UC Irvine

Abstract Convective storms with strong downdrafts create windthrows: snapped and uprooted trees that locally alter the structure, composition, and carbon balance of forests. Comparing Landsat imagery from subsequent years, we documented temporal and spatial variation in the occurrence of large (≥30 ha) windthrows across the Amazon basin from 1985 to 2020. Over 33 individual years, we detected 3179 large windthrows. Windthrow density was greatest in the central and western Amazon regions, with ∼33% of all events occurring in ∼3% of the monitored area. Return intervals for large windthrows in the same location of these “hotspot” regions are centuries to millennia, while over the rest of the Amazon they are >10,000 years. Our data demonstrate a nearly 4‐fold increase in windthrow number and affected area between 1985 (78 windthrows and 6,900 ha) and 2020 (264 events and 32,170 ha), with more events of >500 ha size since 1990. Such extremely large events (>500 ha up to 2,543 ha) are responsible for interannual variation in the overall median (84 ± 5.2 ha; ±95% CI) and mean (147 ± 13 ha) windthrow area, but we did not find significant temporal trends in the size distribution of windthrows with time. Our results document increased damage from convective storms over the past 40 years in the Amazon, filling a gap in temporal records for tropical regions. Our publicly accessible large windthrow database provides a valuable tool for exploring dynamic conditions leading to damaging storms and their ecological impact on Amazon forests. Plain Language Summary Windthrows in the Amazon, the uprooting or breaking of trees by winds, are produced by downdrafts associated with strong convective storms. They are a major natural disturbance that can influence the structure, carbon balance and species composition of forests worldwide. Damage by wind can range from single trees to large forest areas, but only windthrow of large enough size can be detected from satellite imagery. We mapped large windthrows (≥30 ha) occurring between 1985 and 2020 in the Amazon to assess possible trends in their spatial and temporal variability. Large windthrows were more common in the central and western Amazon, with ∼33% of all detected events occurring in only ∼3% of the monitored area. Between 1985 and 2020, the number and total area of forests impacted by large‐scale windthrows increased about fourfold, suggesting an increase in the number of intense storms that can topple trees although there was no obvious change over time in their spatial or size distribution. We did observe interannual variations in the overall mean size (147 ± 13 ha; 95% CI) reflecting the sporadic contributions from infrequent but very large events (up to 2,543 ha). Key Points Windthrows caused by downdrafts associated with convective storms are a natural disturbance that can shape Amazon forests A fourfold increase in windthrow frequency from 1985 to 2020 documents an increase in the frequency of damaging convective storms Roughly 33% of detected events occurred in only 3% of the Amazon, highlighting regional variation on the impact of large‐scale windthrows