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 124

be discerned. Plotting the mean growth rates of individual samples by sample age
(i.e., final tree diameter divided by final age) again reveals a tendency for slowly
growing trees to live longer (Fig. 5.5b ) . The oldest trees have mean growth rates far
lower than the reduction explained by the expected decrease of ring width with age
shown in the curves of Fig. 5.5a . This occurs despite the fact that many subfossil
tree samples lose outer rings because of erosion or degradation of the relatively soft
sapwood.
5.4.3.2 Growth Rate/Longevity Association Distorts RCS Curves
Figure 5.6 illustrates a very simplified example of one distortion that occurs where
a sample of even-sized (radius 26.4 cm) trees of varying age might be cored
(Fig. 5.6a ) . Here there is an assumed constant climate forcing (horizontal dashed
line of Fig. 5.6b ) and the younger trees have progressively larger growth rates. The
mean RCS curve is horizontal up until 240 years (the age of the youngest tree) but
gets progressively lower as the lower growth rates of the older trees dominate. Thus
this is a distortion of the RCS curve caused by older trees having lower growth rates.
Hence individual index series aligned by calendar age then exhibit positive trends
after 240 years (Fig. 5.6c ) , and the calendar-aligned averaged values (Fig. 5.6d )
display an initial (stepped) increasing trend for the first 160 years of the mean
chronology due to the oldest trees having slower growth rates, and a late increasing
trend after 240 years because of distortion of the RCS curve, despite the fact that the
Fig. 5.6 A simple hypothetical example of the distortions to a common underlying forcing sig-
nal when a chronology is constructed by using regional curve standardization (RCS) applied to an
uneven-aged group of similar-sized trees. ( a ) The measurements (mm.) from five trees of unequal
length and of the same final diameter (26.4 cm), each containing the same trend in growth super-
imposed on the differing overall average growth. When aligned by ring age, the average RCS curve
( b ), instead of displaying the expected linear growth trend ( dotted line ) is distorted, showing nar-
rower expected ring width for older trees. ( c ) Series of indices created through division of the
simulated measurements by the RCS curve values, where all but the shortest series are distorted.
( d ) The chronology created by averaging the index series and the desired low-frequency signal
( dotted line )