Large daily variation in 13C-enrichment of leaf-respired CO2 in two Quercus forest canopies

* • The use of the 13C : 12C isotopic ratio (δ13C) of leaf-respired CO2 to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in δ13C of leaf dark-respired CO2 (δ13Cr) under field conditions. * • Here, we explored variability in δ13Cr and its relationship to key respiratory substrates from collections of leaf dark-respired CO2, carbohydrate extractions and gas exchange measurements over 24-h periods in two Quercus canopies. * • Throughout both canopies, δ13Cr became progressively 13C-enriched during the photoperiod, by up to 7, then 13C-depleted at night relative to the photoperiod. This cycle could not be reconciled with δ13C of soluble sugars (δ13Css), starch (δ13Cst), lipids (δ13Cl), cellulose (δ13Cc) or with calculated photosynthetic discrimination (Δ). However, photoperiod progressive enrichment in δ13Cr was correlated with cumulative carbon assimilation (r2 = 0.91). * • We concluded that there is considerable short-term variation in δ13Cr in forest canopies, that it is consistent with current hypotheses for 13C fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for δ13Cr, and that diel changes in leaf carbohydrate status could be used to predict changes in δ13Cr empirically.