Testing long-term summer temperature reconstruction based on maximum density chronologies obtained by reanalysis of tree-ring data sets from northernmost Sweden and Finland

摘要

Here we analyse the maximum latewood density (MXD) chronologies of twopublished tree-ring data sets: one from Tornetr?sk region in northernmostSweden (TORN; Melvin et al., 2013) and one from northern Fennoscandia (FENN;Esper et al., 2012). We paid particular attention to the MXD low-frequencyvariations to reconstruct summer (June–August, JJA) long-term temperaturehistory. We used published methods of tree-ring standardization: regionalcurve standardization (RCS) combined with signal-free implementation.Comparisons with RCS chronologies produced using single and multiple(non-climatic) ageing curves (to be removed from the initial MXD series)were also carried out. We develop a novel method of standardization, thecorrection implementation of signal-free standardization, tailored fordetection of pure low-frequency signal in tree-ring chronologies. In thismethod, the error in RCS chronology with signal-free implementation isanalytically assessed and extracted to produce an advanced chronology. Theimportance of correction becomes obvious at lower frequencies as smoothedchronologies become progressively more correlative with correctionimplementation. Subsampling the FENN data to mimic the lower chronologysample size of TORN data shows that the chronologies bifurcate during the7th, 9th, 17th and 20th centuries. We used the two MXD data sets toreconstruct summer temperature variations over the period 8 BC through AD2010. Our new reconstruction shows multi-decadal to multi-centennialvariability with changes in the amplitude of the summer temperature of 2.2 °C on average during the Common Era. Although the MXD data provide palaeoclimate research with a highly reliable summer temperature proxy, thebifurcating dendroclimatic signals identified in the two data sets imply that future research should aim at a more advanced understanding of MXDdata on distinct issues: (1) influence of past population densityvariations on MXD production, (2) potential biases when calibratingdifferently produced MXD data to produce one proxy record, (3) influence ofthe biological age of MXD data when introducing young trees into the chronologyover the most recent past and (4) possible role of waterlogging in MXDproduction when analysing tree-ring data of riparian trees.