| Vegetation Precipitation Use Efficiency (PUE), a development outcome of Vegetation Water Use Efficiency (WUE), refers to the ratio between dry substances produced by vegetation photosynthesis (net primary productivity, NPP) and precipitation. It reflects the relationship between the process of vegetation photosynthesis and the characteristics of water consumption, and embodies the variable features of water use space as ecosystem distributes along climatic gradient. From regional scale, to discuss and analyze the following three fields of PUE are spatial distributions, time dynamic features and climatic factors influence model, which has more important theoretical and practical significance for the response of the water-carbon cycle in Qinghai-Tibet plateau to climate change.In this study, by means of Remote Sensing (RS) and Geographic Information System (GIS), the data of NPP, PUE and simultaneous temperature and precipitation were all simulated by CASA model during1982-2007. The pattern and the feedback mechanism how varying features of26-year NPP and PUE of Qinghai-Tibet plateau terrestrial ecosystem respond to regional temperature and precipitation changes were analyzed in detail. Preliminary, the changed features of NPP and PUE and the climate drive factors and mechanisms impacting PUE were presented, and the following main conclusions were obtained:(1) Varying Characteristics of NPPThe mean NPP of Qinghai-Tibet plateau for1982-2007was259.39g C·m-2·-1. The spatial distribution of NPP showed an obvious rising trend from northwest to southeast.The rank of varied regions was as follow in order of the26-year mean NPP from greatest to least:Nanyi area in Eastern Himalayan, CangDong mountain region in Western Sichuan, alpine area in Goluonaqu, Qilian Mountain region in Qingdong, Mountain region in southern Tibet, alpine region in Qingnan, alpine region in Qiangtang, Mountain region in Ali Mountain, Mountain region in northern Kunlun mountain, Mountain region in Qaidam, and alpine region in Kunlun mountain. And the order of vegetation types was:broad-leaved forest, cultivated plants, coniferous forest, shrub, meadow, alpine vegetation, grassland, and desert. Particularly, due to its physiological and ecological characteristics, the NPP of broad-leaved forest fluctuated dramatically compared with other vegetation types.(2) Varying Characteristics of PUEDuring26year, the PUE of Qinghai-Tibet plateau vegetation presented an ascendant trend in fluctuation; the mean vegetation precipitation use efficiency was0.751g C#m-2*mm-l.the arrange of vegetation types was as follows in order of the mean PUE from greatest to least: broad-leaved forest, cultivated plants, meadow, coniferous forest, shrub, grassland, desert, and alpine vegetation. The PUE of Qinghai-Tibet plateau vegetation was high in the East, yet it was low in the Midwest.In26years, the PUE in71.33%of Qinghai-Tibet plateau showed an uptrend, with that in 28.67%of region was on the decline. The seasonal spatial changes of PUE were distinct, the PUE in various seasons were totally different and there was no clear rule to follow, which showed that the PUE was likely to reflect the land conditions rather than the growth of vegetation.(3) Correlation Analysis between Vegetation PUE and Climatic factorsOver the26years,the Vegetation PUE of most areas of Tibet Plateau positively correlated with the average annual temperature, and especially, vegetation PUE and temperature showed a strong positive correlation, correlation coefficient of0.5or above in the southwest of Qinghai-Tibet plateau, namely, southern mountain area in Ali Mountain; western region in Qiangtang alpine area; central Qinghai-Tibet Plateau, namely, the Golok Nagqu alpinethe eastern part of the the southern region the western Qingnan alpine region and the Kunlun mountain region of the north wing; as well as part of the region of the mountainous regions of the Qaidam. The west part of the GUoluo Naggu alpine region and the south of the south area of the Tibet were both covered by meadow, which reflected that the lawn PUE was more sensitive to the temperature. In most areas of the south part of the mountainous area of Tibet, the west and the south of the chuan-Tibet and the the central south areas of the Xiantang distrct, vegetation PUE had a negative correlation with the temperature.Over the last26years, the vegetation PUE of the most areas of Tibetan Plateau was in a strong negative correlation with precipitation. Only in the central and southern part of Qiangtang alpine regions and the southeastern region of Kunlun alpine, the Vegetation PUE and precipitation showed a strong positive correlation, the correlation coefficient of0.5or more. |
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