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The aridity influence on oxygen isotopes recorded in tree rings
Kinzie Bailey1,2,3, Paul Szejner4, Brandon Strange1,2,5
1School of Natural Resources and the Environment, University of Arizona, 1064 East Lowell Street, Tucson, AZ 85721, USA.
Tree Physiology
|April 7, 2025
Summary
Stable isotopes in tree rings (δ18Ocell) can reveal past water use, but aridity affects their accuracy. This study shows aridity influences oxygen exchange (apex), impacting how trees record water source and humidity signals.
Area of Science:
- Paleoclimatology and Dendrochronology
- Isotope Geochemistry
- Plant Physiology
Background:
- Stable oxygen isotopes in wood cellulose (δ18Ocell) are used to reconstruct tree water sources and atmospheric humidity.
- Assumptions that δ18Ocell reflects precipitation are often inaccurate in arid regions where trees use deeper water.
- The oxygen exchange proportion (pex) influences the δ18Ocell signal, but is often treated as constant and difficult to measure.
Purpose of the Study:
- To assess how variability in apparent pex (apex) affects the reconstruction of water source and aridity signals in Ponderosa pine δ18Ocell.
- To investigate the relationship between site aridity, apex, and the recording of vapor pressure deficit (VPD) in tree rings.
- To improve the disentanglement of water source and VPD signals in δ18Ocell, particularly in semi-arid regions like the North American Monsoon (NAM).
Main Methods:
- Collected xylem water and wood cellulose samples from seven Ponderosa pine stands in the NAM region.
- Measured δ18O in xylem water and wood cellulose.
- Compared measured and modeled δ18Ocell values to assess the influence of apex variability on environmental signals.
Main Results:
- Arid sites under-represented the vapor pressure deficit (VPD) signal in δ18Ocell, while wetter sites over-represented it.
- Apparent pex (apex) varied with site aridity: high aridity (low precipitation, high VPD) led to high apex, and vice versa.
- The relationship between aridity and apex suggests a complex interplay influencing isotopic signal fidelity in tree rings.
Conclusions:
- Site aridity significantly influences apparent pex (apex), complicating the interpretation of δ18Ocell as a direct proxy for water source and humidity.
- Understanding the aridity-apex relationship is crucial for accurately reconstructing past environmental conditions from tree rings.
- Future research should consider annual ring variations in apex to better differentiate water source and VPD signals, especially in monsoon regions.
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