Are Mongolian rangelands overgrazed? Estimating grassland utilisation and sustainable stocking rates with a vegetation model

Mongolian grasslands are vulnerable systems, prone to degradation, as low precipitation and poor soil fertility limit biomass production, while increasing livestock numbers intensify pressure on rangelands. Although reducing livestock numbers is proposed to relieve pressure, quantitative tools to project forage availability, grassland utilisation, and potential livestock densities over time remain limited. In this study, we applied the LPJ-GUESS dynamic global vegetation model, enhanced with a custom livestock submodule, to simulate historical grassland dynamics (1970–2023, 9 × 9 km) while explicitly accounting for vegetation-livestock interactions. The coupled model was used to analyse spatial patterns of forage availability and grassland utilisation via the Human Appropriation of Net Primary Production (HANPP) framework and to estimate potential livestock densities using a novel optimality approach. The model exhibited reasonable to good spatial and temporal agreement with remotely sensed GPP and NPP. Agreement was strongest in the eastern steppe, where the model also successfully reproduced satellite-based above-ground biomass estimates, which were calibrated with local field data. According to the simulations, HANPP increased from an average of 6% in 1970 to 21% in 2018 across Mongolia's grazing areas. At the same time, livestock reduced NPP by about 9% on average. Model-estimated potential livestock densities were broadly consistent with established methods, demonstrating the approach's potential for application. Reported livestock densities exceeded model-estimated potential densities in 33% of grazing areas in 2019, an estimate subject to substantial uncertainty in desert and desert steppe, where NPP may be underestimated or satellite reference data are lacking. Applied with future climate projections, this method can help policymakers and land managers maintain the ecological balance and productivity of Eurasian grasslands under future climate and grazing scenarios.