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 133

Fig. 5.12 A comparison of regional curve standardized (RCS) chronologies at various sites from
among those used in Esper et al. ( 2002 ) . The alternative chronologies shown for each of the selected
sites illustrated here were produced using different RCS curves: either that used by Esper et al.
based on multi-site data (linear model for Upperwright ( upper line ) and Camphill ( lower line )or
non-linear model for Tornetrask ( upper line ), Tirol ( upper line ) and Gotland ( upper line )) or a
single RCS curve ( thick line ) based only on the data available for that site. The difference between
the two curves at each site represents a potential local bias when all chronologies are averaged to
form a single 'Northern Hemisphere' series. In Esper et al. ( 2002 ) this bias was largely mitigated
because each site chronology was normalized prior to averaging. For clarity all chronologies are
shown as 20-year smoothed series. Sample counts are shown by gray shading
sites used by Esper et al. ( 2002 ) , selected here to show how the use of multi-site RCS
curves, whether linear or nonlinear in shape, can bias the mean of the local indices
with respect to the mean indices that would have been produced by using an RCS
curve derived from, and applied to, the measurement data only for the particular
site. At some sites (e.g., Upperwright and Torneträsk), the bias is generally positive,
while at others (e.g., Camphill) it is negative. At some sites, the mean bias is large
compared to the temporal variance of the chronology (e.g., Tirol). If the regional
chronologies, based on multisite-mean RCS, are simply averaged (as is implied in
Esper et al. 2002 ) , medium-frequency biases could arise in the final chronology as
specific site data contained within it vary through time. In fact (though it is not
apparent in Esper et al. 2002 ) , the individual site chronologies were actually nor-
malized prior to averaging (Ed Cook, personal communication) so that their overall
means are set to a value of 1.0. This operation will mitigate much of the potential
bias and effectively produce site chronologies similar to those that would be pro-
duced by using RCS applied at the site level, though medium-frequency bias will
still arise where the slope of the local and multisite RCS curves differ (e.g., for
Upperwright , Camphill, and Gotland; shown in Fig. 5.12 ) .