Photosynthetic physiology of eucalypts along a sub-continental rainfall gradient in northern Australia

Leaf-level photosynthetic parameters of species in the closely related genera Eucalyptus and Corymbia were assessed along a strong rainfall gradient in northern Australia. Both instantaneous gas exchange measurements and leaf carbon isotope discrimination indicated little variation in intercellular CO2 concentrations during photosynthesis (ci) in response to a decrease in mean annual precipitation from ∼1700 mm to ∼300 mm. Correlation between stomatal conductance and photosynthetic capacity contributed toward the maintenance of relatively constant ci among the sampled leaves, when assessed at ambient CO2 concentration and photon irradiance similar to full sunlight. Leaf mass per area was the most plastic leaf trait along the rainfall gradient, showing a linear increase in response to decreasing mean annual precipitation. The maximum Rubisco carboxylation velocity, Vcmax, expressed on a leaf-area basis, showed a modest increase in response to decreasing rainfall. This modest increase in Vcmax was associated with the strongly expressed increase in leaf mass per area. These results suggest that variation in ecosystem-level gas exchange during the dry season in north-Australian savannas will likely be dominated by changes in leaf area index in response to increasing aridity, rather than by changes in photosynthetic performance per unit leaf area.