East Amazon forest understory temperatures reached a record high in 2023-2024

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 understory temperature distributions were 22% higher compared to 2014–2022 during the 2023 dry season (October) and 25% higher during the 2024 wet season (May). Monthly mean understory air temperature ranged from 21 to 24°C across sites, and site elevation was the main driver of this spatial variability with a lapse rate of -3.8 ± 0.1°C/km. Using locally debiased ERA5-Land 2m 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. We also documented seasonal patterns in canopy buffering effect but did not identify consistent long-term changes. The observed increase by ~ 1°C of understory air temperature in 2023–2024 relative to the 2014–2022 period constitutes a substantial perturbation for forest understory biodiversity, and is discussed in this contribution.