Spatial and temporal scaling of intercellular CO 2 concentration in a temperate rain forest dominated by Dacrydium cupressinum in New Zealand

Seven methods, including measurements of photosynthesis ( A ) and stomatal conductance ( g s ), carbon isotope discrimination, ecosystem CO 2 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 CO 2 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  µ 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 CO 2 ( δ   13 C R ; 236  µ mol mol −1 ) was much lower than other estimates of canopy C i . Partitioning δ  13 C R into four components (soil, roots, litter and foliage) indicated root respiration as the dominant (> 50%) contributor to δ  13 C R . Variable time lags and differences in isotopic composition during photosynthesis and respiration make the direct estimation of canopy C i from δ  13 C R problematic.