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WEED ECOLOGY

Is bitou allelopathic?

 

Our studies have shown that bitou bush creates a new environment on the sand dunes which is different from native habitats. Further work by Emilie Ens for her PhD has suggested that bitou bush releases toxic chemicals into the sand which inhibit the germination and growth of several native plants

We compared extracts from bitou bush roots, leaves and soil to those from the dominant native shrub, Acacia longifolia var. sophorae (coastal wattle) and found that both the hydrophobic (containing oils and resins) extracts from the bitou bush root and soil significantly inhibited the seedling growth of coastal wattle, Actites megalocarpa (coastal dune thistle), Lomandra longifolia (mat rush) and Banksia integrifolia (coastal banksias).

Although the hydrophobic extract of the coastal wattle roots also affected the seedling growth of some species, there was no comparable effect of the soil extract from under Acacia which suggests that these phytotoxic compounds are not released into the sand and therefore do not have the potential to affect the establishment of native species as the bitou bush root and soil extracts did.

 

Figure 1: Banksia seedlings grown in bitou bush root hydrophobic extract (Left) and in water (Right)

To investigate which phytotoxic hydrophobic compounds might be released by the bitou bush into the soil we made up resin bags (see photo below) which were designed to adsorb hydrophobic or volatile compounds that are present in the soil. We put the resin bags out into bitou bush invaded sand, sand below coastal wattle and bare sand and compared the chemical profile of these environments.

 

Figure 2: Resin bags (above) and placing resin bags in the sand under bitou bush, Wyrrabalong NP (right)

We found that in the sand below bitou bush there was a much higher concentration of volatile compounds called terpenes that are known to inhibit plant, fungi and bacterial growth. Some of these compounds were only found associated with the bitou bush. Similarly, we also found that some compounds that were present in the sand below the native vegetation were absent in the bitou bush invaded sand. Therefore bitou bush does change the chemistry of the sand dunes which is likely to inhibit native plant growth and create vacant space for more bitou bush to grow.

Further work is being undertaken by Emilie to isolate, identify and test some of the terpenes from the bitou roots.

Management implications

There will be difficulty in using seed stock to regenerate areas while bitou is still present and shortly after it has been managed.

There is likely to be some residual toxic chemical effects from the presence of bitou bush even after it has been removed. As most of the toxic compounds we identified in the bitou bush invaded system were low molecular weight volatile compounds, they are likely to evaporate from the sand as it heats up following the removal of bitou bush and be broken down by biotic and abiotic factors. Decay rates of the allelopathic chemicals have not been investigated yet, however, we suggest that seed addition may be less successful initially and may need to be repeated after a few months when soil has returned towards its normal chemical state. This will also be facilitated in time by the growth of native plants that will add “native” compounds back into the soil. Planting seedlings may facilitate the process, although we have not tested whether these compounds influence established plants.

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