Invasive Hedychium coronarium inhibits native seedling growth through belowground competition

Invasive plants affect negatively the biodiversity of invaded communities by changing environmental conditions and outcompeting native species. Understand the way in which ecological requirements and compartments (i.e. light and space) modulate interspecific competition is important to elucidate the dominance mechanisms of invasive plants. Here, we tested the effect of an invasive amphibious macrophyte (Hedychium coronarium) on the growth of seedlings of a native pioneer riparian tree (Anadenanthera macrocarpa) under different light levels (direct light and partial shade). We transplanted H. coronarium rhizomes and sowed A. macrocarpa seeds to establish four competition treatments: invasive only, native only, an unmanaged interspecific competition treatment, and an interspecific competition treatment with clipping of invasive aerial parts after four months of seedling growing. All ramets and seedlings were measured soon after clipping and two months later. Both the length and diameter growth of the native seedlings were greater without interspecific competition and they were not affected by light level or by the interaction competition x light level. The removal of invasive aerial parts did not affect the growth of the native seedlings. Hedychium coronarium ramets instead had higher length growth in the presence of native seedlings and under direct light. Our results suggest that the negative effect of this invasive plant on the native seedlings was related mainly to belowground competition rather than to the shading caused by aerial parts. This finding reinforces the contribution of rhizomes to enhance H. coronarium invasion, which may constrain the development of native seedlings and consequently harm the recolonization by native species in riparian invaded areas.