Invasive garlic mustard in a deciduous New England forest

"Biological invasions represent one of the most pressing environmental issues of the present day; the invasive biennial herb garlic mustard (Alliaria petiolata) is of particular concern because it produces allelopathic compounds that negatively affect native plants, fungi, and insects. We report on a 22-year record of garlic mustard population dynamics in three deciduous forest stands in New England and examine how their demographics correlate with conspecific density, environmental conditions, and study site. All three populations showed sustained, unexpected declines in population density after 2016. Prior to this (1998-2015), population densities followed gradually declining trends marked by biphasic population cycles (alternation between high rosette density and high adult density from year to year). The populations’ eventual decline to low levels suggests that controlling new invasions by removing adults prior to seed set may be more productive than managing established populations. Garlic mustard population growth was negatively density-dependent, but density dependence likely only affected seeds and seedlings. Populations benefited from warm winters but also from deeper snow layers and cooler springs, indicating complex relationships which require more study in the field and may affect the species’ invasiveness under future climate warming. Comparisons among the three sites indicated that the mid-successional site was most resistant to invasion, the younger Norway maple site the least resistant, and the late-successional beech-maple site intermediate. Further work will be necessary to explore the factors that may have led to the sites’ differing invasibility."

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