Predicting daytime carbon isotope ratios of atmospheric CO2 within forest canopies

Summary

• 1
    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 (δ¹³C_p) values is influenced by both the variation in δ¹³C of source air (δ¹³C_a) and by photosynthetic carbon isotope discrimination (Δ). However, source air information is often unavailable, limiting the interpretation of δ¹³C_p, particularly in dense stands.
• 2
    In this synthesis we found that about 70% of the observed variation in δ¹³C_p values within the canopy was influenced by changes in Δ, and that about 30% was determined by source air effects. Significant shifts in δ¹³C_a 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 δ¹³C_p measurements are used to calculate Δ and c_i/c_a ratios [ratio of internal CO₂ concentration in the mesophyll airspaces (c_i) to the ambient atmospheric concentration of CO₂ (c_a)].
• 3
    We modelled δ¹³C_a 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 δ¹³C_a values throughout the canopy.
• 4
    In open canopies with leaf area index (L) of <2·5 or at canopy heights ≥1 m, the within-canopy daytime δ¹³C_a differences are negligible, and variations in δ¹³C_p are associated primarily with changes in Δ. Then, one can use the easily available carbon isotope ratio of the troposphere (δ¹³C_trop) as a substitute for δ¹³C_a to calculate Δ to within ±0·4‰.
• 5
    In canopies with L values >2·5, and at canopy heights <1 m, our model is recommended for calculating canopy δ¹³C_a values when direct measurements are not feasible. Although δ¹³C_a is highly variable near the forest floor in those dense forests, our model is more accurate and precise for estimating δ¹³C_a within 1 m above the forest floor than using δ¹³C_trop throughout the canopy (−0·2‰ ± 1·5 versus −1·4‰ ± 1·1).