Data from: Grazing effects on surface energy fluxes in a desert steppe on the Mongolian Plateau

Quantifying the surface energy fluxes of grazed and ungrazed steppes is essential to understand the roles of grasslands in local and global climate and in land use change. We used paired eddy-covariance towers to investigate the effects of grazing on energy balance (EB) components: net radiation (Rn), latent heat (LE), sensible heat (H), and soil heat (G) fluxes on adjacent grazed and ungrazed areas in a desert steppe of the Mongolian Plateau for a two-year period (2010-2012). Near 95% of Rn was partitioned as LE and H, whereas the contributions of G and other components of the EB were 5% at an annual scale. H dominated the energy partitioning and shared ~50% of Rn. When comparing the grazed and the ungrazed desert steppe, there was remarkably lower Rn and a lower H, but higher G at the grazed site than at the ungrazed site. Both reduced available energy (Rn˗G) and H through grazing indicated a “cooling effect” feedback onto the local climate. Grazing reduced the dry year LE but enhanced the wet year LE. Energy partitioning of LE/Rn was positively correlated with the canopy conductivity, leaf area index, and soil moisture. H/Rn was positively correlated with the vapor pressure deficit but negatively correlated with the soil moisture. Boosted regression tree results showed that LE/Rn was dominated by soil moisture in both years and at both sites, while grazing shifted the H/Rn domination from temperature to soil moisture in the wet year. Grazing not only caused a LE shift between the dry and the wet year, but also triggered a decrease in the H/Rn because of changes in vegetation and soil properties, indicating that the ungrazed area had a greater resistance while the grazed area had a greater sensitivity of EB components to the changing climate.

量化放牧与未放牧草原的地表能量通量，对于理解草原在局地与全球气候、土地利用变化中所扮演的角色至关重要。本研究采用配对式涡度协方差（eddy-covariance）塔，于蒙古高原荒漠草原的相邻放牧与未放牧区域开展为期两年（2010-2012年）的观测，以探究放牧对能量平衡（energy balance, EB）各分量的影响，这些分量包括净辐射（net radiation, Rn）、潜热（latent heat, LE）、感热（sensible heat, H）与土壤热通量（soil heat, G）。年尺度下，约95%的净辐射被分配至潜热与感热，而土壤热通量与能量平衡其余分量的总贡献占比仅为5%。感热是能量分配的主导分量，约占净辐射的50%。对比放牧与未放牧荒漠草原可见，放牧站点的净辐射与感热显著低于未放牧站点，而土壤热通量则更高。放牧通过同时降低有效能量（Rn−G）与感热，对当地气候产生了“冷却效应”反馈。放牧在干旱年降低了潜热通量，却在湿润年提升了潜热通量。潜热与净辐射的比值（LE/Rn）与冠层导度（canopy conductivity）、叶面积指数（leaf area index）及土壤湿度呈正相关；感热与净辐射的比值（H/Rn）则与水汽压亏缺（vapor pressure deficit）呈正相关，与土壤湿度呈负相关。提升回归树（boosted regression tree）分析结果显示，两年间两个站点的LE/Rn均主要受土壤湿度调控，而在湿润年，放牧使H/Rn的主导影响因子从温度转变为土壤湿度。放牧不仅引发了干旱年与湿润年的潜热通量差异，还因植被与土壤属性的改变导致H/Rn下降，这表明未放牧区域具有更强的气候抵抗力，而放牧区域的能量平衡分量对气候变化的敏感性更高。