Spatial Patterns of Canopy Decline in Subalpine Eucalypts from Satellite Imagery Analysis

High-elevation subalpine woodlands of south-eastern Australia are characterised by monospecific stands of snow gum (Eucalyptus pauciflora), making this species a keystone in its ecosystem. Although a widespread insect-induced dieback phenomenon associated with the native longicorn beetle (Phoracantha mastersi) currently affects high-elevation snow gum stands throughout the Australian Alps, the spatiotemporal dynamics of this dieback are poorly documented. Using a time series of satellite-derived canopy condition from Sentinel-2 imagery and clustering analysis, we mapped insect-induced dieback with high accuracy (F1-score = 0.82; precision = 0.93; recall = 0.74). We estimated that 11.3% of the subalpine woodlands surrounding Mt Kosciuszko declined during this period, with a higher rate in long-unburnt areas (21.0%) compared to areas previously affected by wildfires between 2003–2006 (6.7%). Greater canopy height, closer proximity to previous dieback-affected areas, and lower elevation are significant predictors of the occurrence of severe dieback, indicating that these stands are more likely to be affected. While post-fire recovery is evident across much of the landscape, long-unburnt stands are facing insect-induced dieback that endangers old-growth subalpine woodlands and their associated ecosystems.