Geoscience Reference
In-Depth Information
-23.0
-23.2
13
C
corr
δ
-23.4
-23.6
-23.8
-24.0
-24.2
-24.4
y = -0.0003x - 23.836
R
2
= 0.01
-24.6
-24.8
-25.0
cambial age (years from pith)
13
C is shown when
13
C ratios, corrected for changes in atmo-
Fig. 6.2
Thejuveniletrendin
δ
δ
13
C
corr
) and aligned to a common mean, are cambial age aligned. The mean of 12
Pinus sylvestris
trees from sites at the northern tree line in Finnish Lapland is shown (the time
series is from Gagen et al.
2007
)
. A linear
trend line
indicates the lack of any long-term trend after
the trees reach ~50 years in cambial age. The juvenile trend, prior to this, imparts a dramatic rise
in
spheric CO
2
(
δ
13
C
δ
As the amount of CO
2
in the atmosphere has also increased, we might expect to
see a direct response to this in trees, because it is the ratio of internal to ambient
CO
2
concentrations that controls
13
C. If trees did not respond at all to the increase
in the availability of CO
2
, and the rates of stomatal conductance and assimilation
remained unaffected, then
δ
13
C
cor
values would show a marked decline since AD
1850, with an acceleration after AD 1950. However, we rarely see such a pattern in
tree-ring
δ
13
C
cor
series, suggesting that trees have actively responded to rising CO
2
by increasing their water use efficiency and keeping the ratio
c
i
/
δ
c
a
approximately
it means that we do not usually need to make any correction over most of the indus-
trial period. There is growing evidence, however, that in recent decades many trees
have reached the limits of this plastic response, so that water use efficiency is no
longer increasing and the
δ
6.4 Progress to Date
Early studies of stable isotopes in tree rings were severely constrained by the lim-
itations of mass spectrometry at the time and, although they made significant steps
towards a mechanistic understanding of the proxy, they did not make a contribution
