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Article

Forest Canopy Changes in the Southern Amazon during the 2019 Fire Season Based on Passive Microwave and Optical Satellite Observations

1
School of Geographical Sciences, Nanjing University of Information Science and Technology, Nanjing 210044, China
2
Earth Observation and Geoinformatics Division, National Institute for Space Research-INPE, São José dos Campos 12227-010, Brazil
*
Author to whom correspondence should be addressed.
Academic Editors: Luis A. Ruiz, Ioannis Gitas, Arnaud Mialon, Nemesio Rodriguez-Fernandez, Mariette Vreugdenhil, Tianjie Zhao and Roberto Fernández Morán
Remote Sens. 2021, 13(12), 2238; https://doi.org/10.3390/rs13122238
Received: 12 April 2021 / Revised: 22 May 2021 / Accepted: 4 June 2021 / Published: 8 June 2021
(This article belongs to the Special Issue Vegetation Optical Depth: Remote Sensing Retrievals and Applications)
Canopy dynamics associated with fires in tropical forests play a critical role in the terrestrial carbon cycle and climate feedbacks. The aim of this study was to characterize forest canopy dynamics in the southern Amazon during the 2019 fire season (July–October) using passive microwave-based vegetation optical depth (VOD) and three optical-based indices. First, we found that precipitation during July–October 2019 was close to the climatic means, suggesting that there were no extreme hydrometeorological events in 2019 and that fire was the dominant factor causing forest canopy anomalies. Second, based on the active fire product (MCD14ML), the total number of active fires over each grid cell was calculated for each month. The number of active fires during the fire season in 2019 was above average, particularly in August and September. Third, we compared the anomalies of VOD and optical-based indices (the normalized difference vegetation index (NDVI), the enhanced vegetation index (EVI), and the normalized burn ratio (NBR)) against the spatiotemporal distribution of fires during July–October 2019. Spatially, the location with a concentrated distribution of significant negative VOD anomalies was matched with the grid cells with fire activities, whereas the concentrated distribution of strong negative anomalies in optical-based indices were found in both burned and unburned grid cells. When we focused on the temporal pattern over the grid cells with fire activity, the VOD and the optical-based indices behaved similarly from July to October 2019, i.e., the magnitude of negative anomalies became stronger with increased fire occurrences and reached the peak of negative anomalies in September before decreasing in October. A discrepancy was observed in the magnitude of negative anomalies of the optical-based indices and the VOD; the magnitude of optical-based indices was larger than the VOD in August–September and recovered much faster than the VOD over the grid cells with relatively low fire activity in October. The most likely reason for their different responses is that the VOD represents the dynamics of both photosynthetic (leaf) and nonphotosynthetic (branches) biomass, whereas optical-based indices are only sensitive to photosynthetic (leaf) active biomass, which recovers faster. Our results demonstrate that VOD can detect the spatiotemporal of canopy dynamics caused by fire and postfire canopy biomass recovery over high-biomass rainforest, which enables more comprehensive assessments, together with classic optical remote sensing approaches. View Full-Text
Keywords: the Amazon; fire; vegetation optical depth; optical indices; canopy changes the Amazon; fire; vegetation optical depth; optical indices; canopy changes
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MDPI and ACS Style

Zhang, H.; Hagan, D.F.T.; Dalagnol, R.; Liu, Y. Forest Canopy Changes in the Southern Amazon during the 2019 Fire Season Based on Passive Microwave and Optical Satellite Observations. Remote Sens. 2021, 13, 2238. https://doi.org/10.3390/rs13122238

AMA Style

Zhang H, Hagan DFT, Dalagnol R, Liu Y. Forest Canopy Changes in the Southern Amazon during the 2019 Fire Season Based on Passive Microwave and Optical Satellite Observations. Remote Sensing. 2021; 13(12):2238. https://doi.org/10.3390/rs13122238

Chicago/Turabian Style

Zhang, Huixian, Daniel F.T. Hagan, Ricardo Dalagnol, and Yi Liu. 2021. "Forest Canopy Changes in the Southern Amazon during the 2019 Fire Season Based on Passive Microwave and Optical Satellite Observations" Remote Sensing 13, no. 12: 2238. https://doi.org/10.3390/rs13122238

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