| In recent years,global scale organic carbon cycle has become a hot topic in geographic research.Net primary productivity(NPP)can indicate the carbon sink function intensity of forests.At the same time,NPP also explores the characteristics of global carbon cycle and ecosystem function and structure.It is of great significance to further explore forest carbon cycle.This paper introduces tree ring chronological data to reconstruct long-term NPP series.The time series of tree ring chronological data is relatively complete and spatially distributed.The information in the chronological data can accurately represent the local climate,water and heat conditions and record them accurately.Therefore,the introduction of tree ring chronological data can effectively reflect the annual NPP status of forest vegetation in historical period.From the point of view of specific tree species,two sample plots of Pinus koraiensis and Larix gmelinii tending to pure forest were selected in Dazhanhe Nature Reserve to make tree ring chronology of the two tree species.Using LANDSAT8,LANDSAT7 and NOAA/AVHRR-NDVI images to extract NDVI data from the sample area,and using geographic information technology software to interpolate the total solar radiation and temperature data of the sample area,the estimation of NPP in the sample area provides a strong guarantee.Using the C-FIX model of the light energy utilization model,the NPP of August 1987 to 2016 in the two research areas was estimated.The 30-year NPP value and its corresponding tree ring chronology were modeled,and the 100-year NPP data in the study area were reconstructed through the tree ring chronology of long time series.The main conclusions of this paper are as follows:(1)Using C-FIX model to estimate NPP value of Pinus koraiensis and Larix gmelinii in the two study areas,and using MODIS 17 product data to compare the predicted NPP value,it is found that the predicted NPP value of C-FIX model is smaller than that of product NPP value,which is similar to other scholars' research.After the statistical correlation analysis of the product value and the predicted value,it is found that the correlation between them is relatively high,which indicates that it is feasible to estimate NPP value by using C-FIX model in Dazhanhe Nature Reserve.(2)Applying C-FIX model to estimate NPP production in the two research areas from 1987 to 2016 in August of 30 years.Statistical analysis of NPP production in August of 30 years and tree ring chronology in the same study area was carried out to determine the function.The results showed that power function was the best choice in Korean pine area and polynomial function was the best choice in Larix sample area.By comparing the average temperature of two tree species in August with that in August,it was found that there was a significant correlation between NPP and the average temperature in August in the Korean pine sample area,but not in the Larch sample area.This indicated that the growth of Korean pine was better when the temperature was higher in the growing season,which was also consistent with the cold resistance of Larix gmelinii and the temperature preference of Korean pine.(3)NPP in August was reconstructed from 175 to 104 years according to tree ring chronology and reconstruction model of Korean pine and larch areas,and NPP reconstruction maps were made.The results showed that there were 5 years of vigorous growth and 7 years of extremely slow growth in Pinus koraiensis region,which accounted for 0.03% and 0.04% of the total year,respectively.The growth of Larix gmelinii was very vigorous for 5 years,accounting for 4.8% of the total year,and very slow for 4 years,accounting for 3.85% of the total year.In the two study areas,there are six years of the same growth extreme,and the corresponding temperature of the previous year and that of the current year are very consistent with the changes of NPP,indicating that the growth of NPP is synchronized by climate factors such as temperature in different tree species.(4)NPP in the two areas was divided into three stages according to the time period.It was found that the three stages of NPP in August in the two areas showed a downward trend.Yield in the first stage to the third stage of Korean pine research area declined a lot,about 0.043 million T.The highest annual decline rate in the first stage to the third stage of Larch research area was about 10.29%,which indicated that the Korean pine sample area was affected by the external environment.It is small and stable,but Larch sample area is greatly affected by the change of external environment.Generally speaking,in Dazhanhe Nature Reserve,the carbon sink capacity of Korean pine forest is stronger than that of Larch forest. |
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