Spring still seems out of reach, so while we’re awaiting balmier days, let’s take a moment to appreciate some of nature’s subtle, yet generous gifts. We owe everything to the natural world and even modest contact with it refreshes and offers solace. While contemplating the obvious things that nature provides—food, water, clean air—it’s easy to overlook the little (and not so little) things.
Plants, the primary producers on this planet, belong to irreplaceable, intricate, ancient ecosystems, within which they support and depend on other species—both flora and fauna— to survive. I like to think of it as everlasting give and take. This post honors one of my favorite Pacific Northwest natives whose gifts are mammoth. Quercus garryana, commonly called Oregon white oak (or “Garry oak” by those in British Columbia and Washington), is a slow-growing, very long-lived, majestic, deciduous tree that, with time, grows beautifully gnarly. As a keystone species, oak trees are vibrant communities in themselves, and support more life-forms than any other trees in North America.
Late last fall, while strolling along a trail at Jackson Bottom Wetlands Preserve (just west of Portland), I was awestruck by the amount of life attracted to the broad canopy of just a single mature Oregon white oak: Visible and audible were multiple white-breasted nuthatches, black-capped chickadees, downy woodpeckers, and red-breasted sapsuckers, all busily going about their foraging business with such enthusiasm that all I could do was look upwards, my mouth agape. The birds weren’t seeking the tree’s highly nutritious acorns, which sustain many other birds, as well as insects, mammals, and reptiles—they were consuming a tasty assortment of insect herbivores, which oak trees are particularly adept at generating. Studies show that the genus Quercus hosts more caterpillars and other insect life than any other genus in the northern hemisphere. This proficiency is especially important during breeding season, when the vast majority of landbirds consume and feed their young highly nutritious insects or their larvae, and other arthropods such as spiders—not seeds or fruit. Other studies show a higher diversity of bird species in oak forests than in nearby conifer forests (although pine forests are quite exceptional as well).
Like other native keystone tree species, Oregon white oak peacefully regulates ecosystem processes like nutrient cycling and energy flow, which provides benefits to wildlife (and the rest of us) that seem endless. Besides the obvious shade, beauty, and exchange of oxygen and carbon dioxide that these trees offer (trees really are the best carbon sink), inconspicuous flowers—which typically bloom in late spring—provide for pollinators like native bees, while the buds of forthcoming rounded, deeply lobed leaves play host to the larvae of gray hairstreak, Lorquin’s admiral, echo blue, California sister, and propertius duskywing butterflies. Speaking of leaves, it typically retains dead leaves on its branches until spring, a process known as marcescense. (It’s believed that marcescense, which is more common on young trees, may serve to protect new buds on branches by discouraging browsing animals from grazing. There’s also speculation that marcescent leaves help oaks create a nutrient-rich mulch when the trees need it most —in springtime. But no one knows for sure.)
In addition, cover, perches, and nesting habitat go to birds such as woodpeckers and vireos, as well as native squirrels. Oaks’ acorns sustain squirrels and other mammals, as well as many bird species. Fallen leaves, which might provide habitat for arthropods, amphibians and reptiles, slowly break down into a rich leaf mold that supports soil-dwelling invertebrates and numerous fungi that allow neighboring plants to thrive. Sugars and carbon are provided for mycorrhizal fungi, which reciprocate with nutrients for growing plants and contribute to the soil carbon pool. Intact bark creates microhabitat for mosses, as well as lichens that supply food, shelter, and nesting material, while loose bark and twigs contribute to nest building as well as browse for deer, which in turn feed carnivores like cougars.
And as oaks deteriorate with advanced age (which can be 500 years), they continue to deliver. Dead trees can last many years as snags, which provide food, nesting material, and housing to cavity nesters like owls, kestrels, woodpeckers and chickadees, as well as bats who may roost in old holes or under loose bark.
How it grows
Elevation, climate, soil, and water persuade Oregon white oak to vary immensely in habit and size. While it thrives in cool, coastal areas and near the edges of streams and wetlands where it tolerates seasonal flooding, it also flourishes in droughty inland sites where it may grow both individually and in groves on low hills surrounded by grasslands. When it occurs on gravelly sites or rocky slopes with thin soils, it often has a shrub-like or scrubby habit. Along the blustery Columbia River Gorge, where it grows with little rainfall and atop hundreds of feet of layered basalt, harshly battered trees grow gnarled but hang on thanks to a very extensive and strong root system. As seedlings, this oak’s root mass may be ten times as large as the aboveground growth.
Within the richer, deeper, riparian soils amongst tapestries of dazzling wildflowers and grasses in the Georgia Basin-Puget Trough-Willamette Valley ecoregion of British Columbia, Washington and Oregon, it acts as a keystone structure, typically growing a very broad canopy, and reaching heights 100+ feet over hundreds of years. Gigantic root systems may grow two or three times wider than the canopy. The ecoregion includes savannas (grassland with trees scattered at least 100 feet apart), upland prairies (another type of grassland), wet prairies, and shady oak woodlands with a continuous or semi-open canopy. I’ll call them, collectively, prairie-oak ecosystems.
To really appreciate an oak, it’s helpful to know something about its unique ecosystems that once provided some of the richest habitat in the world. The historic range of Q. garryana stretches from low elevations of southwestern British Columbia (including Vancouver Island and nearby smaller islands) to California. In Washington, it occurs mainly west of the Cascades on Puget Sound islands and in the Puget Trough, and east along the Columbia River. In Oregon, it is indigenous to the Willamette, Rogue River and Umpqua Valleys, and within the Klamath Mountains.
When pioneers and naturalists encountered prairie-oak ecosystems, they found a breathtakingly beautiful and rich mosaic of plant and animal life. Journals of early Oregonians described massive prairies with five-mile-wide dense forests of ash, alder, willow, and cottonwood that skirted meandering rivers within floodplains. Marshes and sloughs developed during high water periods but often dried out by late summer. At higher elevations within these forest corridors were oak and associated trees. Above the floodplains were upland prairies, filled with herbaceous plants and grasses that could tolerate the parched soil of summer, as well as winter wet. Oak woodlands stood on low hills above the valley floors, surrounded by grasslands, also known as savanna.
But the landscape was not untouched or pristine. Aboriginal peoples managed parts of the ecosystems following the last glacial period, frequently using prescribed burning to boost edible plant productivity, aid the hunting of wildlife, limit the growth of conifers, and facilitate travel, particularly in the northern parts of the oak’s range. Harvesting of plants such as camas (Camassia spp.) and chocolate lily (Fritillaria affinis) also caused soil disturbance, but their eco-cultural manipulations pale greatly compared to what came later.
Since Euro-American settlement, as much as 99 percent of the original prairie-oak communities that were present in parts of the Pacific Northwest have been lost and many rare species dependent on them are at risk of extinction. Extensive destruction and fragmentation began with settlement in the 1850s, with clearing, plowing, livestock grazing, wildfire suppression, and cutting of trees for firewood and manufacturing. Prairie wetlands bejeweled with wildflowers were drained and ditched. Later, subsidies to ranchers encouraged more destructive grazing, while urban sprawl and agricultural use—fueled by human population increase—intensified. Invasion of nonnative species, and the encroachment of shade tolerant and faster growing species—that proliferate with fire suppression—outcompeted oaks and decimated additional native flora and fauna. Prairie-oak ecosystems and associated systems still continue to disappear at human hands, and isolation of the tiny remaining fragments prevents the migration of wildlife and healthy genetic material from one area to another. Other detrimental factors include diseases and parasites, climate change, and the loss of wildlife that cache acorns and perform other essential functions.
Despite continual destruction, there is a renewed and growing appreciation for the diversity and beauty of these habitats, motivated by recognition that we are responsible for what’s been destroyed, an admiration for the interconnected wild species the habitat supports, and a reverence for an iconic, magnificent tree. Intervention has become intensive, and collaborations and partnerships—along with private landowners, who are key partners—are working to reverse the downward trend with preservation, restoration, and management tools, although “a major restoration challenge is restoring wet prairie habitat to a level at which it can maintain resistance to invasive species,” according to the Institute for Applied Ecology.
Regeneration of oak seedlings is essential, but is often difficult. Acorns look tough, but they are viable for only about a year and may be subject to parasitism, weather extremes, and genetic isolation. Consequently, just a small percentage become trees. Two independent studies determined that oak seedlings do best when caged, but protection from other deterrents—drought, competing plants, and rodents—is important, depending on location.
Regional conservation groups include the Garry Oak Ecosystems Recovery Team and the Cascadia Prairie-Oak Partnership.
Try it at home
While the maintenance of only fragments of a past ecosystem is a poor alternative to former richness, if you live in the ecoregion (or other impoverished oak-dominated ecosystem) and want to help, choose this native tree. Even a single isolated tree can be a critical habitat structure on the landscape. It’s the only oak native to Washington and western Canada, and the dominant one in Oregon (black oak—Quercus kelloggii—is another beautiful and valuable large tree that occurs from Lane County, Oregon, south to Baja, at low to high elevations).
An Oregon white oak tree needs a mostly sunny, well-drained site that can accommodate its eventual size aboveground (25-50 feet wide, depending on spacing) and enormous root systems described above. Those grown on poor, dry, rocky sites will grow quite a bit smaller and have a shrubby habit. When planting more than one, space trees 20 to 60 feet apart, using the closest spacing only in dry, rocky terrain. It may be most helpful to visit a nearby natural area and then try to mimic nature’s arrangement.
To maintain genetic integrity, always choose trees or seeds that originated from trees close to your location and from similar terrain. For best results, plant dormant saplings in late fall after rains begin. After watering, apply about three inches of an organic mulch to reduce evaporation and keep weeds (that can steal water and nutrients) down. I prefer low-nitrogen leaf compost, spread out to the tree’s drip line and kept at least a foot from the trunk to prevent rot. Oaks do not need rich soil, so don’t apply synthetic or organic fertilizer because most North American trees don’t need fertilizer and may even respond adversely to it. And don’t use those watering bags that only water at the base of the trunk and may promote rot.
Though this species is drought tolerant, provide ample summer water, deeply and infrequently until established. During the first summer I like to water roughly every five days with about 10 gallons of water that’s applied so that it sinks in slowly. During the second and third summers, water once a week, 10-15 gallons, being sure to water out to the root zone (drip line) and beyond—root spread can be more than twice that of the crown. If severe heat and prolonged droughts appear to be stressing a young tree, provide more water. After the first few years it may do fine on its own, but do water it (deeply) if it appears to be drought stressed. Keep the area well weeded and don’t stake trees unless they are in very windy areas—they’ll grow much stronger if left unsupported. Keep in mind that soil compaction, hardscape, lawns and irrigation systems around water-sensitive oaks are a major cause of their decline in residential areas. Here is more info on how to plant Oregon white oak.
Grab a partner
As with other native species, oaks will function best when grown within a habitat and community type that consists of plants that evolved together and need the same conditions. Figuring out which community occurs in your area requires a walk in a nearby natural area where species, as well as nature’s organization, can be learned. Some associate trees that might thrive with your oak include Oregon ash (Fraxinus latifolia) on moist sites, and madrone (Arbutus menziesii) on drier sites, and Pacific ponderosa pine (Pinus ponderosa subsp. Benthamiana). For shrubs, consider california hazelnut (Corylus cornuta var. californica), osoberry (Oemleria cerasiformis), serviceberry (Amelanchier alnifolia), snowberry (Symphoricarpos albus), oceanspray (Holodiscus discolor), red-twig dogwood (Cornus sericea), and tall Oregon grape (Mahonia aquifolium), depending on your location. Sword ferns (Polystichum munitum), orange or pink honeysuckle (Lonicera ciliosa or L. hispidula), fescues (Festuca spp.), and many wildflowers, including allium (Allium cernuum), camas (Camassia spp.), meadow checker mallow (Sidalcea campestris), western columbine (Aquilegia formosa, pictured right), and shooting star (Dodecathon spp.) associate in different parts of its range.
To find out which habitat type and plant communities would likely have grown in your area, check out this Ecoregional Assessment, or query your county’s soil and water conservation district or native plant society chapter. The following publications may also be helpful:
~ Georgia Basin: Garry Oak Ecosystems Recovery Team
~ Puget Trough: Prairie Landowner Guide for Western Washington
~ Willamette Valley: A Landowner’s Guide for Restoring and Managing Oregon White Oak Habitats
© 2017 Eileen M. Stark
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