Characterisation of harvest residues on New Zealand’s steepland plantation cutovers

Background: Timber harvesting in New Zealand’s plantation forests results in relatively large volumes of woody residues being generated. While a proportion of these residues are concentrated at the landings where the trees are processed, the majority of residues are distributed throughout the cutover. Harvest residues present a biomass market opportunity, however managing un-merchantable residues remains essential as the material can present a mass mobilisation risk. Quantifying cutover residues in terms of volume provides an important step for marketing and for improving post-harvest management. Methods: A refined Line Intersect Sampling (LIS) method was used to measure the cutover residues at 17 recently harvested steepland sites. These covered a range of whole tree harvesting systems, silviculture and geographical locations. The harvesting sites varied in size from 2.3 to 41.1 ha, with an average of 11x 60 m LIS transect plots completed at each site. Woody harvest residues >25 mm in diameter were measured. Results: The median volume of woody residues was 88 m3/ha, ranging from 0 m3/ha in an area swept bare, up to 580 m3/ha in an area severely impacted by windthrow prior to harvest. A distribution of volumes by plot showed a positive skew with an interquartile range of 87 m3/ha. Timber that was considered merchantable as a log at the time of harvest, being >10 cm in small end diameter and >4 m in length, accounted for a median of 11 m3/ha. Residues >10 cm in small end diameter and >80 cm in length that could make a viable biomass product, described as ‘binwood’, accounted for a further 19 m3/ha at the median. Cutovers harvested with cable-based systems had greater median total residue volumes than those harvested with ground-based systems (110 m3/ha versus 68 m3/ha) however the felling method employed made no significant difference to total residue volumes. Conclusions: This study provides cutover residue measurements that can be used to improve post-harvest management, as both a substantial opportunity for improved crop utilisation and also for reducing mobilisation risk. It also provides a contemporary benchmark against which to measure change as harvesting technology or methodology develops.