Résumé
The Amazon forest was struck by an El Niño event with record heat and drought during the second semester 2023, and again in 2024. However, it remains unclear whether this extreme event impacted the forest understory because long-term in situ measurements are rare. Here, we present a time series of understory air temperatures over the 2014-2024 period based on a compilation of 114 ground-based measurements in the Eastern Amazon. We found that understory temperatures increased by up to 1.6 °C in 2023-2024 relative to the 2014-2022 reference period. Understory temperatures increased by 0.04 ± 0.03 °C/yr over 2014-2024, but this increasing trend was mostly driven by the 2023-2024 extreme event. For instance, hourly distributions were significantly warmer than the reference period, reaching + 25% during the 2024 wet season. Monthly mean understory air temperature ranged from 21 to 24 °C across sites, elevation being the main driver of this spatial variability. Using locally debiased ERA5-Land 2 m air temperature, we showed that forest canopies buffered monthly mean temperatures by -2.0 ± 0.5 °C and maximum temperatures by -4.6 ± 1 °C, albeit did not prevent the 2023-2024 anomaly. The observed increase by ~ 1 °C of understory air temperature in 2023-2024 relative to the 2014-2022 period constitutes a substantial perturbation of understory species' theoretical thermal niche, particularly threatening ectotherms.