When and Why to Thin a Timber Stand

By Jennifer Hunt (Content Writer) and Shane Fuller (Certified Wildlife Biologist)

When a timber stand has been planted, foresters just let nature do its thing. On the contrary! Just like in a garden, it’s necessary to “weed out” or “thin” unhealthy seedlings and trees, those that are densely compacted and competing for resources. Timber thinnings are a similar premise, only they are an intermediate cutting that controls the growth of the stand by adjusting its density. This necessary process contributes to overall forest health and resilience. In this blog post, we’ll cover the reasons for thinning a stand, how the thinning schedule is determined, and what happens to trees during a thinning operation.

Tree Thinning Promotes Forest Health and Resilience

There are several reasons to perform a stand thinning, and many revolve around forest health. Overstocked pine stands are susceptible to disease and infestation; for instance, very dense stands may have less resistance to pine beetle attacks, especially those in a period of extreme drought with drier soils. Thinned stands have greater airflow, which dilutes Southern Pine Beetle (SPB) pheromones and can lessen the impacts in the event of an outbreak. Hardier trees will not be competing for resources in a thinned stand, helping them maintain their resilience in the face of an outbreak.

In addition to building resilience against SPB outbreaks, thinning a stand reduces fire risk and enhances wildlife habitat. Uncontrollable wildfires can result from overcrowded stands with excessive fuel loads that reach into the canopy, and the opening in the canopy provides an opportunity for more sunlight to reach the forest floor. This promotes the growth of shrubs, understory plants, and grasses that are beneficial to various forms of wildlife. Additionally, remaining trees will grow faster as there is less competition for nutrients. All of these considerations provide compelling evidence for thinning timber stands and contribute to the overall health of the landowner’s asset. Planning the thinning schedule is an important step, and foresters look for key indicators that a forest is in need of an intermediate cutting.

Signs that a Forest Needs to be Thinned

There are several signs that a forest needs to be thinned, and foresters typically measure by canopy closure, basal area, live crown ratio (LCR), and stand volume.

• Canopy closure: When the canopy is largely closed, sunlight cannot reach shorter trees or the forest floor, which limits the growth of both trees and understory plants that are important to wildlife. Having a full canopy also contributes to wildfire risk, as catastrophic fires tend to be exacerbated by a crowded stand.
• Basal area: Once stands reach a certain basal area density, a thinning is in order. For pine stands, this may be 100-120 ft2/acre. A target range is usually between 60-80 ft2/acre.
• Live Crown Ratio (LCR): The LCR is a ratio of the tree’s live foliage length to its total height, which is normally presented as a percentage. Typically, a stand with an LCR of less 40% needs to be thinned, as a lower percentage LCR is competing too much for sunlight. It also suggests that dominant trees will struggle to reach their full potential.
• Stand volume: Merchantable timber volume or weight is an important consideration for a stand thinning.

What Happens in a Tree Thinning

It’s important to know what to expect during a thinning operation. Bumper trees (designated trees left standing at the edge of a logging skid trail to act as a protective guard) may sustain minimal damage during this time, but this is normal and not usually a cause for concern. While bumper trees are often harvested at the end of the thinning, the other trees that sustain some form of damage may see bark scuff and small areas of exposed wood fiber. If there is concern about exposing the tree to pests, the scars can be painted as a deterrent.

Although you may pluck extra seedlings in a garden and compost them or toss them in a yard waste bin, thinned trees are not wasted. First thinnings tend to produce pulpwood and possibly biomass chips, which can have several uses. Pulpwood can be used for particleboard and fiberboard, and biomass chips can be used in residential wood chip boilers (popular in Europe) and industrial heating. They are also sometimes used for mulch, animal bedding, and erosion control.

Timber stand thinning is a necessary, integral part of forest health. There are numerous benefits, from decreased risk of pests and diseases to lowering wildfire risk and encouraging vegetative understory growth for wildlife. Whether your goal is timber, conservation, wildlife management, or a combination of several objectives, your AFM forester or certified wildlife biologist can help you determine if a thinning is necessary and create a schedule that will maximize forest health and productivity.