One threat, two solutions: Grazing drives divergent adaptive strategies in two dominant Alpine grassland species

Herbivory is a major selective pressure shaping plant evolution, and the responses of plants to herbivory are typically explained by the growth-defense balance hypothesis (GDBH). However, GDBH often oversimplifies the effects of herbivore types and fails to explain why coexisting species adopt different adaptive strategies under the same stress conditions. The intrinsic hormonal regulatory mechanisms and trait network structures driving these strategy divergences remain largely unclear. This study, based on a decade-long (2014–2024) grazing experiment on the northeastern Tibetan Plateau alpine grassland, investigated the adaptive strategies of two coexisting dominant species under three grazing regimes: yak-only grazing, sheep-only grazing, and mixed grazing. We integrated multi-dimensional traits, including macro-morphological, anatomical, chemical stoichiometry, endogenous hormones, and secondary metabolites, to test the hypothesis of adaptive strategy differentiation (i.e., the divergence into distinct tolerance or defense pathways). We found that under grazing, Carex alatauensis S. R. Zhang increased the relative abundance of gibberellins, which mediated an increase in leaf number and the formation of a highly integrated trait network. The defense of C. alatauensis exhibited responses specific to the herbivore identity, selectively enhancing chemical defensive metabolites (e.g., tannins and total phenolics) when confronted with the larger yak. The trait network of Potentilla acaulis L. was highly modular, particularly in response to the more selective grazing by sheep, with a decoupling of morphological traits from chemical defense. This study suggests that plant adaptation to grazing is not merely a trade-off, but rather a complex, species-specific process involving unique hormonal regulation and trait network configurations. Our findings reveal that plant adaptation to grazing extends beyond simple trade-offs; it is a complex, species-specific process mediated by distinct hormonal regulatory pathways and trait network configurations. These mechanistic insights are crucial for understanding community assembly and the sustainable management of alpine grasslands.