Structural complexity biases vegetation greenness measures
2023
Zeng, Yelu | Hao, Dalei | Park, Taejin | Zhu, Peng | Huete, Alfredo | Myneni, Ranga | Knyazikhin, Yuri | Qi, Jianbo | Nemani, Ramakrishna | Li, Fa | Huang, Jianxi | Gao, Yongyuan | Li, Baoguo | Ji, Fujiang | Köhler, Philipp | Frankenberg, Christian | Berry, Joseph | Chen, Min | China Agricultural University (CAU) | University of Wisconsin-Madison | Pacific Northwest National Laboratory (PNNL) | NASA Ames Research Center (ARC) | Bay Area Environmental Research Institute (BAER) | The University of Hong Kong (HKU) | University of Technology Sydney (UTS) | Boston University [Boston] (BU) | Centre d'études spatiales de la biosphère (CESBIO) ; Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3) ; Communauté d'universités et établissements de Toulouse (Comue de Toulouse)-Communauté d'universités et établissements de Toulouse (Comue de Toulouse)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)
International audience
显示更多 [+] 显示较少 [-]英语. Vegetation 'greenness' characterized by spectral vegetation indices (VIs) is an integrative measure of vegetation leaf abundance, biochemical properties and pigment composition. Surprisingly, satellite observations reveal that several major VIs over the US Corn Belt are higher than those over the Amazon rainforest, despite the forests having a greater leaf area. This contradicting pattern underscores the pressing need to understand the underlying drivers and their impacts to prevent misinterpretations. Here we show that macroscale shadows cast by complex forest structures result in lower greenness measures compared with those cast by structurally simple and homogeneous crops. The shadow-induced contradictory pattern of VIs is inevitable because most Earth-observing satellites do not view the Earth in the solar direction and thus view shadows due to the sun-sensor geometry. The shadow impacts have important implications for the interpretation of VIs and solar-induced chlorophyll fluorescence as measures of global vegetation changes. For instance, a land-conversion process from forests to crops over the Amazon shows notable increases in VIs despite a decrease in leaf area. Our findings highlight the importance of considering shadow impacts to accurately interpret remotely sensed VIs and solar-induced chlorophyll fluorescence for assessing global vegetation and its changes.
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