Physiological competitiveness of autumn olive compared with native woody competitors in open field and forest understory

Autumn olive (Elaeagnus umbellata Thunb.) is an invasive shrub known to flourish in abandoned farm fields, road sides and less so in forest understories. Autumn olive is categorized as a shade tolerant plant. The objectives of this research were to describe the rate of autumn olive (AO) advancement in open field and forest understory habitats and to evaluate the leaf-level physiological basis by which AO gains competitive advantage over native woody plants in these two different plant communities. The area invaded by AO in forest understory increased by 26% during a two-year period. More impressively, the density of AO increased six-fold from low (>6 m to the nearest AO plant) to high density (AO plants within 3 m of one another) during the same period. Net assimilation rate of AO was among the fastest of five woody species in the high light intensity field habitat, equal to black cherry and hawthorn, but superior to gray dogwood and black walnut. In the light-limited forest understory, however, AO fixed carbon dioxide at two to three times the rate of the four other co-habiting woody species. The higher net assimilation rate was consistently correlated with leaf chlorophyll content but not water or light use efficiencies in both field and forest environments. While trait plasticity in response to varying light environments is well known, these results demonstrate that AO invasion traits cannot be viewed simply as a species-level property rather they vary in the two different plant community assemblages. The ability of AO to fix nitrogen may contribute to its high net assimilation rate, transpiration rate, and water use efficiency across habitats. These results strongly support the high prioritization of AO control in forest understory habitats for restorative investment because of its competitive physiological traits and rapid rate of encroachment.