Modeling BRF and Radiation Regime of Boreal and Tropical Forest: II. PAR Regime

Abstract

Monitoring functioning of forest ecosystems with remote sensing depends on canopy BRF (bidirectional reflectance function) sensitivity to biophysical parameters and PAR (photosynthetically active radiation) regime. Here, we studied the 3-D PAR regime of tropical (Sumatra) and boreal (Canada) forests, with the DART (discrete anisotropic radiative transfer) model. We considered wide ranges of Sun off-nadir angles (θ s), leaf area index (LAI), and leaf clumping. The BRF of these forests is analyzed in a companion article. Here, we also investigated the possibility to derive simple analytical expressions of PAR vertical profiles: We fitted DART simulated APAR (absorbed PAR) profiles with a modified Goudriaan law (1977) the extinction coefficient of which is multiplied by a factor α that accounts for canopy architecture. Similarly to BRF, the PAR regime is very influenced by canopy structure: for θ s=50°, α≅0.40 for tropical forest, α≅0.56 for coniferous boreal forest (OBS), and α≅0.86 for deciduous boreal forest (OA). Moreover, α strongly depends on θ s and LAI; for example, for tropical forest α decreases from 0.44 to 0.12 if θ s varies from 0° to 80°, and from 0.70 to 0.38 if LAI increases from 3 to 10. α decreases slightly with the increase of leaf clumpiness. The NDVI of tropical and boreal forest sites was rather related to the LAI and fAPAR of the upper canopy than to those of total canopy. Finally, we studied the impact of forest architecture on canopy photosynthesis with the coupling of DART with a leaf functioning model. Neglect of architecture can lead to errors as large as 25% in relative on forest CO 2 assimilation.