Light, allelopathy, and post-mortem invasive impact on native forest understory species

Abstract

Extended leaf phenology (early budbreak and/or delayed leaf drop) and allelopathy are potentially key invasion mechanisms in North American deciduous forests. Because extended phenology confers increased access to light energy and allelochemical production is energetically costly, these traits may interact synergistically to determine invader impact. Garlic mustard (Alliaria petiolata) exhibits both traits, and may also exploit high light in open habitats. We manipulated seasonal light availability to examine effects of light on garlic mustard’s growth, allelochemical production, and impact on native species. Invaded and not-invaded woodland microcosms were exposed to three light treatments: shading year-round (‘extended shade’), shading when the local tree canopy was closed (‘natural shade’), and ambient light year-round (‘no-shade’). Regardless of native presence, garlic mustard biomass was highest under natural shade and, due to apparent irradiation damage, lowest under no-shade. Similarly, growth and fruit production of garlic mustard monocultures were reduced in unshaded conditions. Consistent with these results, garlic mustard reduced the growth of native woodland forbs Blephilia hirsuta and Ageratina altissima most under natural shade, suggesting that extended leaf phenology mediates impact on these herbaceous species. However, garlic mustard growth did not predict reduction of whole-community biomass: invasion reduced native community growth most under no-shade, where invader biomass was lowest but allelochemical production was highest. This result may be driven by a ‘post-mortem’ pulse of allelochemicals from decaying garlic mustard tissue. We conclude that extended leaf phenology may mediate garlic mustard’s impact on some native species, and that light and allelopathy may interact to drive invasion.