BayCEER Workshop 2015

Impatiens glandulifera: Impact of an invasive plant species on the seedling development of co-occurring native species

Biological invasions are a major threat to biodiversity. One possible reason for the success of alien plant species is that they can outcompete native plant species by allelopathy (novel weapon hypothesis). It is known that Impatiens glandulifera (Himalayan Balsam) produces allelochemicals and can suppress the growth of natives. But up to now it is not known, whether and to which extent already seedlings have an impact on the seedling establishment of native plant species and if the Himalayan Balsam itself is immune to its own allelochemicals.

Therefore we investigated the impact of I. glandulifera seedlings on the early seedling development of the own species and four European native plant species, typical for the ecosystems invaded by the Balsam. Germinated seeds were placed on Agar together with seeds of I. glandulifera and radicle growth was measured. In a second trial the seedlings were grown on Agar containing different amounts of material of dried I. glandulifera seedlings or different concentrations of 2-Methoxy-1,4-naphthoquinone, known as an allelopathic agent produced by I. glandulifera.

First results indicate species specific suppression of the natives by I. glandulifera seedlings. Reduced growth was however not due to the allelopathic agent but due to competition. Under influence of the Indian Balsam the natives' radicles were up to 25% shorter. I. glandulifera seedlings also reduced growth intraspecifically.

A high competitive effect of the invader on the seedling development can be highly advantageous when colonizing bare soil, where plant recruitment from seeds is crucial. Co-occuring species may be outcompeted, facilitating the development of pure stands of the Himalayan Balsam.