Assessment of clumping effect for vegetation from ground-based measurements to satellite applications

The clumping issue has been extensively addressed to characterize the statistical distribution of plant stands because the scattering by vegetation canopies is a major topic in terrestrial remote sensing. It has been discussed by many authors using a variety of approaches, including Nilson (1971), Suits (1972), Ross (1981), Hapke (1981), Kimes and Kirchner (1982), Otterman (1983), Gerstl and Zardecki (1985), Camillo (1987), Chen and Black (1991). Nilson (1971) pioneered the possibility to use clumping factors in order to treat the radiant transfer within non-random distributed plant stands. The approach consists to assume that an ensemble of protrusions, each of these protrusions yielding a turbid medium, constitutes a vegetation scene. Instead, John and col. suggested considering a turbid medium but with gaps within. Interestingly, both approaches had led to similar mathematical formalism. Typically two frequencies of gap probabilities can be segregated for forested areas, that is between-crown and within-crown. This was clearly demonstrated by Chen and col., both from an experiment point of view using TRAC instrument and in developing an original modeling approach. Clumping factors were an output of the process. In this regard, John and col. suggested a parameterization of the clumping effect as a function of forest architecture to explain daily variations. This encouraged us to establish the link between canopy architecture and directional effects for further satellite applications. In this respect, the combination between backward (hot spot) and forward (dark spot) scattering has served to map clumping index routinely, in particular using POLDER observations. Most ground segments have now pay attention to the problem and consider LAI correction from clumping.