Dual-wavelength laser scanning for forest health monitoring.

点击翻译按钮获取中文摘要

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.