Spatio-temporal Dynamics Of Net Primary Productivity And Its Driving Forces In The Transnational Area Of Tumen River

Net primary productivity(NPP)refers to the part of the organic matter produced by vegetation photosynthesis that deducts autotrophic respiration.NPP is an important factor in estimating the Earth's supporting capacity and evaluating the sustainable development of terrestrial ecosystems.It plays an important role in global climate change and carbon cycle.At the same time,NPP is also one of the most important ecosystem services provided by terrestrial ecosystems to humans.Based on the CASA(Carnegie Ames Stanford Approach,CASA)model,this study combines land use data,NDVI data and meteorological data to quantify the dynamics of spatio-temporal patterns of NPP in the transnational area of Tumen River from 2000 to 2016.At the same time,combined with the MOD17A3 data and the existing research results,the accuracy of the CASA model was evaluated.Then,using mathematical statistics analysis,trend analysis and coefficient of variation,the temporal and spatial dynamics of NPP in the study area were analyzed.Finally,combined with correlation analysis,landscape pattern index and mathematical statistics,the driving force factors of spatio-temporal dynamics of NPP were analyzed,including climate,topography and land use/cover change.On this basis above,combined with statistical data,the impact of land use/cover change on the three sides of China,North Korea and Russia on food security was assessed.The specific research results are as follows.(1)The NPP data estimated in this study is compared with the existing research results and MOD17A3 data,and its reliability and accuracy are in line with the actual NPP distribution.According to the mean comparison analysis,except for the difference between MODIS NPP and CASA NPP in 2000,2007 and 2010,the NPP values of MODIS in other years are close to those of CASA NPP.At the same time,we obtained the regression of the different types of MODIS NPP data and CASA NPP data,and obtained the good fit of the two data,and the significance was tested by P<0.01.Comparing the estimation results of this study with those of other existing studies,it can be seen that the estimation results of various types of the study are within the effective range and conform to the actual distribution of NPP.(2)The temporal and spatial dynamics of NPP in the transnational area of Tumen River were revealed.In the transnational area of Tumen River,the average NPP from 2000 to 2016 was 558.35gC/(m2·a),and the average NPP on the Chinese side was larger than that on both sides of Russia and DPRK,and the lowest on the DPRK's side.The high value area of NPP is mainly concentrated in the forest area,and its value is higher than 500gC/(m2·a).The low-value areas are mainly distributed near to rivers,coastlines,and urban centers.The average NPP is between 0-200gC/(m2 a).From 2000 to 2016,the total amount of NPP fluctuated around 13TgC.From the spatial variation characteristics of NPP,the overall NPP of the study area from 2000 to 2016 showed an upward trend with an average trend coefficient of 1.63.From the perspective of China,Russia and Russia,the average trend coefficient on the Russian side is greater than that on both sides of China and DPRK.The standard deviation of NPP on the whole region was 43.09,and the mean coefficient of variation was 0.09.The overall NPP showed a steady state.There are also large differences in the coefficient of variation between the three sides of China,DPRK and Russia.The largest value on the Russian side is 0.1 and slightly higher than the mean of the region.There are also significant differences in NPP between different land use types in the transnational area of Tumen River.The NPP of broad-leaved is the highest,the grassland NPP is the second,while the water body NPP is the smallest.The average value of NPP of broad-leaved forest on the three sides in China,DPRK and Russia was the highest,followed by grassland NPP and water body NPP.The NPP of the broad-leaved forest in China showed a trend of rising firstly and then decreasing,while the NPP of grassland was rising all the time.The NPP of the water body first rose and then decreased.The NPP of the broad-leaved forest on the DPRK's side also rises at first and then declines.The NPP of broad-leaved forest and grassland on the Russian side are always on the rise.(3)It reveals the impacts of driving factors of natural and human activities including climate change,topography and LUCC(land use and land cover change,LUCC)on NPP changes.Among the climatic factors,both temperature and precipitation have a significant impact on NPP changes,and precipitation is the main driving factor.Among the topographic factors,elevation,slope and aspect have significant impacts on the change of NPP,and mainly reflected in the spatial pattern distribution.At the same time,the LUCC in the whole region led to a decrease in NPP in the whole region,with a significant decrease on the Chinese and DPRK's side,while a slight increase on the Russia side.In addition,changes in land use have led to varying changes of NPP in regional cultivated land,especially urban land expansion,broad-leaved forests and cultivated land,and inter-transformation between grassland and cultivated land.According to the fluctuations and changes in the NPP of cultivated land,combined with statistical data,there are food security problems on the Chinese side of transnational area of the Tumen River,and it is affected by many factors,such as urban expansion,agricultural modernization,climate change,and national policies.Finally,we suggest that China,DPRK and Russia establish a long-term cooperation mechanism to promote regional biodiversity and socio-economic coordinated development,improve regional NPP,and promote regional sustainability.At the same time,it is reasonable to control the expansion of the transnational urban agglomerations,protect regional cultivated land,and solve food security problems in transnational areas.