Increasing canopy shading reduces growth but not establishment of Elodea nuttallii and Myriophyllum spicatum in stream channels
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Submersed macrophytes are often important drivers of instream structure and function, but can be problematic when overabundant. The establishment success, growth rates, and morphology of submersed macrophytes could be affected by alteration of instream light levels during riparian restoration (via removing or planting canopy-forming vegetation), potentially influencing the success of riparian restoration projects aimed at improving aquatic habitats. To examine the effects of canopy shading on two common submersed macrophytes—Elodea nuttallii (native) and Myriophyllum spicatum (non-native)—I conducted experiments in artificial stream channels in two locations in California, USA. Initial establishment of stem fragments of both species was close to 100% in all shade levels, including shade that reduced incident light by 94%. Growth rates of the two species were similar across shade levels, and lowest in the highest shade. Full light appeared to have a photoinhibitory effect on E. nuttallii at the higher elevation site. Higher shade increased the length:biomass ratio and decreased the branching of E. nuttallii. My findings suggest that altering canopy cover during riparian restoration is unlikely to affect the ability of these species to establish, but higher shade levels should slow their growth and create more favorable conditions for other instream organisms.
KeywordsRiparian restoration Propagule establishment Myriophyllum spicatum Elodea nuttallii Shade Sierra Nevada Aquatic Research Laboratory
I thank Truman Young, Peter Moyle, Eliška Rejmánková, the graduate students of the Young Lab from 2011 to 2013 (Kurt Vaughn, Lauren Porensky, Marit Wilkerson, Jen Balachowski, Kelly Gravuer, Mila Dunbar-Irwin, Steve Fick, Laura Morales, Kristina Wolf, Derek Young, and Grace Charles), Matthew Zimmerman, Alisa Kim, and Elaine Chow for their advice on the manuscript and/or help in the field. Neil Willits gave valuable statistical advice. I am especially Grateful to Andrew Fulks and J. P. Marie of the Putah Creek Riparian Reserve for creating the artificial channel system for the UC Davis experiment. I also thank the staff at the Sierra Nevada Aquatic Research Laboratory and the University of California Natural Reserve System. Fellowship support came from the UC Davis Graduate Group in Ecology and Plant Sciences Department. Additional thanks to two anonymous reviewers for their helpful comments.
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