Examining the role phenotypic plasticity in plant invasions: a study of invasive Japanese knotweed (Fallopia japonica) and native woodland knotweed (Persicaria virginiana)

Background/Question/Methods

The ecological and evolutionary study of plant invasion processes is of exceeding importance in today’s changing environment. Biological invasions may be thought of as natural experiments that researchers can use to study the effects of possibly novel and intense selection pressures on species that are in the process of aggressively colonizing new environments. It has been suggested that there are at least two ways that species can become invasive: through rapid local adaptation and/or through expanded phenotypic plasticity. I have conducted a common garden experiment using eight field populations of invasive Japanese knotweed (Fallopia japonica) and a native, non-invasive relative, woodland knotweed (Persicaria virginiana) to measure their plasticity for traits relating to light acquisition. This experiment followed a study that documented strong, but variable, natural selection pressures in the field on these populations and a reciprocal transplant study that showed no local adaptation of light acquisition traits in either one. In the common garden, all eight populations were subject to two light treatments: ambient and under 50% shade cloth. 

Results/Conclusions

Both species had plastic trait responses to shade, but they followed different plasticity strategies. Woodland knotweed followed a “jack-of-all-trades” approach; it was able maintain its fitness under either light treatment by plastically adjusting stomatal density and growth rate. Japanese knotweed seemed to use the opportunistic “master-of-some” strategy; its trait plasticities (for height and leaf area) allowed it to take advantage of a better quality environment. Examination of associations of each trait with a fitness proxy (biomass) revealed that, depending upon the environmental context, how much fitness variation explained by each trait changes. There was no clear distinction between the native and invasive species studied. Both possess trait plasticity that allows them to thrive in different light conditions, yet woodland knotweed does not display the noxious weed behavior that Japanese knotweed possesses. The opportunistic plasticity of Japanese knotweed is part of what makes it a successful invader, but there are certainly other contributing factors that remain to be investigated to explain its great invasion success.