Oregon White Oak in Silverton

Understanding their biology leads to a recovery plan. First in a series.

part 1 of the Oregon White Oak series

The storm

The February 12, 2021 ice storm targeted the Oregon white oaks of Silverton in a ruthless way. Their branches broke and many toppled from the ground. It seems unfair because these trees sheltered us, and we loved them. They defined Silverton’s skyline more than other trees. At least 61 fell dead in Silverton. For our safety many more will be killed. How old were they and what takes their place? As we formulate plans to replace fallen trees and preserve those that remain it is important to understand the ecology of the oaks. This wider interpretation of the causes to this disaster ensures our reactions are better.

Oregon white oak, known by the Latin binomial Quercus garryana, are most common in two forms. Savannah oaks are ancient and epic, with thick trunks and broad spreading branches. They capture our imagination and attention. Because this species has a high demand for sunlight, when it grows with competition it stretches upward toward the sun. This leads to oak woodlands dominated by skinny tall oaks.

Savannah oaks

Savannah oaks began life in open grasslands scorched by flames. The fires kept faster growing trees like firs, Pseudotsuga menziesii, from strangling the slower growing oaks. Intentional burning of the Valley’s prairies stopped after European settlers arrived in the Willamette Valley of Oregon and southwest Washington. That arrested young oaks growing into the classic ancient savannah shape. And now the savannah oaks are all over 200 years old, many are 400, such as the famous Heritage oak at the Oregon Garden. More occur throughout Silverton. Several make up the groves at the Oregon Garden, and a dozen or more fan out across the town from the central axis of Main Street. The tree on South Water Street between Kensington Court and Olson Road has one of the largest trunks in town and had a very fine crown prior to sustaining heavy damage in the storm. While several at the bottom of Danger Hill were strong under the ice. Many, for several centuries, held the transition of savannah to woodland on the ground of Mark Twain school. All these trees grew and assumed their classic shapes before Silverton existed. Now there are few locations with enough space and the correct conditions to encourage the development of a savannah shaped oak. In shaded conditions or when competing for light Oregon white oaks grow narrow and tall.

Oregon white oaks have a fierce demand for sunlight. This is shown on Silverton’s East slope in the park at Abiqua Heights where there are several grand examples of clustered savannah oaks growing. The crowns of the trees merge and assume the shape and function of one tree. Our developing understanding of below ground ecology tells us that trees growing in a group are functioning as one biological tree. And it begins to demonstrate the blending of a pure savannah into a many trunked oak woodland.

Willamette Valley before conquest

We, as modern residents of the Willamette Valley, understand its productive power. The whole valley is filled with agriculture – alfalfa to zucchini. The Willamette Valley was once so dense with the flowers of camas, whose bulb was a staple food crop for the native American inhabitants, that in 1806 Meriwether Lewis described it as looking like "lakes of fine blue water." With the camas grew native bunch grasses like Festuca roemeri, and Festuca californica. There were other species of flowering perennials too, columbine, checkermallow, Oregon sunshine and Potentilla, narrowleaf mule-ears, and so many other. They supported so many animals and the Fender's blue butterfly Icaricia icarioides fenderi.  Its habitat was in the meadows of the Valley prairies kept open by fire that are now all converted into agricultural fields or homes. The Valley was so thoroughly converted that by 1937 the butterfly was thought extinct. There wasn’t a place for it to survive. Its miraculous rediscovery occurred in 1989. Much effort has been undertaken to save this butterfly from extinction. And the butterfly’s continued tenuous hold in the Willamette Valley demonstrates the continued erasure of the entire ecosystem – oak trees, bunch grasses, perennials, and flowering annuals. Together all these created home and food for the Kalapuya tribes and the plants. And together with fire, the Kalapuya maintained it in working order.

Fire extinguished

Fires set by these people kept trees from encroaching into the savannahs. Douglas firs, Pseudotsuga mertensiana, bigleaf maple, Acer macrophyllum, and Oregon ash, Fraxinus latifolia all grow faster than Oregon white oak. They create shade the oak cannot tolerate, shade the oak will die in. Indeed, the other prairie plants relied on the fires for rejuvenation and the sunlight it brought. Most do not grow well in shady forests.  In Silverton we owe thanks to the Kalapuya people for creating our ancient signature oaks. Their fires kept the prairies productive. The camas flourish in the sunny prairies and in the dappled shade under oaks. The diverse perennials that grow in the prairie feed a greater deer population than can survive in a climax fir forest. Oregon white oaks tolerate a prairie fire, even when many of their leaves burn off. Those fires burn low to the ground and don’t often climb into the crown of the tree. While mature firs are not bothered by ground fires, fire kills young firs. It also killed seedling oaks that would have grown to compete for light with the savannah trees. Fire kept the prairie free of trees that threaten the sunlight needed by the oaks. Few young oaks now grow into savannah form because even when there is space for a broad canopy to form, there isn’t fire to clear away the competition. Some people call them trash oak. It's a measure of the value we assign the juvenile trees that would slowly grow to replace the ancient savannah trees. Our yards do not host young Oregon white oaks and the edges of grass, blueberry and hazelnut fields are more restrictive than they appear at a glance. Farm crops don’t grow well in shade and it is common to see oaks removed from field edges. They interfere with and damage equipment as their branches spread. We have left them no space.

We are familiar with frequent rock outcrops on the Eastern slope of the Willamette Valley, on the toe of the Cascades. In many places the soil is very shallow, and these areas aren’t productive farmland. They are easy to recognize in agricultural fields as islands growing a thick crop of tall weeds, poison oak, hawthorn, and a few oak and firs. Even in this unproductive land the oaks can’t assume their status as the next generation of savannah oaks because there is too much competition. Their growth is slower than the other trees. The ancient savannah oaks you now see in Silverton with houses under their branches predate the town. They are more than 200 years old. They are beaten down by ice and humanity, are maturing and there aren’t new oaks to replace them.

There are many examples of these savannah oaks now surrounded and succumbing in shade. Recently I visited one in a forest up a path, off the corner of Lakeview Drive and Centennial Drive. There the ancient oak is almost dead, swallowed by other trees, all native but nonetheless deadly. Recall fire kept the large trees standing in a savannah of grass and short plants. The dying oak started life in a savannah and will die in a forest. A site to see incredible savannah oaks in a restored prairie is at the Basket Slough National Wildlife Refuge, just past Rickreall on the north side of highway 22. This site is interesting because it also hosts oak woodlands and native plants growing on the ground around the trees.

Oak woodlands

Oregon white oak forests are now the more common representative of the species in Silverton. Indeed, when looking at numbers of trees there are more oaks in the forest than there are as savannah oaks. Forest oaks aren't so iconic. They are easier to ignore because of this. But they are no less important.

The trees grow close together in Silverton’s oak forests, very tall and slender in trunk and branch. Lots of them on the same footprint a single savannah tree would use. Many oak forests have a unique feature; the oak trees in the forest are all about of the same age. This tells a story of their origin and their preferences, or tolerances. The start may have been a decade without fire. It may have been when the Willamette Valley transitioned as settlers expanded out from the Oregon Trail, or more in abandon pastures. Sites with shallow rocky soils unsuited to agriculture and not yet invaded by firs may host oak woodlands. In Silverton at the entrance to Coolidge McClaine Park, stands an oak woodland wedged between old growth firs and the town. The east face of Silver Creek’s valley, called Danger Hill, hosts former oak woodlands and savannah oaks (collapsed in the ice storm) now mixed with homes, huge firs and ornamental trees. Before European invasion fires played a role in some oak woodlands, keeping the forest from moving into the next successional stage. When firs move into oak habitat their shade slowly kills the oak and the forest transitions into a climax fir forest.

It is common to see young oaks growing up into new oak forests. In a century, if allowed to persist, these trees will be tall and narrow. This is because they demand strong sunlight. When oak seedlings begin to grow as a forest, the competition for light causes them to grow upright. Any branches that form, grow near vertical. It becomes a tree with lots of structural faults, the physics of which couldn’t withstand the ice. Sunlight is their only concern. The whole forest acts in unison to support itself. It endures summer and buffets winter’s Pineapple Express with ease.

Habitat

Oregon white oaks create critical habitat for animals and birds, as well as associated plant species. Both forms were unable to sustain the weight of over an inch of ice.

Deer, birds, and squirrels eat the acorns. In the past the Kalapuya people ground the acorns into flour. It was a staple food. Birds rely on the oaks. Western gray squirrels, Sciurus griseus, also make their homes in the branches of Oregon white oak, in preference to ornamental trees. Their populations are declining due to habitat loss and invasive squirrels. Many insects use the Oregon white oak leaves, bark and wood.  Lichen and moss cling with great tenacity to the thick, fissured bark. Because of all these, they are a cornerstone of the ecosystem for other species. While you see many native birds alight on the non-native ornamental trees, they do not use those trees in the same foundational way. The acorn woodpecker and the slender-billed white-breasted nuthatch have become uncommon in the Valley as the oaks they live on become less common and more isolated.

It’s all about the roots

All that explains the oak canopy, but it doesn’t help our understanding of the most important part of the ecological story. Oregon white oaks - all oaks - function in symbiotic relationships with fungi. Oak roots and fungi mycelium, the stringy underground growth of the fungi, join in an economic relationship of sorts in the earth. A marriage of plant and fungi. This is a vast and important web of transportation. Plant leaves capture sunlight and turn it into sugar, a form of useable energy. Until we figured out solar panels, leaves were about the only thing that could do this on the entire planet, (save a few weird branches of the family tree of life.) And you can’t eat a solar panel no matter how smug having one makes you feel. Every other living thing on the planet relies on the leaves of plants manufacturing sugar. The leaf gets eaten. We get fueled up.

Plants push out sugars and other manufactured carbohydrates from their roots into the soil hosting a curious and sophisticated system. This creates a richer habitat than nearby soil and it is favored by soil-dwelling microorganisms. They swarm to the area around the growing root system. Scientists call it the rhizosphere. It is in the rhizosphere that fungi and plants set up shop.

The rhizosphere is like your laptop or phone. It is the interface between you, an example of the tree, and the internet, an example of the fungi. Fungi filaments, mycelium in the soil are much thinner than plant roots and can access water between soil particles where roots cannot go. Fungi are also able to metabolize minerals which are unavailable to but needed by the plants. The water and minerals are delivered through the mycelium to the interface in the rhizosphere. An economic bargain is struck in the little underground shop. Water and minerals traded for sugar from photosynthesis. Jeff Bezos is only a copycat. The relationship functions to feed and irrigate the oak. Tree nurseries take advantage of mycorrhizal fungi and inoculate seedlings. Those inoculated grow larger and stronger with less water and less fertilizer compared to uninoculated seedlings. The mycorrhizal fungi keep the seedlings healthier.

We understand this system to be sensitive. Homes, sewer and water lines, irrigation, fertilizer, roads and driveways, and nonnative plants all affect this complex and sensitive system. It is likely that this creates unbalance in the system and affects the mycorrhizal association between oak and fungi; it is difficult to study hidden in the soil. Unproven but suspected are the negative impacts of human activities on the mycorrhizal relationship and oak root health. Could an irrigated oak be more prone to toppling, even to shattering? Some people think so, but it is not yet proven. Many of Silverton's toppled oaks showed clear signs of root decay. Irrigation came to them 150 years or more into their life and changed the underground ecology. Soil that the oak and fungi live in used to dry, hard, and cracked every summer. When irrigation started the difference was profound and is hard to overestimate. Irrigation for lawns, ball fields and all kinds of aesthetic pleasures flip the drought cycle in the soil of the West. Dry Oregon soil became wet Pennsylvania with irrigation. Mature Oregon white oaks and their fungal associates in the rhizosphere cannot tolerate that. It causes root rot.

Root rot

Trees that live a long time have adapted to survive adverse conditions, rot, and decay for many decades, even a century or more. Root rot doesn’t often kill an oak outright, and never quick. The rot kills and decays, then weakens the whole root mass.  It compromises the tree’s ability to stand up under the massive weight of over an inch of ice. Trees infested with root rot exhibit general unhealth. Growth slows, the canopy thins and their ability to endure decreases. Here, in the Pacific Northwest, a normal summer has almost no rainfall for three months though there are occasional wet, rainy summers. This helps to demonstrate some flexibility and an ability to bounce back from bad conditions. Oregon white oaks live in a different timeline than we do. Their slow approach wins them centuries. We look at them and assume they are fine with irrigation that has been happening for 25 or 50 years and do not consider that as a small fraction of the tree’s life. The timeline of events after irrigation starts begins when its mycorrhizal association grows weak, continues when its roots become infested with rot, and its vigor declines. That process is slow, but trees with roots damaged by rot topple under ice. Still, recovery seems possible if we stop irrigating the spaces under and near Oregon white oaks. It’s a balance we need to become comfortable with to keep our oaks.  

Looking for next steps? Read the second series article.