Stable Isotopes in Dendroclimatology: Moving Beyond ‘Potential’ - Dendroclimatology - page 150

space as regulated by these two rates. Because the climatic signal is usually domi-
nated by the response of either stomatal conductance or photosynthetic assimilation
rate, the same

13 C value can arise from different climatic forcings, in different
locations (Saurer and Siegwolf 2007 ) .
For example, a sunny growing season, at a cool, moist site, may force an increase
in A, resulting in a drop in c i and a larger proportion of 13 C moving through the path-
way to be stored in that year's carbohydrate. In such a situation the carbon isotope
signal would capture sunny summers as high

δ

13 C values. Alternatively, a partic-
ularly dry summer, at a more arid site, might force a reduction in g with the same
effect on c i and

δ

13 C values would record low relative humid-
ity/antecedent precipitation. In both cases, owing to the close association typically
observed between these meteorological variables, it is likely that

13 C. In this case, high

δ δ

13 C would also

δ

correlate with summer temperature.

6.2.2 Stable Oxygen and Hydrogen Isotope Theory

Stable oxygen and hydrogen isotope measurements from tree rings are a proxy for
the isotopic composition of water taken up by the tree's roots (the source water),
overprinted by evapotranspiration in the leaf; the latter signal is dominated by vapor
pressure deficit (relative humidity).
Variability in source water isotopes is of climatological interest because these
isotopes relate to local air mass characteristics (Darling 2004 ) . Isotope fractiona-
tions during the hydrological cycle result in a strong association between isotopes
in precipitation and mean annual surface air temperature, but as part of a com-
plex system (Dansgaard 1964 ) . The water isotope system in trees and the models
describing them (e.g., Barbour et al. 2001 ) are more complicated than those for
δ

13 C. Following uptake by the roots, water is transported through the xylem with-
out fractionation until it reaches the leaves and other non-suberized tissues, where
it undergoes evaporative enrichment via transpiration. Here, fractionation favors
the loss of the lighter isotopes ( 16 O and 1 H), resulting in enrichment of the leaf
waters. Evaporative enrichment in the leaf is diluted by a Péclet effect (Barbour
2007 ) whereby unenriched stem water diffuses into the leaf and replaces transpired
water, attenuating enrichment. Photosynthetic fractionations are also mass depen-
dent and act in addition to leaf-level enrichment (Barbour et al. 2001 ; Roden et al.
2000 ) so that the resulting photosynthate is enriched in 18 O and depleted in 2 H.
Finally, a degree of enrichment is lost during synthesis of cellulose from sugar, due
to re-exchange with xylem water (Sternberg et al. 1986 ) .
Stable oxygen and hydrogen isotope measurements from tree rings thus provide a
mixed signal. The oxygen and hydrogen isotopic ratios of tree-ring cellulose usually
represent a source water isotopic signal modified by evaporative enrichment to a
greater or lesser extent. Strong correlations between temperature and wood/cellulose
water isotopes are sometimes found (e.g., Switsur et al. 1996 ) because temperature
has a direct effect on the isotopic composition of precipitation and also an indirect
effect on evaporative enrichment in the leaf (McCarroll and Loader 2004 ) .