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 122

This mismatch is most apparent for the indices that are produced from earlier
years of the tree lifetime, as the sub-RCS curves for fast- and slow-grown trees
(Fig. 5.4b ) often tend to a common value in old age and the corresponding index
series, produced as quotients from the overall mean RCS curve, display a neg-
ative trend for the early years of a fast-grown tree and a positive trend in the
early years of a relatively slow-grown tree (Fig. 5.4c ) . In an RCS chronology, if
in one period fast-grown trees outnumber slow-growing trees (or vice versa), artifi-
cial medium-frequency trends (i.e., of non-climate origin) might result. It is at the
recent end of a chronology that the influence of downsloping indices, derived from
fast-growing trees, may not, in general, be balanced by the upsloping index series
from slower-growing trees. The result, even under constant climate conditions, is
an overall negative bias, seen in the final century or most recent decades of the
chronology.
5.4.3 'Modern-Sample' Bias
The next bias we discuss has been referred to by Melvin ( 2004 , Section 5.4 ) as
'modern-sample bias.' We consider this of sufficient importance to identify it as a
specific potential bias in its own right, but it arises as a consequence of the pre-
viously discussed bias (i.e., because of different growth rates in contemporaneous
trees) and because of variations in the longevity of trees, allied to common tree
sampling practice.
A naturally grown, uneven-aged forest (even if growing in an unchanging
climate) will contain trees of differing ages that have roughly the same diameter.
The widths of rings of a specific age from trees of the same diameter must be smaller
for the older trees than for the younger trees. A plot of mean ring width for a specific
ring age plotted by calendar year for a specific sampling diameter range (i.e., only
for trees of a similar diameter when cored) will display a steady increase over time,
independent of any common climate signal. If samples are taken only from trees
alive on the sampling date, then the mean growth rate by year (as the average of
each diameter class for any specific age range, which all slope upwards) must also
slope upwards.
5.4.3.1 Relationship Between Growth Rate and Longevity
This bias arises if there is a relationship between average tree growth rate and tree
longevity and generally applies only to trees with full circumferential growth. If we
assume that the probability of tree mortality is related to tree size—i.e., large trees
have a high risk of mortality—then as trees approach the largest size for a given site,
they are much more likely to be killed, perhaps because of some extreme climate
event. Hence, the likelihood that some random extreme event will kill a tree is higher
while it is in the 'near maximum' size category. Rapidly growing trees are more
likely to approach or reach the maximum size than are slower growing trees because
the former need only spend a shorter time in the 'high-risk' (i.e., approaching large)