The traditional emphasis on dense, fast growing, conifer forests in the Pacific Northwest raises questions about the health of dozens of bird and animal species that depend on shrubs, herbs and broadleaf trees, suggests a new analysis by Oregon State University and the U.S. Geological Survey.
Conifers are evergreen trees such as fir and cedar that dominate the forests of the Pacific Northwest.
The study was just published in "Forest Ecology and Management," by 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, which funded the study.
At least 78 vertebrate species have been documented that require the food or habitat provided by non-coniferous vegetation, and may be at risk whenever forest management reduces the prevalence of these shrubs or trees, or targets them for removal.
Among these species of concern are three birds, one amphibian, and five mammals that already have federal or state status as threatened or endangered.
Declines of western bluebirds have been linked to reduction of available nest sites. Similarly, a major threat to the willow flycatcher is destruction of shrubby vegetation.
Mountain quail populations have declined due to loss of upland shrub habitats, plant species diversity and loss of woody vegetation in riparian zones. And a major threat to Columbian white-tailed deer has been removal of "brush" during logging or agricultural development.
In similar fashion, 90 percent of the diet of the northern spotted owl is composed of small mammals that are associated with non-coniferous vegetation.
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 get through the winter. Many species of insects depend on specific host plants, and in turn form the diet for many birds and some mammals. And in the conifer forests of western Oregon, hardwood trees support the abundance of 69 percent of the butterflies and moths.
"Historically, forests contained significant amounts of alder, big leaf maple, white oak or vine maple," Hagar said. "The undergrowth would feature vegetation species such as California hazel, ferns, Oregon-grape, salal, many other types of shrubs and herbs. And this type of vegetation, in turn, provides the habitat and food base for many wildlife species."
Even when the primary goal of a private or public forest is sustainable timber production, Hagar said, the lack of historic tree diversity, shrub and vegetation species may have long-term impacts on forest health, including ability to resist disease, soil function and fixation of nitrogen.
An illustration of this concern is a current epidemic of Swiss Needle Cast, a tree disease occurring in areas that used to have many diverse tree and shrub species, but which have been largely converted to a monoculture of Douglas fir.
Hagar said management options for a wider diversity of vegetation and the wildlife species that depend on it 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. It may take a decade or more, she said, for shrubs, herbs and broad-leaf trees to recover.Environmental News Service
