The Responses Of NDVI-based Vegetation During Growing Season To Climate Factors In The Yellow River Source Region

In the context of global warming, the vegetation-climate interaction, especially the responses of vegetation during growing season as well as growing season starting period to climate change, attracted much attention on the Tibetan Plateau, as one of the world’s most sensitive and vulnerable ecological environment. In the past, the relational researches almost adopted scales of regional, different vegetation types or partial areas (for the meteorological stations center) to define vegetation greening, browning and growing-season length, and then analyzed the correlation between growing-season vegetation and climatic factors. However, there exsited much controversy, which has been thought as the result of different scales and methods on researches preliminarily.This study selected the Yellow River source region (watershed before the Longyangxia), which is located in northeastern of the Tibetan Plateau and known as monsoon fringe, as the study area of vegetation-climate interaction. Based on GIS and and mathematical software, the correlation of NDVI-based vegetation during growing season and climatic factors was discussed in two spatial scales (the region as a whole and partial area) and two temporal scales (the growing season and 16 days) by Kriging interpolation, linear regression analysis, correlation analysis, with Terra/MODIS NDVI data products and temperature and precipitation dataes of 39 meteorological stations in the study area also surroundings over 2000-2013. The main conclusions are as follows:(1) The multi-year averaged growing season starting period of the Yellow River source region was in late May (DOY 145), and the ending period was in mid-October (DOY 273) in the whole area during 2000-2013. With the spatial-temporal heterogeneity of phonological-growing season and climate-growing season at partial area scale, the growing season, of northeast sites was up to 6 months, which of northwest sites was short to 3 months and other parts was in 4-5 months. Furthermore, the growing season starting and ending period of the same site was variable year by year.(2) There existed significant regional differences on multi-year averaged NDVI of growing season, presenting a decreasing from the southeast to the northwest in the Yellow River source region. Overall, the vegetation there tended to improve during 2000-2013. Firstly, the vegetation in more than 2/3 of the study area varied not significantly with a wide distribution. Secondly, the regions vegetation significantly and very significantly increaseing accounted nearly 1/3, which were concentrated on northeast of the Yellow River source region and northwest Zaling and Eling lake surroundings. Thirdly, significant and very significant reduction occurred less than 2% of this area, and mainly distributed around Zaling and Eling lake in small patchy.(3) Growing-season NDVI showed mostly significant positive correlation or not significant correlation with climate factors in the Yellow River source region over 2000-2013, and significant negative correlation areas were very small with scattered patchy distribution. The regions NDVI presenting significant or very significant positive correlation with temperature were that northwest of the study area where was cold and wet, and that southeastern where was warm and wet. NDVI presenting significantly, very significantly negative correlation with the temperature mainly located in the north of the Yellow River source region at some point-like plaques. The regions NDVI presenting significant or extremely significant positive correlation with the precipitation were not only in the northwest and southeast, but in the dry heat northeast corner, where is also an important distribution. NDVI showing significantly, very significantly negatively correlation with precipitation mainly located in the northwest, small and patchy distribution.(4) The growing-season NDVI in most years, relatively weak correlation with precipitation, was significant or extremely significant positive correlation with temperature at the year in the Yellow River source region during 2000-2013. The growing season NDVI were significantly correlated with the preseason (1 month to the growing season) climate factors. Growing-season NDVI laged to climate factors by 16 days with 16-day interval of MODIS NDVI.(5) In most cases, NDVI and climate factors correlated greatly at 10km × 10km spatial extent for the meteorological stations center, which was called "best spatial scale" and varied on different sites or different methods to define the growing season.(6) Growing-season NDVI, which was more sensitive to the growing-season and preseason temperature variety with a lag time to temperature by 16 days, showed a positive correlation to temperature and precipitation in all Yellow River source region sites. Growing-season NDVI laged to precipitation by 32-48 days, and was in response to precipitation rapidly in more arid areas with shorter lag time. In more humid areas, NDVI was in response to precipitation slowly with a longer lag time.(7) It was positive correlation between NDVI during the start period of growing season and temperature and precipitation in all Yellow River source region sites. Vegetation-climate interaction characteristics of phonological-growing season starting period were consistent in all sites, while that of climate-growing season starting period were consistent separately in 5 regions, where were the same warmth respectively. The responses of NDVI during the start period of phonological-growing season to precipitation were roughly identical at stations, where were the same humidity respectively. It was relatively complex of correlation characteristics between climate-growing season starting period NDVI and precipitation.