An acre of land can only grow so many trees. One acre of fertile forestland may be able to sustain 500 small trees, or 100 big trees. Thinning is an effective and powerful forest management tool that promotes tree growth and restores forest health. When thinning, forest operators remove slower-growing or defective trees to provide more space for the remaining trees to grow. Available water and soil nutrients benefit those that remain, resulting in bigger, healthier trees in a shorter period of time.
Federal, state and private forest landowners also use forest thinning to minimize the risk of catastrophic fire. By creating more space between trees, it becomes more difficult for a fire to spread from tree to tree.
Often, forests that are not thinned are filled with small trees, trees with lower-hanging branches, and a greater volume of dry brush and dead logs on the forest floor. Fire can then easily spread up trees on what’s known as a “fuel ladder,” leading to a crown or canopy fire that in most cases kills virtually all the trees in its path.
Science has shown that thinning can positively impact tree growth, species composition and resistance to insects and disease, as well as the quality of forest wildlife habitat.
Thinning out smaller and competing trees, creating standing dead trees (snags) and adjusting canopy diversity can create old-growth forest habitat in 80 to 100 years, while it might take nature more than 200 years to do the same.
Wildfires once regularly burned through Oregon’s dry-climate forests east of the Cascades. The low-intensity fires often killed smaller trees, leaving spaces among ponderosa pines.
Restoration thinning projects on central and eastern Oregon federal forests aim to recreate that natural process. The goal is to bring the forests back to the conditions they were in before wildfire suppression allowed the trees to become overcrowded. Overly dense stands of trees, in combination with species besides ponderosa pine, are fueling larger, more destructive wildfires. Thinning helps create stands that are healthier and more fire-resilient.
Without regular fires, Oregon’s dry east-side forests have grown much thicker. The unnaturally dense forests that exist today are more susceptible to insect and disease outbreaks and are fueling bigger, more intense wildfires that put nearby communities in danger.
Clearcutting is uncommon in central and eastern Oregon, with the exception of its use in lodgepole pine forests infested with mountain pine beetles. The reason thinning is much more common is because of the drier climate east of the Cascade Crest.
The drier forests in the eastern region of the state are fire-adapted ecosystems where, historically, low-intensity wildfires burned every 2 to 20 years. The fires naturally thinned out small trees and burned out less fire-tolerant trees species such as white fir and grand fir. This created pine tree stands that were maintained by fire. Restoration thinning mimics this natural process by taking out some of the non-pine species and alleviating overcrowding.
Another reason thinning is a more common harvest method in eastern Oregon is because the forests are often dominated by ponderosa pine. The canopies of ponderosa pine forests are less dense than those of Douglas-fir, so more sunlight reaches the forest floor. This fosters the regrowth of ponderosa pine seedlings, so there’s no need for commercial timber companies to create a clearing by clearcutting for the forest to regenerate.
As trees grow larger, they need more room to grow. In western Oregon forests, thinning is used to help alleviate overcrowding. By removing selected trees, thinning reduces the competition for sunlight, water and nutrients. This helps the remaining trees stay healthy and grow faster.
Thinning west of the Cascades is often done in younger stands of trees to increase their growth rate, although older trees may need thinning too. Forest landowners may choose to do pre-commercial thinning, which involves thinning out trees that are still too small to sell, or they wait to do a commercial thinning when the trees have grown large enough to produce marketable logs.