Environmental and biotic controls on the invasion of the exotic shrub Elaeagnus umbellata in a temperate forest

Background/Question/Methods

Distinguishing differences between invasive species and those of the invaded native community is important to understand both the mechanisms and the potential impacts of invasion. In this study we ask how the effects of environmental and biotic factors on growth of fourteen native species in a southeast Michigan forest differ from the exotic shrub, Elaeagnus umbellata. Primarily focused on an understory woody plant community, we expected light availability to be a driving factor of growth. To test this hypothesis, we analyzed forest census data to address the strength of these influences on the growth and invasibility of the exotic shrub species in relation to the invaded community. Furthermore, we compared the growth of the invasive species with those of a native sun-demanding tree, a native shade-tolerant shrub, and three shade-tolerant understory trees in the forest community in response to light availability.

Results/Conclusions

Results of the analyses show that the invasive shrub’s adult growth is highly variable in response to environmental parameters, but is controlled by small-scale biotic interactions. In relation to the native understory community, results suggest an increased competitive ability of the invasive shrub over the native shade-tolerant shrub and trees, but not the sun-demanding tree. Results indicate that the spread of Elaeagnus umbellata is limited by shading from taller native tree species. Functionally similar native species could experience negative impacts from the invasion event as the invasive shrub has a higher competitive ability than some of those species as measured by growth in the understory light environment.