Radiation absorption and use by humid savanna grassland: assessment using remote sensing and modelling

Abstract The components of the canopy radiation balance in photosynthetically active radiation (PAR), phytomass and leaf area index (LAI) were measured during a complete annual cycle in an annually burned African humid savanna. Directional reflectances measured by a hand-held radiometer were used to compute the canopy normalized difference vegetation index (NDVI). The fraction fAPAR of PAR absorbed by the canopy (APAR) and canopy reflectances were simulated by the scattering from arbitrarily inclined leaves (SAIL) and the radiation interception in row intercropping (RIRI) models. The daily PAR to solar radiation ratio was linearly related to the daily fraction of diffuse solar radiation with an annual value around 0.47. The observed fAPAR was non-linearly related to NDVI. The SAIL model simulated reasonably well directional reflectances but noticeably overestimated fAPAR during most of the growing season. Comparison of simulations performed with the 1D and 3D versions of the RIRI model highlighted the weak influence of the heterogeneous structure of the canopy after fire and of the vertical distribution of dead and green leaves on total fAPAR. Daily fAPAR values simulated by the 3D-RIRI model were linearly related to and 9.8% higher than observed values. For sufficient soil water availability, the net production efficiency ϵn of the savanna grass canopy was 1.92 and 1.28 g DM MJ−1 APAR (where DM stands for dry matter) during early regrowth and mature stage, respectively. In conclusion, the linear relationship between NDVI and fAPAR used in most primary production models operating at large scales may slightly overestimate fAPAR by green leaves for the humid savanna biome. Moreover, the net production efficiency of humid savannas is close to or higher than values reported for the other major natural biomes.

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