Monitoring downed coarse woody debris through appearance of canopy gaps in urban boreal forests with bitemporal ALS data

The study introduces a method for detailed mapping of downed coarse woody debris (CWD). Bitemporal airborne laser scanning (ALS) data and existing allometric equations were utilized to map the attributes and location of downed CWD. No training or calibration data were used in the process. The CWD locations were determined through canopy gaps detected with canopy height models (CHM) from 2009 and 2012. The attributes of formed CWD were derived through an individual tree detection (ITD) technique. Within the three year interval, 97.8% of all new downed trees were detected and their species group (conifer/deciduous) was classified correctly in 89.0% of the cases (Kappa 0.76). Tree-level CWD diameter and volume were predicted with an RMSE of 8.7 cm and 0.5 m3 respectively. The introduced method allows detailed mapping of CWD and can be utilized in forest planning and operational tasks in urban forest areas.