Abstract
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.
Highlights
The Amazon rainforest is a species-rich biome and is a major component of theEarth’s ecosystem [1]
The results of this study suggests that the vegetation optical depth (VOD) can more accurately capture the general spatiotemporal evolution of canopy changes affected by fires, natural conditions, and forest fragmentation associated with fire relative to optical indices, especially where the fire event is unknown and the precipitation anomalies are not very significant
This study demonstrated the potential of a passive microwave-based vegetation optical depth (VOD) to detect the vegetation change patterns caused by fire over southern
Summary
The Amazon rainforest is a species-rich biome and is a major component of theEarth’s ecosystem [1]. 2021, 13, 2238 lead to significant vegetation changes and an increase in carbon and aerosol emissions, which greatly impacts the carbon cycle and climate conditions over long periods at both regional and global scales [1,6,7]. The synergy between extreme drought and human activities creates the conditions for large-scale forest fires in the Amazon [15,16], which produces a dual threat of fires and climate change in this century [1]. A better understanding of the vegetation dynamics caused by fires during the fire season would help us to improve our regional and global assessments of carbon emissions, carbon cycle variability, and associated climate feedbacks
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