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 134

Fig. 5.13 A simplified illustration of the time dependence of separate potential biases in the grand
average of all local regional curve standardized (RCS) chronologies produced in Esper et al. ( 2002 ) .
The dotted line represents the sum, across all sites (except Mongolia, see text) of the local differ-
ence in the means of alternative chronologies, produced either with a hemispheric-data-based RCS
or a local-data-based RCS curve. The mean of all individual site chronology weightings, according
to site latitude (see Esper et al. 2002 ) , is shown as a dashed line . The product of these two potential
biases is shown in black . Site counts over time are shown by grey shading
Figure 5.13 indicates the changing effective net bias that would be associated
with the application of either the mean linear form or curvilinear RCS curves to
the site measurement data had the data not been normalized prior to averaging.
These biases are simply the sum over all sites of the mean differences between a
chronology produced by using one of the multisite RCS curves and a chronology
produced by using an RCS curve applied only at a site level. (With the excep-
tion of the Mongolia data which were not available for analysis). Normalization
of the individual site series largely removes this potential bias, leaving only the
time-dependent changes in average site-latitude weighting applied by Esper et al.
( 2002 ) according to the site locations (presumably following the weighting nor-
mally applied in regional averaging of gridded temperature records to take account
of the change of area of grid boxes with latitude when the grid is defined according
to fixed latitude and longitude spacings).
5.6.3 Adaption of RCS to Account for Non-climate Bias
In their study of northern Finnish tree growth and climate variability during more
than 7000 years, Helama et al. ( 2005a , b) propose a modification of simple RCS, to
take account of the changing density of forest cover, and hence competitional inter-
actions among trees that they presume are sufficiently strong to alter the shape of
the appropriate RCS curve through time. The progress of ring width decay for open-
grown and close-packed trees will result in differences in the shape of RCS curves
and could lead to bias in RCS chronologies, but can be evaluated only over a com-
mon 'climate' period, and it is inappropriate to disregard the background changes
in the underlying climate forcing of tree growth. They first construct the basis for a
conventional single RCS curve by averaging all age-aligned measurement data. To
this series they fit a negative exponential with an added constant term (following
Fritts et al. 1969 ) oftheform