Optimising interaction between silviculture and harvesting systems

February 13, 2018

By Simon Ackerman and Sally Upfold

The footprint of plantation forestry has changed significantly over recent years, with an increase in mechanisation or machine assisted operations, increased labour costs, changing markets and challenging climatic conditions. These, amongst others, have required forestry managers to rethink the way business is done, applying different operations, systems and technologies to visualise and plan for a changing forestry landscape.

Historically, international and local trends associated with increased mechanisation and improvement of operations have been reflected in increased productivity. This trend is, however, no longer as steep as it once was and in some cases is declining (Figure 1 below). This is due mainly to maturing technology, and often the technology of these systems being far more advanced than can be taken advantage of by operators of these systems in the forestry landscape.

Figure 1: Forestry productivity trends from basic systems to full mechanisation

Understanding and optimising the full forestry value chain is therefore critically important, and particularly developing knowledge around how the two most important functions, silviculture and harvesting, interact and complement each other.

Plantation management occurs over multiple rotations involving harvesting, site preparation, planting, tending and growing, through to harvesting again (Figure 2 below). However, management is often not performed or viewed as a continuum, but rather as discrete independent steps, usually by different individuals or sections within a company and not necessarily aligned with the activities that occur prior to or after each component.

Figure 2: A conceptual understanding of the forestry supply chain (Source: Ackerman 2017)

In South Africa, concerns have been raised around potential operational inefficiencies at the interface between harvesting and silviculture operations. Previously, investigations quantified effects on stand productivity (e.g. the effect of machinery damage during timber harvesting and extraction on stumps and coppice ability) and the impact of harvesting systems on silvicultural operational efficiencies. The effects of silviculture on harvesting are not well understood.

This area has been a focus of the Forest Management project team at the ICFR for the last three years. The work done has sought to understand the operational constraints between harvesting and silviculture, and from this, identify research priorities to address the synergy and interaction between these components.

The main challenges faced when trying to align these two parts of the value chain from both operational and research perspectives, are the following:-

For harvesting on silviculture
Slash and residue management
This aspect is considered to have the largest impact on silviculture productivity. Excessive timber waste and slash impede access to the compartment for silviculture operations (manually and mechanically), limiting planting success, and can lead to high fuel loads and increased fire danger.

It is important to understand how different harvesting systems distribute slash and optimise trees to logs to reduce timber waste. Residues are generally managed by burning; however, alternatives are available that are potentially more sustainable, but at an increased cost. These can, however, be overcome through better residue management with potential long-term soil nutritional benefits over multiple rotations.

Stumps as a legacy from previous rotations (coppiced or not), limit access to compartments by mechanised silviculture machinery as well as contribute to unnecessary timber waste. Ground conditions (rocks and other objects) affect mechanised harvesting, and chainsaw operator technique can influence the extent to which stump heights become excessive.

Compaction and rutting
This factor considers the extent to which rutting and compaction from heavy harvesting machinery occurs and the associated effect on compartment accessibility by mechanised silviculture machines. In extreme cases, this can cause reduced tree growth. Managing this is important to ensure overall plantation sustainability.

Stump coppice-ability
Coppicing is a management intervention used in Eucalyptus rotations, and its success is greatly affected by the ability of stumps to re-coppice. The harvesting system used, or irresponsible travel across the compartment by machinery, can cause a reduction of the ability of the stumps to coppice.

For silviculture on harvesting
Orientation of planting lines
Aligning and orientating tree lines can improve harvesting machinery productivity on slopes.

Traditionally trees are planted across the slope, following contours. However, size and side slope travel constraints of harvesting machines mean that harvesting trees across slopes is not feasible. Orientating tree lines up and down slopes would therefore greatly improve safety and productivity.

Weeding and tending
Poor visibility in compartments being harvested can potentially reduce harvesting productivity.

Maintaining a balance between within-rotation weeding of woody weeds and the cost of the harvesting needs to be done as the safety of operations can be affected.

Tree spacing is generally determined by the species to be planted, site characteristics, required productivity and end-product. Spacing also influences the efficiency of mechanised harvesting operations (i.e. machine access to the compartment between tree rows, and the number of trees in neighbouring rows that can be safely reached by machine booms). Other considerations include the balance between tree size and harvester productivity. Closer spacing leads to smaller more slender trees while larger spacings have the opposite effect. Maximising harvesting productivity requires the system used to be appropriate for tree size.

Coppice management
Harvesting of coppiced compartments is less efficient than that of planted compartments and can lead to higher stumps (with associated loss of wood volume), negatively impacting machinery access.

However, rapid site re-establishment with associated cost savings in silvicultural operations often make coppicing a more desirable and viable management option.

Where do the opportunities lie?
Research opportunities to improve the interaction between harvesting and silviculture should be identified as a matter of priority for improving plantation forestry operations in South Africa. While it is clear that careful management of operations (supervision and reporting) and a cross discipline (understanding the constraints of silviculture and harvesting) knowledge are important, there are also urgent research questions that remain unanswered.

Aspects that require further investigation towards optimising the interaction between harvesting and silviculture include the following:
• Understanding tree spacing for mechanised harvesting and silviculture;
• Developing an understanding of seedling requirements for mechanised silviculture;
• Developing methods to quantify and mitigate stump heights from different kinds of felling techniques;
• The use of multiple use operations such as harvester head chemical application to reduce follow-up manual treatments.

Plantation forestry worldwide is approaching an era of technological advancement in terms of access to big data, advanced forest enumerations as well as system and machinery simulation. Being able to predict outcomes closely approximating reality will lead to reduced cost and more efficient management of forestry systems, with increased machine utilisation, better suited machine application for a particular job and improved timber optimisation and recovery.

This is the aim of forest operations research for South African forestry; to move the sector forward in using technology to understand complex systems, and develop fit for purpose plans that enable budgets to match reality.

Enquiries: Simon Ackerman (simon.ackerman@icfr.ukzn.ac.za);
Web: www.icfr.ukzn.ac.za

*First published in SA Forestry magazine, Dec 2017


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