Divergence pitfalls in tree-ring research

Abstract

,andinto degrees Celsius—forming the cornerstone for proxy reconstructions (Cookand Kairiukstis 1990). The specific subject of the Loehle (2009) contribution is onnon-linear calibration techniques, which have been considered when proxy data in-dicate inconsistent responses to medium and extreme temperature deviations. Non-linear calibration has a long-standing history in tree-ring research (see Fritts 1976 foran introduction), and variants of non-linear response models have been applied in avariety of tree-ring analyses over the past decades (e.g., Carrer and Urbinati 2001;Fritts 1969; Graumlich and Brubaker 1986; Woodhouse 1999). So, while approachesto tackle non-linear tree-ring/climate associations are relevant when dealing withproxy data, it is, however, the motivation that called for re-attention to thesetechniques that is of particular interest: the divergence phenomenon (DP) in tree-ring research.DP was first described over a decade ago by Jacoby and D’Arrigo (1995)andsince then has been reported from a variety of sites mainly concentrated towardsthe Northern Hemisphere boreal forest zone (see D’Arrigo et al. 2008 for a review).DP effectively describes a disassociation of late twentieth century (typically post-1960) tree growth parameters, such as ring width or maximum latewood density,from regional temperature trends. This disassociation does not necessarily comprisea weakening of the high-frequency climate signal. That is, inter-annual tree-ringvariation may be predominantly controlled by temperatures, but the long-term