Full-waveform multispectral lidar to measure vegetation structure and dynamics

测量和监测植被结构和植被特性的季节变化对于更好地了解森林生长和碳循环等关键过程十分重要。传统的直接测量重要的森林结构参数，如叶面积指数(LAI，单位地面面积的单侧叶面积的测量)非常耗时，而间接方法，如基于半球照片的方法，依赖于许多可能引入显著误差的假设。地面激光扫描仪(TLS)能够以前所未有的精度快速、半自动地测量关键的结构参数，有可能给植被冠层结构的测量带来革命性的变化。像LAI这样的变量可以通过记录数百万个激光采样点的反射能量来确定，从而提供植被特征的永久三维记录。然而，商业上可用的TL受到限制，因为只记录到激光束内少量物体的距离，并且无法区分树叶和木材。索尔福德高级激光冠层分析仪(SALCA)是由索尔福德大学和Halo Photonics有限公司开发的一种独特的野外便携式仪器，可以记录每个激光脉冲(全波形)反射能量的完整信号，并使用两种不同波长的激光(近红外和短波红外)来区分树叶和木本植物。该仪器的全波形特性允许检索关于激光光束内的多个物体的信息，以及关于物体性质的更详细的信息，例如它们填充的光束区域的比例。该仪器有可能以前所未有的详细程度提供有关森林树冠结构的信息。SALCA仪器已经在实验室试验中进行了测试，建模技术已经被用来理解从一系列材料记录的反应。拟议的工作旨在通过对该仪器进行实地试验，以评估在一系列英国森林类型中测量结构的潜力，重点是准确量化叶面积指数，从而在这项研究的基础上再接再厉。将开发从SALCA数据估计叶面积指数的算法，并通过与从半球照片和商业激光扫描仪估计的叶面积指数进行比较来进行测试。将在春季和初夏定期重复测量，以评估LAI估计对森林树冠季节性变化的敏感性，并检查在两个不同激光波长的反射率的变化。拟议的工作将使SALCA验证来自卫星和航空传感器的LAI估计的潜力得到评估，并将确定该仪器监测植被特征随时间的变化的潜力，这将有助于更好地解释从碳通量测量塔获得的数据。从开发和测试SALCA中获得的知识有助于今后在航空和卫星平台上开发和应用多波长和全波形激光扫描仪。

Measuring and monitoring the structure of vegetation and seasonal changes in vegetation properties is important in better understanding key processes such as forest growth and the carbon cycle. Traditional direct measurements of important forest structure parameters, such as leaf area index (LAI, a measure of the one-sided leaf area per unit ground area), are extremely time consuming whilst indirect methods, such as those based on hemispherical photographs, rely on a number of assumptions that can introduce significant error. Terrestrial laser scanners (TLS) have the potential to revolutionise measurement of vegetation canopy structure by making rapid, semi-automatic measurements of key structural parameters with unprecedented levels of accuracy. Variables like LAI can be determined by recording the reflected energy from several million laser sample points, providing a permanent 3-dimensional record of vegetation characteristics. However, commercially available TLS are limited by only recording the distance to a small number of objects within the laser beam and by an inability to distinguish leaves and wood. The Salford Advanced Laser Canopy Analyser (SALCA) is a unique, field-portable instrument, developed by the University of Salford and Halo Photonics Ltd, to record the full signal of reflected energy from each laser pulse (full-waveform) and to use two different wavelengths of laser (near and shortwave infrared) to allow discrimination of leaves and woody material. The full-waveform characteristic of the instrument allows retrieval of information on multiple objects within the laser beam and of more detailed information on the nature of objects, such as the proportion of the beam area that they fill. The instrument has the potential to provide information on the structure of forest canopies with unprecedented levels of detail. The SALCA instrument has been tested in laboratory trials, and modelling techniques have been used to understand the recorded response from a range of materials. The proposed work aims to build on this research by conducting field trials of the instrument to assess the potential for measurement of structure in a range of UK forest types, with a focus on accurately quantifying LAI. Algorithms to estimate LAI from SALCA data will be developed and tested by comparison with LAI estimates from hemispherical photographs and commercially available laser scanners. Measurements will be repeated regularly through the spring and early summer to assess the sensitivity of LAI estimates to seasonal changes in the forest canopy and examine the variability in reflectance at the two different laser wavelengths. The proposed work will allow the potential of SALCA for validating estimates of LAI from satellite and aerial sensors to be assessed and will determine the potential of the instrument for monitoring changes in vegetation characteristics over time, which would aid better interpretation of data obtained from carbon flux measurement towers. Knowledge gained from the development and testing of SALCA can contribute to future development and application of multiple wavelength and full-waveform laser scanners on aerial and satellite platforms.