Alliaria petiolata (garlic mustard) is an invasive herb in North America that is known to impact forest understory community structure. Though largely restricted to edge habitats in the home range, garlic mustard invades intact forest understories – a novel habitat type – in the new range. Different environmental conditions between edge and understory habitats could select for different traits, leading to divergence in characteristics and demography over time. Long-term monitoring of active invasions is rare and can provide insight into the invasion process. This novel study examines a metapopulation of garlic mustard distributed across forest edge, intermediate, and forest understory habitat types over a long-term time scale (13 years). We conducted a metapopulation field survey at the Harvard Forest (Petersham, MA) from 2003-2006, and again in 2015 to determine densities and characteristics in each habitat type. Our study investigates whether: 1) functional and fitness-related traits (height and number of fruits per plant) differ over time and between habitat types, 2) edge habitat type supports higher densities of immature and reproductive adults than intermediate or forest understory habitat types, and 3) densities for immature and reproductive adults are increasing, stable, or in decline in each habitat type over a long time scale.
Plants were taller and produced more fruit in the edge habitats, and traits did not differ within habitats from year to year. Immature garlic mustard density was typically higher in the edge habitats than in the forest and intermediate habitats. Reproductive adult densities were generally similar between all habitat types within the same year; unlike immature plants, edge habitat types did not have higher adult densities. There was also high interannual variability in adult densities within habitat types, suggesting the metapopulation is following a biennial life-stage cycle with pulses of seedlings followed by adults in alternate years. Reproductive adult densities in edge habitat types have been similar over the 13-year period, showing little long-term variation, suggesting populations in edge habitats are not in decline. However, we saw declines in reproductive adult densities in forest and intermediate habitats in 2015 compared to earlier years. Since edge habitats also produce the most fruit, we speculate that the forest populations may not be sustainable over the long-term without influx of seed from edge habitats. We will continue metapopulation and functional and fitness trait surveys in 2016 to establish a more complete picture of population dynamics in this species.