Bias in estimated online leaf carbon isotope discrimination due to woody tissues

Abstract

Branch or shoot chamber measurements integrate over both foliar and woody tissue carbon dioxide (CO₂) fluxes and their associated influences on the carbon isotopic composition of atmospheric/headspace CO₂. Here, we quantified the bias introduced by woody tissue carbon isotope fluxes on apparent leaf ¹³C discrimination (Δ¹³C_apparent) estimates, using laser spectroscopy under controlled conditions. CO₂ efflux from woody tissues of defoliated beech saplings in the dark was strongly related to temperature (R² = 0.78), which served as the basis to model light-dependent woody tissue photosynthesis (R² = 0.72). We then quantified the contributions of leaf and woody tissues to leaf Δ¹³C_apparent of foliated beech saplings in the light. Unbiased foliar Δ¹³C was 1.1 to 4.9 ‰ lower than leaf Δ¹³C_apparent, depending on photosynthetic rates of woody tissues. Therefore, we strongly recommend accounting for isotope-related bias due to woody tissues when estimating leaf Δ¹³C_apparent based on branch or shoot chamber measurements.

Keywords:

• assimilation
• branch chamber
• carbon-13
• carbon isotope fractionation
• isotope ecology
• laser spectroscopy
• shoot chamber
• stem CO₂ efflux
• woody tissue

Acknowledgements

We thank Alexander Knohl, Lydia Gentsch and Patrick Sturm for their support concerning experimental design and laser spectrometry measurements. Further, we thank Thomas Baur, Peter Plüss and Patrick Flütsch for chamber construction and technical support and Annika Ackermann and Roland A. Werner for sample analysis at the Isolab, ETH Zürich.

Additional information

Funding

This work was supported by the European Commission under the Marie Curie Excellence Grant [Project No: MEXT-CT-2006-042268]. CHB acknowledged financial support by the Swiss State Secretariat for Education, Research and Innovation [Project No: C09.0159], linked to the COST Action ES0806 “Stable Isotopes in Biosphere-Atmosphere-Earth System Research (SIBAE)”.