Seven methods, including measurements of photosynthesis (A) and stomatal conductance (g(s)), carbon isotope discrimination, ecosystem CO2 and water vapour exchange using eddy covariance and the use of a multilayer canopy model and ecosystem Keeling plots, were employed to derive estimates of intercellular CO2 concentration (C-i) across a range of spatial and temporal scales in a low productivity rain forest ecosystem dominated by the conifer Dacrydium cupressinum Lamb. in New Zealand. Estimates of shoot and canopy C-i across temporal scales ranging from minutes to years were remarkably similar (range of 274-294 mu mol mol(-1)). The gradual increase in shoot C-i with depth in the canopy was more likely attributable to decreases in A resulting from lower irradiance (Q) than to increases in g(s) due to changes in air saturation deficit (D). The lack of marked vertical gradients in A and g(s) at saturating Q through the canopy and the low seasonal variability in environmental conditions contributed to the efficacy of scaling C-i. However, the canopy C-i estimate calculated from the carbon isotope composition of respired ecosystem CO2 (delta C-13(R); 236 mu mol mol(-1)) was much lower than other estimates of canopy C-i. Partitioning delta C-13(R) into four components (soil, roots, litter and foliage) indicated root respiration as the dominant (> 50%) contributor to delta C-13(R). Variable time lags and differences in isotopic composition during photosynthesis and respiration make the direct estimation of canopy C-i from delta C-13(R) problematic.
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