Feller Bunchers and Harvesters (also known as processors) are some of the most important pieces of equipment used in modern forestry. A far cry from the classic chainsaw, these mechanical behemoths make short work of timber with high tech and complex machinery. While both machines cut timber, that is where the similarities end. Both have unique advantages and disadvantages that make them better suited for some tasks than others. In general, feller bunchers are better for hardwood sites and areas with smaller average tree sizes or oversized trees. Harvesters are best used in softwood stands and in more intricate harvests, such as thinning. In this article, we will dive into all the differences of the machines and their optimal applications. First however, here is a bottom-line-up-front summary of the differences between them:
The Difference Between Feller Bunchers and Harvesters
The first step in understanding feller bunchers vs. harvesters is understanding exactly what each machine is and how they harvest timber.
Resembling an excavator, feller bunchers cut trees with a giant rotating saw blade. They then pick up the entire tree with three articulating limbs and drop it neatly into a bunch so it can be picked up and brought roadside by a grapple skidder. Unlike harvesters, feller bunchers cannot delimb felled trees or cut logs to specified lengths. They can only fell trees and “bunch” them, hence the name. Because of the limitations of this machine, it is used in tandem with a grapple skidder and delimber to process wood to a marketable state. Because the end product is an entire tree length instead of cut logs, it is used in what is referred to as a “whole tree” or “tree length” system
Harvesters come with more built-in functionality than a feller buncher. They combine the capabilities of both the feller-buncher and delimber. The head of the machine grabs on to the base of the tree, a saw blade hidden within the machine’s head severs the stem, and then roller wheels move the stem along a set of blades, cutting limbs and allowing the saw blade to cut logs to lengths specified by the lumber mill. Unlike feller-bunchers, which groups entire trees into bunches, harvesters neatly pile logs in pre-processed piles, which are then picked up by machines known as forwarders. The end product is more refined than what is produced by feller bunchers, as logs are cut to desirable lengths. This is why harvester systems are called “cut-to-length” systems.
So why wouldn’t all harvests utilize harvesters if they can do more than feller bunchers? Harvesters come with notable physical and economic limitations that dictate how they can be used. These two limitations are overly big trees and really small, often unmerchantable trees.
Harvesters are more sophisticated machines than bunchers, and so they are also a great deal more expensive, both nominally and on a per-productive-machine-hour basis. Every stem that is severed needs to be processed manually, which takes valuable time. If stems are too small or just barely merchantable, it can be difficult to move enough volume quickly enough to be profitable. Thus, harvesters can simply be too expensive when average stem size is too low or TPA counts are too high. The relationship between stem size and productivity can be seen below.
It should be noted, however, that even feller-bunchers will have productivity problems when harvesting smaller trees, but feller bunchers and the whole tree system in general will be more efficient in such a scenario, as each tree requires less handling time. Even so, in areas where a forest is inundated with unmerchantable, small stems, a feller buncher will be much more able to clear a trail and save time driving through it, whereas the same task can be agonizing or impossible in a harvester.
The other limitation harvesters have is overly large trees. While individual capabilities are going to differ from machine to machine (some Ponsse heads can cut up to 30 inches in diameter, for example), harvesters are generally going to be less capable in the harvest of over-sized trees, especially smaller harvesters. Thus, feller bunchers are better suited for stands where average stand diameter is very large.
Residual Stand Protection
Even if harvesters could harvest larger trees, it wouldn’t be a great idea, as they have little control over where the tree falls. When feller bunchers harvest stems, they physically pick the tree straight up off the stem and move into it into position so it can fall in a safe direction. The result is that residual stems are spared what would otherwise be a massacre of broken stems, branches, and smashed regeneration. Harvesters, lacking the same control, are largely at the mercy of gravity as to how and where the tree falls.
Articulation and Maneuverability
Reading up to this point, you may be wondering why anyone would want to use a harvester at all. Clearly, there are many disadvantages. Well, the key advantage of a harvester lies in its vastly superior head articulation and maneuverability through the woods.
A feller buncher head is fixed and has relatively limited reach and a limited range of motion. It is also large and cumbersome. If it needs to cut a tree in between trails, it must drive up to the tree, which can damage residual stems and lead to unnecessary damage and openings. This is particularly problematic for softwood stands, which tend to be denser and more sensitive to crown openings.
For this reason, harvesters are perfect machines for thinning and other intermediate silvicultural treatments in stands that are not yet regenerated or stands that are dense, yet still merchantable. This can be seen in the photo below. The harvester has the ability to reach in and around trees in this dense stand, whereas a feller buncher would be unable.
Impact on Soils
A second advantage of harvesters is their reduced impact on forest soils. The specific PSI of the wheels on the soil differ from machine to machine, but the real benefit comes from a harvester’s ability to process stems in the trail, thus leaving a bed of brush to drive over. This bed creates a nice cushion that greatly reduces the probability of rutting, compaction, and other soil damage that would otherwise be likely. This advantage makes harvesters a top choice for sensitive sites areas where low-impact logging would be a necessity.
Harvesters and feller bunchers also differ in the trail systems they usually utilize. Because the grapple skidders used with bunchers drag trees behind them, they cannot turn around or turn sharply when loaded. They must use what is called a “herring bone” trail system in which side trails branch off at an angle like the branches of a tree, creating only gradual turns for the skidders. Unfortunately, this system does result in larger overall trail systems and thus an overall less productive forest. The pattern of herring bone trails can be seen in an aerial photo below.
Conversely, the forwarders used with harvesters carry wood off the ground in a bunk, so they are able to make sharp turns and reverse direction while loaded. They are thus able to use what is called a “parallel” trail system in which, as the name suggests, trails are totally parallel from each other. The result is generally a more efficient and more desirable trail system with reduced trail area and less damage to residual stems.
In general, each machine has different site conditions in which they excel.
Harvesters have size limitations and tend to smash up regeneration, but they excel in merchantable softwood stands where finesse is required in moving between trees and there are few concerns about regeneration or limb damage. They also are a top choice for wet or sensitive sites.
Feller bunchers, meanwhile, excel where trees are smaller or overgrown, or where damage to regeneration or residual limbs is a high concern, such as in high value hardwood stands. Like so much in forestry, it is about choosing the right tool for the job.
Believe it or not, it is possible to have the best of both worlds. There are hybrid systems in use in which a feller buncher fells trees and a harvester comes in behind and processes the piles of wood. This allows the feller buncher to gain the soil protection and trail system advantages of a harvester on sites where a harvester would be otherwise inadequate, such as in hardwood stands. As a forester, I saw this system firsthand, and it works well to emphasize the strengths of both machines. Below is a picture of a forwarder moving wood from a hardwood stand that was harvested using such a system.
So which is better, feller bunchers or harvesters? In truth, neither is better than the other. They both serve important functions within their own niche. It is important to understand the strengths and weaknesses of both machines and employ them in the right places at the right time. That may mean using one or the other, or it may mean being creative and combining them in a hybrid system. The choice is yours, but choose wisely. The harvest system used is one of the most important and impactful considerations in forestry.