Black spruce forests dominate the land cover in interior Alaska. In this region, satellite remote sensing of ecosys- tem productivity is useful for evaluating black spruce for- est status and recovery processes. The fraction of absorbed photosynthetically active radiation (FAPAR) by green leaves is a particularly important input parameter for ecosystem models. FAPAR1d is computed as the ratio of absorbed photosynthetically active radiation (APAR3d) to the incident photosynthetically active radiation at the horizontal plane above the canopy (PAR1d, FAPAR1d = APAR3d/PAR1d). The parameter FAPAR1d is scale dependent and can be larger than 1 as a result of laterally incident PAR. We investigated the dependence of FAPAR1d on spatial scale in an open- canopy black spruce forest in interior Alaska. We compared FAPAR1d with FAPAR3d (= APAR3d /PAR3d ), the latter of which considers incident PAR as actinic flux (spheradiance) (PAR3d). Our results showed the following: 1) landscape scale FAPAR3d(30 × 30 m2) was always larger (0.39–0.43) than FAPAR1d (0.19–0.27) due to the landscape heterogeneity and in- cident PAR regime, and 2) at the individual tree scale, FAPAR1d was highly variable, with 34% (day of year [DOY] 180) to 52% (DOY 258) of FAPAR1d > 1, whereas FAPAR3d varied across a much narrower range (0.2–0.5). The spatial-scale dependence of the ratio of PAR3d to PAR1d converged at the pixel size larger than 5 m. Thus, a 5-m or coarser resolution was necessary to ignore the lateral PAR effect in the open-canopy black spruce forest.
