Dual-wavelength laser scanning for forest health monitoring.

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.

The proposed research aims to develop and test a new approach for measuring vegetation stress, indicated by reduced leaf moisture content, with a view to applying the techniques developed in improved validation of remotely sensed imagery and to utilising the knowledge gained to advance the development of airborne or satellite-based dual-wavelength laser scanning. The work therefore has potential to have a high international impact within the remote sensing and space science industry and for government, public sector and other organisations with an interest in monitoring forest and vegetation health and forest degradation.

The UK academic community is currently internationally leading in research into dual-wavelength and multispectral laser scanning for terrestrial monitoring applications. In addition, The UK and European space and remote sensing industry is growing rapidly, with the sector the focus of a number of high level initiatives such as the recent formation of the UK Space Agency and the International Space innovation Centre. Multispectral lidar systems represent an opportunity for this sector to gain a global advantage, with major potential future economic and social benefits (for example, through enhanced access to data for UK and European countries). Before such systems can be further developed, the technology and its advantages for forest, agricultural and vegetation monitoring must be fully demonstrated and the proposed research represents a major step forward in this process.

There is also a strong need for improved data on forest health and for improved methods to map and monitor drought stress, fire risk and disease in forested environments and these needs must be urgently addressed. Remote sensing allows acquisition of data on land cover change and deforestation over large areas and on frequent timescales, but current approaches are less effective for reliably detecting early stages of vegetation stress. Organisations with a responsibility for the monitoring and management of forest resources and for mitigating the impacts of forest pests and disease therefore stand to benefit, both from advances allowing development of airborne or spaceborne dual-wavelength lidar and, more immediately, from improved approaches for making ground-based validation measurements for other remote sensing methods, allowing assessment of the accuracy of results and more informed interpretation of data. In the UK, such organisations include the Forestry Commission and DEFRA, whilst internationally, national forestry agencies and international bodies and initiatives such as the UN Framework Convention on Climate Change - REDD+ monitoring programmes stand to benefit significantly over the longer term. Additional and more immediate benefits may be felt within the precision agriculture industry, through the use of terrestrial and unmanned aerial vehicle dual-wavelength systems for monitoring crop drought stress and focusing use of irrigation.