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 125

underlying forcing signal is constant. The constant mean growth rate is not realistic,
but the bias shown will apply similarly to curvilinear declining ring series.
In order to demonstrate modern-sample bias, it is necessary to use series of tree
indices that have no common climate signal and no common age-related growth
trend. Here, 1724 trees from both Torneträsk (Grudd et al. 2002 ) and Finnish
Lapland (Eronen et al. 2002 ) from the several-millennia period 5400 BCE to AD
1724 are used. Trees that were alive after 1724 are not included so as to avoid any
anthropogenic factors or sample bias arising from the coring of living trees. The
measurement data are all realigned so that their final growth years correspond (i.e.,
they are 'end-aligned' and the end year is nominally set to zero). The mean ring
width of the subset of all 40- to 60-year-old rings from trees with a final radius
between 12 and 14 cm (ring counts shown as gray shading) are plotted by nomi-
nal year for Fig. 5.7a . In order to reach the 12-14 cm final diameter class, the 40-
to 60-year-old rings must be larger for the younger trees than in the older trees
and this curve of mean ring width takes on a positive slope. Provided that there is
some maximum size limit to tree growth, the sum of data for any, and hence all,
size classes will also produce an upwardly sloping common signal. This bias occurs
in the absence of any common climate signal, and is a result of random distribu-
tion of tree growth rates and the fact that sampling takes place at a specific point
in time.
Figure 5.7b shows the yearly means of all RCS indices (based on a single RCS
curve) after alignment by their final growth year (year zero). Even though these trees
grew at various times over a 7000-year period, and should not, therefore, contain
any common external growth-forcing signal when in this alignment, there is still
evidence of a residual bias, represented by a general positive slope as tree counts
increase followed by a slight decline in the final period, when tree counts remain
constant. This decline is likely the result of indices of 'fast-growing' trees, with
their typical negative slope (cf. Fig. 5.4 ) , dominating over the weaker positive trend
from 'slow-growing' trees.
Fig. 5.7 Based on the same subfossil tree-ring dataset used in Fig. 5.5 , but here with all individual
series aligned on the final ring of each series (year zero): ( a ) the mean ring width of the subset of all
40- to 60-year-old rings from trees with a final radius between 12 and 14 cm; ( b ) indices, created
using regional curve standardization (RCS), aligned according to their final year of growth (year
zero) and averaged to form an 'end-aligned' chronology. Ring counts for each year are shown by
the gray shading