Spatio-temporal Patterns Of The Vegetation Greenness Period And Net Primary Productivity And Their Responses To Climate Change In The Middle And Lower Reaches Of Yarlung Zangbo River

The middle and lower reaches of Yarlung Zangbo River is an area with unique and complex geography, topography and plateau climate. The ecosystem of this area is sensitive to climate change and relative fragile. The spatio-temporal patterns of the vegetation greenness period and net primary productivity and their responses to climate change in the middle and lower reaches of Yarlung Zangbo River were studied in this thesis.Based on the SPOT-VGT data and meteorological data (temperature, precipitation, sunshine duration) over the period of 1998-2007, using 3S technology and statistical software and GIS software, the information of vegetation greenness period was extracted, and net primary productivity was calculated and analyzed. Main results were obtained as follows:(1) In the period of 1998-2007, the variation of annual average NDVI in the area did not change much, the average NDVI of 2001-2004>that of 1998-2000> that of 2005-2007. The overall trend increased annually.(2) The mean NDVI values in summer and autumn were relatively high in the study, and that of spring was minimum. The mean NDVI values of May to the highest degree or extent, and the 9th months showed the highest extent of increase annually.(3) In the period of 1998-2007, The start dates of growing seasons of gras,tropical rainforest and coniferous forest were annually early than other vegetation types in the 10-year period in the study area. The start dates of the other vegetation types in late Apirl and early May. The end dates of growing seasons from the end of December to the next January. The green period lasted eight and nine months(4) During the eight years of 1998-2005, whose NPP kept a certain level, expect the year of 2005, whose NPP was slightly lower, the annually accumulated NPP of other years did not change much, in the growing seasons of the 10-year period in the study area. The overall NPP of the study area showed an increase trend annually. The NPP of deciduous forests were hightest, The NPP of evergreen forests were higher than the others vegetations.(5) The average temperature of the study area in 10-year period, showed a gradual increase trend, and the precipitation, decrease. The order of variation range of the seasonal average temperature change was winter>summer>spring>autumn. The precipitation gradually increase from west to east in the study area. The order of the negative growth rate of the seasonal precipitation from fast to slow is summer, autumn, spring and winter.(6) The relationship between the seasonal temperature and the average NDVI was positive correlation in 10-year period in the study area. Correlation between the average temperature of spring, summer, autumn and the average NDVI over the same period>those between the average temperature of winter and the average NDIV over the same period. The average NDVI of winter and the average temperature of autumn was significantly correlated. The correlation between the average temperature of winter and the average NDVI of summer> those of the average temperature of winter and the average NDVI of the other three seasons. The precipitation and NDVI was negatively correlated. The impact of precipitation on vegetation is less than that of the temperature.(7) The tread of increase of the average annual NPP of the vegetation in the study area in the 10-year period was almost kept same as that of temperature, which was opposite with precipitation. The NPP of most vegetation types and the average temperature of the four seasons was positive correlated for many years, but negative correlated with the annual precipitation of the four seasons, and the former correlation coefficient>the latter. The correlation between the NPP of coniferous forest, broadleaved forest, tropical rain forests and other vegetation positively and the mean temperature in spring>those of other vegetation types and the mean temperature in spring. The overall annual NPP and annual precipitation weakly positive correlated.