基于地面LiDAR数据的塔里木河下游胡杨林结构参数反演

以塔里木河下游天然胡杨林为研究对象,利用Riegl VZ-1000型地面激光扫描仪(Terrestrial Laser Scanning, TLS)获取离河道不同距离的8个样方内513棵胡杨的三维点云数据,通过建立冠层高度模型、Hough变换等方法获取单木株数和结构参数,并与传统的每木检尺实测数据和无人机(Unmanned Aerial Vehicle, UAV)低空影像进行对比,验证激光雷达方法的测树精度;对TLS获取的胡杨树形参数进行相关性分析,并建立关系模型;探讨不同水胁迫条件(不同离河道距离,不同地下水埋深)对胡杨单木结构参数的影响;最后按不同径级划分胡杨的年龄,得出各龄级胡杨所占比例。结果表明:(1)TLS能够高精度获取不同密度和长势的胡杨单木株数和结构参数,单株准确分割比率为94%—100%,相对于UAV低空影像更为准确;(2)TLS获取的胡杨树高(Tree height,TH)、胸径(Diameter at breast height, DBH)、冠幅直径(Crown diameter,CD)和冠幅面积(Crown area,CA)与传统实测值拟合度R~2较高,分别为0.95、0.97、0.77和0.84,表明实测数据和TLS获取数据无明显差异;(3)胡杨CD、CA分别与TH呈显著正相关,其相关性系数为0.73、0.67;基于此构建了胡杨TH与CD的关系模型,即TH=2.6274×CD~(0.706),R~2为0.64;(4)根据径级划分胡杨年龄段可知,DBH为15—30 cm的近熟林比例最大,占8个样方内监测胡杨总株数的47%,表明胡杨种群年龄结构相对稳定并总体态势良好,呈现了生态输水对塔河下游胡杨种群恢复有明显的促进作用。总之,激光雷达技术能够客观反映胡杨树形结构参数,可替代耗力、耗费、耗时的传统实测方法,为时时掌握胡杨林生长发育、长势动态以及多尺度、多时相生态耗水研究提供高精度信息,为干旱区荒漠河岸林的有效保护与可持续管理提供科学依据。

Terrestrial laser scanning for retrieving the structural parameters of Populus euphratica riparian forests in the lower reaches of the Tarim River, China

Terrestrial Laser Scanning (TLS) is one of the Light Detection and Ranging (LiDAR) system which has been widely used for measuring three-dimensional (3D) structure of forests, yet it has not been used for the desert riparian forests systematically. In this study, TLS (Riegl VZ-1000, Austria) was applied for measuring the growth parameters of Populus euphratica riparian forests in the lower reaches of the Tarim River, northwestern China. We chose eight forest sample plots, which contained 513 individual trees, with different vertical distances from the river bank and acquired point cloud data. The biometric parameters of every single tree were extracted by establishing crown height model (CHM) and Hough transform methods. At the same time, the parameters measured by TLS were compared with the manual test value and Unmanned Aerial Vehicle (UAV) image for verifying accuracy. The aim of this study is to explore the advantages and verifying accuracy of the TLS for riparian forest survey. We also analyzed the correlation between the parameters and impact of hydro-environmental factors which include the distance from river bank and ground water depth. Our results showed that: 1) TLS could acquire the majority of individual tree structure parameters at high precision about 94%-100%, higher than the results of UAV. 2) The correlation with manual test value of tree height (TH), diameter at breast height (DBH), crown diameter (CD) and crown area (CA) were 0.95, 0.97, 0.77 and 0.84, respectively. It showed that there was no significant difference between the hand-measured data and the TLS-obtained data. 3) There were significant positive correlation between TH and CD, CA of P. euphratica individual trees in our sample plots. The best correlation model is: TH=2.6274×CD^0.706, R²=0.64. 4) According to P. euphratica age group divided by DBH grade, the near mature trees (DBH: 15-30 cm) occupied the highest proportion (47%). It indicated that the age structure of P. euphratica population was relatively stable and the overall situation was satisfying, and demonstrated the hysteresis effects of man-made ecological water conveyance since 2001 and the restoration trends of P. euphratica riparian forest ecosystem along the Tarim River. In short, the LiDAR technology can objectively reflect the biometric and structure parameters of P. euphratica forest, which can substitute the traditional tree measurement methods that are labor-intensive, cost-intensive and time-consuming. TLS can provide high-precision information for monitoring the tree growth and vitality dynamics of P. euphratica riparian forests, and provide a useful tool and scientific basis for the effective protection and sustainable management of desert riparian forests in arid areas.