Vegetation Dynamics And Its Driving Forces In Qingjiang River Basin

Vegetation,as the most basic part of the ecosystem,plays an important role in soil erosion,sand fixation,and climatic regulation.Therefore,the size of the vegetation cover area often reflects the quality of the regional ecological environment and is an "indicator" in the change of the ecological environment.Based on the MODIS NDVI data set and interpolation data,this paper analyzed the temporal and spatial distribution of vegetation cover and climate change at different time scales and spatial scales in the Qingjiang river basin from 2001 to 2015.Then,the relationship between vegetation cover change and climate change was analyzed.What's more,quantitative analysis was used to analyze the relative contribution between climate change and human activities.The main conclusions are:(1)The NDVI changed obviously among the months of the year in the Qingjiang river basin.The trends among different river sections and Qingjiang river basin were the same.The minimum value of NDVI appeared in February and the maximum value appeared in July.However,in general,the NDVI in the middle reaches was larger than that the upstream and downstream.From seasonal point of view,the NDVI in the spring and autumn was the largest in the downstream(0.636 and 0.666,respectively),and the NDVI in the summer was the largest in the middle reaches(0.803).The annual changes of vegetation in spring,autumn,and winter fluctuated greatly.The annual changes of vegetation in spring and the whole river basin were the same from 2012 to 2015.But annual fluctuation of vegetation in summer was small.There wan significant positive correlation between annual NDVI and year(r = 0.70,p <0.01).NDVI in different river sections were also significantly positively correlated with the year,and correlation in the downstream was the largest(r = 0.801,p <0.01).The upward trend of Qingjiang river basin was 0.003 /a.The order of increasing trend of NDV in the upstream,middle reaches and downstream were: downstream(0.0040 /a)> upstream(0.0033 /a)> middle reaches(0.0029 /a).(2)93.1% of Qingjiang river basin's area showed an increasing trend in NDVI from 2001 to 2015,of which 59.77% showed a significant increasing trend,which was mainly distributed in Lichuan and the eastern middle reaches.6.9% of the area showed a decreasing trend,which was mainly concentrated in the urban areas of Enshi city,Yezhou town,and Huoshaoping town.The range of the increasing trend of vegetation cover in four seasons were as follows: autumn(95.6%)> winter(85.8%)> spring(84.4%)> summer(74.2%),of which 50.8% of the area in autumn had a significant increasing trend in NDVI,which was mainly distributed in the eastern middle reaches and downstream.The trend of NDVI was decreased significantly in the urban areas of Enshi,Yezhou town and Huoshaoping town in four seasons.(3)The characteristics of temperature and precipitation in the Qingjiang river basin were obvious during the year.The lowest temperature and the lowest precipitation were in January(4.61 ? and 22.87 mm,respectively),and the highest temperature and the highest precipitation were in July(26.25 ? and 198.55 mm,respectively).The temperature was higher in the downstream and the precipitation was higher in the upstream generally.The overall change law of temperature and precipitation from 2001 to 2015 were not obvious,and the changes were mainly in three stages.The annual trend of the four seasons were various.There was a slight increasing trend in spring and winter(0.016 ?/a and 0.001 ?/a respectively),and a slight decreasing trend in summer and autumn(0.019 ?/a and 0.011 ?/a,respectively).79.55% of the area showed a decreasing trend in temperature during the 15 years.And the maximum decrease trend was 0.05 ? / a.The rainfall increased in 62.30% of the area,and the increasing trend was mainly between 0 and 2mm / a.(4)Vegetation growth in the Qingjiang river basin was mainly affected by temperature among climate factors.6.39% of area was affected by temperature,3.79%of area was affected by rainfall,and only 1.54% of area was affected by temperature and rainfall.From seasonal point of view,temperature in spring,summer and winter played a leading role in vegetation growth,and precipitation in autumn played a leading role in vegetation growth.In the early and late stages of the vegetation growing season(from April to October),the lag of vegetation to temperature and precipitation was mostly 1-2 months,and in the middle of the growing season,the lag of temperature and precipitation could reach 7-8 months.The growth of vegetation was certainly affected by terrain.In areas with an altitude below 400 m and slope below 6 °,the NDVI changed significantly.(5)The vegetation in the Qingjiang river basin from 2001 to 2015 was mainly affected by human activities.Under the trend of increased or decreased vegetation coverage,the relative contribution of human activities were 63.97% and 56.92%,respectively.Climate change had a greater inhibitory effect on downstream's vegetation growth,and human activities had a greater promotion effect on middle reaches' vegetation growth.