The woody residues generated by harvesting plantation forests in New Zealand present significant management challenges, but also opportunities for productive use. Lack of commercially viable demand for the residues results in large quantities left on harvested cutovers and/or stored at landings. The challenges are most acute on steepland sites, typically characterised by aspects such as difficult extraction and longer distance to market. Demand for the residue material is changing; for example as a substitute for fossil fuels. In a market where greenhouse gas emissions are increasingly monetised, woody biomass is becoming regionally important as an energy source for medium-high heat industrial customers.

Forest owners and/or managers’ participation in the developing biomass market requires knowledge of the harvest residues; how much is generated, where it is distributed, and what options are available to manage it. This research aimed to close the divide between producers and potential users and has three main components; (1) using in-field survey, set a current benchmark for residual woody biomass in the steepland harvested cutovers, (2) using geospatial technology, survey residue piles that accumulate at landings and (3) a Delphi survey with industry experts to develop consensus where possible on how best to manage the material. For potential biomass consumers, questions of a regional forest industry’s ability to supply woody biomass are important, with significant capital investments in plant relying on the quality of that information.

Woody residues on harvested cutovers have seen little investigation over the previous two decades. Mechanisation has resulted in significant changes to harvesting systems during this time, with an unknown effect on residue volumes or distribution. For the first part of the study, using a refined Line Intersect Sampling method based on the US Forest Service Down Woody Materials survey method, a total of 17 cutovers were measured across six regions of New Zealand, totalling 185 plots. Plot results for volumes of woody residues >25 mm in diameter ranged from 0 to 580 m3/ha, with a median of 88 m3/ha. Of the 88 m3/ha, 7 m3/ha was older material (‘dead’), the remainder fresh from the harvest (‘sound’). When considering a minimum piece size that might be feasible to extract from the cutover (>0.8 m in length and >10 cm in Small End Diameter), 30 m3/ha was of ‘sound’ quality. Cable-harvested sites carry higher residual woody biomass volumes on the cutover than ground-based harvests (a statistically significant result). Comparing manual versus mechanised felling methods reveals no significant difference in total residue volumes found in the cutover. This study quantifies the opportunity for greater utilisation of large woody residues across New Zealand’s steepland plantation cutovers.

Whole tree harvesting in New Zealand’s steepland plantations also results in relatively large volumes of residues (needles, branches and stem offcuts) accumulating at the landing. This by-product of the log-making process is typically piled near the landing and represents a more readily available opportunity for utilisation than cutover residues. Measurement of residue piles is uncommon, however. This second part of the study provides a contemporary benchmark for volumes of residue piles and new remote-sensing methodologies for collecting volume information. UAV imagery collected from 16 harvested sites was used to compute digital surface models of landing residue piles. Through manual interpolation of the terrain obscured by the residue piles, the average bulk residue volume found was 0.23 m³ per tonne of logs harvested, or 170 m³ per hectare harvested; with results varying from 40 to 350 m³/ha. Piles were also assessed for depth using the surface models, as depth is proposed as a key indicator of a pile’s self-combustion risk. The average maximum pile depth was 2.6 m: the majority of piles achieving accepted best practice in New Zealand harvesting operations (maximum 3 m). The new methodology allows safe and low-cost data capture and could become an increasingly regular part of forest measurement where forest owners need to make informed decisions about the management of the woody biomass resource, as a product or as an environmental hazard.

A further refinement of the photogrammetry method was proposed and tested in a case study of one steepland landing. The refined method enabled an improved render of the terrain surface under the pile by capturing georeferenced data both pre- and post-harvest. Operational benefits from the procedure include being able to accurately inspect pile depth against best practice guidelines and direct pile rehabilitation efforts with more accuracy. Whilst the improved methodology eliminated an estimation procedure, it also required a second visit to the site and more photogrammetry processing, therefore required more resources. The methodology has been demonstrated as a potential tool for operational foresters to support decision making for residue pile management.

Plantation forests in steepland areas have often replaced pastoral farming due to underlying natural and induced soil erosion processes. Under these circumstances conversion to plantation forestry is intended to provide both improved economic returns as well as longer term land stability. Challenges that are inherent to the terrain are transferred with land use change. In addition to adhering to the respective Regional Plans, the forest industry has sought to manage these environmental challenges by implementing Best Management Practices published by the New Zealand Forest Owners Association. Gaining consensus and support for new practice standards can be challenging; although many participants may already be demonstrating suitable standards with the experience gained from exposure to ongoing operations. Focussing on residual woody biomass in remote steepland forests in this third part of the study, a Delphi survey was completed with twenty forest industry experts across New Zealand. The Delphi process was successful in allowing the participants to put forward opinions, unencumbered by affiliations or personal conflict. The outcome improves our understanding of specific practices and knowledge that could inform the advancement of Best Management Practices and how residual woody biomass might be brought to the market (e.g., where it is not currently). The results of the Delphi indicate an intent to participate in the developing biomass market with harvest residues and also a widespread knowledge of practices and processes that lead to woody residues posing risks to operations, the wider environment and communities. Delphi participants identified practices, specific to scenarios near waterways or at the landing, such as retrieving reside piles off slopes steeper than 15-20°, whether on natural ground or engineered fill which give a measurable target for future operations. There was a strong preference for site-specific management of residues, rather than ‘one-size-fits-all’ approaches. An example of this philosophy lies in the management of mature crop trees within waterway margins. Leaving standing trees exposes the riparian margin to the risk of windthrow, while removal of the trees risks soil disturbance and unintentional loading of the waterway with felled woody residues. In the Delphi panel’s opinion, limiting the number of management options available could result in adverse environmental, or economic outcomes.

Woody harvest residues can provide a new income stream for steepland forest owners and new supply chain participants. Their productive use also promises to drive better environmental outcomes for erodible steepland forests. With improved knowledge of the production of residues in New Zealand plantation forests, inventories and forward projections can be made by forest owners to provide security of supply for new biomass customers investing in biomass-specific equipment. Without supply security, long-term investments in high capital equipment are tenuous. Where the market cannot reach, residue management will continue to innovate to meet the environmental and social expectations of the time. This thesis provides answers and direction for both market situations.