Herbivory-Induced Nardus stricta Dominance Enriches Plant Silicon Relative to Phosphorus Enforcing Alternative Ecosystem Stable States in an Alpine Environment

Abstract Herbivory can alter plant stoichiometry generating biogeochemical feedbacks that stabilize or drive transitions between ecosystem states. We report results after 23 years from a grazing experiment in Norway comparing five paired exclosures and grazed plots that were complemented with inclusion of five islands serving as a natural control. We tested the hypothesis that herbivory-driven shifts in plant dominance scale to alter elemental stocks and stoichiometry indicative of decelerated ecosystem biogeochemistry. Domestic sheep, Ovis aries , grazing was associated with dominance of the herbivory-resistant grass, Nardus stricta, rich in silicon (Si), while the ungrazed islands harbored a sheep-preferred grass Avenella flexuosa and forbs such as Solidago virgaurea relatively richer in phosphorus (P). Inside the exclosures, N. stricta had lower abundance than under grazing, but remained dominant at most sites. Its persistence suggests hysteresis on the trajectory to an ungrazed state displayed inside one mainland exclosure and on the islands. N. stricta had a high Si:P ratio, and through its dominance, this stoichiometry scaled to aboveground biomass with a more than doubling of the stock of Si (125%) on the mainland compared to the islands and a 46% increase in the concentration of Si. In contrast, the concentration of P in aboveground biomass on the islands was more than double (122%) the mainland. Herbivory appears to stabilize a high-Si ecosystem state compared to a high-P ungrazed state indicated through the plant Si:P ratio in this alpine ecosystem. This enrichment of silicon relative to phosphorus may cascade throughout the ecosystem with far-reaching implications.