Multiple herbivory pressures lead to different carbon assimilation and allocation strategies: Evidence from a perennial grass in a typical steppe in northern China

Abstract Carbon (C) assimilation and allocation play a crucial role in determining plant responses to environmental stress such as herbivory. However, the pattern how these grasses allocate assimilated C under increasing herbivory intensity hasn't been fully understand. In this study, we quantified photosynthetic C assimilation and allocation of newly assimilated C among tissues and metabolic processes (structural growth, storage, and defense) under different grazing intensities using 13C tracing of a dominant grass species, Leymus chinensis. Light grazing promoted utilization of newly-assimilated 13C (more than 90% 13C allocated to aboveground tissues on first day after labeling), photosynthetic rate, and reduced mean residence time of 13C of L. chinensis . The photosynthetic capacity and regulation of chlorophyll fluorescence thereby C assimilation were constrained under medium and heavy grazing. Light grazing also increased accumulation of non-structural carbohydrates (NSC) in stems for energy supply to leaf regeneration. As herbivory pressure increased, 13C tracing showed preferential allocation of newly assimilated C to belowground tissues (16–27% 13C on first day after labeling), while upregulating leaf defenses by increasing secondary metabolites and in root storage by NSC accumulation. Although the significant changes showed in C allocation to storage and secondary metabolism of L. chinensis , there are no difference in structural growth (defined as structural biomass= biomass - NSC – secondary metabolites) among grazing intensities. Overall, L. chinensis adopted a conservative C-allocation strategy (upregulated C storage and secondary metabolism) that emphasized long-term survival under increasing herbivory. Light grazing was an optimal grazing intensity that promoted structural growth and C allocation of L. chinensis , which could sustain and even increase grassland productivity. These characteristics of the plant’s C allocation strategy provide new insights into the C budget of grassland ecosystems and increase our understanding of the role of C fixation and partitioning when plants respond to environmental challenges.