陕西省植被覆盖时空变化及其对极端气候的响应

基于2001—2018年MODIS NDVI数据，从生态分区视角分析陕西省归一化植被指数(NDVI)的时空变化特征，并结合该地区31个气象站点日值数据，探讨NDVI对极端气温和极端降水指数的响应特征。结果表明：(1)陕西省及其各生态区的NDVI变化均显著上升，整体呈南高北低的分布特点，其中秦巴山地落叶与阔叶林生态区(IV)NDVI值最高为0.86,陕北北部典型草原生态区(I)NDVI值最低为0.38。(2)年际尺度上，陕西省NDVI与极端气温暖极值(暖夜日数)和极端降水指数总体呈显著正相关(P<0.05),在陕西省北部NDVI变化主要受极端降水的影响，南部则对极端气温的敏感度更高。(3)多年月尺度上，各生态区NDVI对极端气温冷极值(最低气温、日最低气温的极低值和日最高气温的极低值)和极端气温暖极值(最高气温、日最低气温的极高值和日最高气温的极高值)存在明显的滞后性，滞后时间多为3个月；与极端降水指数(单日最大降水量和连续5日最大降水量)的滞后时间为2个月，说明陕西省内NDVI对极端气候的响应具有显著的滞后效应。

Temporal and spatial variation of vegetation cover and its response to extreme climate in Shaanxi Province

Vegetation plays an important role in the terrestrial ecosystem. Under the background of global warming, the response of vegetation cover to climate change has become an important research field. Normalized Difference Vegetation Index (NDVI) is commonly used to study the response of vegetation change to temperature and precipitation on a regional scale. In recent years, the frequent occurrence of extreme climate events has increased the risk and vulnerability of terrestrial ecosystems and endangered ecological security. Shaanxi province is located in the hinterland of China, with a large north-south span and obvious regional differences in hydrothermal conditions and surface ecological environment. Therefore, it is necessary to study the dynamic change of NDVI and its response to extreme climate, which can provide theoretical evidence of ecosystem evolution and ecological environment conservation in Shaanxi Province. Based on MODIS NDVI data from 2001 to 2018, the spatio-temporal variation pattern of NDVI was analyzed from the perspective of different ecological areas in Shaanxi Province. The response characteristics of NDVI to extreme temperature and precipitation indices were also discussed by using the daily meteorological observations from 31 stations in this region. The results showed that (1) the variation of NDVI increased significantly and the spatial difference of NDVI value was obvious in Shaanxi Province and its ecological areas. The high value of NDVI distributed in the south, whereas the low value of NDVI distributed in the north. The decreasing order of the value of NDVI of the different ecological areas was as follows:ecological region of deciduous and broad-leaved forest in Qinba Mountain (IV) > agricultural ecological area of Fenwei Basin (III) > agricultural and grassland ecological region of the Loess Plateau (II) > typical grassland ecological area in northern Shaanxi (I). Meanwhile, the lowest value of NDVI was 0.38 and the highest value was 0.86. (2) The correlation between NDVI and extreme climate indices showed that annual NDVI in each ecological area was significantly positively correlated with TN90p and extreme precipitation indices. However, extreme precipitation had an important impact on vegetation growth in the north of Shaanxi Province, and extreme temperature was the main factor in the south. (3) On the monthly scale, the NDVI had a significant lag time of 3 months with extreme temperature indices, including TMIN_mean, TN_n, TX_n, TMAX_mean, TN_x, and TX_x, while had a lag time of 2 months with extreme precipitation indexes in each ecological area. In general, the results effectively revealed the response of vegetation to climate fluctuations in Shaanxi Province.