Farm Forestry New Zealand

Report: Best practices for reducing harvest residues and mitigating mobilisation of harvest residues in steepland plantation forests

By Prof. Rien Visser, with the support of: Dr. Raffaele Spinelli and Dr. Kris Brown, July 2018.

Plantation forestry in New Zealand covers approximately 7% of NZ’s total land area with 28 million m3 of timber expected to be harvested in 2018. The majority of timber harvest is from Pinus radiata (‘radiata’) plantation forests grown 25-30 years. Approximately 40% of the plantation estate is on steeper and or erodible terrain, driven mainly by the lower land values for forest conversion but also the benefits of stabilising erosion prone land with trees. The current preferred harvesting practice in New Zealand is larger scale clear-cutting, based on logistical and economic benefits, but also on planting regimes whereby whole catchment areas are planted in a short time-frame.

Although certainly not new, recent larger scale debris flow events with entrained harvesting residues has caused significant damage to downstream land use. This includes inundation of land with sediment and slash, damage to infrastructure including roads, bridges and homes, or the deposition of woody debris on beaches. These have occurred in Northland, Coromandel, Bay of Plenty, Gisborne-East Coast, and Nelson-Marlborough. These events are prompting Regional Councils to review the acceptability of current forestry practices and to identify opportunities for improvement. Specifically, in June 2018 a larger storm event in the Gisborne region resulted in significant flooding, but also large scale deposition and damage from harvest residues in Tologa, coinciding with recent extensive harvesting in the catchment.

The report focusses on the relationship between harvesting, harvest residues, and the best practices that help mitigate debris flow events and or the delivery of harvest residue. This report was commissioned to answer three primary questions:

1. Wood Generated from NZ Harvesting Operations:
   The harvesting of radiata generates relatively large volumes of harvest residues, typically estimated to be 15% of the total volume. For an average stand with a volume of 500 m3/ha, this equates to 75 m3/ha of residues left on site post-harvest. Volume of harvest residues is particularly high in regions with poor market conditions for short and or small diameter logs, difficult terrain that both increases the level of stem breakage during harvesting and makes retrieving all stems from the site more problematic, but also in harvest areas with poor stand quality, such as those affected by prior storm damage (‘wind-throw’). For the higher production stands in the Gisborne region, and given the difficulty of the terrain, a reasonable harvest residue estimate would be 120m3/ha.
   
   As a general rule, when well spread out over the cutover harvest residues are beneficial to the natural environment. This includes recycling of nutrients from litterfall, and the armouring of the soil against surface erosion. However, if harvest residues are (a) left either in the waterway or floodplain, or (b) left on slopes susceptible to mass movement, then the residues can become entrained during a storm event and are at risk of moving off-site. While all harvest debris might be considered a nuisance, large woody debris is responsible for most downstream infrastructure damage and hence a focus for management activities.
   
   Harvesting systems can be categorised as cut-to-length (CTL) or whole-tree harvesting (WTH). In CTL systems the stems are felled and processed in the stand. This means the residues are evenly distributed, and when placed in front of the harvester are pushed into the ground to minimise disturbance. In WTH the trees are felled only, the extracted to a processing area (called a landing) where they are delimbed, topped, and cut into logs. While some stem and branch breakage occurs during felling and extraction, the majority of the harvest residues are at the landing. In steep terrain, cable yarding systems are used for extraction, whereby CTL operations are not really feasible because of safety risk. While for WTH ground based systems there is the opportunity to at least carry some of the residue back into the forest, in steep terrain the majority of residues will be pulled to, and remain at the landing.
   
   Recovering harvest residue for biomass products at the landing may not be profitable, but will reduce the accumulation of harvest residues, improve operational efficiency and increase post- harvest plantable area. Harvest residues can also be burned to the risk of debris movement, but there are both concerns around visual smoke pollution as well as managing smouldering fires. For operations on steep terrain, cut-to-length as a preferred harvest system is not feasible for both safety, but minimising felling breakage through improved felling and avoiding sweeping debris into gully’s during extraction where it cannot be retrieved are practices that minimise harvest residues.
    
2. Site conditions and forest systems and practices that reduce/exacerbate woody debris generation, and the risk of its off-site movement, focusing on hauler operations in steepland country:
   
   Off-site movement of harvest residue can create a hazard to both the natural and built environment, as well as to subsequent forest operations. Three primary mechanisms for the off-site movement of woody debris are identified: (a) harvesting residues left in gullies, waterways or flood zones than can be flushed out during higher rainfall events, (b) large accumulation of harvest residues, such as birdnests around landings, then can collapse under their own weight over time, and (c) harvest residues on the cut-over that are entrained and mobilised by debris flows.
   
   In steep terrain, felling around waterways can result in higher levels of stem breakage and subsequent cable logging harvest residues can be swept into gullies and waterways. Clearing this debris and placing it in a safe location as harvesting progresses is the most logical mitigation method. For whole-tree harvesting systems the majority of harvest residue will be generated at the landing. Post-harvest this debris can be burned or pulled back on to the landing, but this leaves a residual risk during the harvest period. As such, a preferred solution will be the recovery and or utilisation of residues during harvesting. This can include simply moving the material off-site to a stable location using bins, or developing an integrated biomass strategy.
   
   Debris flows that entrain cut-over harvest residues are infrequent, but high risk events. Forestry activities such as clearfelling, building infrastructure, and creating soil disturbance are all activities that exacerbate the risk of slope failure and should be carefully managed. Restricting the size of clearcut, maintaining infrastructure standards, and setting limits on the level of soil disturbance are all pragmatic ways of mitigating debris flow risk.
   
   Leaving harvest residue on the cut-over is complex and has conflicting goals. Leaving harvest residues has considerable benefit for recycling nutrients, but also for erosion control that will help stabilise the slope. For this, it is important the focus is on minimising the volume of large woody debris that creates the greatest hazard when mobilised
    
3. Guidance on harvest practices and post-harvest strategies that have the potential to reduce woody debris generation and the risk of mobilisation within planted forests:
   
   The larger scale removal of forest vegetation (‘timber harvesting’ or ‘logging’) has an impact on the receiving environment. The biggest and most common concern is associated with the impact on water quality and steam habitat caused by accelerated erosion from harvesting disturbance. Because erosion can occur readily in even small rainfall events, these are extensively studied, well understood, and internationally forestry has relied on the concept of ‘Best Management Practices’ (BMPs) to mitigate or minimise impact from harvesting. BMPs develop over time, responding to both new research and operational experience to define workable solutions to a given issue. However, most BMP guidelines have an element related to harvest residue management.
   
   The issue of larger volumes of harvest residues moving off-site is complex as they occur infrequently, and with a combination of steeper slopes, weaker or more erodible soils, and a larger rainfall events (or smaller event but with saturated soils). New Zealand has a number of documents that support best harvesting practices, but few collate, detail or set standards for minimising debris flow risk. Common BMPs will help maintain overall site stability, especially those related to infrastructure design and maintenance. Specific to debris flows, practices that minimise the risk of their occurrence, and or the severity of their impact are recommended and include (a) limiting the scale of clearcutting (b) use of streamside management zones (c) and avoiding accumulation of harvesting residue through higher utilisation of harvested timber.