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 partial ly attributable to physiological differences to indigenous fynbos. In this investigation we tested the hypothesis that the greater N availability due to the N-2-fixing ability of acacias would result in greater Rubisco activi ty and photosynthetic capacity, relative to fynbos species. The gas exchang e characteristics, leaf N concentrations and photosynthetic N use efficienc ies of Acacia saligna and Acacia longifolia were compared with those of Pro tea repens, Chrysanthemoides monilifera, Dodenea viscosa and Leucadendron s alignum. Compared to the fynbos species, the acacias had larger leaves and higher leaf water contents, but intermediate specific leaf areas. The acaci as had more than 3-fold higher leaf N concentrations, but 50% lower photosy nthetic nitrogen use efficiency than the fynbos species. The acacias showed no clear photosynthetic advantage over the fynbos species, although, the f ormer tended to have higher photosynthetic capacities and water use efficie ncies. The lack of correlation between leaf N concentrations and photosynth etic capacity or Rubisco activity may have been due to differences in the u tilisation 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 .