Grazing effects on energy fluxes in a desert steppe on the Mongolian Plateau
This study investigated the effects of grazing on the energy balance (EB) components: net radiation (Rn), sensible heat (H), latent heat (LE), and soil heat (G) flux in the vulnerable desert steppe of the Mongolian Plateau. Paired eddy-covariance towers were used to directly measure these energy fluxes on adjacent grazed and ungrazed areas from 2010 to 2012.
Near 90% of Rn was partitioned as H and LE, whereas the contributions of G and other components of the EB were 10% at annual scale. H dominated the energy partitioning and shared ~50% of Rn in this dry area. Grazing reduced H consistently both in dry and wet years, with 8% less than that in ungrazed area over the two growing seasons. Grazing reduced the growing season latent heat flux by 11% in the dry year of 2011, but increased it by 13% in the wet year of 2010. Grazing reduced Rn by 5%-10% but increased G by 13% more in ungrazed area, resulting in the available energy (Rn-G) to be was marked lower in grazed area than that in ungrazed area by 10%. There was remarkably 20% higher G/Rn during the day but 60% lower at night in grazed area than that in ungrazed area in a daily scale, but less obvious in yearly scale. There were positive relationships between LE/Rn and canopy conductivity, leaf area index, and soil moisture, between H/Rn and vapor pressure deficit, while negative relationship between H/Rn and soil moisture. Multiple linear stepwise regression analysis showed that H/Rn was controlled by soil moisture in the wet year whereas it was controlled by water vapor pressure deficit in the dry year. Both LE/Rn and H/Rn were changed dramatically in grazed area more than in ungrazed area as the soil moisture changes, indicating that the ungrazed area has a greater resistance to the changing climate.