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 (c_i) 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 c_i 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, V_cmax, expressed on a leaf-area basis, showed a modest increase in response to decreasing rainfall. This modest increase in V_cmax 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.