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具有重要意义。传统的直接测量重要的森林结构参数，如叶面积指数（LAI，单位地面面积单侧叶面积的测量），非常耗时，而间接方法，如基于半球形照片的方法，依赖于一些可能引入重大误差的假设。地面激光扫描仪（TLS）通过对关键结构参数进行快速、半自动的测量，具有前所未有的精度水平，有可能彻底改变植被冠层结构的测量。像LAI这样的变量可以通过记录数百万个激光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TLS仅记录激光束内少数物体的距离，并且无法区分树叶和木材，因此受到限制。索尔福德先进激光冠层分析仪（SALCA）是一种独特的野外便携式仪器，由索尔福德大学和Halo Photonics有限公司开发，用于记录每个激光脉冲（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）来区分树叶和木本材料。该仪器的全波形特性允许检索激光束内多个物体的信息，以及关于物体性质的更详细信息，例如它们填充的光束面积的比例。该仪器有可能以前所未有的详细程度提供关于森林冠层结构的信息。SALCA仪器已经在实验室试验中进行了测试，并使用建模技术来理解一系列材料的记录响应。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LAI。将开发从SALCA数据估计LAI的算法，并将其与从半球形照片和市售激光扫描仪估计的LAI进行比较。测量将在春季和初夏定期重复进行，以评估LAI估计值对森林冠层季节性变化的敏感性，并检查两种不同激光波长下反射率的变化。拟议的工作将使SALCA在验证卫星和航空传感器估算的LAI方面的潜力得到评估，并将确定该仪器在监测植被特征随时间变化方面的潜力，这将有助于更好地解释从碳通量测量塔获得的数据。从SALCA的开发和测试中获得的知识可以有助于未来在空中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.