Is Wood a Renewable Resource?


Trees can grow so slowly it is almost imperceptible. To a casual observer, it can seem like they take centuries to regrow. It’s understandable then, why we might get sad or angry when we see a logging operation or log pile by the mill. Don’t they know how precious trees are? Will I ever see those trees grow back? Can wood even be considered a renewable resource? Fear not! Trees regrow more quickly than one might think, even if a forest isn’t replanted. Properly managed, wood is one of the most resilient renewable resources this planet offers. It’s a good thing too, as wood is the source of a large wealth of the products we need to live comfortable lives. But let’s start by looking into exactly how fast forests grow.

Trees Regrow Faster Than You Think

The growth rate of trees varies wildly depending on species, site, competition, and even individual genetics. However, we do know from diligent data collection by the Forest Inventory and Analysis Program that forests in the United States regrow at a rate of approximately .59 cords of wood per acre per year on average, with individual state growth rates mapped out below.

Forest Growth Rates

These numbers may not mean much by themselves, so let’s put them in context. A cord is a common measurement for wood volume. It is equal to a stack of logs 4’x4’x8‘, or about 3.6 cubic meters for anyone outside the US. If we assume that a “mature” forest holds about 20 cords per acre (this is an arbitrary, but reasonable number), then it takes about 34 years on average for a forest to grow back to a relatively mature state. That may still seem like a long time for the trees to regrow, especially for a renewable resource, but think of it like this: With over 741 million acres of forest land in the US, we would have to harvest more than 22 million acres a year for harvest levels to reach unsustainable rates. That’s an area about the size of the state of Maine.

One should remember, however, that this refers to situations in which forest is left as forest to regrow. If trees are harvested for development or other non-forestry related purposes (like agriculture), the timber does not regrow, and the harvested wood becomes a non-renewable resource. Even so, this all may sound good in theory, but does the practice live up to expectation? Let’s take a look at modern forestry practices to see if wood is truly a renewable resource.

Wood is Harvested With Renewability in Mind

In the modern timber industry, sustainability is a cornerstone. From top to bottom, wood is treated as a renewable resource, and many (if not most) major timber growers are certified by third parties such as Sustainable Forestry Initiative and Forestry Stewardship Council to ensure practices meet certain standards of sustainability and environmental responsibility. These certifications, as well as the companies’ internal policies, focus on all aspects of sustainability and renewability, from growth modeling to silvicultural prescriptions. Let’s look at how forestry enterprises ensure their wood is renewable.

Growth Modeling

Much like the map above (which is based on crude averages), growth rates differ from region to region, site to site, and species to species. Companies invest vast resources into timber measurement and inventory as well as growth modeling to understand the specific data of their land bases. They then craft an “Annual Allowable Cut” that defines the total volume of wood that can be harvested in a year. As long as total timber harvests remain at or below this level, the harvesting remains sustainable, and wood is continuously regenerated.

But It’s not just about the data and harvest levels. Care must be taken in each harvest to ensure trees regenerate properly, so timber companies use what is called “silviculture” to promote desirable outcomes. Silvicultural activities that maximize timber growth and regeneration include planting, shelterwood cutting, and thinning.

Planting

Planting ensures wood remains a renewable resource.
A truck from JD Irving brings tens of thousands of spruce seedlings out to the woods

Planting is by far the most recognizable and easily understood method of regenerating a forest. Seeds are harvested from seed orchards and grown into small seedlings at tree nurseries. They are then brought to a the site of a clearcut or heavy harvest and planted to introduce new growing stock to the forest. From there, trees are carefully monitored over years and prescribed with any additional treatment (such as pre-commercial thinning) that may be necessary to maximize health and growth.

Planting is common all throughout the United States and the world. In the US (as well as Europe), planting activities revolve around softwood species like spruce to be used for dimensional lumber. JD Irving, for example, plants around 14 million spruce trees in Maine and the Canadian Maritimes every year. Weyerhaeuser has similar programs in the western and southern US. Internationally, however, plantations of hardwoods (particularly eucalyptus) are a popular choice for lumber and pulp.

Planting can be particularly beneficial when the it is used to improve the overall quality of the forest. In some cases, forests can be overwhelmed with disease or infested with invasive species. These unhealthy trees are clearcut, and planting is used to reintroduce healthy trees to the site. Planting then becomes not only a means to ensure wood is a renewable resource, but a means to ensure the new forest is healthier than the old.

Shelterwood Cutting

Just because trees are not replanted does not mean land is being deforested or irresponsibly logged. Trees have developed advanced reproductive strategies over millions of years, and so forests are quite capable of regenerating themselves without the need for manual planting. Foresters and other forestry professionals often design harvests using silvicultural methods that optimize trees’ abilities to reproduce and grow. One such method is shelterwood cutting.

With silvicultural prescriptions like shelterwood cutting, foresters help keep wood a renewable resource.
White pines are spaced out to encourage seeding

Shelterwood cutting is a harvest system in which mature, desirable trees are retained in a timber harvest, and the forest canopy is opened up enough to allow light to hit the forest floor. The increase in sunlight allows the seeds of the retained trees to germinate and flourish. In other words, the forest replants itself! After a period of time, the stand will be harvested again, this time targeting the previously-retained overstory. By this time, the understory is firmly established, and the removal of the overstory gives the young trees even more sunlight to maximize growth and vigor. Before long, the forest is returned to its natural, mature state and ready for another shelterwood cut.

Thinning

A Harvester in a thinning block.
A Harvester thins a stand of fir

Harvests don’t have to be solely focused on renewing or regenerating a stand to make the harvest of wood renewable and sustainable, however. In some cases, harvesting wood is done with the sole intention of increasing the growth rates of residual trees and creating a healthier forest. Young forests can be crowded and dark, and the excess of competition leads to slow growth and high stem mortality. This is a situation that should be avoided.

A thinning treatment aims to enter a stand and harvest trees that are likely going to be outcompeted and die. By harvesting these trees for wood products, timber companies not only utilize trees that were likely to be wasted, thus reducing the number of mature trees that need to be harvested, but they create more room for other trees to grow. The result is a much healthier forest that grows back faster and reaches maturity sooner. It is the perfect way of ensuring wood is a renewable resource.

There are, of course, many more methods that are used to regenerate forests, but these are just some of the most common. Foresters have a wide variety of tools in their tool chest that they use to ensure wood harvests remain sustainable and renewable.

Wood Is One of the Best Renewable Resources On Earth

Wood provides us with so much more than just lumber. Wood can be a house, furniture, electricity, jet fuel, and, of course, a cozy fire on a cold winter’s day. Wood is not only a great renewable resource: It is one of the most versatile resources we have. So long as landowners take good stewardship of their timberlands and follow good forestry practices, we will have healthy forests and wood supplies for all to enjoy for centuries to come. Here at The Timberland Investor, that’s what we stand for.

Zachary Lowry

A forester from northern Maine, I spent my early career working for large timberland owners, managing forest land and investments in the form of managing timber harvest operations as well as planning and managing precommercial thinning, planting, and herbicide application programs. These days I work on my own land and help timberland owners large and small manage theirs.

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