Naturalized alien plants experience less negative soil‐legacy effects and gain competitive advantages through spatial heterogeneity

Abstract With the number of naturalized alien plant species continuously increasing, it has become of major interest to understand how they can coexist with or even outcompete native species. Plant–soil interactions and soil spatial heterogeneity are thought to play major roles in the coexistence of plant species. Depending on the strengths of conspecific and heterospecific soil feedbacks, size inequalities between competing plants might either decrease (i.e. promote coexistence) or increase. However, how spatial heterogeneity in soil legacies affects individual growth, competitive balance and thus coexistence of competing plants, and how this depends on the plant origins has never been tested. Here, we first conditioned soil with each of five naturalized alien and five native species separately. Thereafter, we grew all 45 pairwise species combinations under four homogeneous soil‐legacy conditions (including unconditioned soil as one of the homogeneous conditions) and two heterogeneous soil‐legacy conditions. Soil legacies overall had a negative effect on biomass production, irrespective of the origins of the competing species. Biomass inequality between the competing plants was the smallest when they grew on soil conditioned only by the larger one of the two species. Both alien and native plants suffered strongly from conspecific soil legacies. However, alien plants could benefit from another co‐growing alien species on homogeneous‐conspecific soil, which thus mitigated negative soil‐legacy effects, or benefit from heterogeneous soil conditions to increase the competitive advantage over native plants. Our findings show that the performance of competing plants may depend on their origins as well as on soil legacies. This may ultimately affect species competition and coexistence in natural environments where soil legacies are likely to be heterogeneous. Read the free Plain Language Summary for this article on the Journal blog.