光能利用效率(light use efficiency,LUE)是指初级生产力与植被冠层所吸收的光合有效辐射(absorbed pho-tosynthetically active radiation,APAR)之比,它反映了植被利用光能的能力。定量化生产力的时空变化是定量化全球碳循环的重要研究内容,而LUE作为光能生产力模型中的一个重要参数,是定量化生产力时空变化的基础。因此,定量化全球植被的LUE是定量化全球碳循环的重要组成部分。基于MODIS光能利用效率算法,本研究模拟了2004-2005年藏北高寒草甸生态系统的光能利用效率(LUEMODIS),并用观测的光能利用效率(LUEEC)对模型进行了验证。在MODIS算法中,日最低气温(Tamin)和饱和水汽压亏缺(VPD)分别被用来计算温度胁迫因子(Tscalar)和水分胁迫因子(Wscalar)。相关分析和多重逐步回归分析结果表明,相对于Wscalar,Tscalar更能够解释观测的LUE的季节变化。2004和2005年的模拟值分别高估了约14.97%和16.57%的观测值,但配对T检验显示模拟值和观测值差异不显著,即基于MODIS的LUE算法在模拟藏北高寒草甸LUE方面具有较高的精度。相关分析表明,观测的LUE与Tamin的相关性好于观测的LUE与平均气温的相关性,这表明在反应藏北高寒草甸生态系统LUE的季节变异方面,Tamin优于平均气温。总之,基于MODIS算法的LUE模型能够比较准确地定量化藏北高寒草甸生态系统的LUE。
Species richness and diversity indices （Shannon-Wiener index, Simpson dominance index and Pielou evenness index） in alpine grassland ecosystems （alpine meadow, alpine steppe and desert steppe） under grazing-excluded and freely grazed sites were investigated along the Northern Tibetan Plateau Alpine Grassland Transect during summer 2009 and 2010. We found that species richness and diversity have not been significantly altered by short-term grazing exclusion since 2006 at vegetation and regional scales. Species richness and diversity were mainly driven by growing season precipitation （GSP）, which accounted for over 87 % of the total variation observed, Species richness and diversity at grazing- excluded and freely grazed sites appear to respond to growing season precipitation in parallel. Species richness exponentially increased with GSP while diversity indices showed positively linear relationships with GSP. This indicates that GSP on the Northern Tibetan Plateau is crucial in regulating species richness and diversity and should be taken into account in future studies on alpine grassland conservation.
