A harvester is a self-propelled machine with a cutting head attachment that is capable of felling and processing stems.
Harvesters may be wheeled or tracked machines with a processing head attachment. The processing head is capable of felling, delimbing, and bucking a tree to desired lengths.
Wheeled harvesters are purpose built machines having four to eight wheels on an articulated chassis. Some wheeled harvesters have the cab mounted above the engine on the rear articulation while others mount the cab on the front articulation. Some cabs have self-leveling capabilities. The cabs on some machines are capable of rotating like an excavator style cab, while others fixed in place. The boom is either mounted with the cab, or mounted separately on the undercarriage. The boom may have telescoping capabilities that extend its reach.
Tracked machines are similar to tracked feller bunchers with a processor head instead of a bunching head.
One new harvester is a tracked harvester that is based on the wheeled harvester articulated chassis. In place of wheels on the articulations, it has two sets of tracks. This harvester is capable of operating on very steep slopes.
Slope: Tracked harvesters can operate on slopes up to 55%. Wheeled harvesters are generally limited to less than 40% slope. Rough, broken ground will limit the slopes on which they can operate. The tracked, articulated harvester is reportedly capable of operating on slopes up to 80%.
Soil Conditions: Harvesters have the benefit of operating on a slash mat that they can lay down while processing the stems. This allows them to operate on wet or loose soils that would normally restrict operations. Wheeled machines can be fitted with a ‘track’ that fits on over the bogey wheels, thus increasing traction and decreasing ground pressure.
Tree Size: Tree size is limited by the capacity of the cutting head and the weight of the machine. Cutting trees too large for the delimbing knives compromises the ability to process the stems.
Tree Form: Harvesters are limited in their ability to effectively handle and process trees with multiple merchantable stems. Occasional forked tops and small multi limbed species may be safely handled, but large trees with multiple stems are dangerous to handle and difficult to process.
Operating on slopes raises the risk of rollover. This risk can be minimized by selecting equipment capable of operating safely on the given slope.
Harvesters are capable of operating in any type of treatment that permits movement of the machine within the stand. They are not well suited to harvesting hardwood species with multiple merchantable limbs. They operate best in stands consisting of single stem species such as conifers, aspen, and birch.
Working in thinnings requires appropriate room to maneuver the machine and trees. Wheeled harvesters generally have a longer boom, which makes them capable of reaching further into a stand of trees. This may make up for some of their larger size. The size of trees being cut and processed must be balanced with the machine required to perform the work to ensure the equipment can work in the stand. Larger tracked machines will have tail swing issues to work with. Wheeled machines and zero tail swing tracked machines do not have this issue. It is generally true that if they can get between the trees they can operate safely and with little damage to residual trees.
Harvesters were developed in conjunction with forwarders, which are capable of efficiently handling the processed bolts that they produce. They may also be used with helicopter and cable systems where slope permits. In these systems, they can greatly increase the productivity of the system by building full turns in appropriate locations for the extraction method. Processors can also be used at the landing to convert tree length stems into merchantable lengths. In this manner, they work well with a tree length system such as a feller buncher/grapple skidder system.
The standard system, called a cut-to-length system, consists of a harvester, forwarder, and self-loading trucks. While this system contains relatively few pieces of equipment, the harvester and forwarder are expensive to purchase and operate.
The following is a selection of representative research studies and reports done on harvest systems that include harvesters. These reports may be used to get an idea of productivity and impacts of different systems and uses of harvesters as well as some of their limitations. When reading these reports, keep in mind that they describe specific systems and stand treatments. Trying to apply the lessons learned from these reports to systems and treatments outside of the studies’ scope may have unintended or unforeseen consequences.
This is not a complete listing of research on the use of harvesters. Additional information can be found at the USDA Forest Service Treesearch website. This site provides reports on research performed by Forest Service Research and Development scientists and their collaborators.
Title: Productivity and cost of the Ponsse 15-series, cut-to-length harvesting systems in southern pine plantations
Author: Tufts, Robert A.
Description: Machine productivity data were collected for a Ponsse HS-15 harvester and S-15 forwarder from study sites in central Alabama during a second thinning. Tree size, in terms of diameter at breast height and volume, and the number of pieces processed per tree, were the variables with the greatest impact on harvester productivity. The distance from the machine to the tree was also important. Observed productivity averaged about 34.6 m3 per productive machine hour (PMH) and ranged from 8.8 to 65.2 m3 per PMH. For the forwarder, the number of pieces per load and load volume affected loading and unloading times. Travel speed .was influenced by slope and load volume. Predicted productivity was about 29.2 m3 per PMH in pulpwood and 33.8 m3 per PMH in small sawtimber. Machine costs were about $81 per PMH for the harvester and $53 per PMH for the forwarder. The number of trees per hectare harvested, tree size, and the forwarding distance were the most significant factors affecting system productivity.
Title: Forest fuel reduction through energy wood production using a small chipper/CTL harvesting system
Author: Bolding, M. Chad; Lanford, Bobby L.
Description: In the summer of 2000, fire destroyed millions of acres of forest across the United States. This study investigates the feasibility of harvesting to reduce forest fuel buildup and produce energy wood. Cut-to-length (CTL) harvesting coupled with a small in-woods chipper provides a low impact way to harvest pre-commercial trees and tops along with merchantable logs. While CTL harvesting systems have been used successfully with full sized chippers, it requires two or three CTL teams. A smaller, less expensive, chipper which is expected to have similar productivity to a single harvester - forwarder team and have reasonable ownership and operating costs, will allow operations to stay small and efficient. A CTL/small chipper system is projected to be an efficient way of reducing forest fuel loads and less expensive than fire suppression and stand-replacement costs after wildfire. Energy wood from fuel reduction harvesting could be used as an alternative energy source. The benefits of energy wood become more important as fuel prices increase. The feasibility study suggests that if energy equivalent values were obtained, a CTL/small chipper system could provide income rather than expense for site conversion, cleanup operations.
Title: Evaluation of a cut-to-length system implementing fuel reduction treatments on the Coconino National Forest in Arizona
Author: Klepac, John; Rummer, Bob; Thompson, Jason
Description: A Cut-to-Length (CTL) system was evaluated for production and cost while implementing fuel reduction treatments in two stands on the Coconino National Forest in Arizona. Product recovery and fire behavior within each stand after treatment were also examined. Only trees less than 16 inches diameter breast height (DBH) were harvested. After logs were forwarded to a landing, the forwarder for fire hazard reduction removed the remaining slash in each stand. Time-and-motion data collected revealed the harvester produced 364 cubic feet (cf) per Productive Machine Hour (PMH) while harvesting sawlogs and 33 cf per PMH while harvesting biomass. Forwarder productivity averaged 690 cf per PMH while transporting sawlogs and 160 cf per PMH while transporting biomass. System cost, with profit and overhead, was estimated at 208 per Scheduled Machine Hour (SMH). Unit costs were $0.88 per cf while harvesting sawlogs and $9.62 while harvesting biomass.
Title: Performance of a cut-to-length harvester in a singletree and group selection cut
Author: Huyler, Neil K.; LeDoux, Chris
Description: Presents production and cost data for a mechanized and cut-to-length (CTL) harvester used in a singletree and group-selection cut on the Groton State Forest in central Vermont. For trees whose average volume (size) was 7 to 18 ft3, production ranged from 464 to 734 ft3 per productive machine hour (PMH). The cycle time for processing trees into bunches to forward to a landing ranged from about 1 minute to 1.72 minutes per tree. There was little aboveground residual stand damage. The cost of the CTL harvesting system was $146.72 per PMH.
Products and Markets
Harvesters are good at producing maximum value products from the stems. In areas where the market demands this level of utilization, they are a good tool. Harvesters have higher capital costs than other processing machines, thus they are not an effective tool in low value markets.
Harvesters in America are used most commonly where the markets are present to handle cut to length material. Harvesters are common in the Lake States because the paper mills are designed to handle 8-foot lengths of wood. Harvesters work well with the species present in these forests, such as aspen, spruce, fir, and pine. The ground is often wet so that soils are susceptible to rutting damage from wheeled equipment. The ability to create slash mats to operate on and utilize high flotation equipment like harvesters and forwarders is another driver in their utilization.
Another use for harvesters is in prebunching of material for cable and helicopter operations. Helicopters are high productivity, high cost operations that require full utilization of the equipment during operation. Harvesters are capable of processing stems and building turns that match the load capacity of the equipment. They can operate on steep slopes, where soils permit, with minimum soil disturbance.
Another reason to use a harvester is in areas of ground-based extraction where full suspension is required. In this case, forwarders can be used to extract the processed logs.
Harvesters may also be used as processors at the landing in place of other processing options. In this capacity, they are highly productive and capable of good utilization of the material. One benefit of using harvesters in this manner is that they can sort, move, and load logs at the landing in addition to processing.