Investigating holm oak forest dieback through a multiscale approach: physiological measurements and visual assessment data with remote sensing

Drought and heat waves profoundly affect the func@oning of Mediterranean forests. As Quercus ilex dieback has been observed in many Mediterranean stands, it is crucial to develop effec@ve tools for studying this phenomenon across mul@ple scales. Our study was conducted over three years (2019-2021) in declining (D) and non-declining (ND) Q. ilex stands in southern Tuscany, assessing physiological traits such as gas exchange and water rela@ons. Dendrochronological and tree-ring δ13C analyses were combined to inves@gate the effects of previous droughts on tree growth and WUE. Finally, the rela@onships between physiological indicators and satellite-derived Normalized Difference Vegeta@on and Water Indices (NDVI and NDWI) were analyzed. Seasonality had a strong effect on physiological traits, with the main stress occurring during the summer of 2020, as evidenced by the lowest gas exchange values. Furthermore, plants in the ND stand exhibited significantly higher photosynthe@c performances than those in the D stand across mul@ple seasons. The δ13C value at ND significantly exceeded that observed at site D over the last 20 years, revealing more conserva@ve water use. While the high spa@al resolu@on of Sen@nel 2 helped iden@fy the overall Q. ilex decline, these indices were inadequate for analyzing forest stands with patchy dieback. Our study emphasizes the significance of a mul@scale approach that combines in situ physiological measures and satellite data to monitor Q. ilex decline.