The ability to invade new habitat types can be a critical component of the invasion process for many plant species. As the invasive plant Alliaria petiolata (garlic mustard) has spread, it has expanded from sunny forest edge habitats into shaded forest interiors, making eradication increasingly difficult. Populations at the forest edge are a putative source of propagules into the shaded forest understory, but whether and how demographic processes and natural selection differ across habitats to affect broader metapopulation dynamics remain unclear. We conducted a field experiment at the Harvard Forest (Petersham, MA), in which we reciprocally transplanted garlic mustard seeds collected from forest edge, intermediate, and understory habitats into each of the three habitat types. We monitored these plants for two years to determine the impacts of maternal and offspring growth habitats on germination, survival, growth, and reproduction.
We found that there was a combination of maternal and offspring habitat effects on garlic mustard germination, growth, and survival. Germination rate was low among plants originating from edge habitats, and was most suppressed in the intermediate growth habitat, which was also the wettest and warmest. Survival rate was only influenced by growth habitat, and was significantly reduced in the intermediate habitat. There was a slight trend suggesting potential maternal habitat effects on survival. Maternal effects, however, disappeared by the second year of growth, in which the offspring habitat explained all variation in growth, phenology, and reproduction. Similar to the first year’s results, biomass and reproductive output were greatest in plants grown in the edge habitat, and growth and reproduction more suppressed in intermediate habitats than in forested ones.
Asymmetric reproductive output across habitat types suggests that source-sink dynamics contribute to the maintenance of invasion in intermediate and shaded habitats. The results of this study should help inform management strategies, as decreasing reproductive output of edge populations may trigger declines of intermediate and shaded populations. However, maternal effects on germination and early life stages also suggest that plants in the forested habitat could become self-sustaining.