Canadian Forest Service Publications
Determination of Dispersed and Piled Post-harvest Residues in Coastal Douglas-fir Cutblocks Using Unmanned Aerial Vehicle Imagery and Ground-based Surveys. 2017. Trofymow, J.A.; Gougeon, F.; Kelley, J. Natural Resources Canada, Canadian Forest Service, Canadian Wood Fibre Centre, Pacific Forestry Centre, Victoria, BC. Information Report FI-X-015. 39 p.
Issued by: Pacific Forestry Centre
Catalog ID: 38836
Series: Information Report (CWFC - CFS)
Availability: PDF (download)
After forest harvesting, significant amounts of woody residues are left dispersed on cutblocks, with some subsequently piled and burned. Quantification of these residues is required to estimate carbon budgets, billable waste, harvest efficiency, bioenergy potential, and smoke emissions. Currently, various sample-based field methods are used to assess post-harvest residues. Geospatial methods based on LiDAR and high-resolution imagery allow for a complete measurement of a site, enhancing forest stand inventories. These methods could also improve assessment of post-harvest residues. In this study, ground-based and geospatial methods are compared to estimate amounts of woody residues for two cutblocks in the Northwest Bay area on Vancouver Island, British Columbia. Before and after burn pile construction, high-resolution colour photography was acquired using an unmanned aerial vehicle in 2014–2015. Dispersed waste and residue survey plots, and pile or accumulation plots were georeferenced and measured. Images were analyzed with an improved semi-automated log delineation method that better accounted for log overlaps. Image point clouds, digital surface models, and digital elevation models were created to determine bulk pile volumes, packing ratios were calculated for sample piles, and piled wood volumes derived from image point clouds were compared to values derived from field surveys. Analysis showed field methods used to determine pre-piling stratum areas obtained values that differed by 5–20% compared to those obtained using geospatial methods on orthophotos. Analyses of cutblocks that use high-resolution imagery have greatest value in determining the post-harvest areas of the block, roads, and various strata of interest. The semi-automated method was best at determining dispersed residues under well-lit and good flight conditions on logs without obscuring branch foliage. Under such conditions, dispersed wood volumes derived using this method for an entire cutblock were comparable to those derived by the sample-based waste and residue survey method. The semi-automated log delineation method was not suitable for determining residue volumes in accumulation or piles. Field and geospatial method differences in stratum and block-level total residue volumes were affected by the estimates of stratum areas, with the geospatial values more consistent and preferable. For geospatial methods, pre- to post-piling differences in total residue volumes of the pile and accumulation strata were mostly related to the semi-automated log delineation method’s poor estimates of wood in accumulations. Post-piling total residue volumes were more similar among field and geospatial methods than pre-piling, although volume distribution among strata differed with method. Within a method, post-piling volumes were generally less than pre-piling volumes, even though no wood had been removed from the blocks; however, the change in volume was not significant for most methods because of the high variance in plot density or packing ratios. Nevertheless, results suggest the best method for determining residue volumes will require a combination of geospatial and field measurements. Field measurements are still required to determine the site-specific packing ratios used to calculate piled and accumulation residue volumes, and to determine wood species and grade to calculate wood biomass.
Plain Language Summary
After forest harvest significant amounts of woody residues remain and are important for determining C budgets, bioenergy potential, harvest efficiency and billable waste. On two cutblocks on east Vancouver Island, remote sensing methods were developed using high resolution imagery from unmanned aerial vehicles (UAV) to measure volumes of scattered and piled residues. These were compared to values from a ground-based waste and residue survey (WRS). A semi-automated log delineation (SLD) procedure was best at determining scattered residues under well-lit and good flight conditions on blocks with no obscuring foliage. SLD scattered wood volumes for an entire cutblock were comparable to those from the sample-based WRS method. The SLD method was not suitable for determining residue volumes in large or small piles. Processed image point clouds (IPC), derived from overlapping images, were ideal for determining bulk pile volumes, however calculation of piled residue volumes still requires field measurements for site packing ratios. Orthophoto analysis was the most accurate method to determine the cutblock area covered by scattered and piled residues. Results suggest the best method for determining harvest residue volumes will require a combination of remote-sensing and ground measurements.