Facilitating invasion by altering soil conditions: Evidence from Lespedeza cuneata

Background/Question/Methods

Lespedeza cuneata (sericea) is an Asian legume introduced to the U.S. in the 1930s as erosion control and forage, but it can also have strong negative effects on native species and in some cases form dense monocultures. One proposed explanation for this invasive success is that sericea produces allelopathic chemicals that either directly suppresses native species or indirectly alters soil chemistry or microbial communities in ways that give sericea a competitive advantage. Additionally, sericea may have competitive advantages over native species that operate independently or in conjunction with these allelopathic mechanisms. To test these hypotheses, we collected soil from a previous three-year field experiment in which sericea was established in or excluded from randomly selected plots in a field with a common soil type and site history. These soil treatments were partitioned into additional treatments that included autoclaving—to eliminate microbes—and addition of a leachate derived from sericea tissues to test for a direct allelopathic effect. For each treatment combination, a native plant, Sorghastrum nutans (indiangrass), was grown with sericea and alone in a twelve-week greenhouse experiment.  

Results/Conclusions

We found that sericea biomass increased when grown in soil with a sericea history as opposed to a soil with a history of native species (F_{1, 16}=4.79, P=0.04). This may indicate that sericea is able to condition the soil over time to facilitate its own growth. Secondly, autoclaving increased the biomass of indiangrass regardless of soil history (F_{1, 112}=101.2, P=<0.001), and furthermore, the biomass of indiangrass increased more in the autoclaved soil that had a history of sericea rather than the soil with a native plant history (F_{1, 112}=3.9, P=0.049). This may indicate that autoclaving creates a flush of nutrients in the soil that benefits indiangrass, while not having the same benefit to sericea possibly because the latter can fix nitrogen. These results suggest that at least part of the competitive advantage of sericea over native species is linked to alteration of soil conditions.