Abstract
AbstractThe potential for age‐related trends in the stable oxygen isotope ratios of latewood alpha cellulose was investigated in samples of living oak trees and historic building timbers from the UK. When the series are examined individually, it is clear that the strongest trends in individual trees and timbers reflect concurrent trends in climate. Nonclimatic trends are very small and represent random noise that can be removed by averaging. If the same data are analyzed using the more conventional approach of aligning the series by ring number and fitting a regression line, so that the magnitude of the age trend is based on the slope of the mean and the statistical significance on the correlation coefficient, the results are very different. We demonstrate that this conventional approach regularly produces spurious age trends with grossly inflated probabilities, because of offsets in the mean values of series of different length. We conclude that there is no need to detrend stable oxygen isotope series from individual trees or timbers of oak from the UK and that to do so would remove important climatic information. Long isotope chronologies can safely be constructed by combining data from multiple individual trees, or by pooling material from trees prior to chemical treatment and isotopic measurement. Age‐related trends may occur in other species or in other regions, but where they have been identified using the conventional “slope of the mean” approach they should be reassessed using the “mean of the slope” approach.
Highlights
Tree rings provide one of the most valuable archives of paleoclimatic information, but climate reconstructions based on morphological growth proxies, such as ring widths and relative densities, rely on trees whose growth is limited by a single climatic control (Bradley, 2014)
The potential for age‐related trends in the stable oxygen isotope ratios of latewood alpha cellulose was investigated in samples of living oak trees and historic building timbers from the UK
The conventional method for identifying age trends in tree ring isotopes, where the samples are aligned by ring number and the trend quantified by regression of mean isotope value on ring number, aims to capture the average behavior of the individual samples
Summary
Tree rings provide one of the most valuable archives of paleoclimatic information, but climate reconstructions based on morphological growth proxies, such as ring widths and relative densities, rely on trees whose growth is limited by a single climatic control (Bradley, 2014). Perhaps the greatest potential for climate reconstruction in the moist midlatitudes lies in the use of oak (Quercus spp.). Many long oak ring width chronologies have been constructed for archaeological dating purposes (Baillie, 1982; Pilcher et al, 1984), and to calibrate the radiocarbon time scale (Becker, 1993), but it has not proved possible to translate these ring width records into reliable palaeoclimate reconstructions. The problem is that oak trees over most of Europe are not sufficiently limited by a single environmental factor, so that even where a common growth signal can be extracted, it is not clearly linked to a single climate parameter that can be reconstructed
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