PS 76-169
Influence of canopy shading and priority effects on establishment and growth of Eurasian watermilfoil and elodea in artificial stream channels
Availability of resources can influence the invasibility of ecosystems and the competitive interactions between native and non-native invasive species. Light is often a limiting resource in freshwater streams, and can be altered through management practices such as removing or planting overstory riparian vegetation. Invasive plants often outcompete natives in high resource conditions, suggesting that greater light may benefit invasive macrophytes more than natives. I tested the effects of canopy cover on an invasive macrophyte- Myriophyllum spicatum (Eurasian watermilfoil)- and a native macrophyte- Elodea nuttallii (elodea)- in two experiments done in two years. In the first year, I examined how shade levels affected the establishment success and growth of vegetative fragments of both species separately. In the second year, I tested the role of priority effects on growth rates of the invasive species using two treatments: one where both species were planted concurrently, and one where the natives were planted five weeks earlier. These competition treatments were crossed with shade to determine interactions between shade level and priority effects. Both experiments were conducted in the California Central Valley in artificial stream channels using 30%, 60%, and 90% shade cloth and no shade cloth as proxies for different amounts of canopy cover.
Results/Conclusions
In the first experiment, shade level had no effect on the establishment ability of either species: both established with nearly 100% success, even in over 90% shade. The two species performed comparably in each shade level, with both showing decreases in growth at higher shade levels. In the second experiment, I found no effect of native priority on the performance of the invasive macrophyte: growth rates of Eurasian watermilfoil were not significantly different in the two competition treatments in any shade level. As in the first experiment, shade significantly reduced the growth of Eurasian watermilfoil in both competition treatments. Eurasian watermilfoil performs similarly in 0% and 30% shade, suggesting that light is saturating at those levels. Elodea showed much higher variance in response within each shade level, and might be more strongly influenced by factors other than light that may have randomly varied in the experiments (e.g., flow rates and soil nutrients). My results suggest that increasing or decreasing canopy cover is unlikely to influence the ability of either species to establish, though greater shade will limit growth rates. Shade level is also unlikely to alter the competitive dynamics between the species, at least in the short term.