Mechanisms of coexistence: Exploring species sorting and character displacement in woody plants to alleviate belowground competition

Rarely do we observe competitive exclusion within plant communities, even though plants compete for a limited pool of resources. Thus, our understanding of the mechanisms sustaining plant biodiversity might be limited. In this study, we explore two common ecological strategies, species sorting and character displacement, that promote coexistence by reducing competition. We assess the degree to which woody plants may implement these two strategies to lower belowground competition for nutrients which occurs via nutritional (mostly mycorrhizal) mutualisms. First, we compile data on plant traits and the mycorrhizal association state of woody angiosperms using a global inventory of indigenous flora. Our analysis reveals that species in locations with high mycorrhizal diversity exhibit distinct mean values in leaf area and wood density based on their mycorrhizal type, indicating species sorting. Second, we reanalyze a large dataset on leaf area to demonstrate that in areas with high mycorrhizal diversity, trees maintain divergent leaf area values, showcasing character displacement. Character displacement among plants is considered rare, making our observation significant. In summary, our study uncovers a rare occurrence of character displacement and identifies a common mechanism employed by plants to alleviate competition, shedding light on the complexities of plant coexistence in diverse ecosystems.

Methods
All analyses were carried out to the subset of Angiosperms that were classified as woody species. We obtained data on leaf Area, tree height, leaf mass per area (LMA), and seed mass trait values from the Global Spectrum of Plant Form and Function Dataset (Díaz et al., 2022), and wood density information from the International Centre for Research on Agroforestry (World Agroforestry 2023). The selection of plant traits for our analysis was a compromise between feasibility, yielding information for divergent sets of plant species, and covering the three documented sets of autocorrelated traits: the leaf economics spectrum, the wood economics spectrum, and the root economics spectrum. Lists of indigenous species per location were extracted from The Global Naturalized Alien Flora database, a global inventory (van Kleunen et al., 2019). To assess leaf area variability across plant species' distribution ranges, we utilized the SLA dataset from Wright et al. (2017). Mycorrhizal association type information was retrieved at a plant family level as published in Delavaux et al. (2019). We assigned a fixed mycorrhizal state for a species only if at least 40% of the records in Delavaux et al. (2019) matched a single mycorrhizal association type. All analyses focused on woody species, and mycorrhizal association types were reclassified into arbuscular mycorrhiza, non-arbuscular mycorrhiza, mixes (arbuscular mycorrhizal and other mycorrhizal types), and non-mycorrhizal (Fig. 1). Trees not forming any mycorrhiza were excluded from further consideration due to their limited presence in most analyses.