Mowing Modulates the Biotic Filter of Expansive Species

Disturbance and competition are two strong drivers of plant community formation. Disturbances like mowing in semi-natural grasslands enhance taxonomic and phylogenetic diversity by preventing the establishment of expansive species, which act as a biotic filter. However, how mowing and expansive species influence alpha and beta diversity, and how mowing may modulate the effects of expansive species on plant communities, remains largely overlooked. We sampled 61 (0.5 × 0.5 m) vegetation plots in abandoned and mown semi-natural grassland in Central Italy characterised by varying degrees of cover of expansive species. We used Rao's Quadratic Entropy to quantify alpha and beta taxonomic and phylogenetic diversity and we performed multiple regression models to evaluate the effect of management types (mowing and abandonment), expansive species cover, and their interaction. Overall, mown grasslands hosted higher alpha and beta taxonomic and phylogenetic diversity than abandoned grasslands. Alpha taxonomic and phylogenetic diversity decreased with increasing expansive species cover, whereas for beta diversity, only the taxonomic facet is negatively affected. For all diversities except beta-phylogenetic diversity, we detected a significant interaction between the effect of management types and expansive species cover, with their effects being stronger in mown than abandoned grasslands. Mowing represents a management type capable of counteracting homogenisation due to land abandonment at multiple spatial scales, by allowing a higher number of species with distinct lineages to persist locally. Expansive species acts as a biotic filter in reducing the plant diversity within the community, but its effect on homogenising plant communities was weaker. However, the intensity of this biotic filter was more intense in mown grasslands than in abandoned ones, probably due to higher species richness. Our findings highlight how complex these drivers are and that management can modulate the filterering effect of expansive species.