Climatic Research Unit : Papers

Potential bias in 'updating' tree-ring chronologies using regional curve standardisation:
Re-processing 1500 years of Torneträsk density and ring-width data

Thomas M Melvin, Håkan Grudd and Keith R Briffa
The Holocene (2013) 23, 364-373.
DOI: 10.1177/0959683612460791


We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. ad 900-1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of 'growth-rate' based multiple RCS curves is recommended to identify and mitigate the problem of 'modern sample bias'.

Data, Code and Supplementary Files

Melvin_Supp.pdf - Supplementary material including figures S1 to s20

Temperature Reconstructions - The primary reconstructions are in file "Fig5.col" in - for each figure in the main document (Figures 1 to 5) and supplementary material (Figures S1 to S20): '.eps' graphic and .col' plotting data in column format - contains some reports:
UpdateG08MXD.prn Update report for the G08 data
UpdateG11MXD.prn Update report for the G11 data
UpdateG12MXD.prn Update report for the G12 data
UpdateG1112MXD.prn Update report for the combined G11 and G12 data
G08oS88.prn Rescaling G08 old data v S88 data
G08yS88A.prn Rescaling G08 young data v G08 old and S88
G12oS88.prn Rescaling G12 old data v S88 data
G12yS88A.prn Rescaling G12 young data v G12 old and S88 data
Journal Report of regression parameters
Rawstats Some statistics of measurement data files - Measurement data and associated pith offset files. - More measurement data, the climate data used, and some previous published Tornetrask chronologies. - Source Code

The tree-ring chronologies were constructed using the CRUST (CRU Standardisation of Tree-ring data) software. A separate article has been published that describes the processing methods that CRUST implements, and the full CRUST software (including source code) is now available.

We have created a reduced-functionality version of the CRUST software that is specific to the Tornetrask paper (i.e. it does the specific processing necessary to construct the tree-ring chronologies, undertake the various analyses reported in the paper, and to create most of the figures shown in the paper).
This Tornetrask version of CRUST is available in the zip file linked above. It contains all the raw data, together with executable versions for Linux and Windows. It also includes the Fortran source code, from which it can be compiled. We are not able to offer any technical support, but it does include a "makefile" to assist with compiling the software (though we recommend using the pre-compiled, executable versions if they work on your computer system).

The full-functionality version of CRUST is available from the CRUST software page.

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