Light, functional traits, and fall phenology: An explanation of invasion for southeast Michigan shrubs
Invasive species can alter habitat structure, displace native species, and alter ecosystem function of invaded communities. A number of hypotheses of invasion have been proposed, but substantial work remains to determine their generality and fit to specific systems. Within Southeast Michigan, several common shrub species are invasive in the forest understory, where they co-occur with three common native shrubs. Prior work comparing traits of an invasive with those of native species suggested that the invasive species may be “cheating” on the growth-survival tradeoff, allowing it to persist in the understory when its traits suggest otherwise. However, it is also possible that the invasive species with functional traits adapted to a high light environment may be utilizing a less shaded part of the understory. We ask if the suite of invasive and native shrubs share similar traits, similar light environments, and lose leaves at a similar time in the autumn, or if each species is exploiting a unique pathway into the understory by comparing their traits to each other and those of the native community. We utilize a standard suite of plant functional traits, a neighborhood based model of individual light environment, and individual leaf fall to address this question.
In the understory of our research sites in Lenawee County, Michigan, we found substantial populations of Elaeagnus umbellata, Rosa multiflora, Lonicera maackii, Berberis thunbergii, L. morrowii, along with scattered individuals of other invasive shrubs. Native shrubs include Hamamelis virginiana, Ribes cynobasti, and Viburnum acerifolium. We found that all of these species are found in wide-ranging light environments within the understory, and no significant difference between the species in this regard. The invasive species’ traits (Specific leaf area, percent nitrogen, leaf dry matter content) were consistently at the “high light/live fast” end of the leaf economics spectrum, while the native species were at or near the opposite end. For leaf nitrogen content, in particular, Elaeagnus was readily distinguished from all other species, likely due to its nitrogen fixing ability. This apparent contradiction in environmental location and leaf traits may, at least in part, be addressed by their fall phenology. All invasive species tested, showed substantially later leaf senescence and abscission than did any native shrub or tree.