Rapid Loss of the Sierra Nevada's Largest Trees Driven by Fire

A deep learning model called U-Net-ID mapped 6,515,705 large trees across roughly 78.7% of the Sierra Nevada Floristic Province using 2020 sub-meter aerial imagery, according to a study published on arXiv (2609.17925v1). Wildfires, estimated from CAL FIRE fire perimeters, killed 10% of all large trees in the region, with mortality concentrated in the extreme 2020-2021 fire seasons. The model, trained on more than 3.3 million synthetic tree crowns, achieved a median Intersection over Union of 0.602 against an independent dataset of 20,273 crowns.

arXiv:2609.17925v1 Announce Type: new Abstract: Large trees disproportionately contribute to biomass storage, habitat structure, and ecosystem functioning. However, their distribution and health dynamics remain poorly quantified at a regional scale. Here, a deep learning model U-Net-ID and canopy height models derived from sub-meter aerial imagery from 2020 were used to delineate all individual trees with crown area $\geq$ 100 m$^2$ across the Sierra Nevada Floristic Province. The model was trained using more than 3.3 million synthetic tree crowns and achieved a median Intersection over Union IoU of 0.602 when validated against an independent dataset of 20,273 crowns. A total of 6,515,705 large trees were mapped, occurring across approximately 78.7% of the Sierra Nevada Floristic Province. The spatial distribution of large trees showed associations with elevation, temperature, and precipitation. Using Sentinel-2 time series from 2020 to 2025, tree health dynamics were characterized by extracting spectral trajectories for each crown and applying BFAST breakpoint detection algorithm combined with a disturbance classification framework to identify mortality, disturbance, and recovery trajectories of individual trees. Wildfires, estimated from CAL FIRE fire perimeters, were identified as the dominant driver of large-tree mortality, killing 10% of all large trees in the Sierra Nevada, with mortality strongly concentrated during the extreme 2020-2021 fire seasons.