Leaf Area Index: LAI
LAI is defined as one half the total green leaf area per unit horizontal ground surface area (GCOS-138-SUP, Chen and Black, 1992). Green leaves correspond to vegetation matter capable of photosynthesis in ambient conditions. However, this simple definition needs some additional comments when applied to remote sensing observations:
- Leaf/other elements. Leaf (or needles in the case of conifers) should be seen here as a generic term for designing the above ground aeral extent of vegetation. if no distinction is made between leaves (needles) and the other elements, the proper term to use is PAI: Plant Area Index rather than LAI. Note that most indirect methods used to estimate LAI from upward looking canopy transmittance corresponds to PAI rather than LAI.
- Green-non-green elements. Canopies are made of green photosynthetically active and other elements which are not green and therefore non photosynthetically active (senescent leaves, truncs, branches, fruits, flowers...). Since most users are interested in the green active potentially elements, the term GLAI (Green Leaf Area Index) should be used. However, the community uses commonly LAI in place of GLAI. Similarly, the green surfaces are extended to all the green elements, the term GAI (Grenn Area Index) should be used. Note that most remote sensing retrieval methods (therefore the corresponding biophysical products) are based on the specific absorption features of chlorophyll or water, and are consequently mainly sensitive to GAI rather than LAI (or GLAI). GAI may be also estimated from indirect methods based on downward looking measurements of the green fraction (the fraction of green vegetation seen from a given direction from above the canopy). GAI is probably the most pertinent definition
- Flat/convex/concave elements. The early definition of LAI was relative to flat leaves: the one sided leaf area per unit horizontal ground surface area. In case of needles, which are convex objects, the definition was extended to the actual definition: half the total (meaning the developped area) leaf area per unit horizont ground surface area. In the case of concave objects (which are fortunately not common), the definition of LAI should be half the developped surface of the convex hull wrapping each element per unit horizontal ground surface area.
- Apparent/effective/true. The retrieval of LAI (GAI) from indirect methods including remote sensing depends on several components:
- The assumptions used in the retrieval method/model. The retrieval method is either based on an experimental calibration data base or on a radiative transfer model. In both cases assumptions are made, either coming from the experiemental conditions observed in the calibration data base or from the assumptions embeded in canopy structure representation, leaf and soil optical properties or approximations in the radiation transport process.Among these limitations, canopy structure and leaf clumping is probably the strongest one.
- The measurement configuration. The retrieval will likely vary from one observational configuration to another, depending on the particular sensitivity of the reflectance to the variables of interest.
- The uncertainties associated to the measurements. Radiometric measurements are contaminated bu uncertainties, and the retrieved variables are related to the input reflectance measurements in a complex and generally non-linear way. This results in uncertainties and biaises associated to the tretrieved variables.
For these above listed reasons, the retrieved variables, i.e. the products, are likely to be different from the actual LAI (GAI) value, i.e. the 'true' value. The retrieved variables will be called the 'apparent' value, i.e. the one you access through radiometric measurements and the retrieval process. However, since:
- leaf clumping is one of the major limitations in the LAI (GAI) estimation,
- most LAI (GAI) ground measurements used for the validation is based on indirect measurements (gap or green fraction),
the 'effective' LAI (GAI) value was introduced to provide estimates of LAI (GAI) from ground measurements of gap (green ) fraction for a specific configuration, assuming random distribution of the elements within the canopy volume (i.e. no clumping). It is thus proposed to define the effective LAI (GAI) as the LAI (GAI) value of canopy with randomly distributed elements (turbid medium) having an hemispheric gap fraction equal to that of the actual canopy considered. For practical applications, it can be approached by Welles and Norman (1991) approximation in the 30°-60° zenitahl angle range. Equivalently, it also corresponds to the LAI (GAI) value of the same hypothetical turbid medium canopy with a gap (green) fraction at 57.5° equal to that of the actual canopy considered.
Fraction of Absorbed Photosynthetically Active Radiation Absorbed: FAPAR
Solar radiation in the spectral range 400 to 700 nm, known as Photosynthetically Active Radiation (PAR), provides the energy required by terrestrial vegetation to grow. The part of this PAR that is effectively absorbed by plants is called the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). It is a non-dimensional quantity varying from 0 (over bare soil) to almost 1 for the largest amount of green vegetation. Since FAPAR is mainly used as a descriptor of photosynthesis and evapotranspiration processes, only the green photosyntheic elements (leaves, needles, or other green elements) should be accounted for. FAPAR will also depend on the illumination conditions, i.e. the angular position of the Sun and the relative contributions of the direct and diffuse irradiances. Both black-sky (assuming only direct radiation) and white sky (assuming that all the incoming radiation is in the form of isotropic diffuse radiation) FAPAR values may be considered. FAPAR products are currently mainly defined as the black-sky FAPAR value for the same sun position as that observed at the satellite overpass.
Vegetation Cover Fraction: FCOVER
It corresponds to the green fraction as seen from the nadir direction. FCOVER is used to separate vegetation and soil in energy balance processes, including temperature and evapotranspiration. It is computed from the leaf area index and other canopy structural variables and does not depend on variables such as the geometry of illumination as compared to FAPAR. For this reason, it is a very good candidate for the replacement of classical vegetation indices for the monitoring of green vegetation.