Fall is tree thinning time

thin-form-below before and after comparison
Before (left) and after (right) a thin-from-below treatment to improve this forest’s health and resistance to wildfire. Photo: /DNR.

Fall is just around the corner and if you own forested land, or just have a lot a trees on your property, you may want to consider thinning some of those trees. What are the advantages of thinning? Wildfire prevention, wildlife habitat and improved health for the remaining trees are among the leading reasons to consider pruning or thinning woodlands.

Wildfire prevention: Thinning can influence wildfire risk factors such as the species composition of a wooded area as well as the available fuel, fuel moisture levels and other wildfire risk factors. The objective of thinning in wildfire risk reduction is usually to prevent or slow the spread of crown fires by reducing ground level fuels, such as brush, and ladder fuels like low-hanging branches. Thinning that raises the height of the trees’ crowns and breaks up the connectedness of crowns between trees can reduce the tree-to-tree spread of crown fires.

A key consideration in thinning is what kind of trees do you have?

Ponderosa pine, western white pine and western larch all tend to be tall and self-prune (the natural removal of lower limbs that don’t receive enough sunlight to survive). This feature makes them less susceptible to destructive crown fires, which occur when a fire at ground level spreads into low branches and, from there, into the tree’s crown and to nearby tree crowns.

Western white pine and western larch have less volume in their crowns and their branches are typically well above ground-level fuels such as grasses and brush. Because of these attributes, western white pine and western larch do not carry crown fires well and tend not to create ladder fuels (fuels in the lower canopy that carry fire up into the crowns of trees).

In contrast, stands of true firs, Douglas-fir, western hemlock,  or western redcedar do not self-prune well. They tend to have relatively voluminous crowns and may have large branches that hang low. Stands dominated by these species usually support crown fires unless thinned periodically.

Wildlife management: Species associated with fairly open forest canopies and an open forest floor may benefit from thinning. Thinning a stand of trees increases the amount of sunlight reaching the ground level, which stimulates the growth of grasses, wildflowers and native shrubs. Elk, deer and moose will likely benefit from the increase in the quantity and quality of forage that results.

Small mammals such as chipmunks and deer mice may increase in number, particularly after thinning in Douglas-fir and ponderosa pine forests. This may be advantageous to the hawks, owls and eagles that prey on small mammals in open forests and small clearings. Pollinators such as moths and butterflies may also benefit from thinning that increases the amount of light reaching tree foliage and the forest floor.

Timber production: If you are managing your forested land for future timber production, thinning will be an important part of your management plan. Thinning allows the residual trees to get more light water and other important resources for growth. This allows them to allocate more of their energy to diameter growth, which can increase a tree’s volume and, potentially, the market value of the entire tree stand.

Thinning techniques

The tools and methods of thinning are quite varied, and can produce significantly different results. The type of thinning you select may depend on your objectives for your trees and on your tree stand’s individual characteristics, such as its species composition.

When managing for forest health and fuel reduction, private landowners typically use the “thin-from-below” method. Thinning from below consists of removing smaller, low-hanging branches and smaller nearby trees. By removing stagnant, unhealthy trees, the thin-from-below method mimics the tree mortality that is caused by competition from other trees or by surface fires. The tree stand will benefit from having fewer but larger, healthier and fire-adapted trees competing for available water, light, soil nutrients and other resources.

When to thin

Thinning is best accomplished in the late summer and early fall if possible. At this time trees will be least susceptible to damage from the thinning operation and the populations of insects that may be attracted to the slash created will be low. Winter also is an acceptable time to thin, but can lead to soil compaction and erosion if done at the wrong time. Thinning in spring and summer is not recommended as it can attract insects such as bark beetles and can affect wildlife, particularly nestlings.

Find more information about the free services and technical advice that DNR offers to qualifying small forest landowners on the DNR website.

This blog post originally appeared in Forest Stewardship Notes, a free quarterly e-newsletter for forest owners of all sizes. The author, Melissa Fischer, is a forest health specialist in DNR’s Northeast Region. View the full menu of free DNR e-newsletters