Spatio-temporal Dynamic Of NDVI And NPP And Their C Orrelation With Climatic Factors In Southwest China

Climate change is an important impact factor of the interannual variability of vegetation activity。The climatic factors , such as temperature, precipitation are the necessary environmental conditions of plant growth and development, and all the factors have essential affect on the vegetation growth, phenology, internal water relationships and gas exchange. Southwest China stretchs from plain of low elevation to Tibet plateau,which result in the variety of climate and ecosystem types. Therefore, research on the relationships between the growth conditions of vegetation and climate change in this region can help reveal the response of China's vegetation types at different altitudes to climate change at the background of global warming. Based on GIMMS NDVI data and weather data from 1982 to 2006, spatio-temporal dynamics of NDVI and NPP and their correlation with climatic factors were analyzed with ArcGIS, Anusplin, ENVI and related statistical analysis software, eapicially on different vegetation types(marshes, shrubes, bushes, grassland, meadow, needle-leaved forest, needleleaf and broadleaf mixed forests, broad-leaved forest, alpine vegetation, and cultural vegetation). The purpose of this paper is to study the spatio-temporal variation of vegetation and climate characteristics and their correlations in the past 25 years of Southwest areas and provide a scientific basis for coping climate change. The main conclusions are as follows:1) Spatio-temporal dynamic of NDVI, NPP, temperature and precipitation in Southwest ChinaOverall, from 1982 to 2006, temperature has rised significantly, NDVI increased and annual precipitation decreased in Southwest China. Regionally, there was a most significant increasing trend of NDVI at the Brahmaputra Valley in southern Tibet, Southeast along the Nu River Valley, vicinity of Mt. transection. The temperature riesd most significantly in northwestern Tibet. Precipitation increased in central Tibet, the junction of Sichuan and Yunnan and eastern Guangxi and decreased in the other regions. NDVI at Brahmaputra Valley and the Nu River Valley in the southeast of Tibet increased significantly in four seasons, NDVI increased in spring and autumn, and decreased in winter and summer in other regions. From 1982 to 2006, the average NPP in Southwest China was 670.97gC/m~2.a. NPP decreased generally from the southeast to the northwest, and was significantly affected by the climate. There was a high value of NPP at the low-altitude areas in the southwest of Sichuan, Yunnan-Guizhou Plateau, and Guangxi. NPP with a middle-value was distributed at most of Sichuan, southeastern Tibet. NPP in the Sichuan Basin was significantly lower than the surrounding regions. NPP with a low value was distributed at the prairie and meadow of north-western Tibet where the annual precipitation was about 200mm. The average NPP was 1196.38 gC/m~2.a in Yunnan, 1158.42 gC/m~2.a in Guangxi, 1046.40 gC/m~2.a in Guizhou, 934.62 gC/m~2.a in Sichuan, 816.89 gC/m~2.a in Chongqing, and 440.14 gC/m~2.a in Tibet.From 1982 to 2006, it can be divided into two phases by the trend of annual NPP in the southwest of China. Firstly, from 1982 to 1999, NPP increased gradually. Secondly, from 2000 to 2006, NPP showed a decreased trend. In most parts of the Southwest of China, NPP tended to increase from 1982 to 2006. The increasing trend was most evident along the Brahmaputra Valley, Sichuan, and southwest Yunnan.2) Spatio-temporal dynamic of NDVI, NPP, temperature and precipitation of different vegetation typesFrom 1982 to 2006, NDVI increased in all vegetation types, and increased significantly or very significantly in 7 types. The average annual temperature of all the vegetation types increased significantly or very significantly, while precipitation decreased. From the aspect of elevation, the higher the elevation, the increasing trend of NDVI and annual average temperature more obvious, while the decline rate in annual precipitation rate was more slower.In the terms of elevation, the higher the altitudes, the more months that NDVI tended to increase, and the more months that NDVI increased significantly. There was a significant warming in all the vegetation in 2-4, 9-12 months, which was focused on the early spring, end autumn to early winter in which the temperature was relatively low, while in other months temperature increased unobviously. Precipitation decreased in most of the vegetation types, and most obviously in 9-12 months.From the average level, the decreasing order of NPP of different vegetation types in Southwest China was: forest vegetation (broad-leaved forest, broad-needled-leaved forest, needle-leaved forest), cultivated vegetation, shrubs, marshes, meadow, alpine vegetation and grasslands. In 25 years, NPP increased significantly in grassland, meadow and alpine vegetation in the high-altitude area, increasing or decreasing trend in other vegetation were not significant. From January to April, NPP increased significantly in each vegetation of southwest China, the most obvious decreasing trend appeared in August.3) The Correlation between NDVI, NPP and temperature, precipitation in Southwest ChinaIn most areas, annual NDVI was positively correlated with temperature, and the distribution of correlation with precipitation had special heterogeneity. In seasonal level, NDVI was positively correlated with temperature in all seasons. There was a sporadic negative correlation only in winter and summer, which were concentrated in Tibet. The correlation between NDVI and precipitation was mainly negative, the lower the seasonal temperature, the more serious was negative correlation.Annual average NPP was positively correlated to temperature and precipitation in most areas of Southwest China.In Sichuan, Chongqing, Guizhou and eastern Yunnan and the northwestern Tibet, spring and summer NPP were negatively correlated with temperature, and showed a positive correlation in other regions. Autumn and winter NPP were positively correlated with temperature. In Southwest China, a negative correlation of NPP with precipitation concentrated in autumn and winter.4) The Correlation between NDVI, NPP and temperature,precipitation of different vegetation typesIn all vegetation types, NDVI had a significant positive correlation with temperature and a insignificant correlation with precipitation in Southwest China. Inner-year NDVI were positively correlated with the temperature and precipitation, and the correlation with temperature was obviously higher than the correlation with precipitation. The correlation between cultural vegetation, marshs NDVI and the temperature were the highest, followed by the high altitude vegetation. The correlation between meadow and precipitation were the highest, followed by alpine vegetation, marshes, grassland, and other vegetation associated with temperature and precipitation weakly.Monthly NDVI was positively correlated with temperature in most vegetation types, a significantly positive correlation was concentrated in January, February, March and June. The correlation coefficient with temperature was higher in alpine vegetation, meadow and grassland in high-altitude areas. NDVI had a negative correlation with precipitation in most of the vegetation, but significant correlation only appears in individual types.Annual NPP was not significant positively correlated with temperature, and the correlation of needle-leaved forest, alpine vegetation and grassland with precipitation were not significant positive. NPP in all vegetation types were positively correlated with temperature, of which significant positive correlations concentrated in February-June and October. The correlation of 10 vegetation types in Southwest with precipitation showed saltatory.