Willow above, changes below: Seedless tree invader impacts riparian seed bank in the Patagonian ecotone

Abstract Aim The soil seed bank, an ecosystem component with a crucial role in the natural regeneration of plant communities, can be impacted by invasive non‐native plants. The impact of non‐native plants can be direct with the production of their own seeds or indirect without producing seeds, which has been less investigated. We determined the impact of an invasive seedless tree, non‐native willow ( Salix × rubens ), on the soil seed bank in a riparian ecosystem. Location Northwestern Patagonia, Argentina. Methods In autumn and spring 2022, we collected soil samples and estimated understorey cover, respectively, in plant communities invaded by willow and in uninvaded communities with a canopy dominated by native species. Over a year, we recorded the seedling emergence of soil samples to estimate the abundance, richness, and composition of the seed bank. We also compared the composition between the seed bank and understorey. Results Total seed abundance and native woody species seed abundance decreased twofold and fourfold, respectively, in invaded plant communities in comparison with uninvaded communities. The most affected woody species was Ochetophila trinervis , a key tree species, because of its dominance in the canopy and understorey, and its role in various ecological processes (nitrogen cycling). In addition, willow invasion moderately changed the seed bank species composition producing species turnover. For both the seed bank and the understorey, species composition was more similar between invaded and uninvaded communities than between the seed bank and understorey. Willow invasion had more impact on understorey composition than on the seed bank. Conclusions Although willows do not produce seeds regionally, their invasion negatively impacts the soil seed bank by replacing native woody canopy species that supply seeds to the seed bank. These impacts could result in notable changes in the structure of the plant communities and the ecosystem dynamics of riparian areas.