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Evaluating Fire Fuels in Northern California Oak Woodlands using Unoccupied Aerial Systems (UAS)

California’s North Coast hosts a Mediterranean climate with diverse topography, strong climatic gradients of temperature and precipitation, and complex species composition. Unfortunately, historical fire suppression and climate change have led to a significant increase in wildfire frequency and intensity in this unique bioregion. Quantifying forest structure to assess changing wildfire risk factors is critical as California moves beyond the initial stages of short-term disaster recovery and begins to develop mitigation, land management, and resource allocation strategies. Remote sensing offers a valuable method for understanding forest structure and lessening the need for costly field inventory campaigns. Here, we aim to better understand crown fire potential in Northern California oak woodlands by evaluating and estimating forest canopy structure via canopy fuel parameters at plot scale using unoccupied aerial system structure from motion (UAS-SfM) methods. In Summer 2020, images of plot-level forest structure (via Airborne Laser Scanning (ALS) and multispectral UAS) and ground-based measurements of forest fuel metrics were collected at 44 oak woodland plots (20 x 20 m) in Sonoma County, California. Using these field data, we estimated total canopy cover (%), mean canopy height (m), canopy base height (m), canopy bulk density (kg/m 3 ), and tree density (number of trees/plot), from UAS-SfM and ALS data. This study used a tree point cloud-based segmentation method developed by Li et al. (2012) to identify individual trees. trees correctly estimated. trees were compared yielding a was not able to estimate canopy bulk density and canopy base height due to its inability to penetrate dense canopies. For plot-scale canopy cover, mean canopy height and tree density, our research shows that an UAS-SfM approach may be an affordable solution for land managers to accurately quantify important canopy fuel parameters to evaluate fuels management and use as input parameters to fire behavior modeling. This study focuses on 44 plots (22 at Pepperwood Preserve; 22 at Saddle Mountain Open Space Preserve). Pepperwood Preserve hosts a total of fifty 20 x 20 m long-term plots established and maintained by Dr. David Ackerly’s Research Lab out of the University of California, Berkeley. Of the 50 plots, 22 plots were selected for this study based primarily on topographic gradients, ecological heterogeneity, and post-fire burn severity. Unlike Pepperwood, Saddle Mountain Open Space Preserve did not have pre-existing plots. As part of this research, 22 circular plots, with a 11.3 m radius, were established in August of 2020. Plots were selected, using ArcGIS Pro 2.8.1 (Esri 2021, Redlands, California, USA) and field observations, to represent varying forest structure across the preserve’s topographic gradient.