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| Titel |
When isotope signals in tree rings contradict our concepts and interpretations |
| VerfasserIn |
R. T. W. Siegwolf, D. Sarris, M. Saurer, O. V. Sidorova |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250071121
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| Zusammenfassung |
| The use of stable C and O isotopes in tree rings for retrospective climatic and environmental
analyses and reconstructions is well established. The 13C/12C ratio in wood reflects largely
the isotopic signal of the leaves, which is an expression of the balance between the CO2
supply (stomatal conductance) and the Carbon sink strength (photosynthetic rate or demand
function). When the stomatal conductance is reduced (usually under drought conditions) the
leaf intercellular CO2 concentration (ci) is reduced relative to the ambient carbon dioxide
concentration. Thus the conclusion was established the 13C/12C isotope ratio in the leaf is an
indicator for water availability or air humidity. Under dry conditions the 13C/12C isotope ratio
is higher than for conditions when soil water is abundant and the air humidity is
high.
The oxygen isotope ratio is usually considered as a proxy for temperature, since the
condensation temperature of the precipitation water determines the 18O/16O ratio, i.e. the
warmer the condensation temperature of the precipitation water the higher the 18O/16O ratio.
When plants absorb this water the signal is transferred via photosynthesis to the wood, as the
source water from the soil is used during photosynthesis for the sugar synthesis. Transpiration
via leaves either amplifies or reduces this 18O/16O signal. Thus the conclusion that the
oxygen isotope ratio can serve as a paleoclimatic thermometer is plausible and
justified.
Besides numerous successful applications often our concepts and assumptions do not
match the data. E.g. when a tree ring oxygen isotope chronology shows a decrease within the
last fifty years, even though other proxies confirm a continuously increasing temperature.
Or a severe drought period is not reflected in the isotope signals as expected. The
same paradox can be found in air pollution studies, when trees seemingly do not
respond even the heavy pollution loads. At times the explanations for such phenomena
are very plausible and at times we simply do not understand the results. In this
presentation we show results where the expected climate signals are not present or
even show opposite trends and we present various explanations for these findings. |
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