Do the gas exchange characteristics of alien acacias enable them to successfully invade the fynbos?
The invasive success of alien Acacia species in the fynbos could be partially attributable to physiological differences to indigenous fynbos. In this investigation we tested the hypothesis that the greater N availability due to the N2-fixing ability of acacias would result in greater Rubisco activity and photosynthetic capacity, relative to fynbos species. The gas exchange characteristics, leaf N concentrations and photosynthetic N use efficiencies of Acacia saligna and Acacia longifolia were compared with those of Protea repens, Chrysanthemoides monilifera, Dodenea viscosa and Leucadendron salignum. Compared to the fynbos species, the acacias had larger leaves and higher leaf water contents, but intermediate specific leaf areas. The acacias had more than 3-fold higher leaf N concentrations, but 50% lower photosynthetic nitrogen use efficiency than the fynbos species. The acacias showed no clear photosynthetic advantage over the fynbos species, although, the former tended to have higher photosynthetic capacities and water use efficiencies. The lack of correlation between leaf N concentrations and photosynthetic capacity or Rubisco activity may have been due to differences in the utilisation of N for nonphotosynthetic processes and in the activation state of Rubisco. It seems unlikely that photosynthetic characteristics play an important role in the invasive success of alien acacias in the Fynbos biome.