Predicting daytime carbon isotope ratios of atmospheric CO2 within forest canopies

Abstract

Summary While measurements of leaf carbon isotope ratios (13C/12C) in terrestrial ecosystems have become more frequent, interpreting these data can remain a challenge in well developed canopies: the variation in leaf δ13C (δ13Cp) values is influenced by both the variation in δ13C of source air (δ13Ca) and by photosynthetic carbon isotope discrimination (Δ). However, source air information is often unavailable, limiting the interpretation of δ13Cp, particularly in dense stands. In this synthesis we found that about 70% of the observed variation in δ13Cp values within the canopy was influenced by changes in Δ, and that about 30% was determined by source air effects. Significant shifts in δ13Ca occur in canopies with high leaf area, predominantly within 1 m above the forest floor. In complex canopies, particularly in the understorey, source air effects cannot be neglected if δ13Cp measurements are used to calculate Δ and ci/ca ratios [ratio of internal CO2 concentration in the mesophyll airspaces (ci) to the ambient atmospheric concentration of CO2 (ca)]. We modelled δ13Ca of daytime source air for deciduous and coniferous forests in boreal, temperate and tropical biomes. An inverse regression model with easily available input variables accounted for about 90% of the variation in daytime δ13Ca values throughout the canopy. In open canopies with leaf area index (L) of <2·5 or at canopy heights ≥1 m, the within‐canopy daytime δ13Ca differences are negligible, and variations in δ13Cp are associated primarily with changes in Δ. Then, one can use the easily available carbon isotope ratio of the troposphere (δ13Ctrop) as a substitute for δ13Ca to calculate Δ to within ±0·4‰. In canopies with L values >2·5, and at canopy heights <1 m, our model is recommended for calculating canopy δ13Ca values when direct measurements are not feasible. Although δ13Ca is highly variable near the forest floor in those dense forests, our model is more accurate and precise for estimating δ13Ca within 1 m above the forest floor than using δ13Ctrop throughout the canopy (−0·2‰ ± 1·5 versus −1·4‰ ± 1·1).