GCOS Terrestrial ECV T11
Leaf Area Index (LAI)

Definition: Leaf Area Index (LAI) measures the amount of leaf material in an ecosystem, which imposes important controls on photosynthesis, respiration, rain interception, and other processes that link vegetation to climate. Consequently, LAI appears as a key variable in many models describing vegetation-atmosphere interactions, particularly with respect to the carbon and water cycles (GCOS, 2004). The interest in information on LAI distribution and changes has grown substantially in recent decades, due to its intrinsic importance and the emerging capability for LAI estimation over large areas using satellite measurements. (Source: FAO/GTOS)

Introduction: LAI measures the amount of plant leaf material in an ecosystem. It is typically expressed as a non-dimensional value giving the number of square metres of leaf material per square metre of ground. This variable plays important roles in models that represent processes, such as photosynthesis, respiration and rain interception, that couple vegetation to the climate system through the radiation, carbon, and water cycles. Hence, LAI appears as a key variable in many models describing vegetation-atmosphere interactions.
 
LAI can be estimated in situ by destructive sampling or with the help of commercially available dedicated instruments. It is routinely measured at a number of research sites dealing with surface climate, ecological, or agricultural issues. CEOS WGCV is playing a coordinating role in this work. Benchmarking and consistency checking are required for the global archive of LAI measurements. 
 
For reasons set out below, in some parts of the globe (e.g., in the humid Tropics), LAI can only be measured by in situ methods. However, the measurement network is sparse in many regions of the world. It should be maintained and ideally expanded to become much more representative of the diversity of ecosystem conditions. The development and maintenance of reference sites to address this inadequacy should be addressed, as explained elsewhere in this document. Building on existing networks, such as FLUXNET, LAInet and BIGFOOT, is a possible way to improve this situation (see Action T3). The CEOS WGCV has begun to coordinate this through the creation of a centralized database, an activity that should continue. 
 
The retrieval of reliable LAI estimates from space remains difficult. When the canopy cover is sparse, reflectance measurements are dominated by soil properties, and the accuracy of the LAI is low. When LAI values exceed 3 or 4, optical measurements lose their sensitivity to changes in LAI (signal saturation. Also, since the LAI measured by satellites is usually inferred from spectral reflectances in the visible and infrared spectrum, it is de facto coupled to FAPAR estimates, even though both variables are in principle independent and play quite different roles in the climate system. Nonetheless, regular global LAI estimates from space are currently being produced, and this effort should be continued (it requires little extra resource above that required to produce FAPAR). These LAI products have the same spatial resolutions (250 m-1 km) and temporal frequencies (7 to 10 days) as the FAPAR products. Recent research is exploring the feasibility of estimating LAI (and above-ground biomass) from microwave sensors, and these efforts should also be pursued.
 
Benchmarking and comparison of these LAI products is essential to resolve differences between products and to ensure their accuracy and reliability. The CEOS WGCV should lead this activity in collaboration with GCOS and GTOS, exploiting in situ observations from designated reference sites and building on the validation activities currently being undertaken by the space agencies and associated research programmes.

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Changes in land cover are important aspects of global environmental change, with implications for ecosystems, biogeochemical fluxes and global climate. Land cover change affects climate through a range of factors from albedo to emissions of greenhouse gases from the burning of biomass. Deforestation inter alia increases the amount of carbon dioxide (CO₂) and other trace gases in the atmosphere. When a forest is cut and burned to establish cropland and pastures, the stored carbon joins with oxygen and is released into the atmosphere as CO₂. The IPCC notes that about three-quarters of the anthropogenic emissions of CO₂ to the atmosphere during the past 20 years were due to fossil fuel burning. The rest was predominantly due to land use change, especially deforestation. In 2005, a number of developing countries proposed to incorporate deforestation prevention into the Kyoto Protocol, in part through an emissions trading system. The initiative, known as REDD, (Reducing Emissions from Deforestation in Developing countries) would allow developing countries to sell emissions savings from forest conservation. Developed countries would buy the savings to credit against their own emissions targets. IGOS has set up an Integrated Global Carbon Observation (IGCO) Theme (report available from www.igospartners.org ) to develop a flexible, robust strategy for international global carbon observations over the next decade. A key component of IGCO is terrestrial carbon observations aimed at the determination of terrestrial carbon sources and sinks with increasing accuracy and spatial resolution. The IPCC has highlighted an improved understanding of carbon dynamics as vital in tackling one of the biggest environmental problems facing humanity. The IGCO work will be an essential input to the implementation of the United Nations Framework Convention on Climate Change (UNFCCC), particularly on the role of natural sinks in meeting targets under the UNFCCC Kyoto Protocol. Satellite observations allow scientists to map land cover and the dynamics of fire disturbance, and track two key elements of Earth’s vegetation – the ‘Leaf Area Index’ (LAI) and the ‘Fraction of absorbed Photo-synthetically Active Radiation’ (FAPAR). LAI is defined as the one-sided green leaf area per unit ground area in broadleaf canopies, or as the projected needle leaf area per ground unit in needle canopies. FAPAR is the fraction of photosynthetically active radiation absorbed by vegetation canopies. Both LAI and FAPAR are data necessary for understanding how Sunlight interacts with the Earth’s vegetated surfaces.

Multiple types of satellite observations are used in agricultural applications. Space imagery provides information which can be used to monitor quotas and to examine and assess crop characteristics and planting practice. Information on crop condition, for example, may also be used for irrigation management. In addition, data may be used to generate yield forecasts, which in turn may be used to optimise the planning of storage, transport and processing facilities. Classification and seasonal monitoring of vegetation types on a global basis allow the modelling of primary production – the growth of vegetation that is the base of the food chain – which is of great value in monitoring global food security. A number of radiometers provide measurements of vegetation cover, including the ATSR series, AVHRR/3, MODIS, MERIS, SEVIRI and Vegetation. These instruments are helping production of global maps of surface vegetation for modelling of the exchange of trace gases, water and energy between vegetation and the atmosphere. Multi-directional and polarimetric instruments (such as MISR and POLDER) will provide more insights into corrections of land surface images for atmospheric scattering and absorption, as well as Sun-sensor geometry, allowing better calculation of vegetation properties. Synthetic aperture radars (SARs) are used extensively to monitor deforestation and surface hydrological states and processes. The ability of SARs to penetrate cloud cover and dense plant canopies makes them particularly valuable in rainforest and high-latitude boreal forest studies. Instruments such as ASAR, SAR (RADARSAT), and PALSAR provide data for such applications as agriculture, forestry, land cover classification, hydrology and cartography. CEOS and GCOS have concluded that many of the Essential Climate Variables related to vegetation and supported from space will require reprocessing of the moderate resolution historical record (in particular AVHRR) to be of greater value for climate purposes, and appropriate actions have been defined, including the development of enhanced calibration and validation schemes which guarantee long-term stability and consistency over different temporal and spatial scales. Research topics like scaling, and the development of ‘community radiative transfer models’ integrated into sophisticated assimilation schemes, are of paramount importance for an integrated approach. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement)

References:

• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second Report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)
• ECV T11: LAI: Leaf Area Index - Assessment of the Status of the Development of the Standards for the Terrestial Essential Climate Variables (ECV), Version 10, May 25, 2009 (GTOS-66)
• CEOS EO Handbook - Earth Observations Plans by Measurement
• CEOS EO Handbook - Measurement Categories (Updated October 2010)

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