Thinning it Out
Thinnings are a key component in the development of healthy forests, but as in many aspects of the forest industry, a shortage of labour means machines have to play a more important role in this critical process. This is especially true with pre-commercial thinning (PCT), where contractors want to open up the forest prior to most commercial thinning applications without inflicting damage on crop trees. Also referred to as “semi-mechanized strip thinning,” machines specifically designed for this purpose basically mulch the vegetation in front of them as they create narrow strips through the stand that should ideally be less than two metres in width.
November 14, 2011 By Bill Tice
Earlier trials were accomplished with boom-mounted attachments, but over the years manufacturers have developed task specific machines that can cost effectively complete the job. Today, PCT mulchers typically come in three types: wheeled tractors, tracked machines with a non-articulated frame, and articulated machines. All types of machines can work well, but with positives and negatives for each, contractors need to evaluate the individual block where PCT is required to determine which system is best for the specific site conditions.
Wheeled tractors equipped with a brush cutting head are normally the least expensive option. Smaller models are very compact and manoeuvrable, but the power from these machines may not be enough for high-density stands. More powerful machines are available, but they generally cut strips wider than the desired two-metre-wide strip
preferred by foresters.
Non-Articulated Frame Tracked Machines
Tracked machines with a non-articulated frame are also compact and manoeuvrable and are efficient in most operating conditions. Steep slopes and obstacles on the ground can slow down these machines, as can high-density blocks.
Machines with an articulated frame are generally the most expensive option, but they also offer the highest productivity in many cases. The stability, mobility and power of these machines are generally higher than the other types of machines available.
In a 2007 report by FPInnovations Feric Division called “Semi-Mechanized Precommercial Strip Thinning – A Practical Operations Guide,” Feric researcher Michel St-Amour presents the advantages and disadvantages of the various types of machines and discusses the factors that should go into the decision when selecting a machine for the job at hand.
“Stand conditions are the first factor to consider,” he explains. “Stand density influences machine productivity, which decreases as density increases. The productivity decrease is more drastic for machines with a power of less than 100 kW. Articulated machines more powerful than 150 kW are much less influenced by stand density.”
St-Amour also says terrain conditions determine the “trafficability” of a site and influence machine productivity. “Slopes, obstacles on the ground, bearing capacity or a combination of these factors can greatly affect the productivity of a machine and can even render a site inaccessible to some machines,” he says.
The main reason for prescribing PCT is cost reduction, adds St-Amour, especially when you get into higher-density stands that may have more than 25,000 stems per hectare. “By cutting strips, we are taking out part of the vegetation at a relatively lower cost while leaving enough crop trees for the thinner to effectively complete crop tree selection and thinning,” he notes. “In stands of less than 15,000 to 20,000 stems per hectare, you don’t want to use PCT as you may remove too many trees,” he says, while adding that the rule of thumb is to not remove any more than 25% in any block during the PCT process.
St-Amour says timing of PCT can be critical, noting that some damage can be done to the trees on the edges of the cut strips during the strip cutting process. “For this reason, we recommend waiting one year between the strip cutting operation and the final motor-manual crop tree selection and spacing so that you can better evaluate what damage some of the trees on the edges may have sustained,” he explains.
As we look forward, biomass may change the rules for PCT as up until now the vegetation processed by the PCT mulchers has been pulverized and left on the ground to decompose, with the nutrients being deposited back into the soil. “Any source of wood fibre is currently being looked at and scrutinized as a possible source of biomass because quality is not always an issue for biomass sources of fibre,” says St-Amour. “We’re not quite there yet, but there are some mulchers or techniques out there, especially in Scandinavia, with the capability to harvest the thinned material.”
In the near future, we will probably see new technology that will allow for the easy collection of mulched fibre for biomass, but in the meantime, several manufacturers offer mulching equipment adapted for semi-mechanized PCT operations. For more information on the various types of equipment, refer to the manufacturers’ websites listed below.
Mulching Machine Manufacturer Profile
From its 390,000-square-foot facility in Wooster, Ohio, Rayco Manufacturing, Inc. produces a wide range of equipment for mulching, including the new Rayco Predator PM638 mulching head. With 38 fixed teeth, it eats everything in a six-foot (two-metre) path and when attached to the New Rayco C100 Super Crawler, it has the power to get the job done. Both the C100 and PM638 are purpose built. The steel tracked C100 is powered by a 96hp Kubota turbo diesel that goes nearly anywhere with unmatched traction and mobility and features Rayco’s Super Flow, closed loop, hydraulic system, producing 39 gallons/minute at 5,500 pounds per square inch and a forestry cooling package to extend service intervals and keep you running longer, even under the most demanding field conditions.
Once the mulching is done and the site needs to be cleaned up, Rayco’s new line of compact horizontal grinders, including the RH 1754, are ideal for disposing of wood waste without breaking the bank. Now available as either towable or self propelled on a steel tracked undercarriage, the RH1754 is a truly mobile, compact machine designed to grind brush. It can also be used for other applications such as grinding urban waste, including pallets, green-waste, lumber scraps, and construction debris.
Equipment Manufacturer Listings
For more information on PCT mulching equipment, visit the websites of the following manufacturers:
dvanced Forest Equipment
Davco BC6450 & 705HS
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