Geoscience Reference
In-Depth Information
ratios of stable isotopes, primarily of carbon and oxygen, in tree rings as proxy cli-
mate records (Gagen et al.
Chapter 6
, this volume). The issues of sample design,
replication, removal of non-climatic variability, identification of climate signals,
and the properties of networks of isotope chronologies are all being explored, and
appropriate methods are emerging.
In this chapter the discussion of the four key issues mentioned in the previous
section will focus on TRW and MXD because they are the subjects of most of the
existing literature.
2.2.2 Single Site Chronologies
The criteria used to identify sites, species, and trees to be sampled in establishing a
local reference for dating annual rings correspond to those used to capture a clear
and strong climate signal. There is considerable variability in ring growth within
and between trees, and so multiple samples are taken so as to 'distill' the common,
presumably climatic, signal, and to 'dilute' the likely more individual non-climatic
variability or noise (Fig.
2.1
shows raw data from a site where high correlations
between sample series suggest a strong common climate signal). The common sig-
nal, usually expressed as some form of the mean of the individual samples' modified
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1800
1850
1900
1950
2000
1.4
1.4
1.2
1.2
1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0.0
1810
1820
1830
1840
1850
1970
1980
1990
2000
Year
Year
Fig. 2.1
Common pattern of variability shared by trees at one location. Raw, unmodified ring
widths (millimeters) of 18 samples from Douglas-fir in Navajo Canyon, Colorado, each shown by
a single fine line. Note that although absolute growth rates differ, the relative patterns of variability
are similar. The
lower
panels show two 41-year periods with the horizontal scale expanded. Data
provided by David M. Meko
