Post-Fire Vegetation Cover Recovery in Coastal Pine Forests of the Curonian Spit Based on Field NDVI Measurements, Sentinel-2 Data and UAV Imagery

Assessment of vegetation recovery after large-scale forest fires is important for monitoring the resilience of coastal ecosystems. The Curonian Spit is a model area for such analysis, as pine forest sites affected by major fires in different years occur within a single coastal sandy landform. This enables comparison of vegetation cover at different stages of post-fire recovery. This study aims to assess vegetation cover in post-fire sites of coastal pine forests of the Curonian Spit by comparing field NDVI measurements, Sentinel-2 NDVI, and structural interpretation of UAV imagery.

The study was carried out in April 2026 at three post-fire sites differing in time since fire and affected area: Smiltynė (1995 fire, ~60 ha; 31 years post-fire), Alksnynė (2006 fire, ~236 ha; 20 years post-fire), and Juodkrantė (2014 fire, ~120 ha; 12 years post-fire). In total, 122 georeferenced field NDVI points obtained using a handheld GreenSeeker sensor were used: 32 at Smiltynė, 50 at Alksnynė, and 40 at Juodkrantė. Sentinel-2 NDVI values for April 2026 were extracted for the same coordinates.

Preliminary results show that post-fire recovery does not follow a simple linear trajectory. Mean Sentinel-2 NDVI was highest at Smiltynė (0.57), remained relatively high at Juodkrantė (0.56), and was lowest at Alksnynė (0.47). Field GreenSeeker NDVI showed a different pattern: Juodkrantė (0.41), Smiltynė (0.34), and Alksnynė (0.28). This difference indicates that Sentinel-2 NDVI reflects the integrated signal of a 10-m forest pixel, including the tree canopy, whereas GreenSeeker NDVI is more sensitive to local lower vegetation and ground cover. Since the measurements were conducted in April, the obtained NDVI values reflect the early phase of seasonal vegetation activity rather than its maximum. Integrating field measurements, Sentinel-2 data, and UAV imagery allows a more accurate assessment of spatially heterogeneous recovery of coastal pine forests on sandy substrates after fires.