Not invasive status but plant overstory matters: open shrub canopies support greater plant and arthropod diversity and more complex food web structures compared to shady tree canopies

Plant invasions threaten native biodiversity, but detailed information on patterns and mechanisms of diversity changes across multitrophic levels remains unknown. Alien plants can reduce richness of native plants, thereby negatively affecting arthropods and their multitrophic interactions. In particular, tall invasive trees may limit native understory plants and associated arthropods more than small invasive shrubs, which may support light-demanding taxa under their more open canopies. In this study, we investigated how two non-native highly invasive species (shrub Sorbaria sorbifolia and tree Amelanchier spicata ) and two native species (shrub Rubus idaeus and tree Prunus padus ) distributed in monodominant patches along roadsides in SW Finland affect arthropod-feeding guild biomass and food webs. Under trees, regardless of their origin, the diversity and biomass of understory plants and arthropods living in the canopy and on the ground was significantly reduced compared to shrubs, which had higher biodiversity and more significant interactions between feeding guilds and understory plants. The higher biomass of native understory plants increased the abundance of herbivores under the shrubs, and thus indirectly predators. The species richness of understory plants determined the richness of saprophagous organisms in native stands. We conclude that more open roadside shrub canopies support greater plant and arthropod biodiversity and more complex food web structures compared to shady tree stands, and that alien shrubs do not necessarily reduce arthropod biodiversity. The study opens up the possibility of predicting arthropod composition and biomass using functional attributes of understory vegetation and the origin, physiognomy and species identity of overstory dominants.