Spatial and temporal scaling of intercellular CO2 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 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.