不同降水梯度下草地生态系统地表能量交换

通过对不同降水梯度下的蒙古中部针茅草原(KBU)、内蒙古羊草草原(NM)、海北高寒灌丛草甸(HB)和当雄高寒草甸草原(DX)4个草地生态系统的能量通量连续4-5 a的测定,分析了影响青藏高原和蒙古高原草地生态系统生长季中地表能量交换的主要因素。研究表明:相对于KBU、NM和DX,HB高寒灌丛草甸NDVI(0.58)和土壤含水量(28.3%)最大,因而地表短波反射率(αk)最低(0.12),从而获得了最大的净辐射(Rn)。KBU、NM和DX 3个草地生态系统生长季中αk随着植被的生长而降低,在生长季末期,随着植被的凋落而增加;HB的αk季节变化趋势与其它生态系统相反。从蒙古高原(KBU和NM)到青藏高原(HB和DX),随着降水量的增加,波文比(β)逐渐减小(2.25-0.53),即生态系统与大气的能量交换从显热(H)占主导转变为潜热(LE)占主导。植被状况对草地生态系统与大气之间能量交换的季节动态有重要的调控作用,在NDVI较低的时候,4个生态系统H/Rn都大于LE/Rn,LE/Rn随着NDVI的增加而增加,而H/Rn呈现出与LE/Rn相反的季节变化趋势。

Surface energy exchanges in grassland ecosystems along a precipitation gradient

This paper explore surface energy exchanges and their underlying mechanisms during the growing season in four grassland ecosystems in Qinghai-Tibetan Plateau and Mongolian Plateau through analyzing the 4-5-year continuous flux data obtained by the eddy covariance technique. The four ecosystems are a temperate steppe ecosystem in central Mongolia (KBU), a temperate steppe ecosystem in Inner Mongolia (NM), an alpine shrub-meadow ecosystem (HB) and an alpine steppe meadow (DX) in Qinghai-Tibetan Plateau along a precipitation gradient. The results show that the surface reflectivity (albedo) for short-wave radiation (α_k) was lowest (0.12) in HB because of highest NDVI (0.58) and volumetric soil water content (28.3%) compared with those of other three ecosystems. The α_k values in KBU, NM and DX decreased with the growth of plant, and increased as the plant senesced during the late growing season; but the seasonal variation of α_k in HB exhibited an opposite trend to those at other three sites. The Bowen ratio decreased from 2.25 to 0.53 from the Mongolian Plateau to Qinghai-Tibetan Plateau with the increase of precipitation, suggesting that there is a shift of the sensible heat flux (H) dominated energy partitioning to the latent heat flux (LE) dominated energy partitioning. Vegetation growth played an important role in controlling energy exchange between grassland ecosystems and the atmosphere. The evaporative fraction of net-radiation (LE/R_n) was lower than H/R_n when NDVI was low; but with an increase in NDVI, LE/Ro increased whereas H/R_n decreased.