Above‐ground and below‐ground productivity shows divergent compensatory responses to grazing in an alpine grassland

Abstract The grazing optimization hypothesis suggests that moderate herbivory can stimulate plant growth and increase productivity, a phenomenon referred to as overcompensation. However, most evidence for this response is based on above‐ground responses, while below‐ground responses remain uncertain. Below‐ground productivity accounts for more than half of total grassland productivity, making it essential to assess whole‐plant responses. We conducted a 3‐year yak grazing‐intensity experiment, manipulating four levels of herbivory, and quantified compensation indices for both above‐ground (ANPP) and below‐ground (BNPP) net primary productivity relative to ungrazed controls. We complemented this with a controlled simulated‐grazing experiment to isolate the effects of defoliation, trampling and excrement on these compensation responses. Across light, moderate and heavy grazing treatments, ANPP increased by 28%–43% relative to ungrazed controls, suggesting clear overcompensation in above‐ground production. In contrast, BNPP showed little response to grazing intensity and exhibited only limited compensatory responses, such that total net primary productivity (NPP) showed only weak overcompensation. Over time, the magnitude of ANPP overcompensation declined, whereas no temporal attenuation was detected for BNPP or total NPP. Results from the simulated‐grazing experiment further showed that defoliation alone increased ANPP, while trampling and excreta return had negligible effects on ANPP, BNPP or their associated compensation indices. Synthesis . Our findings suggest that reliance on aboveground‐only measurements systematically overestimates the degree of overcompensation in grazed grasslands, underscoring the necessity of whole‐plant assessments for accurately evaluating grassland productivity.