Assessing Ash (Fraxinus excelsior L.) Dieback Dynamics in the Białowieża Forest, Poland, Using Bi-Temporal High-Resolution Remote Sensing Data

The ash dieback epidemic, caused by the fungus Hymenoscyphus fraxineus, has been a significant issue in Europe for over 20 years, severely affecting ash tree populations (Fraxinus excelsior L.). In the Białowieża Forest, ash trees now represent less than 1% of the species composition, with a sharp decline observed over the past several decades. This study aims to map the dynamics of ash mortality in the Białowieża Forest and assess the influence of habitat and stand factors on the severity of mortality. We utilized bi-temporal high-resolution remote sensing data from 2015 to 2019 to track the decline of ash trees and to identify factors affecting mortality. The analysis employed a combination of Boosted Regression Trees (BRTs) and hotspot analyses. Our results show that between 2015 and 2019, 29% of the living ash trees in the canopy layer of the forest died. The findings indicate that ash mortality was most pronounced in stands with a high proportion of ash trees, particularly where dead deciduous trees were already present. Intensive dieback of other deciduous trees was also noted in these stands. This study provides valuable insights into the factors influencing ash mortality dynamics and demonstrates the potential of remote sensing for large-scale monitoring of tree health. The results have important implications for forest management and conservation, offering baseline data that can inform strategies to manage ash dieback and guide targeted interventions in affected forest areas.