Satellite LiDAR enhancement of Forest Inventory and Production Forecast Capabilities

了解储存在植被中的碳分布是报告碳储量变化所必需的一个重要因素，并在国际协定中得到承认，例如1997年《联合国气候变化框架公约京都议定书》。林木蓄积量的量化对于森林管理监测林分性能和评估商业潜力也非常重要。此外，植被的垂直结构可以提供有关林地作为物种栖息地的质量的有用信息。来自卫星的光学数据可以提供陆地等级分布的二维透视，因此可以根据大面积的反射特性来划定森林和林分。然而，从光学数据量化植被的估计依赖于基于其在不同波长的反射率的间接假设。光探测和测距(LiDAR)提供了一种利用植被本身的结构特性来估计植被高度、垂直剖面、体积和冠层覆盖度的直接方法。全波形激光雷达利用从地面、机载或卫星平台发射的激光脉冲穿透植被树叶之间的缝隙。能量从照明区域(占地面积)内的所有截获表面反射并返回到传感器，这意味着返回的波形既代表树冠结构，也代表表面地形。在传感器处检测返回能量所花费的时间可以使用光速转换为距离，从而允许计算截获表面之间的高差，从而提供垂直树冠轮廓。搭载在冰云和陆地高程卫星(ICESat)上的地球科学激光高度计系统(GLAS)每年三次提供近全球覆盖，使用大约直径的激光雷达足迹对地球表面进行采样。因此，这颗创新的卫星为在区域到全球范围内提取季节性生物物理参数提供了前所未有的机会。作为国家森林调查的一部分，林业委员会将在本研究过程中制作确定林区的土地覆盖图。这些将被用来定义解释森林类型的区域(即广泛的植被类别，包括针叶、阔叶、新种植的林分等)。利用光学遥感数据将其进一步划分为物种类别。然后，返回的激光雷达波形将用于这些类别的样地，以建立与植被分析中使用的重要森林参数的关系。这将包括与植被高度分布、树冠覆盖率和树干材积的直接关系，以及对林业应用中使用的参数的间接估计，如平均直径分布和断面积。然后，这些模型将被用于将估算扩展到分类区域，从而生产出全国范围的地图产品。这对于森林调查特别有意义，因为虽然有关于公共林地的全面信息，但对于私人所有的土地则不是这样。由于私有土地占英国60%的林地和80%的木材生产，验证这些地区植被参数估计的方法将有助于减少碳核算中的不确定性。此外，该产品的三维视角将使生境结构与土地覆盖类型和物种分布相关，以便更全面地分析生境特性和碎片化。垂直植被结构也可以用来更好地了解生态系统的通量以及人类和环境对这些通量的影响。

Knowledge of carbon distribution stored within vegetation is an important factor necessary for reporting changes in carbon stock and recognised in international agreements such as the Kyoto Protocol to the United Nations Framework Convention on Climate Change 1997. Quantifying tree volume is also of great importance for forest management to monitor stand performance and assess commercial potential. Additionally, the vertical structure of vegetation can provide useful information regarding the quality of woodlands as species habitats. Satellite-derived optical data can provide a two-dimensional perspective of land class distribution and therefore permits the delineation of forests and stands according to the reflective properties for large areas. However, estimates to quantify vegetation from optical data rely on indirect assumptions based on its reflectivity at different wavelengths. Light Detection and Ranging (LiDAR) provides a direct means of estimating vegetation height, vertical profile, volume and canopy cover using the structural properties of the vegetation itself. Full waveform LiDAR uses the ability for laser pulses, emitted from terrestrial, airborne or satellite platforms, to penetrate gaps between vegetation foliage. Energy is reflected and returned to the sensor from all intercepted surfaces within the illuminated area (footprint), meaning that the returned waveform represents both the canopy structure and surface topography. The time taken for the returned energy to be detected at the sensor can be converted into distance using the speed of light which allows elevation differences between the intercepted surfaces to be calculated, therefore providing a vertical canopy profile. The Geoscience Laser Altimeter System (GLAS) aboard the Ice, Cloud and land Elevation Satellite (ICESat) provides near global coverage three times annually, sampling the Earth's surface using approximately 64 metre diameter LiDAR footprints. This innovative satellite therefore offers an unprecedented opportunity for seasonal biophysical parameter retrieval at regional to global scales. As part of the the National Forest Inventory, land cover maps identifying forested areas will be produced by the Forestry Commission during the course of this research. These will be used to define areas of Interpretive Forest Types (i.e. broad vegetation classes including conifers, broadleaves, newly planted stands, etc.) which will be further segmented into species classes using optical remotely sensed data. Returned LiDAR waveforms will then be used for sample plots of these classes to establish relationships with important forest parameters used in vegetation analysis. These will include direct relationships with vegetation height profile, canopy cover and stemwood volume as well as the indirect estimation of parameters used in forestry applications such as mean diameter distribution and basal area. These models will then be used to extend the estimates across classified areas leading to the production of a nation-wide cartographic product. This is be of particular interest for forest inventory purposes as, whilst comprehensive information is available relating to public woodland, this is not the case for privately owned land. As private land accounts for 60% of Britain's woodlands and 80% of timber production, methods of validating estimates of vegetation parameters for these areas would serve to reduce uncertainty in carbon accounting. Furthermore, the three/dimensional perspective of this product would allow habitat structure to be related to land cover type and species distribution for a more comprehensive analysis of habitat properties and fragmentation. Vertical vegetation structure can also be used to better understand ecosystem fluxes and the effects of humans and the environment on these.