Sustainability is at the core of modern forestry. Foresters and landowners take great strides to ensure timber harvests are healthy and sustainable. Over the last century, the science of forestry has developed a number of types of timber harvests and silvicultural systems to help accomplish just this. These harvest systems include clearcuts, shelterwood cuts, seed tree cuts, and selection harvests. Out of all the methods, what is the most sustainable way to harvest timber? In truth, any method of timber harvest can be completely sustainable as long as the level of overall harvesting is kept in check and all dimensions of sustainability are accounted for. As long as this is the case, harvesting remains sustainable.
In this article, we’ll look the different dimensions of sustainability, what it means for a harvest to be sustainable, and the most sustainable way to harvest timber.
Any Type of Harvest Can Be Sustainable
Timber harvesting can be complex proposition. With so many different styles of harvesting with vastly differing outcomes and visual aesthetics, it can seem like some are better than others. Nothing can be further from the truth! Clearcuts can be visually unappealing, for example, but they allow timber companies to plant trees, and it can vastly improve forest quality over time. Selection harvests can look great, but a more frequent harvest schedule can damage roots and soils, leading to long-term problems. Additionally, selection harvests are more prone to high-grading, which can be an incredibly damaging practice. Each type of harvest has its own unique benefits and drawbacks that make it better suited for certain forest types and situations than others.
The sustainability of a timber harvest is not determined by a single harvest or type of harvest, but rather how these harvests are used in relation to the forest at large. Determining what harvests to use, where to use them, and when to use them is largely the discipline of forestry. Foresters generally use three dimensions of sustainability to examine the nuances of each harvest and choose the best method for each. The three dimensions of sustainability are growth, ecological, and economic sustainability.
Growth sustainability is the most basic and obvious form of sustainability. It boils down to one basic question: Are you growing as much or more timber than you cut? However, ensuring the answer to this question remains “yes” goes far beyond the scope of a single harvest. Instead, growth must be analyzed on a large-scale basis, and individual harvests need to be planned within the scope of the larger picture. Luckily, foresters have numerous tools and strategies to do just this.
Annual Allowable Cut
The first tool available to measure growth sustainability of a timber harvest is the annual allowable cut, or the maximum amount of timber that can be harvested from a given acreage without depleting the resource.
The growth rate of a forest is expressed in cords per acre per year (or using another unit of volume). Below is a map of average forest growth rates by state:
An annual allowable cut allows foresters and landowners to take growth rate data like this and extrapolate it over an entire land base. For example, if growth rates are determined to be .5 cords/acre/year, and a landowner owns 1000 acres, then the annual allowable cut for that property would be 500 cords per year. Any harvests that are done, regardless of the type or style of harvest, must remove on average 500 cords a year or less to remain sustainable.
Optimizing Growth Rates
Interestingly, however, growth rates can be effected by levels of harvest, as creating a younger forest can increase the overall growth rates. Because of this, a more slow-growing mature forest may support higher harvest levels despite having a lower growth rate per acre due to the introduction of younger stems after harvest. The effect can be seen in the sigmoid-shape curve of a tree’s growth:
Growth is substantially lower at the beginning and end of a trees life, so keeping the mean age of a forest in the mid-range can drastically improve growth rates, making each timber harvest more sustainable. Additionally, planting offers the opportunity to plant fast-growing trees in the forest, which can have a positive impact on sustainability as well.
Unfortunately, optimizing and measuring growth rates requires a lot of work, planning, and ultimately guesswork, which is prone to error. Especially for smaller landowners, a better method to ensure a timber harvest is sustainable involves area control. Area control, simply stated, assumes a rotation age (or maximum age of a forest) and harvests based on that interval. For example, if a landowner who owned 1000 acres wanted to grow trees to 100 years, then they would schedule 10 acre harvests annually or 100 acre harvests every 10 years. This, on average, will ensure that trees are grown to be 100 years old. Because it is based on area and not volume or growth, no guesswork is involved. Harvests are inherently sustainable.
There is more to the equation than just growth sustainability, however. True sustainability requires that future growth remains economically viable. The economically viability of future growth is a function of both the species, form, and size.
Retaining Valuable Stems and Species
Not all trees are created equal. There is a wide range in stumpage prices (the price paid to the landowner) between different product classes and stem form. Pulpwood, for example, is considered the lowest quality product and fetches the lowest premium. Sawlogs, which are straight, free of defects, and come from a species like maple or oak, fetch a much higher premium.
For a timber harvest to be sustainable, it must take care so that the future growth of the stand is as valuable or more valuable than its current condition. Otherwise, even if it grows at a sustainable rate, the future economics of a harvest will be significantly worse.
Sustaining a Merchantable Size
Along with the quality of the wood, maintaining a valuable and merchantable stem size matters. As mentioned earlier, younger wood grows faster. Thus you can be growing 1 cord per year spread across thousands of little stems, or you can be growing .25 cords a year spread across a handful of large trees. Which is better? While the higher growth rate is better on paper, smaller trees can make for worse economics.
Larger trees are easier and cheaper to process for both loggers and sawmills. Thus, they command higher premiums. Over-optimizing growth rates to the point where most trees are below an optimum size can damage the economic sustainability of a harvest. To prevent this outcome, foresters use the measurement of quadratic mean diameter to measure how average tree diameters change over time. Careful harvesting practices will lead to a stable or increasing QMD.
Of course, a forest is much more than a bunch of trees. It is a diverse biological community of plants, fungi, and animals. For a timber harvest to be sustainable, the ecology of the forest must be accounted for.
Diverse Biological Communities
The maintenance or improvement of wildlife habitat must be a consideration for sustainability. Unfortunately, this can be a difficult task because of the wide variation in habitat preferences. Animals like the snowshoe hare prefer forests of younger softwood. Meanwhile, the spotted owl famously prefers mature stands of softwood. Every animal has unique considerations. It can be difficult if not impossible to account for each and every one, so it is the goal of sustainable timber harvest to create or maintain a diversity of stand types and conditions across the entire forest to promote these communities to the greatest extent.
Soil and Hydrology Disturbance
Even with efforts to promote biological diversity, timber harvesting can be a incredibly disruptive to ecosystems. Heavy logging equipment can leave ruts and damage roots and soils, and harvesting around streams can lead to erosion and sedimentation if care is not taken. Luckily, a variety of small scale logging equipment and best management practices for low-impact logging exist to help reduce this threat.
A Sustainable Timber Harvest Requires a Long-Term and Multi-Dimensional Approach
Clearly, the concept of sustainability is a complex idea. There is no hard and fast definition, and what may on the surface be a sustainable practice may in truth be deeply destructive in the long term. Instead of basing sustainability on the method of timber harvesting or single factor, a wise forester or landowner takes a multi-dimensional approach over time. All factors of sustainability must be accounted for if a timber harvest is truly to be in harmony with the forest.