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 x 30 m(2)) 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 FAPAR(1d) > 1, 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.