| With global warming, the earth’s hydrosphere, atmosphere and biosphere are affected deeply. As scholars pay much attention to global warming, the Qinghai-Tibetan plateau, with its unique geographical location and the sensitivity of the response to global change arouse great interests. Vegetation plays an important role in ecosystem, and phenology could reflect the adaptation of vegetation to climate change. Green-up date, as one indicator of phenology,has received much attention. Earlier green-up date of vegetation may result in extending the length of the plant growing season which could affect the gross and net primary production and carbon balance, and may also influence the grazing which is related to economic activity.Recent studies showed changes of green-up date varied geographically. At the same time, the regions of Qinghai-Tibet Plateau especially its southeastern regions are also the important forest area in our country. This study will also provide important reference to forest sustainable management under the background of climate change.Qinghai-Tibetan Plateau has experienced significant climate changes in recent decades.However, the results are heavily biased in the trend of green-up date and its reasons. On the basis of previous studies, this study proposed the following scientific hypothesis: due to the unique ecological environment of Qinghai-Tibetan Plateau, the change of its green-up date has high complexity. The discrepancy appears in different regions and different elevation. The dominant factors are also different in different regions. Based on the scientific hypothesis, the purpose of this paper is to calculate the Qinghai-Tibetan Plateau green-up date and analyze the related reasons.We used multiple datasets(remote sensing-based Normalized Difference Vegetation Index(NDVI) data: Global Inventory Modeling and Mapping studies(GIMMS) from 1982 to2006, Systeme Probatoire d’Observation dela Tarre(SPOT) from 1999 to 2013 and Moderate Resolution Imaging Spectroradiometer(MODIS) from 2001 to 2013) to figure out changes of green-up date. We used the software of TIMESAT to figure out the green-up date in Qinghai-Tibetan Plateau, and used the software of ARCGIS to analyze the spatial distributionof green-up date. Then, we used the software of SPSS to analyze the relationship between the green-up date and other factors like temperature, precipitation, humid, snow and grazing.Primary conclusions can be drawn as follows:(1) The spatial distribution of green-up date is that earlier in east and later in west. In theeastern region the green season mainly began in about 130 days, while the western mainlybegan in about 150 days.(2) From 2000 to 4700 meter, the green-up date has delayed along with the increasing of altitude. From 4700 to 5900 meter, the green-up date has advanced along with the increasing of altitude.(3) We further indicated the green-up date is negatively correlated with spring temperature,previous winter(from previous December to February) precipitation and previous winter snow depth, and is positively correlated with previous winter temperature and spring precipitation. We also found the green-up date in high latitude was more sensitive to precipitation.(4) Interestingly, human activities influenced the onset of green-up in a more complex way.Increased number of livestock could lead to spring phenology postpone, but grazing in adaptable regions may help vegetation green-up.(5) The combined effect of these factors caused the changes of green-up date, and the results also showed the main forcing driver varied during different periods. |
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