LIMITATIONS

Mesophyll conductance, gm, was estimated from measurements of stomatal conductance to carbon dioxide transfer, gs, photosynthesis, A, and chlorophyll fluorescence for Year 0 (current-year) and Year 1 (1-year-old) fully sunlit leaves from short (2?m tall, 10-year-old) and tall (15?m tall, 120-year-old) Nothofagus solandrii var. cliffortiodes trees growing in adjacent stands. Rates of photosynthesis at saturating irradiance and ambient CO2 partial pressure, AsatQ, were 25% lower and maximum rates of carboxylation, Vcmax, were 44% lower in Year 1 leaves compared with Year 0 leaves across both tree sizes.

Although gs and gm were not significantly different between Year 0 and Year 1 leaves and gs was not significantly different between tree heights, gm was significantly (19%) lower for leaves on tall trees compared with leaves on short trees.

Overall, Vcmax was 60% higher when expressed on the basis of CO2 partial pressure at the chloroplasts, Cc, compared with Vcmax on the basis of intercellular CO2 partial pressure, Ci, but this varied with leaf age and tree size. To interpret the relative stomatal and mesophyll limitations to photosynthesis, we used a model of carbon isotopic composition for whole leaves incorporating gm effects to generate a surface of ‘operating values’ of A over the growing season for all leaf classes. Our analysis showed that A was slightly higher for leaves on short compared with tall trees, but lower gm apparently reduced actual A substantially compared with AsatQ.

Our findings showed that lower rates of photosynthesis in Year 1 leaves compared with Year 0 leaves were attributable more to increased biochemical limitation to photosynthesis in Year 1 leaves than differences in gm. However, lower A in leaves on tall trees compared with those on short trees could be attributed in part to lower gm and higher stomatal, Ls, and mesophyll, Lm, limitations to photosynthesis, consistent with steeper hydraulic gradients in tall trees.