Extended leaf phenology, allelopathy, and inter-population variation influence invasion success of an understory forest herb

Abstract

Extended leaf phenology (ELP) may commonly drive invasion in Eastern deciduous forests of North America. ELP may confer an advantage in competition, and may interact with other invasion factors. For example, ELP may interact with allelopathy (release of toxins) if exposure to seasonal light influences allelochemical production. Here, we examine ELP and its interaction with glucosinolate (allelochemical) production in invasive garlic mustard (Alliaria petiolata). To test ELP’s role in invasion, garlic mustard was grown in monoculture or polyculture with native species under natural and extended shade regimes. Consistent with an ELP invasion mechanism, garlic mustard survival was higher in natural shade than extended shade, although invader biomass and native responses did not differ between light treatments. While garlic mustard leaf glucosinolate concentration was higher in natural than extended shade during September of its first year, this pattern did not hold at three other time points. Stronger support for the role of ELP in driving garlic mustard invasion emerged from direct manipulation of germination phenology, with higher garlic mustard survival and biomass resulting when germination occurred earlier in the season. Analysis of allelochemical production across eight populations of garlic mustard revealed significant inter-population variation in glucosinolate responses to light availability. Overall, results of these three experiments indicate that ELP may facilitate garlic mustard’s survival in invaded communities. We did not find strong evidence for a synergistic relationship between ELP and allelopathic potential, possibly due to high levels of inter-population variation in the relationship between allelopathy and light.