Geoscience Reference
In-Depth Information
6.1 Scope and Background
When trees grow, they assimilate carbon from atmospheric carbon dioxide, and
hydrogen and oxygen from soil water. The stable isotope ratios of these three ele-
ments carry signals that can be interpreted in terms of past climate because isotope
ratios are climatically controlled by the tree's water and gas exchange budgets.
Mechanistic models are available that describe the fractionation of these isotopes
the climate signal, but climate reconstruction still relies on the statistical techniques
used in other branches of dendroclimatology.
The traditional tree-ring proxies form the most widespread and arguably the most
valuable of the high-resolution climate archives. In his review of dendrochronol-
isotopes worth the extra effort? He points to the climatological effectiveness of the
traditional tree-ring proxies and suggests that to be a useful complement to them,
stable isotopes would need to show coherent patterns of variability on spatial scales
of hundreds to thousands of kilometers and contain different kinds of information.
Hughes goes on to state that 'there is reason to believe that such a major effort may
be worthwhile' and points to emerging evidence that these conditions could be met.
Here we demonstrate that progress in recent years has shown that it is indeed worth-
while, and that stable isotope dendroclimatology has unique strengths, and is now
making novel and specific contributions.
In this chapter we describe what is involved in measuring tree-ring stable iso-
topes, provide a brief review of progress to date, and attempt to answer Hughes
used to provide something new. We conclude that the technique can sometimes be
used to provide stronger climate signals than the traditional proxies, particularly
where sample replication is limited. Stable isotopes can also be used to access dif-
ferent climate signals in trees, providing a more synoptic view of past climate and
may also have the potential to provide a greater proportion of the lower-frequency
climate signal that is difficult to retain during statistical detrending of tree-ring width
series. The use of stable isotopes may also pave the way to extracting climate sig-
nals from ringless tropical trees. The time has come for isotope dendroclimatology
to move beyond papers that simply demonstrate 'potential' and actually start to
reconstruct the climate of the past, and this must be done in collaboration, not in
competition, with traditional dendroclimatology.
6.2 Theoretical Background
There is a robust theoretical framework linking stable isotopes to climate, which is
provided by models describing how carbon, oxygen, and hydrogen are fractionated
by plants. These models provide the theory for expecting a climate signal from the
stable isotope ratios of wood, the explanation for why the climatic signal varies from
place to place, and why it is stronger in some areas than in others.
