Effectiveness of the Reconstructed MODIS Typical-Angle Reflectances on Forest Biomass Estimation

Multi-angle optical reflectance measurements such as those from the NASA moderate resolution imaging spectroradiometer (MODIS) are sensitive to forest 3D structures, potentially serving as a useful proxy to estimate forest structural variables such as aboveground biomass (AGB)-a potential theoretically recognized but rarely explored. In this paper, we examined the effectiveness of the reconstructed MODIS typical-angle reflectances-reflectances observed from the hotspot, darkspot, and nadir directions-for estimating forest AGB from both theoretical and practical perspectives. To gain theoretical insights, we first tested the sensitivities of typical-angle reflectances to forest AGB through simulations using the 4-scale bidirectional reflectance distribution function (BRDF) model. We then built statistical models to fit the relationship between MODIS multi-angle observations and field-measured deciduous-broadleaf/mixed-temperate forest AGB at five sites in the eastern USA, assisted by a semivariogram analysis to determine the effect of pixel heterogeneity on the MODIS-AGB relationship. We also determined the effects of terrain and season on the predictive relationships. Our results indicated that multi-angle reflectances with fewer visible shadows yielded better AGB estimates (hotspot: R-2 = 0.63, RMSE = 54.28 Mg/ha; nadir: R-2 = 0.55, RMSE = 59.95 Mg/ha; darkspot: R-2 = 0.46, RMSE = 65.66 Mg/ha) after filtering out the effects of complex terrain and pixel heterogeneity; the MODIS typical-angle reflectances in the NIR band were the most sensitive to forest AGB. We also found strong sensitivities of estimated accuracies to MODIS image acquisition dates or season. Overall, our results suggest that the current practice of leveraging only single-angle MODIS data can be a suboptimal strategy for AGB estimation. We advocate the use of MODIS multi-angle reflectances for optical remote sensing of forest AGB or potentially other ecological applications requiring forest structure information.

Automated summary

In this study, the effectiveness of using multi-angle optical reflectance measurements, specifically those from the MODIS instrument, for estimating forest aboveground biomass (AGB) was examined. The results showed that multi-angle reflectances, particularly in the NIR band, were sensitive to forest AGB after accounting for complex terrain and pixel heterogeneity. The accuracy of the estimates also varied with the acquisition dates or season of the MODIS images. Overall, the study suggested that incorporating multi-angle reflectances in optical remote sensing can improve the estimation of forest AGB.