Metro is working to protect oak habitat, which has declined dramatically throughout the Willamette Valley.
Major restoration project under way at Willamette Narrows natural area near West Linn
On a warm June morning, a Pacific wren warbled complex melodies high in a canopy of craggy oaks. Seed-heavy native grasses like blue wild rye and brome bowed in a breeze. Atop rocky outcrops, prairies bore delicate blooms of purple brodiaea, yellow hawk's beard and native Triteleia, a white lily – the latest in a succession of color at Metro's natural areas in the Willamette Narrows.
This sun-dappled oak woodland is a spectacular landscape, encompassing steep Willamette River cliffs and rocky islands. But scientists know that something is amiss. Douglas fir and Oregon ash are rapidly shading out the Oregon white oak and other plants that are part of the oak community.
At Metro's 16,000 acres of natural areas and parks, oak habitats have become a top priority for protection and enhancement. Many residents of the Portland area are surprised to learn that much of our region, so associated with Douglas fir, Western red cedar and hemlock, looked very different just a few hundred years ago. Much of today's conifer-rich landscape was then light and open oak woodlands and prairies.
Better mapping is showing scientists how dramatically oaks have disappeared, said Jonathan Soll, manager of Metro's science and stewardship division. "Compared to 15 years ago there's a much larger awareness of the decline of oak habitats in the Willamette Valley," he said.
One map, developed by the Nature Conservancy and the Oregon Natural Heritage Program, used surveys from the 1850s to define historic vegetation zones. It and other investigations indicate that as much as 1 million acres in the 3.3 million-acre valley were covered in Oregon white oak savanna and woodland prior to settlement in the mid-nineteenth century. Today, only 8 percent remain.
The Willamette Narrows natural area, which spans more than 500 acres, contains some of the largest known contiguous expanses of Oregon white oak trees in the Portland area. That's why Metro is taking steps this year to make sure they thrive.
Oak management: an ancient practice, revived and updated
Marked with an "S" for "snag," this Douglas fir tree in the Willamette Narrows will have the top cut off; the rest will be left as a source of food, rest and shelter for wildlife. "With compass in hand, we looked at every tree and its sun exposure," said Metro scientist Elaine Stewart, project manager for the Narrows restoration. "Not every fir will be cut. We wanted to target tree removal."
Since settlement began, nearly all of the valley's oak habitats have been converted to agriculture or urban uses, a trend that continues. Settlement also brought fire suppression, causing further declines in oak acreage. For centuries, indigenous people had nurtured oak habitat, setting fires in oak woodlands and savannas to improve hunting and the production of plants they relied upon for food. These frequent, low-intensity fires killed oak competitors but left the thick-skinned oaks to thrive. Natural fires also helped ensure conditions hospitable to the oak. Without fire to burn off competing vegetation, faster-growing Douglas firs eventually overtop and shade out oaks.
That's the case at Willamette Narrows, where land hasn't been managed for decades. Competing trees are shading out the once light-filled forest of oaks and native habitat that depends on them, such as madrone trees and white rock larkspur. Invasive, non-native plants, including spiny dogtail grass and Scotch broom, are outcompeting natives such as blue wildrye and yellow wood sorrel.
Metro's goal at the Narrows is to "release" the oaks — in other words, eliminate their competition, minimize invasive species and maintain a plant community rich in native biodiversity. Technicians have already selectively sprayed and hand-removed non-native plants. The most significant aspect of the work comes this fall, when Metro will fell or top 651 trees that stand between the oaks and the sun. Eliminating these trees will free up water, nutrients and light for the oaks on more than 100 acres of the Narrows properties.
"We're going to benefit a group of species that depend on this habitat, such as the white-breasted nuthatch," said Metro's Elaine Stewart, project manager for the Willamette Narrows restoration work.
With three years of bird surveys at the site, Stewart is excited to observe the results of this fall's work. In 2008, Metro thinned trees in a similar landscape at Canemah Bluff Natural Area, across the Willamette River in Oregon City. "After the oak release at Canemah," Stewart said, "we had an immediate response; birds came that first year that hadn't been there before."
Metro's approach supports the Oregon Conservation Strategy, the state's blueprint for conserving at-risk species and habitat, said Susan Barnes, conservation biologist with the Oregon Department of Fish and Wildlife. She described Oregon white oak as a habitat "of highest conservation concern."
"A suite of species – plants and animals – is associated with Oregon white oaks," Barnes said. "We need to take actions to protect what is left, to re-establish oaks through planting of new trees and to enhance other areas by oak release."
Metro scientist Elaine Stewart was thrilled to find this white rock larkspur – Delphinium leucophaeum, a federal species of concern – in the Willamette Narrows in mid-June. "I don’t think there’s another place in the region with so many delphinium," she said. "The numbers here are remarkable."
Besides contributing to statewide conservation efforts, Metro's work on this and other oak habitats fills a void. When landowners or agencies build on or otherwise alter oak woodlands or prairies, they are not required to offset the loss of habitat by restoring, preserving or enhancing another nearby site.
"Oak is an upland habitat for the most part," said Elizabeth Ruther, North Willamette district habitat conservation biologist with the Oregon Department of Fish and Wildlife. "And, unlike wetlands and waterways, which are protected by the Clean Water Act, upland habitats are not protected. That's a big problem."
For Metro, managing natural areas is an ongoing commitment
Metro's Willamette Narrows project will outlive intensive efforts early this fall to release oaks from the shade of faster-growing competitors.
Data gathering and analysis will continue, as will a good amount of hand labor. In late October, staff will sow seeds of native wildflowers and grasses collected by volunteers from Metro's Native Plant Center in newly opened areas; in 2014, after curing in piles, the land's high fuel load will be burned off in controlled burns.
Metro is committed to "adaptive management" – a flexible style recognizing that each natural area is different, and can react differently to the same series of actions. One thing is certain, however: invasives come back. Many of their seeds remain in the soil. New seeds will be brought on the wind, via animal droppings and the soles of shoes.
The region, like everywhere on the planet, is increasingly vulnerable to exotic species that arrive by boat, in nursery stock, on shoes and in tire treads. Invasives are the number one reason ongoing management is vital for natural areas.
"For millennia, this place and other habitats were in equilibrium," said Stewart, the Metro scientist working on Willamette Narrows restoration. "Now, the land cannot take care of itself; invasive spaces, without action by humans, take over native plants."
By approving two natural areas bond measures, voters have supported Metro's work to protect and care for native landscapes – a job that enlists expert partners and citizen volunteers, too. Any curious nature lover can experience the results firsthand, Soll said, by going for a walk among the oaks.
"In my experience, when people hike in an oak savanna or oak woodland for the first time, they are amazed," Soll said. "They often don't realize these kind of places exist here … that a significant portion of the Portland area was once oak woodland and prairie. Oaks are part of the history of Oregon."
The Willamette Narrows Complex is not open to the public, but Cooper Mountain Nature Park and Canemah Bluff Natural Area share the same oak community of plants. Both are open to the public.