Assessment of the Initial Pine Grove Fire Recovery

Abstract

The Pine Grove Fire of August 2021 burned approximately 65.39 km2 on the southern end of the Fort Belknap Indian Reservation in the Little Rocky Mountains in north-central Montana, an area already affected by legacy disturbances from the Zortman–Landusky gold mines. My study evaluates vegetation recovery from 2021–2023, using imagery from 2020 as a pre-burn baseline,covering a period from 2020-2023.The purpose of my study is to use Landsat-8 imagery processed in Google Earth Engine, I derived annual August composites of the Normalized Difference Vegetation Index (NDVI) and Normalized Difference Moisture Index (NDMI) for the full burn perimeter and three proximity zones: mine-adjacent (Zone 1), transitional (Zone 2), and Pine Grove (Zone 3). Digital elevation models, climate data, and Principal Component Analysis (PCA) were used to relate recovery patterns to topography, moisture, and substrate conditions.Pre-fire NDVI indicated moderately dense vegetation (mean ≈ 0.70). Immediately after the fire, the mean NDVI decreased by about 0.37, with the lowest values in the central and southern sectors. By 2023, NDVI recovered to ≈0.53, and NDMI also increased, indicating substantial but incomplete regrowth under semi-arid conditions and recent drought. Zone-level summaries show that Zone 1 generally maintained the lowest NDVI and NDMI, whereas Zone 3 exhibited the highest values; however, one-way ANOVAs detected no statistically significant differences in mean August NDVI or NDMI among zones over the four years. Recovery was instead strongly structured by topography and moisture: north- and east-facing, low-slope, and drainage positions showed faster regrowth than steep, south-facing slopes. PCA, which explained 99.38% of spectral variance in the first three components, highlighted persistent barren or sparsely vegetated patches near the mine complex and more extensive rangeland and shrub recovery in central and northern portions of the burn. Overall, the results suggest that while legacy mining influences recovery at patch scales, broad late-summer greenness and moisture have largely converged across the burn perimeter. The integrative NDVI–NDMI–PCA framework provides a reproducible approach for long-term post-fire monitoring and can inform restoration planning and tribal land-management decisions on Fort Belknap and in similar semi-arid, disturbance-affected landscapes.