Monitoring and modeling short-wave radiation in a broad-leaf forest

Among the plant factors, canopy geometry plays a crucial role in deciding the driving force for leaf biological processes since it modulates canopy interception of short-term radiation fluxes and it controls their distributions throughout the foliage (Monteith and Unsworth, 1990; Jones, 1992). In particular, it is recognized the biological role of PhAR (400-700 nm waveband) in driving the photosynthetic process and the role of global radiation (300-2500 nm waveband) in driving the transpiration activity (Nobel, 1991). Unfortunately, focusing on a forest stand, a fast and convenient method for the characterization of the canopy geometry remains elusive. Only innovative methodologies, e.g. laser-based and imagery remote sensing seem to be promising (Wulder and Franklin, 2003). Consequently, a spatial distribution of photon and radiant flux densities throughout the foliage is often modeled assuming canopy geometrical simplifications. In particular, Beer’s law has been and is still widely applied, requiring the assumption of foliage geometrical homogeneity, to model canopy downward attenuation of quantum flux density (Monsi and Saeki, 1953). Given this context, our study objectives were: 1) to measure photosynthetic photon flux density (PPFD, μmol ms) in a broad-leaf mixed forest stand by using a simple and inexpensive technique proposed by Parker et al. (1996): a custom-built balloon filled with helium was used as a platform to lift a radiation sensor throughout the foliage; 2) to describe the distribution of radiation flux densities at increasing canopy height levels; 3) to process the collected radiation data using Beer’s law (after Monsi and Saeki, 1953) to estimate spatial distribution in vegetation.