The influence of elevational differences in absolute maximum density values on regional climate reconstructions
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We found that indiscriminately using tree-ring MXD data with inhomogeneous temporal distribution from different elevations might cause biased chronologies. A mean-adjusting method was developed to overcome this bias.
Here we analyse maximum latewood density (MXD) of Pinus sylvestris L. (Scots pine), from deadwood (dry) and subfossil wood (from lakes) collected along an elevation gradient in and close to the central Scandinavian Mountains, in the province of Jämtland, Sweden. Focusing on two common time periods (900–1150 CE and 1300–1550 CE), the mean absolute MXD of deadwood samples showed an inverse relationship with elevation, i.e. the absolute MXD decreases with elevation. However, the MXD values of the subfossil samples did not show such a consistent relationship with elevation. It was also noted that the differences in mean absolute MXD values among sites of different elevations in a given period were larger than among sites of similar elevation between the two time periods, where the former was assumed to be warmer than the latter. Using a theoretical model and a real example, it was shown that indiscriminately using MXD data with inhomogeneous temporal distribution from different elevations may cause biased chronologies, which can have significant effects on subsequent interpretations of past climate variability. A mean-adjusting method was developed to overcome this bias, and its usefulness was demonstrated by comparing two chronologies built on mean-adjusted and unadjusted MXD samples. It was concluded that unadjusted samples from different elevations with inhomogeneous temporal distribution can distort the long-term trend in a final chronology, while this bias can be alleviated if mean-adjusted samples are used.
KeywordsElevation Tree-ring MXD Mean-adjustment Reconstruction bias
Author contribution statement
PZ sampled and prepared parts of the data, designed the analysis, visualized the results and contributed to the writing; JB and HL sampled parts of the data, and contributed to the analysis and the writing.
We acknowledge the County Administrative Boards of Jämtland for giving permissions to conduct dendrochronological sampling in the area, and Mauricio Fuentes, Petter Stridbeck, Riikka Salo, Emad Farahat and Eva Rocha for their help in the field. We also thank Björn Gunnarson, Laura McGlynn and Håkan Grudd for assistance with the MXD measurements and Andrea Seim for suggestions on revising the manuscript and helping out with GIS. This work was supported by grants from the two Swedish research councils (Vetenskapsrådet and Formas, grants to Hans Linderholm) and the Royal Swedish Academy of Sciences (Kungl. Vetenskapsakademien) (grant to Peng Zhang). This research contributes to the strategic research areas Modelling the Regional and Global Earth system (MERGE), and Biodiversity and Ecosystem services in a Changing Climate (BECC). This is contribution # 34 from the Sino-Swedish Centre for Tree-Ring Research (SISTRR).
Conflict of interest
The authors declare that they have no conflict of interest.
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