| As the most active factor in terrestrial ecosystem,vegetation reflects the characteristics of regional land cover change to a certain extent,and has a very good indicator for the ecological environment change.At present,the study of vegetation cover change is mostly concentrated in arid and semi-arid areas and less rivers,and the scale is large and the detailed study of the differentiation characteristics is lacking.In fact,vegetation cover in warm and humid areas is also affected by water and temperature.The study of vegetation cover and its influencing factors in humid areas is also helpful to reveal the law of vegetation evolution,the planning of Vegetation Ecology and sustainable development.It is also of great significance.In recent years,the ecological environment of the middle and lower reaches of the Hanjiang River basin is undergoing profound changes due to the South-to-North Water Transfer,rapid urbanization and global warming,and has become one of the sensitive zones in China.Therefore,it is instructive to study the temporal and spatial changes of vegetation cover and its influencing factors in the basin.Based on the data of MOD13Q1-NDVI,precipitation,temperature and DEM from 2001 to 2015 in the middle and lower reaches of the Hanjiang River,the temporal and spatial variations of vegetation cover at different elevations in the middle and lower reaches of the Hanjiang River and their influencing factors were analyzed in detail from three time scales of year,season and month by using the methods of linear fitting,partial correlation analysis and residual analysis based on pixels.The main conclusions are as follows:Topography determines the spatial distribution and change track of vegetation.The terrain of the middle and lower reaches of the Hanjiang River is complex.There are many low mountains in the northwest and plains in the southeast.The vegetation cover in the basin shows a decreasing spatial pattern from northwest to southeast.The annual average NDVI value in the growing season is 3.56.During 2001-2015,vegetation degradation and improvement coexisted in the whole basin,showing a general trend of degradation,and vegetation degradation at the rate of 4.8×10-3/a.From different elevations,the vegetation in the area with elevation below 200 m is degrading at the rate of 8.7×10-3/a;the vegetation condition in the area with elevation between200 and 1500 m is getting better,and the change speed is 5.4×10-3/a;the vegetation condition in the area with elevation above 1500 m has little change and keeps stable.The vegetation cover in the watershed is affected by the combination of water and heat and human activities in the horizontal direction,but the influence of each factor on vegetation has spatial heterogeneity.The vegetation degradation area in the middle and lower reaches of the Hanjiang River is a low-altitude farming area,which is mainly affected by human activities,and the intensity of vegetation degradation is gradually weakening.The vegetation at 200-1500m altitude is improved,and the vegetation type is grassland and woodland.The vegetation in this area is mainly affected by temperature,and the time lag effect of temperature on vegetation growth is about 2months.With the warming of temperature,the vegetation degradation intensity in this area is gradually decreasing.The vegetation condition is getting better;the vegetation in forest areas with elevation more than 1500m is mainly affected by precipitation,and the time lag effect of precipitation is one month;for vegetation in areas with elevation more than 200m,although the impact of human activities is small,it promotes the growth of vegetation in this area,and the promotion effect is increasing year by year. |
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