1120 SW Fifth Avenue, Portland, OR 97204
The 100-acre South Escarpment Unit extends from the Sellwood Bridge north to the Ross Island Bridge and includes Oaks Bottom Wildlife Refuge, which is publicly-owned and managed by Portland Parks & Recreation. The wildlife refuge is a focus area for fire reduction projects along the South Escarpment Unit. About 75 acres of this unit are on public lands, within the wildlife refuge and Sellwood Park. The south unit also includes additional State of Oregon and Metro parcels, and several privately-owned properties.
Oaks Bottom Wildlife Refuge is a 170-acre wetland at the base of the southern Willamette Escarpment along the east bank of the Willamette River. It is rare for such a natural wetland to exist in the heart of a city. The refuge also protects remnant riparian cottonwood forest growing along the river bank. The upland bluffs of the escarpment are an important complement to the riparian and wetland woodlands on the refuge floor.
Changes at Oaks Bottom began with two sets of railroad tracks on trestles that crossed the site in the early 1900s. As the trestles deteriorated, a dirt berm was constructed, which cut off natural flooding from the Willamette River. The City acquired the first 115 acres of the existing refuge in the late 1950s. At that time, about 20 acres in the south end of the wetland had been used as a refuse dump, filled with 10-15 feet of waste, and covered with a thin layer of soil. In the 1960s, 30 acres of the north end of the floodplain wetland was filled with 10-15 feet of excavated soil from the Stadium Freeway (I-405) project by the Drake Company, which sold its 50-acre holding to the City in 1969. A 5-foot culvert under the railroad berm still allows flows from the Willamette River into the center of the wetland. Portions of the site were heavily used by off-road vehicles in the 1960s and 1970s.
These changes in hydrology, along with disturbing the native vegetation and soils, resulted in a landscape choked with weeds. The north and south ends of the floodplain were converted into dryer upland habitat with grasslands that burn more often and that are also in close proximity to the upland forests along the bluff, a combination that allows more naturally frequent grassland fires to spread up into forest canopies.
Conservation efforts began in 1972, after a campaign by several groups. The site was surveyed by the state and maintenance by several groups began in 1984. In 1988 Oaks Bottom became Portland's first officially designated wildlife refuge. A management plan written by Oaks Bottom Coordinated Resource Management Group (CMRG) was adopted. Restoration projects have expanded native habitat remnants including the oak/madrone/maple woodland along the escarpment overlooking Oaks Bottom, which is home to many species including salamander and frog species. Projects have included invasive plant removal, native plant establishment, and creating vernal pools. In 1991 the water control structure proposed by the CMRG was installed to control invasive vegetation, enhance wildlife habitat, and control floodplain mosquitoes.
WILDFIRE RISK REDUCTION & HABITAT RESTORATION
Although there have been considerable restoration efforts within the Oaks Bottom Wildlife Refuge over the past 15 years, a vegetation inventory conducted by PP&R from 2002-2003 shows that much of the Willamette escarpment along Oaks Bottom is still dominated by invasive plants such as English ivy, clematis, Himalayan blackberry, and non-native grasses. Clematis and Himalayan blackberry increase wildfire risk by acting as ladder fuels from the ground into tree canopies. As invasives with few or no environmental controls, they produce more leaves and stems than native vegetation, providing more medium and fine-sized fuels that can act as tinder, helping fires to start and spread to larger fuels. Finally, the volume combined with their more flammable nature also contributes to a higher wildfire risk. Other highly flammable plants along the south escarpment include Scot's broom, thistle, and invasive non-native grass species.
Fire plays a necessary role in the health of forests and grasslands by reducing fuels that build up over time, which can raise the risk of large wildfires that threaten homes, particularly where neighborhoods meet natural areas. Using fire to reduce wildfire risk also prepares the site for seeding with native grasses and perennials that will, in time, result in grassland with higher habitat values and lower wildfire risk.
Prescribed burns have been used in Oaks Bottom by the City of Portland since the mid-1990s as a management tool to reduce fuels, kill invasive plant seed banks, and return nutrients to the soil. Funded by the 2006-2009 FEMA Wildfire Risk Reduction grant, the City of Portland conducted prescribed burns in Oaks Bottom Wildlife Refuge in September 2006, August 2007, and August 2008. The 2006 burn was conducted on an 8-acre grassland area in the south end of Oaks Bottom Wildlife Refuge. The 2007 burn was conducted in the north end of the refuge. The north end was burned again in 2008.
DESIRED FUTURE CONDITION
Through a collaborative process with land management agencies, homeowners, and other interested parties, the Desired Future Condition (DFC) has been identified along with steps needed to reach these goals. PP&R has developed a generalized DFC map based on the natural history of Oaks Bottom, the 1988 Coordinated Resource Management Plan, and its current environment.
The City will continue to manage vegetation to reduce wildfire risk. On the refuge floor, techniques will include planned burns. On the upland slopes where fire cannot be safely used, fuels will be managed using other methods such as cutting, mowing, and herbicides to reduce the potential for grassland fires to move up into the forested slopes.
The City will also provide support to private land owners to implement management projects on their properties with the financial assistance and professional guidance through the Wildfire Risk Reduction Project.
For additional information about the FEMA Wildfire Risk Reduction Project, please contact:
Mark Wilson, PP&R Ecologist