Extracting long-period climate fluctuations from tree-ring chronologies over timescales of centuries to millennia



For a long time, tree-rings have been thought of as containing almost no variation at timescales of centuries and millennia, i.e. at low frequencies. Here, we show that this might be an issue of data analysis rather than an actual lack of variability. A data set of subfossil and living Scots pines from northern Fennoscandia was examined by means of their ring-width time series. The premise was that the growth trends of individual time series could be quantitatively determined and decomposed into their different elements. It was shown that not all the components of growth trends were invariant over long periods of time, and that consequently the use of a single-curve standardization (i.e. Regional Curve Standardization, RCS) may result in temporally inflated and deflated indices of ring-widths. Observed non-climatic bias in tree-ring indices was probably due to gradually changing conditions in the pine population of the forest-limit ecotone. Changes in population density seem to have hampered the previous attempts at palaeoclimate reconstruction by masking the actual low-frequency climate variability. A new approach, expected to yield unbiased tree-ring indices, was proposed. The new chronology constructed by this approach showed consistency with multi-centennial variations that are based on independent palaeoclimate evidence. Copyright © 2005 Royal Meteorological Society.