Monitoring vegetation phenology in farming-pastoral zone using SPOT-VGT NDVI data

Abstract: Vegetation phenology dynamics reflect the response of biosphere to global climate change and terrestrial hydrological cycle mechanism changes. It is connected well to ecosystem primary productivity of terrestrial and carbon cycle. As the best indicator in monitoring the influence of climate on vegetation, plant phenology has become the key point of global change research. The transition zone between cropping area and nomadic area is sensitive to climate change, thus the changes of vegetation phenology in this area has become an essential issue. Based on the Savitzky-Golay filtering and dynamic threshold method, the spatial-temporal pattern of vegetation phenology in farming-pastoral zone in Northern China was analyzed using SPOT-VGT NDVI data from 2001 to 2010. The results showed that vegetation phenology in the study area generally started from mid-April to late May, and ended in late September to late October. The spatial patterns of vegetation phenology were significantly different in study region. Growth stages of vegetation phenology in the southwest started earlier than the northeast, while ended later. Therefore, length of vegetation growth season in the southwest was longer than those in the northeast. However, greenup dates of vegetation in the Yellow River coast were earlier than vegetation in the same latitudinal zone because of the effect of water. Compared with natural vegetation, growth stages of crops were slightly different due to management. To study the trends of inter-annual variations of the SOS and EOS over the study area, linear regressions are fitted to the start of greening and end of season dates for each pixel of the study area from 2001 to 2010. The results showed that start dates of growth stages were advanced approximately 1-10 days in most study areas for all years. However, the end dates of vegetation growing seasons had no significant changes from 2001 to 2010, except for partial areas. In some regions, the length of seasons (LOS) extended, while decreased in other areas. The phenology of vegetation extracted using SPOT-VGT NDVI datasets in this paper was generally concordant with many reported researches. This paper can provide a reference for the evaluation and protection of ecological environment in farming-pastoral zone.