Research On The Response Of Vegetation To Climate Change And Human Activities On The Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau(QTP)is a sensitive area of climate change,which has a significant impact on regional and even global climate.In recent decades,the QTP has been getting warmer and wetter.However,as an important part of terrestrial ecosystem,vegetation is extremely sensitive to the warming and wetter.Normalized Difference Vegetation Index(NDVI)and Net primary productivity(NPP)are important variables representing vegetation status,which can well reflect the response characteristics of Vegetation to climate change.Based on the GIMMS NDVI(1982-2015),SPOT NDVI(2000-2018)and MODIS NPP(2000-2018)remote sensing data,this study analyzed the spatial and temporal distribution characteristics and change trends of vegetation on the QTP.Based on the temperature and precipitation data,the spatial distribution characteristics of temperature and precipitation and the warming and wetness trend of the QTP were analyzed,and the response of vegetation on the QTP to climate change was revealed by various correlation analysis methods.Finally,NPP simulated by the LPJ model(Lund-Potsdam-Jena Model)was used for residual calculation by MODIS NPP,and different scenarios of vegetation change were constructed.The relative effects of climate change and human activities on vegetation change on the QTP were analyzed,and the main conclusions were as follows:(1)The vegetation on the QTP showed an improving trend from 1982 to 2018.The annual mean of GIMMS NDVI,SPOT NDVI and MODIS NPP over the QTP showed a spatial pattern of gradual increase from northwest to southeast,and showed an increasing trend.The GIMMS NDVI of forestland mainly showed a decreasing trend,while the grassland generally showed an increasing trend.However,the SPOT NDVI of cultivated land and grassland in most areas showed a significant increasing trend,with an area proportion of 58.78%,75.47%and 39.22%,respectively.The MODIS NPP of cultivated land,forest land and grassland mainly showed an increasing trend.From 1982 to 2015,the annual mean of GIMMS NDVI showed a fluctuating slightly upward trend,with an annual mean value of 0.35.From 2000 to 2018,the annual mean of SPOT NDVI showed an insignificant and continuous upward trend,with an annual mean of 0.35.The MODIS NPP averaged 143.33 g C·m²·y^-1 from 2000 to 2018,and the overall variation was small.(2)The QTP showed a warming and humid trend from 1982 to 2018.The spatial distribution of mean temperature over the QTP from 1982 to 2018 was unbalanced.The eastern and southeastern parts of the Plateau had higher mean temperature above 0?,while the central and western parts had lower mean temperature below 0?.Around 2000,the area of significant warming has shifted to some extent,showing a significant warming trend as a whole.The multi-year average temperature of the QTP is-1.7?,and it is warming at a rate of 0.5?per decade.The spatial pattern of precipitation over the QTP are distinct between wet and dry,and the annual precipitation decreases gradually from southeast to northwest.The areas with high annual precipitation value are mainly distributed in the southern Nyingchi region and the westen Sichuan and eastern tibet region(>1000 mm).The regions with an annual precipitation of more than 400mm are mainly distributed in the Qilian Mountains,the Golonaqu Plateau and the alpine mountains of South Tibet.The annual mean value of the QTP is 398.87 mm,and the interannual variation shows a fluctuating increasing trend,which indicated the Plateau is slowly getting wetter.(3)Vegetation change on the QTP was mainly in positive response to temperature.The GIMMS NDVI and MODIS NPP were positively correlated with air temperature over the QTP.The SPOT NDVI was positively correlated with both temperature and precipitation,but the correlation coefficient between SPOT NDVI and temperature was higher than that of precipitation.According to the partial correlation analysis,the partial correlation between GIMMS NDVI and air temperature from 1982 to 2015 was more obvious,with a positive partial correlation in 44.59%of the regions,and a significant or extremely significant positive partial correlation in 8.78%and7.29%of the regions.SPOT NDVI showed a positive partial correlation with air temperature(49.26%),which was mainly distributed in the eastern region of the Plateau.MODIS NPP showed a positive partial correlation with air temperature(50.44%)and a negative partial correlation with precipitation in 45.49%of the regions.(4)Forestland and grassland are the main land cover types in the major ecological engineering areas on the QTP.Since the implementation of the Natural Forest Protection Project,the Three-north Forest Protection Project and the Grain for Green Project on the QTP,the area of woodland and grassland in the project area has decreased slightly.In term of the Grazing Withdrawal Project,the area of woodland decreased slightly,while the area of grassland increased.In addition,the construction land has increased significantly.The land use type transfer in the QTP was as follows:the transferred area of grassland was the largest,with a net grassland transfer area of 3,122 km²from 1980 to 2000 and-2,217 km² from 2000 to 2015.(5)By constructing different scenarios of vegetation change on the QTP,the residual method was used to quantitatively separate the relative effects of climate change and human activities on vegetation change.The results showed that the vegetation restoration in the QTP was mainly dominated by climate factors during 2000-2018,and the total NPP increased by 23.28 Tg C,accounting for 64.85%of the total area.The main influencing factor of vegetation degradation was human activities.The total amount of NPP decreased by 6.92 Tg C,accounting for 25.4%of the total area.Human activities have a great impact on the area below 3000 m on the QTP.The vegetation degradation was mainly dominated by human activities.The total amount of vegetation NPP decreased by 4.17 Tg C,accounting for 50.29%of the area.In this study,vegetation change and its driving factors on the QTP were analyzed and evaluated,but the accuracy of the assessment,uncertainty quantification and the temporal and spatial resolution need to be improved.In addition,more environmental and social factors should be incorporated into future studies to improve the accuracy of model prediction,improve the evaluation framework,and provide reliable suggestions for policy and decision-making.