While timber is a totally renewable resource, it can take a long time for timber to grow back once cut. It can take so long, in fact, that many mistakenly believe that once timber is cut, it can never be cut again in one’s lifetime. While that is a total myth, it comes from a fair question: How often can you harvest timber? From seedling to final harvest, trees can be harvested every 30-100 years depending on species, climate, and more, but one does not need to wait this long for another harvest. Partial harvesting such as thinning and selection cutting can easily cut these time frames in half and even benefit the forest! Moreover, with proper planning and a wise forest management plan, timber can be harvested every year as long as the amount removed is consistent with long-term growth. But as with anything in forestry, there are a great deal of factors to consider.
The question of harvest frequency is fascinating and nuanced, and perhaps the most important question in forestry. In this article, we will attempt to shed some light on this question and the myriad of considerations that it raises. Hang tight!
Understanding Timber Rotation Ages
The entire lifespan of a stand of timber, from seedling to final harvest, is known as the stand’s “rotation age.” While there can be harvesting in between this time period, the final harvest tends to be the largest of the harvests in terms of timber removal, and it tends to be the most profitable for the landowner. Thus, when answering the question of how often one can harvest timber, the rotation age gives the clearest answer.
However, what one must understand about rotation ages is that it represents neither the age at which the stand dies nor the age at which the stand becomes re-forested. Instead, rotation ages signify the point at which the size and volume of the timber becomes economically optimum for harvest. That is an important distinction, as the number then not only relies on the growth of the stand, but also markets and the economics of harvest. Added together, there are many factors that influence the rotation age and how often one can harvest timber.
What Affects Rotation Age
A stand’s rotation age is how often one can harvest is going to be dictated by a myriad of factors, including species, site quality, climate, management, and economics.
One of the most important factors that determines rotation age is the species the stand is comprised of. Some species, certain species will grow much faster than others. Generally speaking, softwoods like spruce and pine will grow substantially faster than hardwoods like oak or maple, so while a stand of maple might have an 80 year rotation age, spruce may only have a 50 year rotation age. However, some hardwoods such as aspen grow substantially faster than softwoods and can be harvested in only 20 years.
Of course, site quality is also important. well-drained, deep, and nutritious soils are always going to result in faster growth compared to poorly-drained, shallow, and nutrient-drained soils. Even slow growing species can put on growth fast if they are grown on the right sites.
Climate also plays a significant role in how fast trees grow and how often one can harvest the timber. Warmer climates have longer growing seasons, which of course will result in larger trees in a shorter amount of time. But moisture and precipitation can also be a limiting factor to growth. Climates that experience prolonged and severe droughts can inhibit the growth of species as well as increasing fires that can slow down net growth on average.
Forest management regimes can also dramatically alter rotation ages. Managed stands, such as one that are planted, pre-commercially-thinned, or commercially thinned can reduce the competition for crop trees, thus allowing them to grow bigger in a shorter amount of time. In these situations, rotation ages can be drastically reduced, and the introduction of thinning and other partial-harvests allows for smaller harvests between final harvests, allowing for more frequent timber harvesting.
Finally, economics is a major factor in how often wood can be harvested. The first impact of economics is the minimum merchantability of the wood. If the wood is too small and cannot be used for any products, than the wood cannot be harvested. However, even if the wood is big enough to harvest, it still may not be a wise idea to harvest. That is because the wood at its smallest, minimally merchantable state is usually not its most valuable. The table below shows the relationship between tree size and value. Once wood reaches certain size thresholds, it becomes more valuable on average.
Thus, while trees of pulpwood size can technically be harvested, it is usually much wiser to grow them at least to sawtimber size. For this reason, rotation age is largely an economic decision. To learn more about when to harvest timber, we have an article on the subject here.
Average Rotation Ages in the United States
That all having been said, we can devise a metric for an average age to timber maturity, which can act as a proxy to typical rotation ages. However, it requires that we make certain assumptions.
First, we have to define what a mature stand is, and here we will (somewhat arbitrarily) define a mature stand as having 30 cords to the acre.
Second, we will assume average state gross forest growth rates as measured by FIA inventory data.
Mapping those assumptions gives us a geographic view of how often one can harvest timber in the United States.
Nothing about those assumptions makes sense in all cases, but as you can see, it does a decent job of showing us trends in timber growth across the US. Warmer climates where plantation silviculture dominates produce the shortest rotations. On the other hand, temperate climates with more hardwood forests take a bit longer. Drier, more arid regions as well as agricultural states where most remaining forests grow on poorer soils produce the longest rotation ages.
Typical Rotation Ages By Species
Of course, as previously mentioned, even within a single geographic area, rotation ages can vary a great deal for a variety of reasons, not the least of which is the dominant species in the forest. Keep in mind these are only typical rotation ages, and timber can be harvested sooner or later depending on individual management goals and financial objectives.
Hardwoods (trees with broad leaves) typically grow slower than softwoods, resulting in longer rotations, but this is not always the case. Below is a list of common hardwoods and typical timeframes for how often you can harvest timber for each.
Maple (particularly sugar maple) is among the slower-growing of hardwoods. Typically, you can harvest maple trees every 70-90 years, but maple is conducive to partial harvests, allowing for more limited harvests more frequently.
Birch trees, including white birch, yellow birch, and grey birch are among the faster growing of the hardwoods. Typically, you can harvest birch every 30-50 years, but yellow birch may have a longer rotation age, as yellow birch sawtimber is more valuable.
While Oak trees are among the faster-growing of the dense hardwoods, owing partially to the warmer climates where it grows. Oak trees can be harvested approximately every 50-70 years.
Black Walnut can be harvested about every 60-80 years, but because of the uniquely high value of black walnut, it may make sense to wait longer to ensure optimum values.
Hickory trees grow similarly to oak and share similar growing sites, thus, hickory trees can be harvested every 60-80 years.
Black cherry is a relatively fast growing species, know for pioneering cleared land. However, because this tree is valuable for its heart wood, which comes in maturity, and due to its propensity to grow to large size longer rotation ages are appropriate. Black cherry is best harvested every 80-100 years.
Aspen is an incredibly fast-growing species that thrives on frequent harvests. Moreover, these trees are used almost exclusively as pulpwood for OSB, so one does not need to wait for it to reach sawtimber sizes. Accordingly, aspen can be harvested in as little as every 20 or 30 years.
Softwoods (trees with needles) typically grow faster than hardwoods, and because their value is mostly based on volume instead of stem size or quality, softwoods are usually grown with shorter rotation ages. The primary softwoods are pine, spruce, and fir.
Pine is a genus with many different species with drastically differing rotation ages. White pine in the northeast is typically harvested every 70-100 years, but yellow pine, slash pine, and loblolly pine in the American south can be harvested in a much shorter time period, often 20-30 years. On the other hand, ponderosa pine, which can grow very large and take a long time to grow, is often grown to 100 year rotations or longer.
Spruce is a faster growing softwood, and while there are many species, they mostly grow similarly, with the largest limiting factor of their growth being site quality. On a nice site, spruce trees can be harvested every 50-70 years.
There are three primary types of fir in the United States: Balsam fir, Fraser fir, and Douglas fir (which is not a true fir). Balsam and Fraser fir can be harvested every 40 to 50 years, but Douglas fir can be harvested every 60-90 years.
Thus far, we have been discussing rotation ages, which are useful to use and discuss as an analog to harvest frequency, but they don’t truly answer the question because how often you can harvest timber is 100% dependent on how much standing volume you harvest. For example, if trees were to grow on a 100 year rotation, then you could harvest 1% of the volume every year, or 10% every ten years, etc. These types of harvests are known as partial harvests, and they primarily take two forms: Thinning and Selection cuts.
Thinning is a common method of partial harvesting that involves intentionally reducing the density of a stand of timber to reduce competition and allow selected crop trees to grow bigger in shorter amounts of time. A typical progression of a thinning harvest can be seen in the diagram below.
An important distinction of thinning from other partial harvests is that thinning is done as a part of even-aged management regimes with the intention of a final harvest. Only a small portion of the stand is harvested during a thinning with the intention of improving the size and quality of the residual timber so the final harvest later is more profitable. Thus, while thinning does provide a return to the landowner, it is usually small in comparison to the final harvest. In some cases, the landowner may even need to pay to have a thinning done.
Selection cutting is a process of continuously removing a portion of the stand over time, removing older, mature stems and giving younger stems space and time to grow. A typical progression of selection cutting can be seen in the diagram below.
Unlike in thinning and other partial harvests, there is never a final harvest with a selection cut regime. Instead, the stand retains a diverse mix of age groups, the growth of which is optimized during harvest so a certain percentage of the stand is removed at regular intervals. For example, it is possible with a selection cut to harvest trees every 10 years despite only harvesting trees that are 100 years old. It only requires cutting just the 10% oldest trees.
Unfortunately, one downside of selection cut harvesting is that repeated and frequent entries into the stand risks excessive damage to residual stems, branches, and roots, lowering the value of the forest overall. When selection harvesting is done, it is important to use lower-impact machinery with skillful operators.
Area and Volume Control for Consistent Harvests
Much of the question of how often one can harvest timber is greatly dependent on the size of the acreage. So far, we have discussed the implications on a single stand or small parcel of timberland. However, with larger forests and parcels, it is easier to continuously harvest wood using either area control or volume control approaches.
The first means of creating consistent harvests over a larger land base is known as area control. Essentially, this works by creating a harvest schedule based on a percent of the acreage. For example, if a landowner wanted to grow trees out to 100 year rotations, and they owned 1000 acres, then they could plan on harvesting 10 acres a year, or 100 acres every 10 years, or whatever other schedule they find best suits their financial objectives.
The benefit of area control is that it is more objective and easier to measure, so it is a simple method of long-term forest management. However, because each acre may not have the same growth rate or volume, the return from each harvest may not be so consistent.
The second means of creating consistent harvests is volume control. With this system, a certain amount of volume is removed over a given timescale as is consistent with the forest’s estimated growth rate. If a landowner has 1000 acres, for example, and each acre grows .5 cords per acre per year, then the land can support the removal of 500 cords every year.
The benefit of this system is that it allows for consistent returns because the volume removal remains relatively consistent. However, growth rates as well as estimates of the volume removed can be difficult to estimate or model accurately, so it can be prone to over or underestimation and requires more sophisticated tracking mechanisms.
There is no objective answer to how often timber can be harvested, but the best answer is truthfully “As frequently or infrequently as the landowner desires.” As with much in the world of forestry, the best answer requires an extensive knowledge of the nuances of the forest and a careful examination of one’s own priorities in respect to the management of the land. While brief in comparison to the grandiosity of the subject, hopefully this article has helped you gain a little more insight into the considerations of harvest frequency on a timberland property.