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Predicting daytime carbon isotope ratios of atmospheric CO 2 within forest canopies
Author(s) -
Buchmann N.,
Brooks J. R.,
Ehleringer J. R.
Publication year - 2002
Publication title -
functional ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.272
H-Index - 154
eISSN - 1365-2435
pISSN - 0269-8463
DOI - 10.1046/j.0269-8463.2001.00591.x
Subject(s) - canopy , daytime , atmospheric sciences , temperate deciduous forest , isotopes of carbon , biome , deciduous , taiga , leaf area index , δ13c , stable isotope ratio , understory , environmental science , biology , ecology , ecosystem , total organic carbon , geology , physics , quantum mechanics
Summary1 While measurements of leaf carbon isotope ratios ( 13 C/ 12 C) in terrestrial ecosystems have become more frequent, interpreting these data can remain a challenge in well developed canopies: the variation in leaf δ 13 C (δ 13 C p ) values is influenced by both the variation in δ 13 C of source air (δ 13 C a ) and by photosynthetic carbon isotope discrimination (Δ). However, source air information is often unavailable, limiting the interpretation of δ 13 C p , particularly in dense stands. 2 In this synthesis we found that about 70% of the observed variation in δ 13 C p values within the canopy was influenced by changes in Δ, and that about 30% was determined by source air effects. Significant shifts in δ 13 C 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 δ 13 C p measurements are used to calculate Δ and c i / c a ratios [ratio of internal CO 2 concentration in the mesophyll airspaces ( c i ) to the ambient atmospheric concentration of CO 2 ( c a )]. 3 We modelled δ 13 C 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 δ 13 C 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 δ 13 C a differences are negligible, and variations in δ 13 C p are associated primarily with changes in Δ. Then, one can use the easily available carbon isotope ratio of the troposphere (δ 13 C trop ) as a substitute for δ 13 C 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 δ 13 C a values when direct measurements are not feasible. Although δ 13 C a is highly variable near the forest floor in those dense forests, our model is more accurate and precise for estimating δ 13 C a within 1 m above the forest floor than using δ 13 C trop throughout the canopy (−0·2‰ ± 1·5 versus −1·4‰ ± 1·1).