Plant functional traits suggest novel ecological strategy for an invasive shrub in an understorey woody plant community

1. Understanding ecological strategies of invasive species relative to the entire native community is important in understanding and managing both the mechanisms and the potential impacts of invasion, but few studies have taken this approach. 2. We utilize advances in plant ecology to compare functional traits of an invasive shrub species, autumn-olive Elaeagnus umbellata, to those of the understorey native woody plant community in a southeast Michigan forest. We estimate species trait distributions for six leaf functional traits (specific leaf area, leaf laminar area, leaf dry matter content, leaf nitrogen content, leaf carbon: nitrogen ratio and delta N-15) using a kernel estimator. We then use pairwise trait distance and overlaps to describe the ecological strategy of the invasive shrub relative to that of the native understorey woody plant community. 3. The overlap of the invasive shrub's trait distribution with those of the native species was found to be smaller than the overlap of a native shrub's distribution with other native species, consistent with the empty niche hypothesis of invasion. However, the invasive species' mean leaf functional trait values typically fall outside of the range of most native species' mean trait values, suggesting the invasive shrub extends trait space for understorey individuals of the native woody plant community. 4. The invasive shrub's leaf trait values are characteristic of a sun-demanding species, contradicting its presence in the shaded forest understorey community and its persistence in an empty niche. We propose that this novel use of leaf functional traits by the invasive shrub could indicate that the invasion is facilitated by additional mechanisms such as release from natural enemies, association with nitrogen-fixing bacteria, use of an empty temporal niche (namely the light available to the understorey early in the growing season), bird dispersal or some synthesis of these factors. 5. Synthesis and applications. This study develops plant functional trait theory by studying an invasive species within the context of the entire native woody plant community. The approach can improve our understanding of the potential mechanisms of an invasion event and suggest ecological consequences and related management strategies for the maintenance of native forest communities. Specifically, this study, along with known data on the invasive species, suggests aggressive management in sunny gap and edge habitat provides the best strategy for long-term control.