| Site Selection and Sample Collection |
Tree ring samples are collected in the field by using a hand held increment borer to remove a cylinder of wood roughly 5mm in diameter along the radius of a tree. The investigator selects the site in order to maximize a particular signal. For example, a steep, rocky, south facing slope may be selected to find trees under maximum water stress, such that growth rates can become a strong proxy for precipitation. Therefore, it is essential to be familiar with the characteristics of the site, as well as the factors regulating tree growth, in order to correctly interpret the results. For reliable statistical analysis, a rule of thumb is 20 trees per site, but this will vary according to the strength of the climate signal in the trees and the purpose of the collection. Two samples are generally collected per tree to facilitate cross-correlation and accurate dating of the annual rings.
The samples are returned to the laboratory, where they are mounted and finely sanded to allow cross-dating and measurement of the widths of the annual rings. (In some cases wood density is also measured, which may provide a more reliable growth signal and additional information). The ring widths are measured to the nearest 0.01mm or .001mm and recorded in computerized data files.
A statistical evaluation of the crossdating has been conducted for most ITRDB sites using the COFECHA program contained in the ITRDB Program Library .
Note: In the Southern Hemisphere, all collections are dated such that the calendar year represents the year in which tree growth began.
The raw ring width data from the samples collected at one site are standardized and the results are averaged into a site chronology. The standardization process involves fitting a curve to the ring-width series, and then dividing each ring-width value by the corresponding curve value (or calculating the difference between the ring-width values and the curve value) to generate a series of growth indices. This process allows samples with large differences in growth rates to be combined, and can be used to remove any undesired growth trends present. For example, a typical sample might display exponentially declining growth with age, the classic biological growth curve. Standardizing this sample using a negative exponential function results in data values which represent the departure from the "expected" value for a given year. The series of standardized growth indices is then used to interpret a proxy environmental signal in the data.
Many of the chronologies in the ITRDB were created with Program ARSTAN, developed by Dr. Edward R. Cook at the Tree-Ring Laboratory, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York (Cook 1985). Program ARSTAN produces chronologies from tree-ring measurement series by detrending and indexing (standardizing) the series, then applying a robust estimation of the mean value function to remove effects of endogenous stand disturbances. Three versions of the chronology are produced, intended to contain a maximum common signal and a minimum amount of noise. The standard chronology, designated sitecode.crn, is processed as above without autoregressive modeling. The residual ("whitened") chronology, designated sitecodeR.crn, is additionally processed using autoregressive modeling to remove autocorrelation. The third version, or ARSTAN chronology, designated sitecodeA.crn, is calculated by reincorporating the pooled autoregression (persistence) into the residual chronology.
The index values are unitless, with a nearly stable mean and variance, allowing indices from numerous trees to be averaged into a site chronology. The statistical methods for accomplishing the standardization can be complex. For a more complete discussion of methods, see References
The chronology represents the departure of growth for a given year vs. the series mean, and is expressed as a 3 or 4 digit integer with 1.000 (listed without the decimal point as 1000 or 100 in the data files) representing the long term mean. Higher or lower values for a given year represent proportionally higher or lower tree growth for that year. A researcher can combine knowledge of the individual site and tree species to interpret the growth variations in terms of climate or other environmental factors.
The chronologies recorded here are those calculated by the original investigator. The raw ring widths are also archived to allow reprocessing of the chronologies.
Format for Tree-Ring Data Files
Raw Data Files (File Extension .RWL)
These are measurements in units of .01mm or .001mm of the thickness of tree ring width for each year. Each file consists of all the measurements for a given site. Fifty or more Core ID numbers and data series may comprise one (site) file. Missing value code is 999 or -9999. The 10 values following the decade are the 10 annual measurements for the 10 years of that decade. First and last decade rows for each core may contain less than 10 values. The standard format is:
Core ID Number columns 1-6
Decade columns 9-12
Data Values columns 13-73, 6 columns/measurement, 10(I6)
Optional Site ID columns 74-78
Processed Data Files (Site Chronologies, File Extension .CRN)
These are the standardized tree-growth indices from a stand of trees, representing the mean growth observed for each year over the entire stand. Site chronologies are used in climate analysis. Data are stored as 3 or 4-digit numbers, with a value of 1000 representing mean growth, a minimum value of 0 (no growth), and no defined maximum. There is only one time series per file, in contrast to the raw data files. Missing value code is 9990. Site information is stored in the first 3 records of the file.
Format for chronology header records:
Record #1: 1-6 Site ID, 10-61 Site Name, 62-65 Species Code, optional ID#'s
Record #2: 1-6 Site ID, 10-22 State/Country, 23-30 Species, 41-45 Elevation, 48-57 Lat-Long, 68-76 1st & last Year
Note: lat-lons are in degrees and minutes, ddmm or dddmm
Record #3: 1-6 Site ID, 10-72 Lead Investigator, 73-80 comp. date
Chronology Data, Records 4-??
Site ID# column 1-6
Decade column 7-10
Index Value-Sample Number* pairs of values, columns 11-80, 10(I4+I3)
TRL ID#(optional) column 82-88
*Index Values, columns 11-14,18-21,25-28,32-35,etc
# of samples used in calculating chronology, columns 15-17,22-24,29-31,36-38,etc.
Example:1450 670 171018 17 897 18...
Here, 670 is the ring-width index value for the year 1450, with a sample size of 17;
1018 is the ring-width index value for the year 1451, with a sample size of 17;
897 is the ring-width index value for the year 1452, with a sample size of 18
Chronology Statistics, Last Record, Optional:
Site ID# column 1-6
Number of Years column 8-10
First Order Autocorrelation column 13-16
Standard Deviation column 19-22
Mean Sensitivity column 25-28
Mean Index Value column 29-35
Sum of Indices column 37-44
Sum of Squares of Indices column 46-53
Max# of series column 62-63
ITRDB Species Codes
The complete list of species codes was compiled by Henri D. Grissino-Mayer, and is available via his Ultimate Tree-Ring Web Pages.
Key to Data Type Codes
The majority of ring measurements in the ITRDB are of the width of the annual ring. However, some of the series are measurements of portions of the annual ring, namely Earlywood or Springwood, and Latewood. The Earlywood appears light in color due to less dense cell walls, and is formed in the earlier part of the growing season. The Latewood, darker due to more dense cell walls, is formed later in the growing season. Some researchers measure wood density in addition to width, and density measurements can also made on the Earlywood and Latewood portions of the annual ring, as well as the as maximum and minimum density over the entire ring. Below is a table of the codes used to designate the type of measurement contained within a data file. The filename convention is a site code, optionally followed by a measurement and/or a chronology type. Default values are ringwidth measurement type and standard chronology (as described above), and these types are not coded into the filenames. For example, the ringwidth measurement file and standard chronology file for site CO512 are co512.rwl and co512.crn. The earlywood measurements file is co512e.rwl, and the latewood arstan chronology file is co512la.crn. A table of the codes used:
|Tree Ring Measurement Type Codes|
|Code Measurement Type|
|D Total Ring Density|
|E Earlywood Width|
|I Earlywood Density|
|L Latewood Width|
|N Minimum Density|
|R Ring Width|
|T Latewood Density|
|X Maximum Density|
|P Latewood Percent|
|Tree Ring Chronology Type Codes|
|Code Chronology Type|
|P Low Pass Filter|
|W Re-Whitened Residual|
|N Measurements Only|
Cook, E.R., and Kairiukstis, L.A., eds. 1990. Methods of Dendrochronology. Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061 USA, or P.O. Box 17, 3300 AA Dordrecht, The Netherlands.
Cook, E.R. 1985. A Time Series Approach to Tree-Ring Standardization. PhD Dissertation. University of Arizona, Tucson, AZ, USA
Esper, J., Cook, E.R., Krusic, P.J., Peters, K., Schweingruber, F.H. 2003. Tests of the RCS method for preserving low-frequency variability in long tree-ring chronologies. Tree-Ring Research 59(2): 81-98.
Fritts, H. C. 1976. Tree Rings and Climate. Academic Press LTD., 24/28 Oval Road, London NW1, or 111 Fifth Avenue, New York NY 10003.
Hughes, M.K. et al. 1982. Climate from Tree Rings. Cambridge University Press, Cambridge, UK.
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