Fire causes rapid loss of largest Sierra Nevada trees
Rapid Loss of the Sierra Nevada's Largest Trees Driven by Fire
Computer Vision and Pattern Recognition
Summary
Large trees play an important role in storing carbon and providing homes for wildlife, but no one had measured their numbers and health over a big area. The authors used detailed aerial pictures and a deep learning model to find and map every big tree in the Sierra Nevada region. They then used satellite images over five years to watch how these trees grew or died. The study found that wildfires were the main reason for the death of 10% of all large trees, especially during the extreme fires in 2020 and 2021.
What this means in practice
- •For forest management teams: Map and monitor large tree populations to prioritize wildfire protection and recovery efforts in Sierra Nevada forests using combined aerial and satellite data analysis.
- •For environmental monitoring organizations: Track individual large tree health over time to assess ecosystem impacts of wildfires across large regions with a remote sensing and AI-based workflow.
Authors
Fabien H. Wagner, Dan J. Dixon, Christopher W. Woodall, Mayumi C. M. Hirye, Felipe Saad, Griffin Carter, Ricardo Dalagnol, Lorena Alves, Cynthia Creze, Stephen C. Hagen, Zhihua Liu, Christopher Mihiar, Adugna Mullissa, Le Bienfaiteur Sagang Takougoum, Bryan Shaddy, Yan Yang, Dafeng Zhang, Sassan Saatchi
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.