Remote Sensing Estimation Of Vegetation NET Primary Productivity And Its Response To Climate Change In The Yellow River Basin From 1992 To 2015

The Yellow River Basin spans arid,semi-arid and semi-humid areas in China,and its ecosystem environment is fragile.Assessing the productivity of the Yellow River Basin can provide scientific references for managing ecological resources,and improving and protecting the ecological environment.Based on meteorological data?NDVI?elevation and vegetation classification data,the CASA model and BP neural network model are used to estimate the vegetation net primary productivity?NPP?in Yellow River Basin,respectively.In addition,by means of linear regression analysis,partial correlation analysis and R/S analysis,the spatial and temporal changes of vegetation NPP and its response to climate changes in the Yellow River Basin from 1992 to 2015 are studied in different spatial and temporal scales.The main results are as follows:?1?The feasibility of using the CASA model and BP neural network model to estimate vegetation NPP in the Yellow River Basin is verified.?2?The annual average vegetation NPP has a strong spatial differentiation and decreases from south to north.The annual average vegetation NPP in the upper,middle and lower reaches of the Yellow River Basin show obvious gradient distribution.That is,the upper reaches of the Yellow River<the middle reaches of the Yellow River<the lower reaches of the Yellow River.The vegetation NPP in spring,summer,autumn and winter are different,and their contributions to the annual average NPP are 22.85%?48.47%?18.14%and10.53%,respectively.The accumulation of vegetation NPP is mainly in summer.?3?From1992 to 2015,the annual average vegetation NPP in the Yellow River Basin as a whole fluctuates from 227.780 to 283.407 g C?8)^-2?(6^-1,and shows a significant increasing trend.The vegetation NPPs of the upper reaches,middle reaches and lower reaches of the Yellow River Basin show slight increase,significant increase and slight decrease,respectively.?4?The changes of vegetation NPP in the Yellow River Basin during 1992-2015 are spatially heterogeneous with an increasing trend.The vegetation NPP of 71.20%regions has obvious change trends.The coverage of vegetation in the upper and middle reaches of the Yellow River Basin are improved as a whole,and the areas with improving and degrading vegetation in the lower reaches of the Yellow River Basin are comparable.The change of vegetation NPP has a weak sustainability,and the future development direction is mainly weak disadvantage and weak advantage,accompanied by uncertainty.?5?On the multi-year scale,vegetation NPP in the Yellow River Basin is less restricted by the temperature,and is more closely related to precipitation.The correlation between NPP and precipitation in the upper and middle reaches of the Yellow River is higher,while that in the lower reaches of the Yellow River is lower.On the annual scale,vegetation NPP is negatively correlated with temperature and positively correlated with precipitation,but there are fewer areas passed the significance test.The correlation between vegetation NPP and precipitation is higher in the upper and middle reaches of the Yellow River,while the correlation between vegetation NPP and precipitation or temperature is lower in the lower reaches of the Yellow River.The correlation between monthly average NPP and monthly mean temperature or monthly precipitation is higher than that of interannual level,which is positively correlated with monthly precipitation.?6?The average annual NPP of different vegetation types in the Yellow River Basin is in the following order:deciduous broad-leaved forest>evergreen broad-leaved forest>evergreen,deciduous broad-leaved mixed forest>shrub>cultivated vegetation>grassland>evergreen coniferous forest>deciduous coniferous forest,and the annual total NPP was the following order:grassland>cultivated vegetation>deciduous broad-leaved forest>evergreen broad-leaved forest>evergreen broad-leaved mixed forest>shrub.Grassland contributes most to vegetation NPP,and occupies half of the total.The vegetation NPP of grassland and shrub have high positive correlations with precipitation.The relationship between cultivated vegetation and temperature is not significant,and it is moderately positively correlated with precipitation.Except deciduous coniferous forests,the correlation coefficients of forest ecosystem vegetation with precipitation and temperature are low.