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

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Laura M. S. Hancock, Department of Environmental Conservation, University of Massachusetts Amherst, Amherst, MA and Kristina A. Stinson, Department of Environmental Conservation, University of Massachusetts, Amherst, MA

Background/Question/Methods
Long-term population dynamics across heterogeneous environments can be a major factor in determining species’ ability to expand their ranges and persist in novel environments. If – and how – metapopulation dynamics over time and across heterogeneous habitats impacts invasion success is poorly understood. Though 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, where it is known to impact above and below ground community composition. In this study we ask: 1) if growth microhabitat significantly affects reproductive capacity, growth, and biomass allocation in A. petiolata, and 2) if there are demographic differences among populations of A. petiolata growing in three microhabitats over a decadal time scale. Specifically, 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.
Results/Conclusions
Adult plants in the edge were taller and branchier, produced more fruits, and had higher total and reproductive biomass than plants in the intermediate and forest microhabitats. Over time, seedling density remained highest in the edge microhabitat compared to the forest and intermediate microhabitats, which supported similar densities. Reproductive adult densities were similar among all microhabitats at the beginning of the study, but a decade later, all microhabitats exhibited a decline in the number of adult plants they supported. Populations in the intermediate microhabitat displayed the steepest decline in reproductive adults between sampling periods but still supported more adult plants than the forest microhabitat. 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), with the edge and intermediate populations still growing (λ>1). Since edge and intermediate patches had higher densities of adult plants which produced the most fruit and had larger reproductive biomass, it appears that the edge populations, and possibly the intermediate populations, have sustained the low-density forest populations through source-sink dynamics at our study sites. Mitigation of A. petiolata populations in the edge microhabitat could thus be an effective management strategy for reducing populations across microhabitats.