Plant-soil feedbacks along a chronosequence of invasion

Background/Question/Methods

Plants alter soil microbial communities and in turn these influence plant growth, termed plant-soil feedbacks. Positive/negative feedback occurs when a given plant species has greater/smaller fitness in home versus away soil (i.e., “trained” by another plant species). Positive/negative feedback can be due to a net balance between mutualists/pathogens. Exotic invasive plant species commonly show positive feedback in their invaded ranges and negative feedback in their native ranges, purportedly due to enemy release. However, broad scale studies have shown negative feedback can increase in exotic invasive species over time, which is theorized to be a result of the local enemies adapting to the novel plant.

Using a combination of fully replicated field, glasshouse, and lab experiments along an invasion chronosequence, we investigated fine scale temporal changes in plant-soil feedbacks and root-associated fungal communities in a highly invasive exotic plant in eastern North America, Vincetoxicum rossicum (Apocynaceae). We hypothesized that plants grown in soil from recently invaded sites (<12 years) would demonstrate positive feedback, but negative feedback would increase with invasion time (50-100 years). In addition, we used 454 pyrosequencing to determine the structure of fungal communities in roots of plants across the invasion chronosequence, expecting more putative pathogens at older sites.

Results/Conclusions

Contrary to our hypothesis, we found evidence that negative feedbacks declined with increasing invasion age. Specifically, a generalized linear mixed model with a priori contrasts showed that plants grown in soil from recently invaded sites had lower shoot biomass than plants grown in soil from where V. rossicum has been present for at least 100 years (P<0.05). Further models showed that plants grown in soil from recently invaded sites had significantly fewer leaves and fewer flowers (P<0.05) than plants in soil that had been invaded for 50-60 years (intermediate invasion age). Contrasting with these findings, more plants in the intermediate invasion age showed no signs of root necrosis compared to recently invaded sites (P<0.05).

Several known fungal pathogens were detected in the roots of V. rossicum (Cadophora sp., Colletotrichum sp., Macrophomina phaseolina, Ilyonectria radicicola, Phialophora sp., Plectosphaerella cucumerina, Phaeocytostroma sp., Rhizopycnis vagum). However, their degree of specialization on this plant species is unknown. Vincetoxicum rossicum may eventually decline in abundance as negative soil feedback caused by these fungi increases. However, more than 100 years of invasion does not appear to have been sufficient for root enemies to cause declines in this exotic invasive plant.