The negative effects of an allelopathic invader on native plant photosynthesis are amplified after tree canopy closure

Many invasive plants produce antimicrobial allelopathic compounds that disrupt plant–fungal symbioses of native species in the communities they invade, influencing nutrient and water provisioning that support photosynthesis. Previous studies have linked these disruptions to reductions in photosynthesis and stomatal conductance, but whether these effects are tied to reductions in photosynthetic capacity remains unclear, limiting inferences about the mechanisms driving physiological responses of native species. Furthermore, how these responses vary temporally across the growing season is unknown. To investigate the temporal dynamics of native plant responses to allelopathic invasion, we measured gas exchange in two understory native species ( Trillium spp. and Maianthemum racemosum ) at two points during the growing season—before and after tree canopy closure. Plants were measured in a long‐term field experiment where Alliaria petiolata , an allelopathic invader known to disrupt AM fungal symbioses, has been removed or left at ambient levels since 2006. Both native species exhibited significantly reduced net photosynthesis rates under ambient A. petiolata levels compared with the weeded treatment. This response was due to a reduction in apparent photosynthetic capacity in Trillium spp. and a reduction in stomatal conductance that increased stomatal limitation in M. racemosum . Photosynthetic responses to A. petiolata in both species were amplified after tree canopy closure. Alliaria petiolata reduced native plant net photosynthesis either by increasing nutrient stress, as indicated by the reduction in apparent photosynthetic capacity ( Trillium spp.), or by increasing water stress, as indicated by the reduction in stomatal conductance ( M. racemosum ). These responses were amplified only after the tree canopy closed. Our findings highlight the importance of seasonal changes that can regulate plant physiological responses to allelopathic invaders and demonstrate the diversity of mechanisms by which allelopathic invaders can influence native plant physiology. Read the free Plain Language Summary for this article on the Journal blog.