Forest stress analysis due to heatwaves in Araucarias State Park, Brazil, using remote sensing

Forest decline has been increasingly observed across different biomes in Brazil and worldwide, with climate change identified as one of its main drivers. Among climatic factors, heatwaves often associated with El Niño events stand out for inducing physiological stress and degradation in plants. Understanding the causes of such stress and forest responses to these events is essential for developing mitigation strategies. This study aimed to characterize the occurrence and severity of heatwaves between 1998 and 2023, to relate climatic data to the Normalized Difference Vegetation Index (NDVI), and to assess stress in native vegetation within the Araucarias State Park, Paraná, Brazil. Climatic data were obtained from the Paraná Meteorological System (SIMEPAR), and satellite images from LANDSAT 5, 7, 8, and 9 were analyzed. Significant seasonal variations were detected, with NDVI values ≤ 0.279 indicating possible water stress due to heatwaves. Winter and spring showed the highest incidence of heatwaves, with 1999–2011 identified as the driest and hottest period. El Niño events intensified heat stress by approximately 60%, but an increasing trend in heatwave severity was observed independently, driven by global warming. NDVI data were statistically correlated with climatic stress indicators, allowing identification of the most affected seasons and the delineation of stressed versus healthy forest conditions. NDVI proved to be a valuable methodological tool for assessing heatwave impacts when associated with the Forest Stress Index (FBS). For the Araucarias State Park, NDVI ≤0.279 marked the threshold distinguishing stressed forests. Continuous monitoring is recommended to anticipate potential forest decline or even large-scale mortality. • Seasonal Stress: Winter and spring see the most severe heatwaves; 1999–2011 was the hottest and driest period recorded. • Climate Drivers: El Niño spiked heat stress by 60%, but global warming is driving severity independently of ENSO events. • NDVI Threshold: An NDVI below 0.279 was set as the critical threshold for identifying water stress in the park's forests. • Forest Health: Araucarias show persistent climatic stress and low physiological growth, with NDVI values around 0.30. • Remote Sensing: NDVI and FBS are vital tools to monitor vegetation behavior and predict large-scale forest mortality.