A Closer Look at Regional Curve Standardization of Tree-Ring Records: Justification of the Need, aWarning of Some Pitfalls, and Suggested Improvements in Its Application - Dendroclimatology - page 142

at that time and perhaps, the comparative magnitude of warmth in medieval as
compared to modern times.
This is not to say that biases are manifest in all RCS chronologies produced up to
this time. However, it is hoped that drawing specific attention to these potential prob-
lems will stimulate a more routine approach to investigating their likely extent. This
should lead to a circumspect interpretation of RCS-based climate reconstruction and
provide impetus for further work aimed at improving the RCS method.
Acknowledgements The authors are very grateful to Ed Cook, Connie Woodhouse, Malcolm
Hughes and Samuli Helama for their thoughtful reviews and suggested modifications to the original
manuscript. KRB acknowledges support from the UK Natural Environmental Research Council
(NERC) (NER/T/S/2002/00440) under the Rapid Climate Change Program. TMM acknowledges
current support from The Leverhulme Trust (A20060286). KRB also acknowledges travel support
from the organizers of the Tucson conference.
Appendix: Signal-Free Standardization
In this review, we make several references to the 'signal-free' method in tree-ring
standardization (see Sections 5.4.1 , 5.5 , 5.6.1 , and 5.7 ) . A more detailed discus-
sion of the topic (in the context of 'data-adaptive' standardization involving 'curve
fitting' to individual measured series) can be found in Melvin and Briffa ( 2008 ) .
However, for the convenience of the reader, a brief description of the rationale and
application of the signal-free approach is provided here.
Background and Rationale
The signal-free concept stems from the observation that individual tree-ring mea-
surement series represent a mixture of potential growth influences, among which
are included first, that of climate variability through time and second, that of chang-
ing allocation processes and tree geometry that both affect the size of annual stem
increments. Standardization has always aimed to remove or reduce the allocation
bias; e.g., the signal of reducing ring width with age, so that the remaining variabil-
ity in ring width indices over time provides a clearer representation of the influence
of climate variability. Developing standardization curves from measurement series
that contain the climate signal, where the standardization curve may track the cli-
mate signal, at least to some extent, will lead to the removal of some climate-related
variance and so bias the resulting chronology. This bias is well known with respect
to the removal of variance representing timescales longer than the typical life span
of the sample trees (Cook et al. 1995 ) . However, it can also arise where a common,
externally forced growth signal influences the more localized fit of a standardiza-
tion curve, resulting in the partial, or even complete, loss of a relatively short-term
climate signal (in the case of more flexible standardization curves) and the distor-
tion of medium-term climate trends in adjacent periods (where less flexible, but still
'fitted,' standardization functions are employed).