Plant-herbivory feedbacks and selective allocation of a toxic metal are behind the stability of degraded covers dominated by Brachypodium pinnatum in acidic soils

Perennial tall-grasses have experienced a successful expansion in the last decades leading in some cases to extremely degraded communities, which remain in an alternate stable state. This research focuses on the mechanisms of persistence of the spreading native Brachypodium pinnatum in acidic soils. We hypothesize that plant-herbivory feedbacks and an adapted response to soil constraints, promote species cover stability. Eight on-site populations of the species growing in two contrasted situations (high-diversity grazed and low-diversity ungrazed communities) were sampled during a growing season and analysed for nutrient (N, P and K), metal (Al) and C allocation, biomass production and tissue quality. The species showed a high capacity for belowground accumulation of Al and an efficient reallocation and resorption of nutrients, which may explain its success in acidic, poor soils. The lack of attractiveness to herbivores of the species-poor stands prevented biomass removal, which strengthened some effects and enhanced the build-up of a thick layer of recalcitrant tissues, an effective barrier (aboveground and belowground) for the establishment of sympatric species. Density-dependent feedback mechanisms (herbivory attractiveness), and the particular attributes of B. pinnatum (i.e. powerful rhizomes with a selective performance for nutrients and aluminium) may lead to critical processes of community domination and degradation.