Stable Isotopes in Dendroclimatology: Moving Beyond ‘Potential’ - Dendroclimatology - page 162

where there are plenty of trees available that is no real advantage, given the extra
cost and effort required for IRMS. One of the great challenges for isotope dendrocli-
matology is to extract climate signals from areas where traditional measurements do
not perform well, such as the moist midlatitudes where, for example, climatically
sensitive ring width series are often limited to high-altitude sites. Climate recon-
structions from such areas are needed to redress the bias towards high latitudes
in hemispheric temperature reconstructions and to test general circulation models
(GCMs).
Long tree-ring chronologies already exist for many areas in the moist midlat-
itudes, typically constructed for dendrochronological dating, or to calibrate the
radiocarbon timescale rather than for climate reconstruction (e.g., Pilcher and
Baillie 1980 ; Suess 1980 ; Becker 1993 ; Friedrich et al. 2004 ) . There must be some
common (climatic) information in these ring width series; otherwise, they would not
crossdate, but it is rarely strong enough to be useful for climate reconstruction. The
common signal in isotope series is also likely to be weaker away from the ecological
boundaries, and so the required replication will be greater, but preliminary results
suggest that it may be possible to produce acceptable climate reconstructions based
on stable isotopes (Saurer et al. 1995 ; Switsur et al. 1996 ; Loader and Switsur 1996 ;
Robertson et al. 1997a , b; Saurer et al. 1997 ; Hemming et al. 1998 ; Loader et al.
2008 ) . This would be a valuable contribution to climatology.

6.5.2 Different Climate Signals

The climate reconstructions that have so far been produced have not been based on
mechanistic models; they have used the same statistical techniques that are used in
the more traditional approaches to dendroclimatology. However, we know that this
is a simplification of reality, and that none of the isotopes is really controlled by a
single climate parameter in a linear fashion. Of course this is true for all proxies,
and there are many factors that can influence the annual growth increment of a
tree, for example. The advantage with stable isotopes, however, is that the range of
parameters that can control fractionation and cellulose formation are both limited
and relatively well understood.
This does not help, of course, in interpreting a single isotope series in isolation;
all we can say is that there is more than one potential change in climate that could
explain a rise or fall in the stable isotope values in a particular tree, in the past.
However, when we combine two or more stable isotope series together, and also
include the growth proxies, then we can begin to define a much smaller range of
conditions under which a particular suite of proxies will covary through time.
Stable isotopes almost certainly perform best in combination with each other;
there are numerous different climatic conditions that might lead to a particular
δ

13 C value, but a much smaller range of conditions will account for a specific
combination of

18 O values (McCarroll and Loader 2004 ) . The potential of
this 'multiproxy' or 'multiparameter' approach to dendroclimatology looks very
promising. For example, McCarroll et al. ( 2003 ) combined stable carbon isotopes

13 C/

δ δ