Deciduous broadleaf forests green FPAR and its relationship with spectral vegetation indices
The fraction of absorbed photosynthetically active radiation (FPAR) is directly related to the primary productivity of photosynthesis, and is widely used to estimate ecosystem primary production. The forest canopy can be divided into photosynthetically active vegetation and non-photosynthetic vegetation according to their photosynthetical function. In this study, the scattering by arbitrary inclined leaves (SAIL) model was used to partition the PAR absorbed by canopy components into two parts: PAR absorbed by PAV and PAR by NPV. The characteristics of green FPAR (the fraction of PAR absorbed by PAV) and the relationships between green FPAR and spectral vegetation indices (NDVI, EVI, EVI2, SAVI) were analyzed. The results showed that the green FPAR varied with the canopy structure. In broadleaf deciduous forests with high coverage, the green FPAR was close to the total FPAR, and the contribution of NPV to the total FPAR was very low. Plant area index had more important impacts on the green FPAR than the proportion of PAV in the canopy and optical properties of PAV. The green FPAR had significant relationships with four spectral vegetation indices, but the correlation coefficient between green FPAR and EVI was the largest. Therefore, compared with other three vegetation indices, EVI may be more suitable to estimate forest green FPAR.