Practical Tools for Fire Risk Assessment in Mediterranean Open Woodlands Using Handheld Laser Scanning and Unmanned Aerial Vehicle Photogrammetry

Abstract Mediterranean forests are becoming increasingly vulnerable to wildfires. Given this, there is an urgent need for accurate, high-resolution data on canopy fuel characteristics to facilitate fire behavior modeling and guide risk mitigation efforts. This study assesses the performance of handheld mobile laser scanning (HMLS) and low-cost unmanned aerial vehicle digital aerial photogrammetry (UAV-DAP) in estimating key canopy fuel parameters and stand structural attributes, including canopy base height, stand height, canopy fuel load, canopy bulk density, and canopy cover. Field data were collected from 12 circular plots in open woodlands ( dehesas ) dominated by Quercus suber or Quercus ilex in the southwest of the Iberian Peninsula. Each plot was surveyed using traditional forest inventory, HMLS with a GeoSLAM ZEB Horizon scanner, and UAV DAP with RGB imagery processed using structure-from-motion techniques. In both cases, the resulting point clouds were processed using LiDAR360 software to extract canopy fuel parameters. HMLS yielded the most accurate estimates for most variables, with relative root mean square error (RMSE%) values ranging from 4% for canopy fuel load to 23.3% for canopy base height. UAV DAP showed a wider range of RMSE% values, from 5.9% for stand height to 41.2% for canopy bulk density, and was notably less effective for variables that require the characterization of subcanopy structure, such as canopy fuel load and canopy bulk density. These findings suggest that HMLS is a reliable and efficient tool for under-canopy data acquisition for small-scale forest managers to assess fire risk and provide reference data for high resolution fuel mapping, while UAV DAP may be a viable option for managers operating in areas that are difficult to access or require low-cost inventories.