Do plant soil feedbacks impact Ardisia elliptica invasion in Hawaii?

Background/Question/Methods

Impacts of plants on soil communities and in turn impacts of soil communities on plant invasiveness are often identified as ‘plant-soil microbe feedbacks.' Invasive species may experience positive plant-soil feedbacks, which contribute to their invasion success. Additionally, invasive species may suppress the performance of native vegetation by releasing chemicals novel to the invaded communities, disrupting mutualistic associations, and/or cultivating native soil pathogens. We investigated the role that soil feedback plays in the success of Ardisia elliptica (Primulaceae) a dominating invasive tree in Hawaii.

We measured plant species richness in areas invaded by Ardisia elliptica and areas not yet invaded by it at two sites on the island of Oahu using haphazardly placed 1x1m quadrats. We investigated plant-soil feedbacks by planting seedlings of A. elliptica, and the native herb Bidens sandwicensis (Asteraceae) in soil cones in a two by two factorial design with sterilized or unsterilized soil and soil from A. elliptica rhizospheres or topsoil from sites with few or no A. elliptica. Seedlings were grown for 4 weeks and plant performance was measured as increase in root and shoot length.

Results/Conclusions

A two-way Analysis of Variance showed that invaded sites had lower species richness, averaging of 3.2·species·m^-2 compared to 6.3·species·m^-2 at adjacent uninvaded plots (F=35.6;df=1,55;P<0.001). Species richness did not differ between the two study sites (F=2.3;df=1,55;P=0.135). At the dryer site, Ardisia elliptica root growth in sterilized soil was 7.0cm, versus 4.4cm in unsterilized soil (F=12.7;df=1,28;P=0.001), and this positive effect of sterilization was observed for soils taken from either the A. elliptica rhizosphere or adjacent uninvaded areas. There was no treatment effect of soil sterilization on A. elliptica root growth in soils collected from the wetter site (F=2.08;df=1,27;P=0.652). A. elliptica shoot growth was negligible across all treatments. Native Bidens sandwicensis roots grew an average of 8.2cm and shoots an average of 0.3cm but growth did not depend on either soil sterilization (F=0.161;df=1,22;P=0.692) or A. elliptica invasion (F=1.19;df=1,22;P=0.286).

Ardisia elliptica invasion in Hawaii may not be explained by below-ground direct positive effects mediated by soil biota. Contrary to expectations, soil biota had a negative effect on A. elliptica performance at one site. We did not find suppressed performance in native Bidens sandwicensis grown in A. elliptica soil despite there being lower species richness in A. elliptica stands. Nevertheless, further examination of soil feedback pathways, introduction of novel chemicals, soil fertility and/or accumulation of soil pathogens might help to explain A. elliptica’s dominance.