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Why old trees matter

Posted by Hannah Meganck on June 8, 2017

Blue Mountains Restoration Strategy spotlights the value of old trees in creating resilient landscapes

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Ochoco National Forest. Photo credit: U.S. Forest Service

Restoring our landscapes to more resilient conditions requires us to consider the mixture of tree species, spacing of those trees, and diversity in age class present across a landscape. A multitude of mid-seral closed forest conditions and overly dense forest stands across the Blue Mountains pose risks to the valuable ecosystem services that the forests provide, especially in the face of a changing climate. Restoration treatments in priority watersheds present an opportunity to be thoughtful about where we conduct active management. This includes mechanical thinning and prescribed fire to conserve and promote wildlife habitat, increase tree species diversity, and improve spatial pattern and variation to create more resilient forests.

Addressing degraded forest conditions and improving forest resilience will only be effective if there is an intentional effort to promote the re-establishment of large, old trees. The Blue Mountains Restoration Strategy and Forest Resiliency Project strives to retain old trees as part of the existing large tree structure in restoration planning because they are underrepresented in many sub-watersheds.     

In the Blue Mountains, ponderosa pine and western larch are two species forest managers look to conserve because both are fire resistant. These tree species have thick bark, which helps protect them from fire. When a wildfire does come through the area, ponderosa pine and western larch are more likely to survive than other tree species. After a disturbance, the surviving trees are an important seed source for regeneration. Historically, these fire resistant trees persisted from one disturbance to the next. Selective harvesting of fire tolerant species like ponderosa pine and western larch, combined with fire suppression and the impacts of current regulatory requirements, have resulted in a shift in tree species composition in the Blue Mountains towards more shade-tolerant species that are generally younger in age and smaller in size. Today, there is a lack of open stand conditions with large, old fire tolerant trees in the Blue Mountains ecosystem.

Large, old trees provide important habitat to many wildlife species, such as the Northern Goshawk. This migratory forest raptor is a powerful hunter capable of killing a variety of prey, including tree squirrels, hares, grouse, corvids, woodpeckers, and large passerines such as American Robins. The Goshawk prefers mature forests with large trees for nest stands and a surrounding landscape of diverse forest and habitat types for hunting. Creating a resilient landscape includes creating openings and different forest types and structures. These not only provide important habitat for the many birds and mammals the goshawks hunt, but the forest complexity and spatial variations help alter fire behavior, protecting the stands of old trees and mature forest.

Ochoco National Forest. Photo credit: U.S. Forest Service
Ochoco National Forest. Photo credit: U.S. Forest Service

It is easy to tell if a tree is “large,” but how do forest managers determine what is an “old” tree? Various tree species show their age in different ways. Size is not always an indicator of age, particularly for a species like ponderosa pine that can grow in a wide range of conditions, from rocky cliffs to moist habitats. The size of a ponderosa pine may reveal little about its age, but many other features of the tree are good indicators. For example, bark characteristics provide clues to the age of a ponderosa pine. According to Van Pelt (2008), orange-yellow bark, comprised of small, puzzle-piece looking flakes, is characteristic of an older tree. Younger ponderosa pine trees have bark that is nearly black in color. As the tree ages the outer layers of the bark ridges begin to flake off, revealing the reddish brown color characteristic of mature trees. With age, the outermost bark continues to flake off, causing the colorful plates of outer bark to get wider, while the width of the dark cracks in between remain relatively constant. 

There is not always a straightforward way to determine the age of a tree just by looking at it. Since old trees are a unique and valuable part of ecosystems, an important part of restoration planning is recognizing tree characteristics that are an indicator of age.

The Forest Resiliency Project provides an opportunity for land managers to take a landscape-level approach to creating a more resilient Blue Mountains ecosystem. The Forest Resiliency Project is designed to set up our forests to be adaptable and resilient with natural disturbances in the face of a changing climate.  By strategically reducing stand density across large landscapes (thinning the number of trees on an acre), promoting species composition more in line with historical conditions (altering the species of trees and other vegetation growing on a site), and creating mosaic forest patterns, we can reduce the risks of uncharacteristic wildfires and allow fire to play its natural role on the landscape.

This article series Features from the Blue Mountains Restoration Strategy is brought to you by Sustainable Northwest and the U.S. Forest Service Blue Mountains Restoration Strategy Team.