The Edge: Partial harvest systems to meet specific management objectives within boreal mixedwood and pure softwood stands

by | Apr 25, 2024 | 2024, Logging & Sawmilling Journal, March/April 2024, The Edge

The Canadian Wood Fibre Centre (CWFC) technical development group of the Canadian Forest Service (CFS) and their partners have a history of developing innovative partial harvest systems that address issues of forest managers in the Prairies.

Here are a few options available to industry to address the current and anticipated consequences of a changing climate, as well as to help maintain a sustainable fibre supply as Canada continues to transition to managed forests.

These partial harvest systems—although heavily weighted to mixedwood stands featuring trembling aspen and balsam poplar with white spruce because of the type of industry and forests in the Boreal Plains—can also be applied in pure conifer white spruce and lodgepole pine stands. They also have adaptability to both natural and managed stands.

“There are a variety of reasons to consider partial harvest systems as we witness the greater impact of a changing climate on the commercial forest while we also experience a growing demand for fibre,” says Derek Sidders, Program Manager, Technology Development and Transfer at CWFC. “It’s important to consider ways to maximize the values and health of the forest, which is where partial harvest systems play a significant role.”

Furthermore, it’s also important to design a harvest pattern that not only results in diverse values but also recognizes the technology of the day that is operationally and economically realistic for both initial and follow up entries.

Sidders adds that fibre supply and sustainability can be heavily influenced and enhanced with the use of partial harvest systems where multiple harvest entries can selectively manipulate species distribution and values recovered from the stand, while also investing in future values that will be recuperated in 20 to 40 years, depending on the management objective.

For more information about these partial harvest systems, contact Derek Sidders at derek.sidders@nrcan-rncan.gc.ca.

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Ecosystem Management Using Variable Retention in Mixedwood Stands

This partial harvest design for mixedwood sites was developed by CFS in conjunction with Alberta industry and the University of Alberta to try to harvest wood fibre in a way that mimics natural disturbances in the forest.

It features a harvesting system where there is both individual tree and group retention that mimics the aftermath of a wildfire. The Ecosystem Management Emulating Natural Disturbance (EMEND) research site, located in the Clear Hills Upland, Lower Foothills Eco-region of Alberta approximately 90 kms northwest of Peace River, is a good example of where this partial harvest system was deployed.

The basic design specification of this partial harvest system features concentrated machine corridors operating parallel to each other. Whether using feller bunchers and skidders or harvesters and forwarders, the distance between the width of each machine corridor provides access to each individual commercial stem in the stand from the corridor, as well as a route out of the stand for harvested trees or stems.

Based on the species composition and the management plan, the partial harvest objective using this method can vary. The goal could be to achieve a variable retention percentage of perhaps 20, 50 or 75 per cent, to remove specific species from the mixedwood stand or to strictly harvest fibre by vulnerability, value or diameter class.

If the goal is to harvest a certain amount of volume in a defined basal area, it is possible to use a systematic pattern of individual tree removal. First, consider the removal from the machine corridor from the total stand composition which is commonly 25 per cent of the stand, and then based on the prescribed retention level, individual trees can be removed systematically from the remaining strips which continue to occupy 75 per cent of the land base. For example, by harvesting one tree and leaving two trees, 50 per cent retention is achieved and for 20 per cent retention, three trees are harvested with one retained.

The accompanying graphic demonstrates this pattern.

“The management objectives using this partial harvest method can be anything from a preference for final species diversity for multiple values such as ecological and long term production values,” says Sidders, “or the objective could strictly be to provide commercial access to a species that grows faster in a mixedwood stand, with the intent of removing as much of that commercial species as possible, while releasing the shade tolerant and slower growing species in the understorey.”

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Selective Harvest to Reduce Pest and Fire Vulnerability While Maximizing Fibre Recovery

This partial harvest technique, which is similar in design to the ecosystem management method, can be applied in a mixedwood stand, mixed conifer stand or pure conifer stand. Based on the species distribution and size of the trees of each species in the block, preferential harvesting is conducted to achieve specific value outcomes.

For example, to reduce stand vulnerability, the objective could be to remove larger stems, create space within dense stands, protect preferential species with low vulnerability or to remove dead and dying stems to eliminate any type of long term potential pest, pathogen or wildfire impact. The key to successful deployment of this design is the effectiveness of the pre-treatment sampling, the harvest pattern for access to all trees within the retention strips from the machine corridor, and, of course, a clear description of the trees to be removed and retained.

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Commercial Coniferous Thinning Partial Harvest System

This method applies a design of either concentrated machine corridors using conventional logging equipment or ghost trails with a cut-to-length system in a mature natural coniferous stand where the fibre volume can be 400 to 550 cubic metres per hectare and the white spruce is well established at 1200 to 2000 stems per hectare.

The goal is to harvest commercial value while opening up the site to allow retained stems to take full production advantage of increased sunlight and nutrients while maintaining species health, diversity and keeping 50 to 70 per cent of the fibre volume intact.

Stems are harvested to meet the anticipated basal area removal level relative to the productive potential of the retained stems, with thinning conducted from above and below with the goal of meeting that 50 to 70 per cent retention level.

“Also, the higher the removal level, the greater the potential for the establishment of a natural understorey of white spruce originating from the overstorey that’s retained. Enough light penetrates the stand for soil heating, germination, and access to micro-sites created from the first entry harvest,” says Sidders. “Fifteen to twenty years later, you could have full juvenile spruce occupancy with trees above 1.3 metres in height where there is ground disturbance, as well as a significant increase in the retained stems to justify another entry.”

Second entry is determined by the overall management objective, such as retained stem growth and established spruce regeneration, as well as the local forest manager’s knowledge of historical growth response from each individual site.

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Understorey Protection in Mixedwood Stands

The stands targeted for this partial harvest application are mixedwood stands featuring a commercially mature stand of trembling aspen/balsam poplar with an established and well-distributed understorey of immature white spruce of at least 60 to 80 per cent stocking, but with larger conifer stems vulnerable to wind throw or sun exposure that require wind and light protection.

A machine corridor pattern is used so that the feller buncher can reach the mature deciduous component within the remaining stand while retaining the conifer understorey. The corridors are established so that there is a buffer within the remaining stand out of the reach of the feller buncher, creating, for example, a 5- metre wide wind buffer every 35 to 40 metres. North/south machine corridors are recommended since prevailing winds typically come from the west in the Boreal Plains.

“The conifer stems benefit from being retained (increased light, heat, water and nutrients) and having been given breathing room through this first entry,” says Sidders. “They will expand their size to commercial value within 20 to 25 years post entry, assuming the spruce are healthy and 8 to 12 metres in average height.”

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Adaptive Understorey Protection

A new design method within this partial harvest system developed by the CWFC technical development team is called adaptive understorey protection. It uses the variable retention logging corridor model which eliminates wind buffers but keeps and distributes an additional 20 to 30 per cent of the deciduous stems within the retention strips to protect the understorey. This results in a more evenly distributed overstorey that maximizes growth potential of all retained softwoods in the understorey. It also allows the remaining deciduous to continue to occupy natural gaps within the retained stand and machine corridor, which maintains a diverse mixedwood stand and is significantly advantageous from an environmental and changing climate perspective while adding more value to the second entry harvest.

There are numerous examples of these methods available for viewing both in commercial and research applications throughout the Boreal Plains region.

Tony Kryzanowski

Author

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