Environmental and Biotic Controls on the Invasion of the Exotic Shrub Elaeagnus umbellata in a Michigan Forest

Abstract

Distinguishing the 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, I 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, I expect light availability to be a driving factor of growth. To test this hypothesis for the invasive shrub, I carried out a transplant experiment of seedlings of the invasive plant along light gradients. In a complementary analysis, I use growth data from forest censuses carried out five years apart to study how growth and invasibility of the exotic shrub species relate to the invaded community. From the experimental work, I found that seedling growth of the invasive shrub is strongly affected by soil water content and initial size, whereas light provides a marginal effect, contradicting common assumptions on how this species responds to resources. Results from the analysis of the census data show that the invasive shrub’s adult growth is highly variable in response to environmental parameters, but is controlled by small-scale biotic interactions, like competition for space or limited resources. The model outcomes distinguish the growth of the invasive species from those of a native shade-intolerant tree and a native shade tolerant shrub in response to light availability. Predictions from the empirical model suggest an increased competitive ability under high light conditions of the invasive shrub over the native shrub (Hamamelis virginiana), but not over the shade-intolerant tree (Sassafras albidum). When comparing these results to a community analysis of plant functional traits, I find similar results to the response to light; however, plant functional traits fail to reveal biotic controls noted in the demographic study. Ultimately, I conclude the spread of Elaeagnus umbellata is restricted by moisture and herbivory during early establishment and limited by proximity to taller individuals as an adult. However, despite the identification of controlling factors on the growth of Elaeagnus umbellata, it is still likely that functionally similar native species will experience negative impacts from the invasion event as the invasive shrub has a higher potential competitive ability than some of these species as measured by growth in the understory light environment.