Minimal Tree Mortality Occurred in Southwestern USA Sky Islands During an Extended Drought

Persistent drought since the early 2000s has increased forest vulnerability to disturbances like insect outbreaks, wildfire, and hydraulic failure across the southwestern United States (US). Sky islands, characterized by geographic and climatic isolation and rich biodiversity, face heightened risks due to limited connectivity, restricted migration potential, and upslope range contraction of high-elevation species. This study quantified tree mortality rates across southwestern US sky island ecoregions, focusing on gradual disturbances (e.g., drought, insect colonization) rather than abrupt events (fire, thinning), and developed a 30-m predictive map of cumulative mortality (1997–2023). We manually interpreted high-resolution aerial imagery from 1,076 forest stands to quantify mortality rates, then combined these data with Landsat time series and Random Forest modeling to create a 30-m predictive mortality map. Changes in Landsat shortwave infrared (B5; 1.55–1.75 μm) were the top mortality predictor. Annualized mortality rates (0.4%) fell within typical western US background levels. Low mortality (<30%; ~1%/year) occurred in 94% of the area, while higher mortality (>80%; ~2.5%/year) was concentrated in fragmented high-elevation patches. The model had low-moderate accuracy (R²=0.28, RMSE=18%), highlighting challenges in capturing gradual disturbance effects using Landsat imagery. This study emphasizes the need for continued monitoring of background tree mortality rates throughout southwestern US sky islands and provides insights for monitoring tree mortality in other vulnerable, remote forest ecosystems. The files included in this archive, combined with publicly available data sources described in the paper and statistical code, can be used to replicate our analyses, and may be useful for related studies for Southwest USA sky islands and elsewhere. Files compressed in .7z format using 7zip v. 19.00 on Windows v. 10.0.26100. Approximate download size is 250 MB. For more information see "README.md" file