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

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 (Δ13Capparent) 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 Δ13Capparent of foliated beech saplings in the light. Unbiased foliar Δ13C was 1.1 to 4.9 ‰ lower than leaf Δ13Capparent, depending on photosynthetic rates of woody tissues. Therefore, we strongly recommend accounting for isotope-related bias due to woody tissues when estimating leaf Δ13Capparent based on branch or shoot chamber measurements.