T. Kraaij et Md. Cramer, Do the gas exchange characteristics of alien acacias enable them to successfully invade the fynbos?, S AFR J BOT, 65(3), 1999, pp. 232-238
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
.