One mechanism explaining the success of invasive weeds may be the production and release of allelopathic compounds by the invader that, due to a lack of co-evolutionary history, have harmful effects on plant neighbours in the introduced range.
We partially tested this hypothesis by growing seven competing native European plant species either with the introduced Solidago canadensis s.l., one of the most successful invasive plants in Europe or on soil pre-cultivated with S. canadensis. We added activated carbon to the soil to neutralize organic chemical compounds with putative allelopathic effects. Furthermore, we added
unsterilized soil inocula from the introduced (Switzerland) or native (USA) range to the soil to test potential confounding effects of soil microbes on invasion success. Untreated sterilized soil served as control.
Five out of the seven native species were more competitive against the invasive species in soils with activated carbon than without, supporting the allelopathy hypothesis. However, competitive outcomes were also influenced by the two sources of soil inoculum and by interactive effects of soil inoculum and Solidago origin suggesting that soil microbes alter allelopathic effects.
Achillea millefolium, the species least affected by the presence of S. canadensis and with no response to the activated carbon treatment is the only species used in this experiment reported to grow within Solidago stands in Europe, whereas the other European species tested tend to grow at the periphery of invasive Solidago stands.
Chemical analysis by LC-MS of Solidago root extracts revealed four main secondary chemical compounds with potential allelopathic effects. Root exudates of Solidago
showed a significant inhibitory effect on growth of the model plant species Arabidopsis thaliana. The magnitude of
inhibition increased with increasing concentration of the extract.
Synthesis. Levels of the four compounds were lower in
Solidago populations from the invasive range than in populations of the same ploidy level from the native range. This suggests lower investment of invasive plants into these secondary compounds, possibly because of a higher
susceptibility of plant competitors in the invasive range to these substances.