| Tibetan Plateau (TP), owning the largest area and the most unique type of alpine grassland ecosystem, as well as nearly half lakes in China, is an important ecological barrier. Since the plateau is a sensitive region to global climate change, significant changes of grassland vegetation and lake in TP have taken place due to the climate warming in recent years. Therefore, studying the variations of vegetation activity and lake as well as their influence on climate change plays a vital role in understanding ecological environment and regional sustainable development. Based on multisource remote sensing data and grid interpolation data of mean monthly temperature and monthly accumulative precipitation from 1981 to 2010, we analyzed the temporal-spatial distribution and trend of grassland phenology from 1981 to 2010, the variation in lake water level from HY2003-2010, and the response of grassland phenology and lake water level to climate changes, respectively. In addition, taking Nam Co basin as a typical example and combining with data from field compaign, the linkages between climate change, lake expansion and grassland vegetation and the influence of snow cover on grassland vegetation growth were systematically examined. Relevant results are as follows:1) Grassland phenology has shown an obvious regional difference from southeast to northwest in TP. The grassland vegetation in eastern and northwestern part of plateau turned green earlier and brown late, with a relatively longer growth season than other regions. The changes of grassland vegetation phenology from 1981 to 2010 in the east and west regions were also remarkable in TP. The start of growth season (SOG) was in advance in eastern region. Grassland in western region had delayed greening and shortened length of growth season (LOG). The end of growth season (EOG) showed a delaying trend in the southwestern of TP and eastern of Qinghai province. The onset of withering period had an advanced trend in the part of western and eastern area.2) From the perspective of time, temperature and precipitation in spring has a certain promoting effect on grassland vegetation greening. In these regions, with an advanced EOG and shortened LOG, EOG and LOG were limited by summer temperature. However, in the area of delayed EOG and prolonging LOG, EOG and LOG were determined by temperature and precipitation. From spatialdistribution, the onset of grassland vegetation greening in eastern of TP, the onset of withering in eastern and western of TP and length of growth season in northern of TP were affected by temperature and precipitation. Nevertheless, in the other areas, greening and withering were mainly influenced by temperature. The length of growth season was influenced by precipitation much more than temperature.3) Most of lakes mainly distribute in the central and western region of TP, and their lake water levels are mainly between 4500 to 5000 meters, with a severely changes such as Selin Co, Nam Co and Kekexili. Lake water level of Tak kyel and Yamzhog Yumco in the southern of TP as well as Panggong in the western of TP showed a decreasing trend. Analysis of the relationship between lake water level and climate factor indicated that lake water level in different basin showed different trends because of varied temperature and precipitation. The increasing trend of lake volume was not only dependent on direct supply of precipitation, but also effected by melted water from glaciers and snow due to climate warming.4) In recent years, it is found that the increasing temperature was the main reason for both the vegetation deterioration and lake expansion. The enhanced vegetation index (EVI) in Nam Co basin negatively correlated well with the lake’s area, the correlation showed gradual decrease as distance away from the lake’s shoreline. Two main factors might have contributed to this:(a) lake expansion buried grassland vegetation in zone A, and (b) more gravel buildup and soil erosion due to runoff from snow melted water in zone A than in zones B and C.5) Snow cover fraction had positive relationship with start of growth season in Nam Co basin, but negative relationship with the length of growth season. Without considering the influences of snow cover on vegetation phenology, there was no significant relationship between the temperature and vegetation phenology, which illustrated that the influence of the vegetation phenology on mean snow cover fraction rate during April and May, was more sensitive than that of the spring temperature. The grassland vegetation growth condition in the basin has become worse with the growth season prolonging and advanced greening, because the advancement of shallow snow melted may lead to vegetation exposure under a relatively lower temperature environment, and caused frost-damage. |
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