Dual-wavelength laser scanning for forest health monitoring.

森林在全球碳循环中发挥着至关重要的作用，既是潜在的碳源，也是潜在的碳汇。它们还在水循环中发挥重要作用，森林冠层的蒸散作用和树叶对降雨的拦截影响着水分的供应。然而，大气温室气体水平上升、森林病虫害全球蔓延加剧以及人类活动和土地利用的变化造成的区域气候变化，对许多森林生态系统构成了重大威胁。特别令人关切的是，在许多重要的森林地区，如亚马逊盆地，预计干旱的频率和严重程度会增加。高毁林率和森林退化率（例如，通过选择性采伐或火灾的影响）也是一个主要问题，最近在2011年哥本哈根气候会议上商定的“减少发展中国家毁林和森林退化造成的排放”（REDD+）等国际倡议反映了这一点。这些倡议的成功依赖于对大面积森林覆盖和条件的频繁监测。因此，如果要在早期阶段发现森林退化，无论是由于人类活动、气候变化还是疾病造成的，就迫切需要精确和敏感的卫星或航空监测（遥感）方法，以便林务人员开展缓解活动或当地社区进行适应。现有的遥感技术在绘制大面积土地覆盖变化（例如森林砍伐）方面非常有效。然而，目前的方法对环境条件变化或疾病引起的林冠层压力的早期迹象（如叶片含水量减少）不太敏感。这是因为背景土壤和下层植被特性的高度变异性以及森林结构的变化可以掩盖叶片生化特性对卫星测量的反射率的影响。这项研究通过实验方法和模型的使用，测试了一种测量森林冠层含水量的新方法的潜力，这种方法可以作为干旱压力的衡量标准和疾病指标。通过使用最近开发的独特的地面激光扫描仪，索尔福德先进激光冠层分析仪，可以直接测量叶片含水量，基于检测水对两种不同波长激光脉冲反射率的影响。这种仪器有可能提供树冠含水量的三维测量，将树冠的响应与地面的响应分开，并提供独立于太阳光照背景变化的测量。该项目将改进对卫星传感器对森林冠层应力估计的准确性的评估，并将证明双波长激光扫描系统监测森林健康的能力，促进进一步发展作为机载或卫星仪器使用的技术。这种系统将代表负责监测和管理森林的国家和国际组织（如英国林业委员会或联合国REDD+计划）能力的重大进步，并将使英国和欧洲的航天工业在这项技术的前沿处于国际领先地位。

Forests play a vital role in the global carbon cycle, acting as both a potential carbon source and sink. They also play an important role in the water cycle, with evapotranspiration from forest canopies and interception of rainfall by leaves influencing water availability. However, changes in regional climate due to rising atmospheric greenhouse gas levels, increased global spread of forest pests and disease and changes in human activity and land use, represent major threats to many forest ecosystems. Of particular concern is the increased frequency and severity of droughts predicted in many important forested regions, such as the Amazon basin. High rates of deforestation and degradation (for example, through selective logging or the effects of fires) are also a major concern, as reflected by recent international initiatives such as Reducing Emissions from Deforestation and Forest Degradation in developing countries (REDD+), agreed at the Copenhagen climate conference in 2011. The success of such initiatives relies on frequent monitoring of forest cover and condition over very large areas. Accurate and sensitive satellite or aerial-based methods of monitoring (remote sensing) are therefore urgently needed, if forest degradation, whether due to human activity, climatic changes or disease, is to be detected at an early stage, allowing mitigation activities by foresters or adaptation by local communities. Existing remote sensing technology is highly effective at mapping land cover change (for example, deforestation) over large areas. However, current methods are less sensitive to the early signs of forest canopy stress (such as reduced leaf water content) resulting from changes in environmental conditions or disease. This is because high levels of variability in background soil and understorey vegetation properties and changes in forest structure can mask the influence of leaf biochemical properties on the reflectance measured by a satellite. This research tests, through experimental approaches and the use of models, the potential of a new approach to measuring the water content of forest canopies, as a measure of drought stress and an indicator of disease. Through the use of a recently developed and unique ground-based laser scanner, the Salford Advanced Laser Canopy Analyser, leaf water content can be measured directly, based on detecting the influence of water on reflectance of laser pulses at two different wavelengths. Such instruments have the potential to provide three-dimensional measurements of canopy water content, separating the response of the tree canopy from that of the ground and providing a measurement that is independent of background changes in solar illumination.The project will allow improved assessment of the accuracy of estimations of forest canopy stress made from satellite sensors and will act as a demonstration of the ability of dual-wavelength laser scanner systems for forest health monitoring, facilitating further development of the technology for use as an airborne or satellite instrument. Such systems would represent a major advance in capability for national and international organisations responsible for monitoring and managing forests, such as the UK Forestry Commission or the United Nations REDD+ programme and would allow the UK and European space industry to take an international lead at the fore-front of this technology.