A Closer Look at Regional Curve Standardization of Tree-Ring Records: Justification of the Need, a Warning of Some Pitfalls, and Suggested Improvements in Its Application

Abstract

Some background describing the rationale and early development of regional curve standardization (RCS) is provided. It is shown how, in the application of RCS, low-frequency variance is preserved in the mean values of individual series of tree indices, while medium-frequency variance is also preserved in the slopes. Various problems in the use of the RCS approach are highlighted. The first problem arises because RCS detrending removes the average slope (derived from the data for all trees) from each individual tree measurement series. This operation results in a pervasive 'trend-in-signal' bias, which occurs when the underlying growth-forcing signal has variance on timescales that approach or exceed the length of the chronology. Even in a long chronology (i.e., including subfossil data), this effect will bias the start and end of the RCS chronology. Two particular problems associated with the use of RCS on contemporaneously growing trees, which might represent a typical (i.e., modern) sample, are also discussed. The first is the biasing of the RCS curve by the residual climate signal in age-aligned samples and the undesirable subsequent removal of this signal variance in RCS application. The second is the 'differing-contemporaneous-growth-rate' bias that effectively imparts a spurious trend over the span of a modern chronology. The first of these two can be mitigated by the application of 'signal-free' RCS. The second problem is more insidious and can only be overcome by the use of multiple sub-RCS curves, with a concomitant potential loss of some longer-timescale climate variance. Examples of potential biasing problems in the application of RCS are illustrated by reference to several published studies. Further implications and suggested directions for necessary further development of the RCS concept are discussed.