An improved CASA model with MODIS NDVI along with meteorological data was used to estimate the net primary productivity (NPP) of grassland vegetation in pasture season in July each year in 2000, 2005 and 2008, and to analyse its spatial and temporal dynamics. The results showed that: (1) The fundamental spatial pattern of annual NPP in Gannan pastoral was characterized by low NPP levels distributed in the west-southwest and northwest corners, then the NPP levels increased gradually from the two comers into the arid inland regions. This distribution was bound up with different hydrothermal conditions and variation of zonal of vegetation. In addition, it was significantly different between the average NPP of grassland vegetation in July and the total NPP of grassland vegetation in July, which was closely related to each county area. The sequence of the value of average NPP of grassland vegetation in July in each county was Luqu ＞Maqu ＞ Hezuo ＞ Xiahe, while the sequence of the value of total NPP of grassland vegetation in July in each county was Maqu ＞ Xiahe ＞ Luqu ＞ Hezuo. (2) The value of NPP of grassland vegetation in July of 2008 was much smaller than that of 2000. From 2000 ～ 2008, the NPP of grassland vegetation of the entire study area in July increased first, and then decreased. The main reason was the changes of temperature, precipitation and wettability, while wettability change was a major limiting factor to affect the growth of pasture in Gannan plateau.%以改进的CASA模型为基础,结合MODIS NDVI数据、气象资料,估算了甘南牧区草地2000、2005、2008年牧草生长旺季7月净初级生产力,并分析了其时空动态.结果显示:(1)甘南牧区草地净初级生产力(NPP)的空间分布基本格局是西南偏西和东北两区域最低,然后由西南-东北方向为轴心逐渐向内陆中心部分递增,该分布规律与各县市的水热条件差异和植被类型的地带性分异规律紧密相关.另外,各县市草地7月平均NPP与7月NPP总量有明显的不同,这与各县市的面积密切相关.7月平均NPP值大小顺序依次为:碌曲县＞玛曲县＞合作市＞夏河县;而7月NPP总量的值的大小顺序依次为:玛曲县＞夏河县＞碌曲县＞合作市.(2)2008年较2000年,研究区7月草地NPP的值明显减少,但在2000～2008年间整个研究区7月草地NPP呈现先增加、后减少的趋势,其中,2005年较2000年显著增加,2008年较2005年明显较少.其主要原因是气温、降水量、湿润度的变化所致,其中湿润度变化是影响甘南高原牧草发育生长的主要限制因素.
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