Thermal stress in degraded forests in the Brazilian Amazon Arc of Deforestation

2 36 Abstract37 Understanding thermal stress in tropical forests has taken on new urgency in light of38 accelerating climate change and expansion of deforestation and forest degradation. Degraded39 tropical forests in particular may be approaching critical temperature thresholds even more40 rapidly than intact forests, with implications for tree survival and ecosystem recovery. We41 investigate thermal stress in degraded tropical forests within the Brazilian Amazon Arc of42 Deforestation. Using land surface temperature data from the ECOsystem Spaceborne Thermal43 Radiometer Experiment on the international Space Station (ECOSTRESS), we compared canopy44 temperatures of intact, selectively logged, and burned forests in Feliz Natal, Mato Grosso, Brazil.45 Even after 1-5 years of recovery, upper canopy temperatures in previously burned forests were46 4.1% higher (mean = 36.5°C) and 50.9% more variable compared to intact and logged forests,47 which showed remarkably similar temperature distributions (means of 34.9°C and 35.1°C,48 respectively). Modeled leaf temperature distributions based on canopy temperature49 measurements from the warmest day in a 2-year dry season record indicated that 87% of leaves50 in the warmest burned forest patches exceeded the temperature threshold where respiration51 surpasses photosynthesis, compared to approximately 72-74% in intact and logged forests,52 respectively, in the same time period. After controlling for environmental factors, burned forests53 were predicted to be 3.1°C warmer on average than intact forests at low canopy heights (10m),54 with this effect decreasing to ~2°C at 40m height. Critically, the probability of leaves exceeding55 temperatures that cause permanent leaf damage was five times higher in burned forests, with56 implications for the future of degraded forest regeneration in water-limited regions of the57 Amazon basin. In particular, the combination of higher mean temperatures, greater temperature58 variability, and more frequent exposure to damaging thermal thresholds implies that burned59 tropical forests will experience substantially higher mortality rates and slower biomass recovery60 compared to intact forests, especially in water-limited regions where trees cannot rely on61 evaporative cooling to moderate canopy temperatures.