From White Oak to Blue Oak: Street Tree Transition in Eugene

Friends of Trees is cranking up for the season, and now's the time to start thinking about planting trees!

Beyond their environmental and aesthetic benefits, street trees have useful roles in traffic calming. They are full win all the way around.

This year's schedule

Friends of Trees have offices and staff in Portland and Eugene, and it was interesting to learn more about their work in Eugene recently. A recent tree walk on the southeast side of campus followed part of the Fairmount streetcar line from 1907.

Eugene's Fairmount Streetcar line 1907

On Moss Street between 17th and Fairmount tracks were still visible in much of the street.

Vestigial tracks and wide curb strip on Moss Street at 17th
(tree group three houses up)

Trees, not streetcars or other transportation, was the focus, but the walk's guide did direct attention several times to the variable widths of the curb or parking strip. Here on Moss the right-of-way is quite wide for a residential street, and the curb strip also very wide. In other places the sidewalk more nearly hugs the street, and the strip is but a slight margin. Unsurprisingly and obvious to most anyone who walks, root systems caused a lot more disruption to the sidewalks on the narrow strips. Matching root system to soil type and to strip width and depth has become part of basic tree selection now.

Our street tree list highlights the curb strip width

More interesting was discussion of a nascent shift from Oregon White Oak to California Blue Oak at some sites.

Friends of Trees on tree selection and
 climate disruption (Summer 2013 newsletter)

The Blue Oak's "native range extends from the Central Valley foothills into the interior valleys of southern California." It's a tree for a much hotter and drier climate!

The Eugene Friends of Trees was actually planting these Blue Oaks in places where they would have put a native White Oak not long ago, and we saw some recently planted young Blue Oak trees. They are thinking not about current conditions, but about the tree's maturity in two or three generations and the shift in climate we are facing. It's not like they are doing some wholesale, comprehensive shift, but they are mixing in these non-native Blue Oaks and reducing the proportion of Oregon White Oaks.

That was an elegiac moment of climate realism and accommodation. Maybe such a shift is also happening in Salem, but if so it has not got much visibility. Our street tree list does not include Blue Oaks and, at least in an overt and obvious way, does not make any reference to accommodating warmer and drier summers in the future.

A recent dissertation on this, "Implications of Urbanization and Climate Change for Oregon White Oak (Quercus garryana): Regeneration, Planning, and Management in the Pacific Northwest," is cautiously optimistic:

Current evidence suggests that, at a broad scale, the Willamette Valley climate will remain suitable or even improve for Q. garryana, at least into the 2080s. There is unanimous agreement among climate projections that temperatures will increase but are very likely to remain within the general range tolerated by this species. There is less certainty associated with precipitation, but there is good model agreement that fall, winter and spring will be wetter and summers will be drier. These changes are unlikely to disadvantage Q. garryana and may benefit this species....

[Still,] The consensus from the climatic niche model projections shows the majority of the Willamette Valley converting from Oregon coastal conifer forest and Oregon deciduous and evergreen forest to Cascade-sierran montane conifer forest.

Our White Oaks may not be quite as iconic as Salmon, but they are a central part of the there there here, a sine qua non for Salem. Changes in the insect or bird population are noticeable, but may not fundamentally reshape the landscape in obvious ways the loss of White Oaks would. (Though of course changes in insects and birds will still affect the health, pollination, and seed dispersal of trees.)

And the centrality of the Oaks raises questions: How much of our attempts to restore native Oak savanna and plantings may need adjustment? Are the Oak groves, like those at Bush Park and the Fairgrounds, going to need intervention? At the State Hospital where we are replacing Walnuts with Oregon White Oaks, is this possibly short-sighted? Do we need now to be transitioning to, or mixing in, Blue Oaks or other species from California and places south? The way we talk about Oregon White Oaks now may not be responsive enough to the lifespan of a tree and the facts of a changing climate.

Like California wineries purchasing Oregon vineyard land as a hedge, with a view towards grafting over or replanting to warmer climate grapes in a generation (or less!), it may be that our tree strategies also will need to look to California species and cultivars.