OSU: Managed Northwest conifer forests threaten many species

September 3, 2007

Forest management that prioritizes dense, fast-growing, conifer-dominated forests in the Pacific Northwest may threaten the health of dozens of animal species that depend on shrubs, herbs and broad-leaf trees, according to an analysis by Oregon State University and the U.S. Geological Survey.

At least 78 vertebrate species were documented that require the food or habitat provided by non-coniferous vegetation. These species may be at increasing risk whenever forest management reduces the prevalence of these shrubs or trees, or targets them for removal.

"Wildlife species that depend on the resources provided by non-coniferous vegetation may not persist in forests where these components are scarce," the report said.

Natural forests of the Pacific Northwest are dominated by conifers, said Joan Hagar, an affiliate faculty member of the Department of Forest Science at Oregon State University, and a wildlife biologist with the Forest and Rangeland Ecosystem Science Center of the U.S. Geological Survey.

But natural forests also provide a continuity of trees and plant life that are:

• Young and old, short and tall.
• Diverse shrubs, especially in the early stages of forest re-growth.
• Gaps, snags and cavities.
• A dozen or more hardwood tree species.
• Possibly hundreds of grass and herbaceous plant species.

In contrast, the report said, a managed forest is planted very densely with conifer trees, which dominate the forest within a short time. Broad-leaf trees and shrubs are often suppressed with herbicides or other management techniques. Even when new forestry techniques are used to encourage a diversity of trees with different sizes and ages, the undergrowth trees are usually conifers.

"Historically, forests contained significant amounts of alder, big leaf maple, white oak or vine maple," said Hagar. "The undergrowth would feature vegetation species such as California hazel, ferns, Oregon-grape, salal and many other types of shrubs and herbs. And this type of vegetation, in turn, provides the habitat and food base for many wildlife species."

When the primary goal of a private or public forest is sustainable timber production, said Hagar, the lack of historic tree diversity, shrub and vegetation species may have long-term impacts on forest health, including the ability to resist disease, soil function and fixation of nitrogen.

An epidemic of Swiss Needle Cast, a tree disease occurring in areas that previously had many diverse tree and shrub species, but which have been largely converted to a monoculture of Douglas fir, is seen as an example of the impact of reduced tree diversity.

Hagar reviewed the life history accounts for forest-dwelling vertebrate wildlife species, and identified 78 vertebrate species in Oregon and Washington that are associated with non-coniferous vegetation, often the foundation of major food webs.

Among these species of concern are three birds, one amphibian and five mammals that have special federal or state status:

• Declines of western bluebirds have been linked to reduction of available nest sites.
• A major threat to the willow flycatcher is destruction of shrubby vegetation.
• Mountain quail populations have contracted due to loss of upland shrub habitats, plant species diversity and loss of woody vegetation in riparian zones.
• A major threat to Columbian white-tailed deer has been removal of "brush" during logging or agricultural development.

Many species rely on a diversity of grass, herb, shrub and tree species for their energy needs, the report said.

• Fruits from deciduous trees and shrubs are a critical resource for migrant birds.
• Rodents cache seeds and nuts to survive the winter.
• Many species of insects depend on specific host plants, and in turn form the diet for many birds and some mammals.
• In the conifer forests of western Oregon, hardwood trees support the abundance of 69 percent of the butterflies and moths.

Additionally, 90 percent of the diet of the northern spotted owl is composed of small mammals that are associated with non-coniferous vegetation.

While supporting wildlife species, non-coniferous vegetation also makes important contributions to nutrient cycling, carbon sequestration, soil fertility and aquatic food webs, the report said.

Management options that include slower rates of conifer re-establishment, less-dense conifer plantations, more thinning of over-stocked forests, and less control of shrubs or other vegetation that would contribute to a wider diversity of vegetation and the wildlife species, said Hagar. However, it may take a decade or more for shrubs, herbs and broad-leaf trees to recover.

The study was published in Forest Ecology and Management.

Source: Oregon State University.

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