| The extraction of forest structural parameters,such as tree height,breast diameter,crown width,and three-dimensional structural information,is a fundamental prerequisite for quantifying the structure,pattern,and function of forest ecosystems.Improving the accuracy of remote sensing extraction of forest structure parameters plays an important role in the study of forest succession,carbon cycle,and forest productivity.In recent years,Li DAR(Light detection and ranging)has shown superiority for high-precision forest three-dimensional parameter extraction.With a diverse working mechanism than other passive remote sensing,forest structure parameter inversion has been successfully implemented.In particular,ground-based Li DAR provides accurate three-dimensional information related to tree structures with millimeter-level accuracy and small light spots.Due to the harsh natural conditions of the desert riparian forest in the lower reaches of the Tarim River,traditional field forest surveys are time-consuming and labor-intensive,and the vertical structure of Populus euphratica trees is complex.Therefore,based on the above content,this article has carried out the following aspects of research work based on high-precision point cloud data of ground-based Li DAR:(1)Taking Populus euphratica forest on the desert bank in the lower reaches of the Tarim River as the research object,five monitoring plots(50 m×50 m)(a total of 284 P.euphratica trees)with different distance to the river were selected for scanning.The multi-site stitching error is controlled within 2 mm,and the ground-based Li DAR point cloud data is used as the data source to classify the ground points and non-ground points with a progressive encryption triangle filter,algorithm and on this basis,samples for digital elevation model(DEM),digital surface model(DSM)and canopy height model(CHM)are generated.(2)The breast diameter and position in batches,and the height of the single tree with the vertical distance of the ground along the highest point of the canopy were extracted using the Hough transform circle fitting method,and the convex hull algorithm was used to obtain the projected area of the canopy and verify the fitting accuracy with measured values.The results show that:1)The tree height and DBH are well extracted,and the Pearson correlation coefficient(R~2)are 0.957 and 0.978respectively;2)There is a significant correlation between the LAI of VCP model and the measured data(R~2=0.84).(3)Five isolated trees were selected from each quadrat for omni-directional scanning.A three-dimensional voxel model of P.euphratica was established by using the method of canopy analysis(VCP).The leaf area index(LAI)of P.euphratica was retrieved from the canopy structure P.euphratica and verified by fitting with the measured value.The results show that the LAI of VCP model is significantly correlated with the measured data(R~2=0.959).(4)The correlation between various parameters of P.euphratica and the influences of regional water environment conditions were analyzed.The results showed that the height of P.euphratica increased gradually and then decreased within 200 m away from the river;the DBH did not change significantly with the distance from the river,and the DBH was greater than 50 cm within 1000 m away from the river is dominant;the size of tree crown structure decreases with the increase of river distance;there is no significant linear correlation between tree height and DBH,DBH and tree crown structure;there is certain correlation between tree height and tree crown structure;there is no significant correlation between LAI of P.euphratica and tree height,DBH,different river distance,but there is a significant correlation between LAI and groundwater depth(R~2=0.923).In this study,a complete technical process for extracting structural parameters of P.euphratica in desert riparian forest by ground-based three-dimensional Li DAR was established.Due to the sparsity of vegetation on the desert Bank of the lower reaches of the Tarim River and the absence of trees blocking each other,it is convenient for the layout of 3D laser scanner stations on the ground.Under the effect of ecological water transportation in the lower reaches of the Tarim River,the secondary crown phenomenon of Populus euphratica has some limitations on DBH fitting.It is inconvenient for the single tree segmentation to some extent,but the overall effect is excellent.Ground-based Li DAR is expected to replace some heavy and high-cost field work,and it can accurately and quickly obtain the information of single tree structure parameters in the sample,providing reliable theoretical basis and technical support for future research on the topics of photosynthesis,transpiration,biomass,carbon storage estimates. |
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