陕西省退耕还林植被覆盖度与湿润指数的变化关系

Relationship between fractional vegetation cover and humidity index after returning farmland to forest in Shaanxi Province

使用MODIS-NDVI数据和气象站点资料,通过GIS遥感技术和数理统计等方法,分析了陕西省退耕还林后(2000—2012年)植被覆盖度与湿润指数的时空变化规律及两者变化的关系。结果表明,陕西省植被覆盖度和湿润指数都呈现由南向北递减的分布规律并且有明显的季节变化特征。2000—2012年,陕西省植被覆盖度在波动中呈现大幅增加的趋势,陕北地区增加最为显著,生态环境得到明显改善,然而部分城市周边地区植被有退化的迹象。2000—2012年湿润指数年际变化波动较大,有上升的趋势,陕南地区增加显著。空间分布上随着植被覆盖度的增加湿润指数呈指数变化趋势,相关性与植被覆盖度面积取值范围有关,范围取值越大相关系数越高。植被覆盖度的年际变化受到气候和人为因素影响,陕南地区植被覆盖度与湿润指数的相关性较显著,而受到人为影响比较明显的陕北、关中地区相关性不显著。

Vegetation is an important part of a terrestrial ecosystem, and climate condition determines the growth and development of plants. At the same time, vegetation also has a regulatory role on climate. Since 1999, a project to return farmland to forest has been ongoing in Shaanxi Province, leading to the mitigation of the deterioration in the local ecological environment. Therefore, an analysis of the vegetation and climate conditions in Shaanxi Province, as well as a study of the changed relations between the two, may provide an important theoretical basis for response to global climate changes, and should improve the further development of ecological environment construction in Shaanxi Province. In this study, MODIS- NDVI data were used to calculate the fractional vegetation cover, and the humidity index was taken as the climate indicator in order to reflect the degree of regional dryness and wetness, and the hydrothermal balance status. In addition, GIS and remote sensing technology and mathematical statistics were used to analyze the spatiotemporal variation laws and the changed relations between fractional vegetation cover and humidity index after returning farmland to forest in Shaanxi Province （2000--2012）. The following observations were made： 1 ） The fractional vegetation cover in Shaanxi declined from south to north, and the fractional vegetation cover in southern Shaanxi was the highest, followed by those in the Guanzhong region, Huanglong Mountain and Ziwu Mountain, whereas in the north, the Yan＇an region was the lowest. The fractional vegetation cover showed clear seasonal variations, reaching a peak in August. From 2000 to 2012, the fractional vegetation cover in Shaanxi Province showed an extremely significant increasing trend in volatility, and this was most significant in northern Shaanxi. Vegetation degradation occurred within the vicinity of some cities, especially on the border between Xi＇an and Xianyang. 2） The spatial distribution of the humidity index was similar to the fractional vegetation cover, which also decreased from south to north. The southern Shaanxi region is a wet area, the area from Guanzhong to Yah＇an in northern Shannxi is a relatively arid region, and Yulin has the highest drought levels. The humidity index also showed clear seasonal changes, and was at its highest in late summer and early autumn. From 2000 to 2012, the humidity index for Shaanxi Province had larger inter-annual fluctuations. The index for southern Shaanxi showed a clear rising trend, while those in the other areas had insignificant rising trends. 3 ） The fractional vegetation cover and humidity index showed an extremely significant spatial correlation. The humidity index showed an exponentially increasing trend as fractional vegetation cover rose. This correlation was related to the area of average fractional vegetation cover, i.e. the higher the area value is, the greater the correlation coefficient will be. Vegetation growth was influenced by hydrothermal conditions, and the 16 d fractional vegetation cover changes had an extremely significant correlation with humidity index. The inter-annual variability for fractional vegetation cover was also affected by climate and human factors. The correlation between fractional vegetation cover and humidity index was significant in southern Shaanxi, but insignificant in northern Shaanxi and Guanzhong, despite the fact that they have relatively significant human influences.