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 128

(i.e., partly subfossil) chronology, with long-timescale variation maintained in the
means of each index series, this is an 'end effect.' However, where the chronology
comprises one set of currently coexisting trees, as in many modern samples, the
overall slope of the chronology representing the external (i.e., climate control of tree
growth) will be removed. Had the chronology of Fig. 5.6 had an overall downward
slope, the resultant chronology would still display an upward slope (due to modern-
sample bias) because the downward slope of the chronology would be removed
by RCS.
Even where an uneven-aged sample of trees covers a wide time span, a localized
coincidence in the temporal spans of many samples at roughly the same stage in
their life span will locally bias the RCS curve. This bias is more likely near the 'old-
age' section of the RCS curve, where typical low replication of very old trees leads
to greater uncertainty in the RCS curve. This bias is potentially large for modern
chronologies and seriously limits the application of RCS where trees come from the
same time period (Briffa et al. 1996 ) .
In a typical 'modern' dendroclimatic sample collection, the earliest measure-
ments will come from the oldest trees cored, which tend to be slow growing.
Faster-growing trees that may have been contemporaneous with the old trees in the
early years will likely not have survived long enough to be included in the modern
sample. Similarly, the most recent section of the chronology produced from these
sampled trees would not contain data from young, slow-growing trees because these
trees would not be of sufficient diameter to be considered suitable for coring. Any
relatively young trees sampled would likely have to have been vigorous and growing
quickly enough to allow them to attain a reasonable size in a short time. This leads
to a situation where a 'modern sample' may exclude the fastest-growing trees of the
earliest period and also exclude the slowest-growing trees of the most recent period.
Such a sample of uneven-aged trees will be less susceptible to trend-in-signal bias,
but still prone to contemporaneous-growth-rate bias, with smaller indices at the start
of the chronology and larger ones at the end, imparting a positive bias on the overall
chronology slope.
Figure 5.9 illustrates the use of RCS on a 'modern' chronology and the way in
which a recent (ca. 1920) increase in the radial increments can influence the shape
of the RCS curve. This set of 100 measurement series (each an average of data
from multiple cores) from Luosto, north Finland (Melvin 2004 , Section 3.2 ), has
a wide age range. Pith-offset estimates are available for all of these data. First, an
RCS curve and corresponding chronology were produced from them (thin lines in
Fig. 5.9a , b ) . The signal-free method (see Appendix) was used to create an unbiased
(signal-free) RCS curve and corresponding chronology (thick lines in Fig. 5.9a , b ) .
The removal of the common signal from the measurements changes the shape of the
RCS curve, removing the influence of the post-1920 growth increase from recent
data (see values for ages 200-240 and 340 onwards in Fig. 5.9a ) and produces a rel-
ative increase in the expectation of early growth (up to age 120), and a smoother, less
noisy RCS curve. The resultant chronology will still suffer from bias (see Sections
5.4.1 , 5.4.2 and 5.4.3 ) , and the overall chronology slope is to some extent 'arbi-
trary' as described above. However, provided there is a wide distribution of tree