Spatial Scale and Landscape Heterogeneity Effects on FAPAR in an Open-Canopy Black Spruce Forest in Interior Alaska

Black spruce forests dominate the land cover in interior Alaska. In this region, satellite remote sensing of ecosystem productivity is useful for evaluating black spruce forest status and recovery processes. The fraction of absorbed photosynthetically active radiation (FAPAR) by green leaves is a particularly important input parameter for ecosystem models. FAPAR_1d is computed as the ratio of absorbed photosynthetically active radiation (APAR_3d) to the incident photosynthetically active radiation at the horizontal plane above the canopy (PAR_1d, FAPAR_1d = APAR_3d/PAR_1d). The parameter FAPAR_1d is scale dependent and can be larger than 1 as a result of laterally incident PAR. We investigated the dependence of FAPAR_1d on spatial scale in an open-canopy black spruce forest in interior Alaska. We compared FAPAR_1d with FAPAR_3d( = APAR_3d/PAR_3d), the latter of which considers incident PAR as actinic flux (spheradiance) (PAR_3d). Our results showed the following: 1) landscape scale FAPAR_3d(30×30 m²) was always larger (0.39-0.43) than FAPAR_1d (0.19-0.27) due to the landscape heterogeneity and incident PAR regime, and 2) at the individual tree scale, FAPAR_1d was highly variable, with 34% (day of year [DOY] 180) to 52% (DOY 258) of , whereas FAPAR_3d varied across a much narrower range (0.2-0.5). The spatial-scale dependence of the ratio of PAR_3d to PAR_1d 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.