Spatiotemporal variation of grassland vegetation NDVI in the middle and upper reaches of the Hei River and its response to climatic factors

Vegetation dynamics and its coupled relation with the climate is a hot spot in the study of how terrestrial ecosystems respond to climate systems.Analysis of grassland NDVI(normalized difference vegetation index) spatiotemporal dynamics and exploration of key climate factors has important significance for research of grassland cover change.Based on the RS,GIS and SPOT NDVI data and monthly meteorological data from 1999 to 2007,the spatiotemporal variation characteristics of grassland vegetation NDVI in the middle and upper reaches of the Hei River and the correlation between NDVI and four climate factors were analyzed.1) The annual average NDVI and growing season NDVI increased in the middle and upper reaches of the Hei River.The area of grassland NDVI that increased accounted for 71.53% of the total grassland area,while that which decreased accounted for 19.43%.The grassland vegetation NDVI in typical steppe,alpine steppe and meadow of the Tuolai Mountain,Southern Mountain of Hexi corridor,Lenglong Ridge and the middle of the Hexi corridor increased,while the decreased regions were located in the zone between desert grassland and non-grassland of the Tuolai Mountain,Southern Mountain of the Hexi corridor and Lenglong Ridge.2) The grassland vegetation NDVI of the Hei River shows a single peak curve within the yearly changes of different grassland types,and the value of NDVI reached a peak in July.3) There was an extremely significant positive correlation between grassland NDVI and average temperature(R=0.942,P0.01),and a significant correlation with average precipitation(R=0.922,P0.05).The coefficient of correlation between grassland NDVI and relative humidity was positive but not significant,while the correlation with sunshine hours was negative.The limiting factor was different in the four grassland types.In recent years,with the shift from warm-dry to warm-humid climate in Northwest China,grassland vegetation coverage in the middle and upper reaches of the Hei River significantly increased and temperature was the important factor for grass growth.