Summer grazing change fitness in a Tibetan lotus

Abstract

It is believed that grazing has an effect on the functioning and performance of flowers, as well as their association with pollinator preferences and plant reproduction. However, the effects of grazing time on nectar secretion and pollen removal are still not fully understood. We hypothesized that seasonal grazing influences pollinator visitation and pollen removal by altering above-ground biomass and nectar secretion. In this study, we conducted field experiments to measure above-ground biomass, flower number per capitulum and capitulum number per plant, nectar volume and concentration, pollinator visitation of male- and female-phase flowers, pollen removal (e.g., number of pollens exported and deposition on a pollinator body and stigma), seed set ratio, and seed number between summer and winter grazing pastures of Saussurea stella. Our findings demonstrated that summer grazing reduced the above-ground biomass, nectar secretion, and pollinator visitation of male-phase flowers, thereby decreasing pollen export. There was a positive correlation between nectar secretion and above-ground biomass, as well as between pollinator visitation and nectar secretion, the number of pollens exported, and pollinator visitation of male-phase flowers. Experiments involving the removal of leaves revealed that a decrease in above-ground biomass was associated with a decrease in nectar secretion, pollinator visitation of male-phase flowers, and pollen removal. However, plants growing in summer and winter grazing pastures had comparable nectar secretion, pollinator visitation of female-phase flowers, seed number, and seed set ratio. Our results suggested that seasonal grazing influences nectar rewards, pollinator preferences, pollen removal, and male fitness. These findings would shed new light on the influence of herbivores on plant-pollinator interactions in the Tibetan meadow.