Bitou bush is a weed of national significance
and has been declared as a key threatening
process in NSW. We aimed to strengthen the scientific
understanding of the mechanisms of invasion by
investigating potential allelopathy and indirect soil
Our study compared whether extracts of bitou
bush leaves, roots and soil had a different effect on the
seedling growth of a range of native species compared
to comparable extracts from an acacia, the native dominant
in the non-invaded system. We found that bitou
bush roots released significantly higher concentrations
of sesquiterpenes into the soil, compared to the acacia.
Corresponding bitou bush root and soil extracts were
found through lab based bioassay studies to inhibit
seedling growth of four native species. Moreover,
acacia seedling growth was significantly inhibited only
by the bitou bush soil extracts, suggesting an indirect
soil chemical effect.
We also found that the acacia soil had higher
concentrations of phenolic compounds than the bitou
bush invaded soil. Several acacia root, leaf and soil
extracts also inhibited the growth of native seedlings,
although to a lesser extent than the bitou bush extracts.
These results suggest that although there appeared to
be allelopathic effects between co-evolved plants,
the exotic bitou bush inhibited the seedling growth
of all five native plant species studied, including the
dominant acacia. This finding suggests that bitou bush
dominance could at least partially be due to allelopathy
or indirect soil effects.