OREGON STATE UNIVERSITY

Southwestern Oregon streams: a flawed view of the past

11/23/2011

CORVALLIS, Ore. – The hands-off approach to riparian zones in southwestern Oregon is perpetuating congested, conifer-dominated streams that bear little resemblance to pre-European settlement conditions, and may be detrimental to long-term ecosystem health and some goals of the Northwest Forest Plan, a new study concludes.

Contrary to common assumptions, stream sides in this region were an open forest dominated by a mix of conifers and hardwoods until the 1850s, according to an Oregon State University study.

The forest cover prior to settlement varied, ranging from an open savannah at low elevations to a low-density forest at mid-elevations, with grass and shrubs in the understory that provided more food for fish. There was likely less shade and woody debris than the conifer-dominated streams seen today.

In a study published in Forest Ecology and Management, researchers said there is a flawed perspective about the historic nature of these streams before settlers arrived. At least some thinning and use of prescribed fire may be necessary to regain those conditions, they said.

“The major changes in these streams at first had little to do with logging or fire suppression, and actually started around 1850,” said David Hibbs, a professor in OSU’s Department of Forest Ecosystems and Society.

“Settlers brought in grazing animals, cleared land, and Native American populations declined,” he said. “These and other changes allowed more Douglas-fir to move in and displace the oak, madrone and other hardwood trees and shrubs that used to be a major part of the forest. At mid-elevations, white fir has even begun to replace the Douglas-fir.”

Before settlement, these riparian zones were actually more similar to the upland areas, with well-spaced trees, grasslands, and shade-intolerant tree species that could deal with more frequent fire, researchers said.

“They were quite open compared to what you see today,” Hibbs said. “And with our current policies of fire suppression and no timber removal, they are moving even further from their historic condition. Our vision of what these lands used to be is inaccurate. The question is where to go from here.”

In the days before European settlement, Hibbs said, the streams probably had less shade to cool them than many of them do today, more and higher quality leaf litter, and less woody debris. Both water temperature and woody debris are important to stream health. But deciduous trees and shrubs also provided greater volume and higher quality of litter that feeds insects and ultimately fish.

This description probably applies to forests of southwestern Oregon and parts of the northern Sierra Nevada range in California, Hibbs said, but not to streams in other locations that are not as hot or dry.

The researchers did a historical analysis of this region by analyzing existing tree species, tree rings, fire scars and other evidence to gain a better perspective on what these stream ecosystems looked like when affected only by Native Americans.

The report observed that:

  • Many of these stream ecosystems developed with more frequent but low- or mixed-severity fire and were dominated by a mix of conifers, deciduous trees and shrubs.
  • Major changes in forest composition began in the late 1800s – of the live conifers in some areas, five times as many germinated between 1850 and 1900 as in the 280 years before that.
  • Fire exclusion in the 20th century has allowed the changes begun in the 1800s to continue.
  • The change in tree and shrub composition is now reducing food and habitat for both aquatic and terrestrial wildlife, including a primary food source for the northern spotted owl.
  • A century and a half of fire exclusion and, more recently, regulations against any tree removal have created congested forests that may be moving even further away from historic conditions and ecosystems.

“The policy for riparian management areas in the Pacific Northwest, though well-intentioned, may be detrimental to the long-term health of riparian forests in regions shaped by fire,” the researchers wrote in their conclusion.

If historic and more natural conditions are desired, they said, partial harvest treatments and use of prescribed fire in some riparian areas would help to gradually restore them.