The north-flowing Willamette River and its tributaries form the Willamette Valley as we know it today in the northwestern quarter of Oregon. During its geologic history, the Willamette Valley has seen both fire and water. First fire, with lava flowing into the valley from erupting fissures in eastern Washington around 16.7 to 15.6 millions years ago (Ma). Successive lava flows covered a vast area of around 80,000 square miles that included much of the Willamette Valley. Second, and much later came multiple catastrophic Missoula (or Bretz) floods around 15,000 to13,000 years ago (ka).
This Bucket focuses on a very small area of 250 acres at the north end of the valley — the Graham Oaks Nature Park. I came across this park while on a quick work project in the Portland area. Graham Oaks was near where I was working and staying for a night. I planned on spending a little time there in the morning before driving home to Seattle. I spent all of one and one half hours walking the park trails. How much can we see in that short timeframe?
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Graham Oaks Nature Park officially opened in 2010. Before it became a park, it was farmland and from what I observed, some of it was a Christmas tree farm with trees growing in neat little rows, but now intertwined and grown tall beyond anything that would fit into a normal house. Other farming included seed grass and a 6-acre filbert orchard.
The park includes a small tract of native Douglas fir-bigleaf maple woodland, a few pocket wetlands, but it is mostly wide open savannah-in-the-making. From what I’ve read in researching this diary, 15,000 Oregon white oak saplings have been planted in an effort to restore the oak savannah habitat. These saplings won’t mature for another 100 years. At this winter season, the wide open spaces of the park were covered in dormant brown grasses that appeared mowed and the surface lightly disc harrowed. In addition to the oak saplings, 150,000 native trees and shrubs have been planted along with native seeds. In spring, this park will be gloriously ablaze with green grass, wildflowers, bees, butterflies, and dragonflies.
Underlying the surface here are sedimentary deposits laid down by the Missoula floods. These deposits are named the Willamette Silt Formation. The formation is composed of lacustrine fine sandy silt and clay. Lacustrine means that the silt was laid down in a lake bottom environment. When the floods occurred, they were constricted downstream on the Columbia River at the Kalama Gap. The constriction caused the floodwater to pool up into the Willamette Valley forming successive temporary lakes with each outburst flood. Dozens of these floods backed up into the Willamette Valley as ice dams were formed and breached multiple times. No outcrops or exposures of the Willamette Silt Formation are visible here except where creeks have cut down into the soft formation, exposing it in the stream banks.
The following are excerpts of the geologic map of the local area around Graham Oaks.
To start my walk, I wandered through through the open savannah areas. The most common birds were California scrub jays. I also spotted an American robin, a northern flicker, and an American kestrel. The photos aren’t the best but provide a sense of the local bird life in the open areas.
As I walked the path, this little vole caught my eye as it darted out and then quickly back into its burrow.
Part of Graham Oaks is a wooded tract of native Douglas fir and bigleaf maple. I took a path wandering through these woods. I found a good variety of moss, fungi, and lichens. I had read that much invasive English ivy was cleared from these woods allowing the native species to flourish.
And then surprised by a golden-crowned kinglet.
The main feature of Graham Oaks is this legacy Oregon white oak, named the Elder Oak. It’s estimated at from 150 to 200 years old. The roots fan out beyond the drip line. The split-rail fence protects the shallow roots from too many treading feet that would compact the soil. This is the only mature oak in the park. The 15,000 planted saplings will come along in another 100 years.
Some information on the Oregon white oak from OSU - Oregon Wood Innovation Center.
General Characteristics
Oregon white oak, a member of the beech family (Fagaceae), is one of only four deciduous oaks native to the West Coast. The massive, branching trunks and broad crowns of old white oaks are characteristic features of valley woodlands in the Pacific Northwest.
Geographic Range
Oregon white oak has a wide latitudinal range from Vancouver Island (lat 49° N) to southern California (lat 34° N), although it takes on a shrub form toward the southern end of this range. It is well distributed throughout the valleys west of the Cascade Range and Sierra Nevada on inland slopes of the Coast ranges.
Ecological Role
Oregon white oak is a persistent climax or sub-climax species on dry sites or under regimes of periodic fire. Large oaks have thick bark and are resistant to fire. Smaller trees are generally killed or badly injured by fire. Oregon white oak is an early successional species on better sites, where it is replaced by Douglas-fir and bigleaf maple in the absence of fire. Historically, periodic fires were a major factor maintaining Oregon white oak woodlands. After a century of fire exclusion, many acres have progressed from open Oregon white oak, to closed white oak, to Douglas-fir. Fire prevention is probably causing continued decline in the extent of Oregon white oak type forests. The white oak type will continue to diminish without periodic fire.
This last line: “The white oak type will continue to diminish without periodic fire.” I wonder how the park will manage the oak savannah habitat without fire as a management tool. The park is surrounded on three sides by residential subdivisions and a school. So, fire is not an option.
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