Monitoring holm oak forest dieback combining tree physiological and visual assessment data with remote sensing

In the context of climate change, drought and heatwaves affect Mediterranean ecosystems by deteriorating tree health, leading to species die-off, with consequences for biodiversity and ecosystem functioning. Therefore, it is crucial to identify effective tools for monitoring forest dieback after extreme climatic events. We monitored changes in tree health status induced by severe drought in a holm oak forest located in Southern Tuscany by integrating field measurements and Sentinel 2 data over two years (2020-2021). We analysed the relationships between field-assessed crown defoliation and satellite-derived Normalized Difference Vegetation and Water Indices (NDVI and NDWI) in six holm oak forest plots characterized by different degrees of defoliation and tree mortality. In addition, chlorophyll fluorescence and the DUALEX® chlorophyll index were monitored in two of these plots showing contrasting degrees of defoliation. The relationships between physiological indicators of plant stress and remote sensing indices were evaluated. A strong relationship was observed between NDVI and NDWI indices and crown visual assessment for the plot characterized by the highest holm oak mortality rate. Although the high spatial resolution of Sentinel 2 (10 m) allowed for the detection of general vegetation decline after the extreme summer drought of 2017, these indices were ineffective for the detailed analysis of forest stands characterized by a patchy dieback. Positive correlations among NDVI, NDWI and physiological parameters were observed in plots with lower defoliation in all seasons. In conclusion, Sentinel-2 satellite indices are efficient tools for monitoring holm oak forest status only when tree health conditions are severely and homogeneously impaired. Our study highlights the importance of multiscale analyses that integrate in situ physiological measurements and satellite data to effectively monitor holm oak forest health status and responses to extreme climatic events.