Using UAV (drone) technology to measure the dynamics of <i>Abies religiosa</i> (Kunth) Schltdl. &amp; Cham. decline in the Monarch Butterfly Biosphere Reserve, Mexico, and its relationship to climate change

This study examines the dynamics of Abies religiosa (Kunth) Schltdl. &amp; Cham. decline in the Monarch Butterfly Biosphere Reserve (MBBR) of Mexico, and its relationship with climate change, using remote sensing data acquired by unmanned aerial vehicles (UAVs), also named drones. Carried out between April and October 2023 over a 25 ha area within the MBBR core zone, this study integrated LiDAR and multispectral imagery to detect and geolocate individual trees, assigning vitality classes based on the normalized difference vegetation index (NDVI). We also introduced a new metric, the Monthly Dryness Index (MDI), which combines temperature and precipitation to capture short-term fluctuations in dryness conditions and evaluate their influence on tree vigour. The results indicate a statistically significant decline in NDVI values in June 2023, coinciding with an increased MDI of 0.47 together with a +2.7°C heat wave anomaly, suggesting an extreme drought event. A transition in tree vigour was observed, with the proportion of healthy individuals decreasing from 37% in May to 18% in June, and weakened trees increasing from 56% to 72%, indicating a critical shift in stand health. Regression models showed that MDI effectively explained variations in the number of trees by vitality class, with consistently high coefficients of determination ( R² ≥ 0.94) across vitality classes (healty, weakened, highly weakened, and dead trees), highlighting the species’ vulnerability to heat waves during the dry season.