Canopy architecture and remote sensing of the fraction of photosynthetically active radiation absorbed by boreal conifer forests

Measurements of the fraction of photosynthetically active radiation (FPAR) absorbed by the forest overstory were made at 20 sites in black spruce (Picea mariana) and jack pine (Pinus banksiana) boreal forests located in Saskatchewan and Manitoba, Canada. Canopies of both species have similar vertical tree crown structure but different branch and shoot architecture. Intensive investigation was made on the effect of these canopy architecture on the penetration of total visible radiation into the canopy at various solar zenith angles θ, quantified using the projection coefficient G_t(θ). Based on experimental evidence, constant values of G_t(θ) and the above- and below-canopy PAR reflectivities are suggested for these two species for the calculation of daily green FPAR. The calculation then requires only a single stand parameter: the effective green leaf area index (LAI) L_eg, which is similar to the effective LAI L_e measured using optical instruments but reduced by a small fraction to remove the contribution of woody material to the total above-ground plant area. Daily green FPAR of the sites was correlated with the Simple Ratio (SR) and the Normalized Difference Vegetation Index (NDVI) obtained from Landsat 5 TM images. The correlation was better in late-spring than in mid-summer, suggesting spring images are more useful for obtaining FPAR of the overstory. Comparisons of the present with previous results suggest that the background (understory and ground cover) signal and the tree crown shadows are important in satellite measurements of FPAR