| In order to develop Net Primary Productivity（NPP）estimated methods,this study used the Random Forest（RF）method to simulated NPP from 2001 to 2018 in Hexi Corridor based on observed NPP,remote sensing data,DEM and land use types,and this study also analyzed the spatial-temporal changes in NPP in Hexi Corridor.According to importance of each variables in the Random Forest,the main driving factors for the changes of NPP including fAPAR（fraction of Absorbed Photosynthetic Active Radiation）,precipitation,relative humidity,temperature,potential evapotranspiration（PET）,altitude,slope,and aspect in the Hexi Corridor were analyzed by correlation coefficient of the relationship with NPP.This study quantified contribution of NPP IAV（Interannual Variability）with each cells,land use types,and the areas or land use types that play leading role in NPP IAV were discussed.This study calcuated the contribution of NPP IAV within each grid,and which areas or land use type played the controlling role in NPP IAV in Hexi Corridor was discussed.The results showed that RF is one of the effective methods to estimate NPP in arid and semi-arid regions and NPP showed an increasing trend in Hexi Corridor from 2001 to 2018.The main conclusions are as follows:1.Temporal and Spatial analysis of vegetation NPP in Hexi Corridor based on Random Forest.（1）In this study,the NPP simulated using the Random Forest has a high correlation with the observed NPP,with R2 of 0.8434.Therefore,the NPP result simulated by the Random Forest has a high accuracy in the Hexi Corridor.（2）The average NPP showed a spatial pattern of decreasing from southeast to northwest in Hexi Corridor from 2001 to 2018.The high-value area was mainly distributed in the Qilian Mountain of upstream and the oasis of middle and lower,while the low-value area was distributed in the sandy land and gobi of downstream.The average annual NPP in Hexi Corridor was 130.03 gC·m-2·a-1,and the total NPP was 32.17 TgC·a-1（1 Tg=10122 g）.As for the three major river basins,the order of vegetation productivity was:Shiyang River Basin>Heihe River Basin>Shule River Basin.（3）The annaul NPP varied from 106.79 to 151.97 gC·m-2·a-1,and the overall trend was rising with an average annual growth rate of 2.18 gC·m-2·a-11 in the Hexi Corridor from 2001to 2018.The NPP in the Shiyang River Basin has fluctuated widely during 18 years,followed by the Heihe River Basin,and the NPP has a relatively stable fluctuation in the Shule River Basin.In 2001,the total NPP was the lowest with 26.42 TgC·a-1,and the total NPP was the highest with 37.60 TgC·a-11 in 2018.（4）For the land use types in the study area,the mean and total NPP were fluctuating upward from 2001 to 2018.The annual average NPP of the forest land was the highest in the Hexi Corridor,707.92 gC·m-2·a-1,with the largest interannual variation.The total NPP of grassland were the highest,11.96 TgC·a-1.The average annual increment of NPP in forestland was the largest,with a value of 7.41 gC·m-2·a-11 and the average annual increment of NPP in unused land was the smallest,with a value of 1.27 gC·m-2·a-1.2.Analysis of NPP driving factors in Hexi Corridor（1）Land cover type is the most important factor affecting NPP,and it is related to the physiological characteristics of vegetation itself.Among the climate impact factors,annual average fAPAR has the greatest impact on vegetation NPP,followed by the precipitation,relative humidity,temperature and annual potential evapotranspiration.For the topographic factors,the order of importance was in order of aspect>altitude>slope.（2）The NPP was positively correlated with the annual average fAPAR,total precipitation,annual minimum temperature and PET in Hexi Corridor.The correlation between NPP and annual average relative humidity was negatively in Hexi Corridor.（3）The annual average NPP of the Hexi Corridor has increased firstly and then decreased with the increment of altitude and slope.For the aspect,the average NPP of the shady aspect（Northeast and North aspect）and semi-shake aspect（East aspect）was significantly higher than that of the sunny aspect（South and Southwest aspect）and semi-sunny aspect（Southeast and West aspect）.3.Analysis of NPP IAV contribution in Hexi Corridor（1）The average contribution of NPP to NPP IAV of each grid was positive in the study area.Separately,the regions with more positive contributions were mainly distributed in the Qilian Mountains and oases in Hexi Corridor,while the negative contributions found mainly in the oases and deserts.（2）For the basins,the overall contributions of the three basins are in order from high to low:Shiyang River Basin>Heihe River Basin>Shule River Basin.Therefore,Shiyang River Basin plays a leading role in the entire Hexi Corridor NPP IAV.（3）The administrative divisions have significant regional differences in the contribution of the whole Hexi Corridor NPP IAV.The largest contribution of NPP IAV is Sunan County,which is 13.17%.Sunan County,Subei County and Tianzhu County（the total contribution is37.07%）together dominated the NPP IAV of Hexi Corridor.（4）Among the land use types,the total contribution of grassland was the highest,with32.06%.The negative contribution of unused land type was the highest,with-12.05%.In terms of the land use types,grassland generally dominated the NPP IAV of Hexi Corridor. |
