Verification of the biomass transfer hypothesis under moderate grazing across the Tibetan plateau: a meta-analysis

Aim We aimed to explore the general response patterns of plant biomass allocation to grazing disturbance and to test two important hypotheses, optimal partitioning and isometric allocation, for explaining potential mechanisms by which grazing controls biomass distribution in an alpine grassland on the Tibetan Plateau. Methods We identified 57 relevant papers about grazing on the Tibetan Plateau, from which 366 data sets suitable for the meta-analysis were extracted. Effect sizes were assessed by computing natural log-converted response ratios of response variables. Percentage change relative to control was used for each estimate of grazing effects. Results The aboveground biomass, soil water content (SWC), soil organic carbon, soil total nitrogen, and soil total phosphorus significantly decreased with increased grazing intensities, while plant species richness (SR), soil bulk density (SBD) and the ratio of root to shoot exhibited the opposite tendency. Belowground biomass (BGB) showed no significant differences under light and high grazing intensities while apparently increased under moderate grazing intensity (MG) that verifies the biomass transfer hypothesis. BGB was positively related to SBD and SR but was negatively associated with SWC. Conclusions The biomass transfer in MG supports the optimal partitioning hypothesis that plants partition biomass among various organs to maximize growth rate responding to environmental stress. The findings suggest that the primary mechanisms leading to the enhancement of BGB in MG are compensatory growth of individual plants, a dwarfing tendency within the plant community, a significant increase in species richness, and changes in soil microbial communities resulting from grazing.

Automated summary

The study found that aboveground biomass, soil water content, soil organic carbon, soil total nitrogen, and soil total phosphorus decreased significantly with increased grazing intensities on the Tibetan Plateau. On the other hand, plant species richness, soil bulk density, and root to shoot ratio displayed an opposite trend. Belowground biomass did not differ significantly between light and heavy grazing intensities but increased notably under moderate grazing intensity, supporting the biomass transfer hypothesis.