Patterns of tree damage and fuels around powerlines in Northern California

Rural powerlines are critical for delivering electricity to communities far from urban areas but present a significant hazard when located in forested regions, where damaged trees can fall onto high voltage lines. In four diverse Mediterranean forest types in Northern California, USA, we evaluated the stand-level conditions driving levels of tree damage (i.e., structural damage, biotic damage, and beetle presence) and fuels (i.e., litter and duff depths, fine woody fuel loads, and coarse woody fuel density) using generalized linear modes in paired plots (adjacent to powerline corridors and within the nearby interior forest) at each site (50 total sites). Tree structural damage was inversely related to stand-level structural diversity in yellow pine plots (p < 0.001) and beetle presence was lower in plots adjacent to powerline corridors in Douglas fir (Pseudotsuga menziesii; p = 0.039) and deciduous oak (Quercus sp.; p = 0.003) plots. Further, deciduous oak plots had relatively high levels of structural (p = 0.013) and biotic damage (p = 0.003). Litter depth decreased (p < 0.001) and duff depth increased (p = 0.015) with greater aridity. Powerline-adjacent plots also had lower levels of fine woody fuels (p = 0.011) and higher levels of coarse woody fuels (p < 0.001) than paired plots in interior forest. For diverse Mediterranean regions such as Northern California, our study demonstrates how risk assessments to prevent outages and powerline-ignited wildfires can be informed by site-level conditions such as dominant vegetation type, forest structure, aridity, and proximity to powerline corridors.