Remote Sensing Estimation And Simulation Of Major Tree Species Aboveground Biomass And Density In The Forest Region Of Northeast China

Forest ecosystems are main component of terrestrial ecosystem and play important roles in global carbon cycle.Temperate and boreal forests act as major sinks for atmospheric CO₂ and have received increasing attention because mid-and high-latitudes are predicted to have much greater temperature increase than low-latitudes.Obtaining the spatially accurate information on aboveground biomass of temperate and boreal forests is crucial for accurately estimating regional carbon storage.Northeast China is located in the sensitive zone of mid-latitude with rich forest resources,and is the most important forest carbon pool in China.Forest region here includes cold-temperate coniferous forest and temperate coniferous and broad-leaved mixed forest and they are unique forest types in China.However,previous studies on aboveground biomass in the forest region of Northeast China mainly focused on the estimation of total aboveground biomass lumping all tree species.No study has been conducted on the estimation of the species-level aboveground biomass and density for entire Northeast China.It is the species-level aboveground biomass and density that can provide important information for understanding forest dynamics.In addition,due to the different sensitivity of major tree species to climate change in Northeast China,the composition and structure of tree species in the region may change significantly in the future with the global climate change.Therefore,quantitative assessment of the aboveground biomass and density of the major tree species in Northeast China and prediction of the future dynamic changes of tree species composition under different climate scenarios will help to grasp the response and feedback mechanism of the major tree species to climate change in Northeast China,and provide scientific basis for the region and nation to formulate forest management policies to cope with climate change.In this study,we established 630 k-Nearest Neighbor?kNN?biomass estimation models integrating forest survey,remote sensing,and environmental data in the forest region of Northeast China.We selected the optimal kNN model for estimating aboveground biomass of 17 major tree species through model evaluation.We discussed the relationship between the spatial distribution characteristics of each tree species and environmental factors.Then,based on the estimated aboveground biomass of the 17 tree species,we estimated the distribution density of each tree species in each diameter and age class,and analyzed the distribution characteristics of diameter at breast height?DBH?and age of each tree species.Finally,we used the forest ecosystem process model?LINKAGES v3.0?to predict dynamics of tree species composition over Northeast China under the current and three warming climate scenarios?RCP2.6,RCP4.5 and RCP8.5?for the next 100 years?2001-2100?,and then analyzed the potential spatial distribution of the major tree species in different simulated periods.The main conclusions are as follows:?1?Among the designed 630 kNN biomass estimation models based on 6 different distance metrics,15 k values,and 7 sets of multi-temporal MODIS data and environmental variables combinations,we found that the models with random forest?RF?as the distance metric showed the highest accuracy.Compared to the use of single-month MODIS data for September,there was no appreciable improvement for the estimation accuracy of species-level aboveground biomass by using multi-month MODIS data.Although the accuracy of the kNN biomass estimation model increased as the value of k increased,when k>7,the accuracy improvement of the RF-based kNN models using the single-month MODIS predictors for September was essentially negligible.Therefore,the kNN model using the RF distance metric,k=7,and single-month?September?MODIS spectral reflection variables combined with environmental variables was the optimal model to impute the species-level aboveground biomass for entire Northeast China.?2?The aboveground biomass of the major tree species in Northeast China and their spatial distribution were significantly affected by environmental variables.The aboveground biomass of larch and white birch was significantly negatively correlated with temperature,distributed most widely in Northeast China,and were therefore the two dominant tree species in Northeast China currently.The aboveground biomass of spruce and fir was positively related to elevation,and therefore,they are mainly mapped in areas of relatively high elevation.The aboveground biomass of Korean pine,ribbed birch,mono maple,basswood,ash,elm,Amur corktree,and walnut showed positive correlation with precipitation,temperature and solar radiation,which were mainly distributed in Lesser Khingan Mountains and Changbai Mountains with sufficient humidity and temperature,but rarely distributed in the severe cold Greater Khingan Mountains.Scotch pine had a high tolerance of drought and low temperatures,and consequently was mainly distributed on sunny slopes and ridges in the northern edge of Greater Khingan Mountains.Willow was mainly distributed along the river.The aboveground biomass of willow and Amur corktree were very low due to their limited distribution over the study area.Overall,the aboveground biomass in the Greater Khingan Mountains was lower than that in the Lesser Khingan and Changbai Mountains.Accuracy of the estimation results improved obviously from the stand scale up to the ecotype scale.?3?By combining the two-parameter Weibull probability density function with the kNN model,the distribution density of the major tree species at each diameter and age class in Northeast China can be reasonably estimated.The total stand density of 17 tree species in Northeast China estimated in this study was between 700-3000 trees/ha,and the average value of the total stand density in the whole study area was 1464 trees/ha.Overall,the total stand density in the Greater Khingan Mountains was higher than that in the Lesser Khingan and Changbai Mountains.The DBH distribution of aspen,Scotch pine,spruce,ash,Amur corktree and willow were generally showed an inverted "J" curve,with the majority of trees in the small diameter classes.The DBH of other tree species showed a left-unimodal mountain shape distribution,indicating these tree species were mostly belong to small and medium diameter classes.In addition,the age of the major tree species in Northeast China was mostly distributed in the form of left-unimodal mountain shape.Although for most of the tree species,young and middle-aged forests accounted for a large proportion,some of them?such as Mongolian oak,mono maple and ash?also had a large proportion of near-mature or mature forests.On the whole,the composition and the DBH and age structure of the major tree species in Northeast China were relatively complex.?4?In the next 100 years,there were little difference in the composition of tree species in Northeast China among different climate scenarios.However,the potential response of each tree species to climate change was significantly different.In the next 100 years,the major broad-leaved tree species in Northeast China would be dominated by Mongolian oak,aspen,white birch and mono maple,accompanied by a relatively small proportion of basswood,black birch,elm,ash and ribbed birch,and the proportion of walnut under all climate scenarios,while Amur corktree and willow were almost negligible.Coniferous species would be dominated by Korean pine and larch,accompanied by a small amount of Scotch pine,spruce and fir.For individual tree species,the current climate condition has been close to marginally suitable conditions for survival of larch,spruce,fir,and ribbed birch.The further rapid rise of temperature would cause these tree species unable to adapt to environmental changes in the short term and face degradation.By the end of the 21 st century,Mongolian oak and Korean pine would become the most dominant broad-leaved and coniferous tree species in Northeast China,respectively.With moderate temperature increase in the three climate warming scenarios,most of the temperate broad-leaved tree species and Korean pine in Northeast China had the trend of northing in spatial distribution,and the greater the warming amplitude,the more obvious the moving trend.