LAUSR

BACKGROUND Despite many studies on the importance of competition and plants' associations with mutualists and pathogens on plant performance and community organisation, joint effects of these two factors remain largely unexplored. Even less is known about how these joint effects vary through plant's life in different environmental conditions and how they contribute to species long-term coexistence. 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's growth in the first and second vegetation season and fruit production. We also estimated effects of different treatments on overall populations growth rate of the species and predicted their potential coexistence. RESULTS Soil conditioning played a more important role in the early stages of 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 plants' 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 the plants' life. Our study suggests that we cannot easily predict the effects of soil conditioning on long-term coexistence of species using only data on performance in single occasion as commonly done in PSF studies. We also show the importance as realistic environmental conditions as possible (such as drought stress experienced in dry grasslands) to draw reasonable conclusions on species coexistence.