a mean chronology of tree-ring indices. Each set of tree ring measurements must be standardized differently depending on the statistical properties of the data series. The expected signal derived from the tree rings must be determined prior to selecting the method of standardization. Once the growth signal has been defined, it is then possible to assign the most appropriate method of standardization to each series (Cook and Kairiukstis, 1990).
Once you have chosen the most appropriate method of standardizing your ring-widths, ARSTAN can be used to develop your chronology (Cook, 1985; Grissino-Mayer et al. , 1992). Several files are provided in this program depending on the options chosen in ARSTAN. Obtaining the most updated version of the manual for ARSTAN may be useful before running the program.
COFECHA, ARSTAN as well as a number of other dendrochronological programs may be obtained free of charge, from The Laboratory of Tree-Ring Research, University of Arizona: http://www.ltrr.arizona.edu/
For a list of the signature years in chronologies developed from the southwest Yukon, click here.
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Technical questions or comments about this site? Last updated: 2009.11.23