Functional traits and environmental characteristics drive the degree of competitive intransitivity in European saltmarsh plant communities

Competitive intransitivity, the existence of loops in competitive hierarchies, is one mechanism that can promote the local coexistence of competitors and maintain high local species diversity, although its prevalence and importance remain largely unknown. A full understanding of local community assembly needs knowledge of how transitive and intransitive competitive interactions are linked to species functional traits and the strength of biotic and abiotic filters. We apply a recently developed statistical tool to quantitative data on central European inland saltmarsh plant communities to infer causal relationships between soil characteristics, species occurrences and functional traits, and we estimated coefficients of competition. We found a predominance of intransitive competitive hierarchies. The proportion of such hierarchies was positively correlated with local species richness and compositional variability. Average soil characteristics were not correlated with competitive intransitivity, whereas high soil pH and the high variability in local pH and soil salinity decreased the overall impact of competition on community composition. In pairwise comparisons of species, dissimilarity in morphology, resource demand and reproductive phenology was significantly negatively correlated with differences in competitive performance, while higher environmental dissimilarity was particularly linked to intermediate degrees of competitive superiority. Our results suggest that habitat filtering for similar traits might intensify competitive interactions, but might also give rise to intransitive competitive loops that subsequently promote species coexistence and permit species' functional equivalence. Intransitive competition appears to increase local diversity and small scale-species turnover. The observed local differences in competitive structures suggest frequent competitive plasticity and context-dependent competitive interactions. Finally, our results support the view that local abundance distributions can be used to infer the strength and outcome of competitive interactions.Synthesis. Our results confirm that intransitive competitive interactions might be a strong force structuring local plant communities. Intransitivity needs to be considered when studying plant community assembly and species co-existence.