Invasive species present a substantial risk to biodiversity and ecosystem function. Understanding mechanisms of invasion is important to prevention of invasion and remediation of existing species. A number of hypotheses have been proposed but there is substantial question as to generality or applicability of these hypotheses to specific systems. Invasive shrubs, in particular, demonstrate traits consistent with a sun-demanding, fast-return on investment position on the leaf economics spectrum. However, these species are commonly found invading the understory of closed canopy forests. This research tracks four species of common invasive shrubs and three native species of shrubs in southeast Michigan forests. Specifically, we ask 1. Do the native and invasive shrubs share similar traits among species in each class? 2. Do invasive species consistently have a longer growing season due to later leaf fall? and 3. Do invasive shrubs harbor fewer fungal species than native species? To address these questions, we collected leaf functional trait data (SLA, LNC, LDMC, Delta 15N) from 20 individuals of each study species and tracked fall phenology for these species. We also grew out leaf fungi from each species on potato agar plates, and used morphotyping to determine the number of types of fungi infecting each species of shrubs.
Results/Conclusions
Substantial populations of Elaeagnus umbellata, Rosa multiflora, Lonicera maackii, Berberis thunbergii, L. morrowii, Rhamnus cathartica were found invading the forest understory. Native shrubs include Hamamelis virginiana, Ribes cynobasti, Cornus foemina and Viburnum acerifolium. All species occurred in a range of understory light environments, showing no mean difference in light levels in which they occurred. Consistent with prior work, invasive species showed high light/fast return on investment strategies with regard to the leaf economics spectrum, while native species ranged over a greater part of the spectrum, tending toward the shade-tolerant, slow return end of the LES. Invasive species exhibited an extended fall phenology, but this was a continuum, with Elaeagnus and Lonicera holding leaves into December, while other invasive species only had a 2-3 week extended growing season over native species. Furthermore, native shrubs had an over 2 fold increase in fungal endophytes over invasive species, suggesting that perhaps reduced fungal load is providing a sort of enemies release for invasive shrubs. Combined, these results suggest that these invasive shrubs are succeeding not becasue of a single "Magic bullet", but rather due to a range of factors that together give them a substantial advantage over native species.