Scale dependence of absorption of photosynthetically active radiation in terrestrial ecosystems

The fraction of photosynthetically active radiation absorbed by plant canopies (fAPAR) is a critical biophysical variable for extrapolating ecophysiological measurements from the leaf to landscape scale. Quantification of fAPAR determinants at the landscape level is needed to improve the interpretation of remote sensing data, to facilitate its use in constraining ecosystem process models, and to improve synoptic-scale links between carbon and nutrient cycles. Most canopy radiation budget studies have focused on light attenuation in plant canopies, with little regard for the importance of the scale-dependent biophysical and structural factors (e.g., leaf and stem optical properties, leaf and stem area, and extent of vegetation structural types) that ultimately determine fAPAR at canopy and landscape scales. Most studies have also assumed that nonphotosynthetic vegetation (litter and stems) contributes little to fAPAR. Using a combined field measurement and radiative transfer modeling approach, we quantified (a) the relative role of the leaf-, canopy-, and landscape-level factors that determine fAPAR in terrestrial ecosystems and (b) the magnitude of PAR absorption by grass litter and woody plant stems. Variability in full spectral-range (400-2500 nm) reflectance/transmittance and PAR (400-700 nm) absorption at the level of individual leaf, stem, and litter samples was quantified for a wide array of broadleaf arborescent and grass species along a 900-km north-south Texas savanna transect. Among woody growth forms, leaf reflectance and transmittance spectra were statistically comparable between populations, species within a genus, and functional types (deciduous vs. evergreen, legume vs. nonlegume). Within the grass life-form, spectral properties were statistically comparable between species and C₃/C₄ physiologies. We found that tissue-level PAR absorption among species, genera, functional groups, and growth forms and between climatologically diverse regions was statistically similar, and for fresh leaves, it represented the most spectrally similar region of the shortwave spectrum. Subsequent modeling analyses indicated that the measured range of leaf, woody stem, and litter optical properties explained only a small proportion of the variance in tree and grass canopy fAPAR. However, the presence of nonphotosynthetic vegetation (e.g., stem and litter) had a significant effect on canopy fAPAR. In trees with a leaf area index (LAI) <3.0, stem surfaces increased canopy fAPAR by 10-40%. Standing grass litter canopies absorbed almost as much PAR as green grass canopies. Modeling the radiation regime in plant canopies should therefore account for the absorption of PAR by nonphotosynthetic plant components. Failure to do so may lead to overestimates of primary production, especially in woodlands, savannas, and shrublands dominated by species with optically thin canopies and in grasslands that accumulate senescent material. Further sensitivity analyses revealed that the extent and LAI of vegetation structural types (trees and grasses) were the dominant controls on savanna landscape-level fAPAR, accounting for 60-80% of the total variation. Variation in leaf-level and all other canopy-level factors contributed individually to explain only a small proportion (<11%) of the variance in landscape fAPAR; however, when considered as a group, they accounted for 20-40% of the variation in landscape fAPAR. These results emphasize the need for more mechanistic analyses of canopy-level radiative transfer, and subsequent carbon flux and trace gas processes, in plant canopies and across landscapes comprising heterogeneous mixtures of plant growth forms and life-forms.