Could riparian restoration increase stream invasibility to submersed macrophytes? Effects of shading on Elodea canadensis and Myriophyllum spicatum

Background/Question/Methods

Recent decades have seen a surge in restoration of riparian zones. Riparian restoration has been shown to benefit multiple taxa (e.g., fish, birds), but effects on instream communities of macrophytes (aquatic plants) are rarely addressed. My research investigates the role that riparian restoration practices, through their alteration of overstory canopy light levels, may have on the establishment and growth of vegetative propagules of two common macrophyte species. I chose a native, Elodea canadensis (common elodea), and an invasive, Myriophyllum spicatum (Eurasian watermilfoil) as study species because I am particularly interested in how the alteration of a limiting resource (light) affects the dynamics between native and invasive species. I conducted experiments in two systems of artificial stream channels, using shade cloth as a proxy for different levels of riparian canopy cover. Simulating aquatic propagule dispersal, I staked vegetative fragments of these two species to the substrate within plots covered with 90%, 60%, 30% or no shade cloth, and measured their morphology and biomass after 7-8 weeks.

Results/Conclusions

Vegetative fragments of both species were able to establish and grow in all four light conditions. However, light level had significant effects on the growth rate and morphology of the plants. In general, lower light was associated with reduced biomass, less branching, and fewer flowers, although E. canadensis appeared to experience photoinhibition at full light levels in one system. Conversely, greater shade was associated with longer stems (and therefore less biomass per cm of stem), suggesting an etiolation response that could have implications for fragment dispersal. Myriophyllum spicatum grew larger than Elodea canadensis, but shade negatively impacted the invasive M. spicatum more than the native E. canadensis (85% vs. 76% biomass reduction, respectively, from full light to 90% shade). These results imply that in full light conditions, which could be created in natural systems through the common restoration practice of removing invasive riparian vegetation (e.g., eucalyptus, tamarisk), the invasive M. spicatum may be better able to outcompete the native E. canadensis. Conversely, at higher shade levels, which could be created by restoring overstory canopy cover in riparian zones, this competitive advantage may be significantly reduced.