Trait variation and long-term population dynamics of the invasive Alliaria petiolata (garlic mustard) across three microhabitats

Long-term population dynamics across heterogeneous habitats can be a major factor in determining species’ ability to expand their ranges and persist in novel environments, though this is largely unstudied in relation to invasion success. While largely restricted to disturbed semi-shaded microhabitats in its home range, the invasive herb Alliaria petiolata (garlic mustard) successfully invades intact forest understories – a novel microhabitat – in its introduced range. To test the hypothesis that source-sink metapopulation dynamics may be promoting A. petiolata ’s incursion into the forest understory, we conducted two multi-season field surveys approximately a decade apart to evaluate trait variation, biomass allocation, and long-term population demographics of A. petiolata growing at the forest edge, within the intact forest understory, and in the intermediate transition zone between the two. Our results show that plants in the edge microhabitat had highest performance and reproductive capacity, as well as the highest density across most life stages in both survey periods. Populations in all microhabitats were predicted to grow (λ > 1) at the onset of the study. A decade later, declines in population size were only predicted in the forest understory (λ < 1). Since edge patches had the highest densities of adult plants which produced the most fruit and had larger reproductive biomass, edge populations may have sustained the lower-density forest populations through source-sink dynamics. Within a management context, eradication of A. petiolata populations in edge microhabitat could thus be an effective management strategy for reducing populations in the forest microhabitat.