Bitou bush invasion is facilitated by soil chemistry changes which inhibit the growth of native plants

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

chemical effects.

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.