Lonicera maackii (Amur Honeysuckle) is an invasive shrub that is widespread throughout the eastern United States. Lonicera invasion is often associated with loss of diversity and declines in overall forest health. Lonicera’s competitive advantage is largely attributed to its quick growth and ability to crowd out its neighbors. However, this advantage would be limited to mature plants. Lonicera maackii has been shown to produce root exudates and leaf litter that likely alter the soil microbiome. Impacts on soil biota may help to explain why Lonicera maackii is so successful, raising the questions at the core of this study: Does the soil microbiome vary enough between Lonicera maackii and the native constituents to confer a competitive advantage to Lonicera seedlings? and Does the soil microbiome of Lonicera have antagonistic effects on native species? To explore these questions Lonicera maackii and Acer negundo (a common native tree) were grown in isolation and together in co-culture. For each experiment, replicates were grown in soil inoculated with either Lonicera or Acer soil. A control was grown with sterilized inoculum to account for abiotic heterogeneity. Plants were harvested, dried and weighed after three months.
Results/Conclusions
Biomass data revealed that the differences in the soil microbiome between Lonicera maackii and Acer negundo significantly affected plant growth. This was true both when plants were grown in isolation and in co-culture. When the plants were grown in isolation, each microbial community had either a negative or nonsignificant impact on plant growth. Interestingly, the strongest negative interactions were between each plant species and its own soil microbes, suggesting self-inhibition. When grown in co-culture, soil microbes had vastly different effects. When grown in co-culture, the relationship between each plant and microbiome varied widely from strongly antagonistic to mildly positive. The differences between plants grown in isolation and co-culture indicate that soil microbes moderated the effects of competition. When a positive plant-microbiome relationship was seen in co-culture, it much likely due to a stronger microbial inhibition of a competitor when compared to the uninoculated control. These results differ from expectations of direct microbial inhibition, yet still provide an explanation of Lonicera maackii’s advantages in establishment.