Vegetation change and its response to climate change in the Yarlung Zangbo River basin in the past 18 years
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Abstract:
Climate change is a hot topic in recent decades,and the global ecosystem has already been affected by it.Climate warming has a significant impact on the vegetation change process in country.Vegetation is an important part of the terrestrial ecosystem.It not only affects the hydrology,climate,and energy balance of the ecosystem,but also is subject to change the hydrological,climatological process.Therefore,by monitoring the vegetation status,analyzing the characteristics of temporal and spatial changes of vegetation cover and the correlation of climate factors,soil and other factors with the vegetation changes are of great significance for revealing the intrinsic relationship between climate change and vegetation changes. Firstly,based on MODIS NDVI data,DEM data,and 1∶1 million Chinese vegetation type map,the MannKendall trend and mutation point test method were used to analyze the temporal change characteristics of NDVI at different altitude gradients and vegetation types from 2001 to 2018 in the basin.Slope value of MODIS NDVI for each pixel and the characteristics of the vegetation NDVI spatial change were calculated from 2001 to 2018 in the Yarlung Zangbo River basin.Secondly,spatial distribution characteristics of the correlation coefficient between NDVI and monthly precipitation and monthly average temperature under the four lag time conditions of the current month,the previous month,the first two months,and the first three months were analyzed to explore the lag of climate factors on NDVI.Finally,the response time corresponding to different vegetations was calculated based on precipitation and temperature change and then correlation between NDVI changes of different vegetation types and climate factors were analyzed. (1) From 2001 to 2018,NDVI showed an increasing trend in 80% of the Yarlung Zangbo River basin.Among them,the downstream area showed the most significant increase in NDVI.The 6 types of altitude gradients showed a gradual increase in NDVI.Vegetation NDVI in areas with an altitude greater than 4,500 m has a significant growth trend.From the perspective of vegetation types,the results are similar to those of different altitude gradients,and both show a gradual significant increase trend;(2)The area where the vegetation NDVI is moderately correlated with precipitation and temperature is 80 % of the Yarlung Zangbo River basin,and it is found that the correlation between vegetation NDVI and precipitation in 5296% area of the basin is weaker than that of temperature,but 3369% area is highly correlated with precipitation than that of area highly correlated with temperature;(3) From the perspective of the same planting type in the Yarlung Zangbo River,except for grassland and meadow,the correlation coefficients of vegetation NDVI with temperature are all higher than precipitation.The response time of grassland,meadow,and alpine vegetation to temperature is longer than precipitation,and the response time of other vegetation to the changes of the two climatic factors shows that precipitation response time is greater than temperature. (1)Vegetation growth in the Yarlung Zangbo River basin has improved in the past 18 years.The vegetation NDVI at different altitude gradients and vegetation types has shown an increasing trend in 2015.It may be related to the state′s environmental protection and ecological construction policies in Tibet during the "Twelfth FiveYear Plan" period after 2013.(2) The NDVI of the vegetation in the Yarlung Zangbo River basin is greatly affected by precipitation and temperature.The correlation of vegetation with precipitation and temperature displayed that temperature is slightly stronger than precipitation;(3)The temperature in the Yarlung Zangbo River basin has a stronger impact on vegetation than precipitation,and the vegetation has a shorter response time to temperature changes.
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