Disturbance by livestock impacts ground temperatures in marginal permafrost areas in Central Mongolia

In the Central Mongolian mountains, the presence of permafrost is intimately linked to local topography and ecosystem setting. Permafrost in these areas is generally found on north-facing slopes, and it is an important element of the hydrological regime, contributing to water availability and downstream ecosystem function. Currently, these linked systems are under increased pressure from both intensified land use and climate warming. This includes disturbances from herd animals, who modify the surface energy balance at grassland sites by changing vegetation structure in summer and snow cover in winter. However, the effect of livestock grazing and trampling on the ground thermal regime in these marginal permafrost areas is largely unknown. In this study, we investigate how semi-nomadic pastoralism impacts surface cover and associated ground temperatures at grassland sites in Central Mongolia. We survey vegetation and snow cover in summer and winter and monitor ground surface temperatures (GSTs) over 14 months at plots featuring different grazing intensities (intensely and ungrazed), as well as topographic aspects (north- and south-facing). We find that plots subject to intense grazing feature lower vegetation density and height, reduced snow cover and an absence of surface litter layers. Overall, intensely grazed plots display an intensified annual cycle of GSTs compared to ungrazed plots, with GSTs at a south-facing site up to +5.1°C warmer in summer and up to -5.4°C colder in winter. We further find the impact of grazing on GST to depend on topographic aspect, and at a north-facing site we observe lower seasonal differences in GST of +1.4°C and -2.5°C between grazed and ungrazed plots. At our study sites the seasonal differences in GST largely cancel each other out, with the net effect depending on spring and autumn conditions. These results suggest that surface conditions at grassland sites can be managed by regulating the disturbances caused by livestock, which in turn can modify the ground thermal dynamics. For example, a cooling of ground temperatures can possibly be achieved through shielding areas from grazing during the growing season while allowing or even promoting grazing and trampling in the snow season. Such livestock management could potentially offset current and future surface warming in marginal permafrost areas, contributing to sustained local water availability and ecosystem function.