The role of plant-soil feedback in long-term species coexistence cannot be predicted from its effects on plant performance

Abstract

Background: Despite many studies on the importance of competition and plants' associations with mutualists and pathogens on plant performance and community organization, the joint effects of these two factors remain largely unexplored. Even less is known about how these joint effects vary through a plant's life in different environmental conditions and how they contribute to the long-term coexistence of species.

Methods: We investigated the role of plant-soil feedback (PSF) in intra- and interspecific competition, using two co-occurring dry grassland species as models. A two-phase PSF experiment was used. In the first phase, soil was conditioned by the two plant species. In the second, we assessed the effect of soil conditioning, competition and drought stress on seedling establishment, plant growth in the first and second vegetation season, and fruit production. We also estimated effects of different treatments on overall population growth rates and predicted the species' potential coexistence.

Results: Soil conditioning played a more important role in the early stages of the plants' life (seedling establishment and early growth) than competition. Specifically, we found strong negative intraspecific PSF for biomass production in the first year in both species. Although the effects of soil conditioning persisted in later stages of plant's life, competition and drought stress became more important. Surprisingly, models predicting species coexistence contrasted with the effects on individual life stages, showing that our model species benefit from their self-conditioned soil in the long run.

Conclusions: We provide evidence that the effects of PSF vary through plants' life stages. Our study suggests that we cannot easily predict the effects of soil conditioning on long-term coexistence of species using data only on performance at a single time as commonly done in PSF studies. We also show the importance of using as realistic environmental conditions as possible (such as drought stress experienced in dry grasslands) to draw reasonable conclusions on species coexistence.

Keywords: Bromus erectus; Inula salicina; Janzen–Connell hypothesis; Plant–soil (below-ground) interactions; coexistence; germination; moisture treatment; population dynamics; population growth rate; target–neighbour design.