Direct transformation of tree-ring measurements into palaeoclimate reconstructions in three-dimensional space

Abstract

Here we propose a new method for dendroclimatic reconstructions. This DIrect REConstruction Technique (DIRECT) performs modelling of the age-dependent association between climate and growth in three-dimensional space and directly transforms the raw tree-ring values (on the scale of original growth units) into climate variability while considering the effect of age in this relationship. DIRECT was applied to maximum latewood density (MXD) data from Finnish/Swedish Lapland. The new DIRECT reconstruction was compared with a reconstruction produced using regional curve standardization (RCS), a method that is often used to preserve the low-frequency variability in tree-ring proxy data. Comparison of the DIRECT and RCS methods showed that both produced summer temperature reconstructions with approximately equal amounts of low-frequency palaeoclimate information. The DIRECT method avoids traditional detrending techniques by considering the cambial age of each tree when building the statistical climate–proxy model and identifying non-linear relationships between climate and tree growth. DIRECT provides objective consideration and removal of differential growth rates and age-related noise without losing the low-frequency portion of palaeoclimate variability. It is anticipated that DIRECT will prove to be a useful tool in regions where strong climate control enables the use of tree rings for palaeoclimate reconstructions.