Simulation of canopy FPAR and its relation with vegetation index: A case study of evergreen coniferous forests

The fraction of absorbed photosynthetically active radiation (FPAR) is an important biophysical variable, and is widely used in satellite-based production efficiency models to estimate GPP or net primary production. To the forest, the vegetation canopy is composed primarily of photosynthetically active vegetation (PAV) and non-photosynthetic vegetation (NPV). But only the PAR absorbed by PAV is used for photosynthesis. Current methods of measuring absorbed PAR in fields measure total PAR absorption. The Scattering by Arbitrary Inclined Leaves (SAIL) model was modified to simulate canopy reflectance and to separate the PAR absorbed by PAV and NPV of the evergreen coniferous forest in two scenarios. The total FPAR, green FPAR and FPAR determined by NPV were calculated and the relations between them and two vegetation index (NDVI and EVI) were analyzed. Our results showed the variation of canopy architecture influenced the canopy absorption of PAR. In high coverage forests, the green FPAR was close to the total FPAR of vegetation canopy and the contribution of NPV to the total FPAR was low. While, in low coverage forests or open forests, the situation was contrary. The significant relationships were found between two vegetation indices and FPAR, but compared with NDVI, EVI was more suitable to describe the variation of green FPAR.