What Portland’s Doing to Plant Climate-Resilient Trees
For a host of reasons, procuring climate-forward trees for Portland is no easy matter, but hope may lie ahead.
By Kyna Rubin
Incense cedar (Calocedrus decurrens). Photo by John Ruter, University of Georgia, via Bugwood.org.
Baker cypress, incense cedar, Natchez and Tuscarora crape myrtles, strawberry tree, Chinese pistache, a wide range of evergreen oaks. These are some of the tough, drought-tolerant trees that the City of Portland and the nonprofit Friends of Trees have found to perform well in our rising summer heat and droughts (see article end for more such species).
Urban foresters have known for some time the need to diversify tree palettes to help ensure canopy survival. Trees such as birch, ash, and maple that many cities historically have planted in abundance--sometimes to replace trees like elm or chestnut that years ago succumbed en masse to invasive pests and disease--are now facing similar threats. Following the science, Portland has removed those trees from its approved street tree list. The bronze birch borer has already killed birches here; the lethal emerald ash borer, found in Oregon in 2022, will threaten Portland’s ash population in the next decade; and those two species plus maples and a slew of others lining our streets will die if the Asian long-horned beetle reaches our port city. This is possible because it landed, in the 1990s, in New York and Chicago via wood shipping crates from China.
But climate change has greatly magnified the need to diversity Portland’s tree palette because trees stressed by heat and drought are even more susceptible to pests and disease. Scorching summers and drought are already weakening even some iconic natives. Western redcedars, Douglas-firs, bigleaf maples, and cascara are showing vulnerability, depending on sun exposure and soil moisture, says Matt Krueger, lead staff for the Bureau of Environmental Services (BES) Tree Program. City planters are having to take more care where they site these trees.
Both BES and Parks & Recreation’s Urban Forestry division plant trees across the city. BES plants 600 to 800 trees a year, says Krueger, mostly on private property in commercial, industrial, and multi-family zones since the end of its street-tree (right-of-way) programmatic permit with Urban Forestry. BES plants mainly north of Killingsworth Street, including in industrial areas along the Columbia corridor from St. Johns east to Troutdale, and in East Portland east of 82nd Avenue.
Urban Forestry gave away 2,000 yard trees this fiscal year and through its opt-in street-tree pilot program is planting 1,100 street trees in North and East Portland. It also replaces park trees as needed, according to Tony Mecum, supervisor of Urban Forestry’s Planting Program. However, Urban Forestry will soon be planting lots more trees than it did in the past. It assumed responsibility for planting street trees after ending its contract with Friends of Trees, and it is poised to play a large role in the Portland Clean Energy Fund (PCEF)’s Equitable Tree Canopy program. Currently that program plans to spend $40 million over five years to plant 15,000 to 25,000 street and yard trees. Urban Forestry will likely handle the planting contracts and partnerships needed to plant so many trees.
“Much of what’s long been planted here, such as red maple, is native to the eastern U.S. and is not well adapted to our climate.”
Though generally no longer contracted to plant street trees in Portland, Friends of Trees (FoT) is now planting about 1,300 yard trees through contracts with Oregon’s Department of Environmental Quality in north and northeast Portland. As part of two remediation cases, the state is requiring an Owens-Brockway glass recycling facility and Malarkey Roofing Products to fund the tree planting. This year the Portland nonprofit’s goal is to plant a total of some 2.300 trees in Vancouver, Gresham, Milwaukie, Oregon City, Wilsonville, and Beaverton.
All of these planting entities seek to plant more species proven to stand up well to our hotter temperatures and prolonged droughts. However, all face challenges sourcing these trees.
Early Nudging for More Climate-Resilient Trees
Strawberry tree (Arbutus unedo).
BES’s Krueger, who has been with BES since the start of its tree planting program in 2008, says that early on “we were just looking for better trees, climate change aside.” BES moved away from severely overplanted red maples. And from crabapples, hawthorns, dogwoods, and cherries, which the bureau found were struggling from heat and disease, not holding up over time, and not providing the stormwater function BES seeks. “We wanted to protect the investment,” he says.
BES prioritizes trees that are low-maintenance and will last many decades. As climate became a prominent topic, “our earlier efforts, which had nothing to do with climate, aligned with that effort,” says Krueger.
“ Most often local nurseries target larger markets across the country with greater purchasing power than Portland’s.”
BES and Friends of Trees were inspired years ago in part by renowned local plant guru and iconoclast Sean Hogan, owner of Cistus Nursery. Since the mid-1990s, Hogan, who has been working on projects small and massive in and outside of Oregon, including on Apple’s new Cupertino, California campus, has long argued that many of Portland’s streets are lined with trees that don’t belong here. Much of what’s long been planted here, such as red maple, is native to the eastern U.S. and is not well adapted to our climate. These trees don’t thrive without supplemental water over our increasingly dry summers. Drought and extreme heat are stressing these trees, stunting their growth and making them more prone to insects and disease. For practical reasons, Portland needs to expand its tree palette, Hogan told me in 2018.
According to Krueger, BES was planting evergreen (sometimes called live) oaks as early as 2009 and was successful in getting the City to add to its approved street tree list crape myrtles, strawberry trees, and evergreen oaks, many from Mediterranean climates similar to ours.
These trees have thrived here and are now valued as the kind of climate-resilient trees Portland needs more of (see farther below for more such species).
Sourcing Challenges
Young coast live oak trees (Quercus agrifolia).
Procuring climate-resilient tree species in desired sizes and quantities isn’t easy.
Urban Forestry is learning the problem.
It’s very challenging to find “the numbers we want” of trees like the evergreen oaks that provide the stormwater management and drought tolerance the City is looking for, says Urban Forestry’s Tony Mecum. Contractors and Parks staff, he says, “are often left with very limited availability of trees that meet our caliper standards for various zoning types.”
Several factors are at play. To be profitable, most local nurseries, especially the larger ones, prioritize volume, demand, and the costs of growing various species. Most often they target larger markets across the country with greater purchasing power than Portland’s. Boring, Oregon-based J. Frank Schmidt, for instance, sells across the globe, including to China. These customers prefer trees adapted to their humid, summer rainfall climates. They are also willing to enter into nursery grow contracts signed years in advance for planning purposes. Because these clients are typically not ordering the species Portland now wants, most local nurseries aren’t growing them.
“We don’t have much influence with what local nurseries grow,” says Krueger, because even the few thousand trees a year that BES and FoT had been planting until recently were insufficient to sway big regional producers from primarily growing what sells, nationally and locally. And what sells are trees like dogwoods, cherries, and Japanese snowbells, not the more robust, long-lived, climate-forward, more environmentally useful (but less sexy) species Portland needs.
Crucially, it costs nurseries more time and money to grow such trees because they grow more slowly than the popular ornamentals. However, their denser wood makes them sturdier. An evergreen oak crop could take five years to grow to transplantable size, says FoT’s Andrew Land. Flowering pears can be harvested in two, but they live fast and die young. By contrast, oaks and many other large climate-resilient trees tend to have lifespans double or triple those of smaller trees.
Building new sources to expand the planting palette took BES substantial effort, says Krueger. Lack of local availability of the “better” trees BES seeks led his team years ago to create growing arrangements and relationships with growers farther south, in Salem, Eugene, and California. But shipping trees, especially large ones, to Portland can be costly. In 2018 he told me his goal was to order smaller tree stock from California and grow them to a larger size up here. The problem with that, he says, was that the smaller, more economical stock didn’t meet Urban Forestry’s minimum standards for trunk size (known to arborists as caliper).
The Caliper Size Question
As Mecum says, the caliper size of a young tree plays a role in nursery availability. Defining optimal caliper standards for planting in a tough (for trees) urban environment is tricky because of the trade-offs in cost, vulnerability, and establishment speed among various caliper sizes. Opinions about optimum caliper size vary.
Urban Forestry hews to the caliper standards in Title 11.60.020, the tree code. According to Angie DiSalvo, Urban Forestry’s science, outreach, and planting manager, the rational for these standards, which range from 1.5 to 2.5 inches depending on the site, is that larger caliper trees reduce a newly planted specimen’s vulnerability to vandalism. Also, larger stock typically branches higher up, offering pedestrian clearance sooner than smaller caliper trees do. And because of their bigger size they require less structural pruning and training in their early years. These factors are especially important for street trees.
“Urban Forestry wants contractors to know they can now source trees farther away if necessary.”
Trees for Life Oregon’s Jim Gersbach, who once worked for Urban Forestry and founded the Linear Arboretum in NE Portland, has been a keen observer of trees he and others have planted around Portland for thirty years. He concurs with DiSalvo that larger caliper trees are more likely to already have been structurally pruned by the grower. “This was especially important at the time the tree code was developed,” he says, “because Urban Forestry had limited funds for maintenance,” including early structural pruning. He argues that with today’s infusion of Parks levy and PCEF funds, and the existing infrastructure of tree stewards, neighborhood tree teams, and other volunteers the City and FoT spent years training to prune small trees, this particular argument for favoring larger caliper now makes less sense.
Silverleaf oak (Quercus hypoleucoides).
One of the downsides of putting larger caliper trees in the ground is transplant shock, says Gersbach. Bigger trees have more extensive roots that can suffer damage when being wrapped into a burlap root ball for transporting. They can take more time to adjust to their new home.
“Smaller caliper trees in front of homes in most low- to medium-density residential areas aren’t usually vandalized and do quite well,” he observes.
It’s quite likely, says Trees for Life Oregon’s Bruce Nelson, a retired arborist, that, absent vandalism, “a younger and smaller tree of the same species will more rapidly grow a better anchored root system and within 10 years will likely be healthier and larger than a tree that was planted with a much larger caliper.” Another issue is cost, says Nelson. Larger caliper trees are much more expensive, since they require more time and labor to get to that bigger size.
No longer working through an Urban Forestry contract to plant street trees, BES now has the flexibility to plant smaller stock. Besides costing less to purchase, says Krueger, smaller caliper trees need less watering, are easier to plant, and establish faster. FoT’s Andrew Land agrees. But for ease of procurement he sticks to ordering 1.5-inch caliper trees for both streets and yards, because it satisfies street tree requirements across the region.
Actions to Get Trees of the Right Species, Size, and Quality
With so many variables affecting availability, what are City staff and Friends of Trees doing to meet tree-sourcing challenges?
It’s been years since Krueger first started going farther afield to procure the species BES wanted for diversity reasons. But the local nursery industry hasn’t changed, he says. He has found a few nurseries that will hold and care for trees BES ships in from elsewhere until contractors come to plant them. But he still has to be assertive about finding sources, planning ahead, getting the desired quantities and quality, and shipping them to Portland.
BES is no longer sourcing as much from California as it was when it was planting street trees, though Krueger is still seeking contractors with connections in northern California. At present most of BES’s trees come from the Salem area on up to Portland. BES is careful to focus on quality stock and works with nurseries that know the geographic origin of the seeds and acorns they sell. For instance, BES gets ponderosa pines, one of its “workhorse conifers,” from a nursery that collects seeds from pines native to the Willamette Valley that are adapted to both wet winters and dry summers. Increasingly, Krueger is ordering from Cistus Nursery, on Sauvie Island, because right on site it is growing the types of trees BES wants, and owner Sean Hogan can tell him which part of which California valley or hillside a tree came from.
Every year Urban Forestry will go to BES for 10 to 20 evergreen oaks or other specialty trees they traditionally haven’t sourced easily, says Krueger. As Urban Forestry starts beefing up its procurement of these species, this may no longer be necessary.
To improve Urban Forestry’s pilot street tree program, and to prepare for the larger volume of street trees it will be planting, Urban Forestry planned to revise the language in its March 2023 RFP for planting contracts, says Mecum. Earlier contract wording, before his time, directed procurement within the Portland area. Urban Forestry wants contractors to know they can now source trees farther away if necessary. He says that his staff work closely with colleagues at Parks & Recreation’s horticulture services and that the latter have reached out to nurseries in California and Washington (which, despite being north of us, is growing more climate-smart species).
“FoT is noticing willingness among a few regional growers to risk raising some climate-resilient trees.”
Urban Forestry’s new and current planting specialists will focus on opportunities to expand tree quantity and quality, says Mecum. His office will be exploring “all options” to procure what’s needed. Notably, they would consider grow contracts with nurseries, though at present he knows that Portland can’t compete with the larger volume of demand local nurseries have from elsewhere.
Baker cypress (Cupressus bakeri).
With hopes of figuring out how to change this situation, Mecum meets regularly with Scott Altenhoff, program manager for the Oregon Department of Forestry (ODF)’s Urban & Community Forestry office. Altenhoff recognizes the procurement challenges. No set plan is yet in place, says Mecum, but he is excited about a potential ODF effort to form a united front of Oregon municipalities that would increase their purchasing power and thereby have more influence over what regional nurseries grow. The support needed for such an effort is one of the things that ODF has requested in its proposal for funds from the federal Inflation Reduction Act, according to Jim Gersbach.
Friends of Trees, which has declared this “the year of the oak,” inspired by Douglas Tallamy’s book The Nature of Oaks, is noticing willingness among a few regional growers to risk raising some climate-resilient trees. Andrew Land is seeing more availability of silverleaf oak, for instance. And this summer he found 55 Chinese evergreen oak (Quercus myrsinifolia) at Blue Heron Nursery, in Corvallis. That nursery is now reaping the benefits of planting acorns of these trees five years ago, when Land says some people were asking the nurseries to consider growing them. FoT lacks the money to enter into growing contracts; the City of Portland is better positioned to do that.
“Creating enough critical mass of demand within Oregon to motivate nurseries here to grow what we need to remain livable in climate crisis is a crucial step.”
Over the years, FoT has developed what Land calls a symbiotic relationship with its primary tree source--the small, family-owned, mostly BIPOC-staffed Rigert Nursery, in Aloha. Inclusion, as it relates to trees, matters to Land. Although FoT turns to dominant regional player F. Frank Schmidt to buy some of the familiar, commonly available trees that many residents still want for their yards, Land strongly values the more personal relationship FoT has developed with Rigert. This small nursery supplies FoT with some of its harder-to-find climate-forward species, and has been very accommodating, even allowing FoT to return end-of-season unused stock at no cost, as does the much larger J. Frank Schmidt.
The Way Forward
One sign of change is the fact that the April 2023 issue of Digger Magazine, published online by the Oregon Association of Nurseries, is devoted to stories about nursery owners’ thinking about climate when determining what trees to grow.
Crape myrtle (Lagerstroemia 'Natchez').
Creating enough critical mass of demand within Oregon to motivate nurseries here to grow what Portland and other cities need to remain livable in climate crisis is a crucial step. Toward that end, it may be time for Portland bureaus to consider being more flexible about caliper size. With $40 million of PCEF money about to be invested in planting and maintaining thousands of new trees in high-need neighborhoods to buffer heat islands, sticking too closely to pre-determined caliper standards in a tree code that was developed years ago may no longer be in the best interest of creating a healthy, diverse, climate-resilient urban forest.
Over time, it may also not be practical to rely solely on City staff to do the structural pruning that thousands of new and existing young trees, including climate-resilient trees, need to survive. Trees for Life Oregon hopes that the City will find ways to reap benefit from the longtime investment it made in training community volunteers to prune young trees by incorporating them into its tree planting and maintenance work. Further, it should consider once again involving nonprofits to hold community pruning events as Friends of Trees had done in Portland and is now doing successfully in cities elsewhere in the region.
Based on BES monitoring data, after four years, top performers--those at a 95
percent or greater survival rate--include Baker cypress (Hesperocyparis bakeri), a
fairly rare tree from southwest Oregon that Krueger says they’ve been planting for
some time. BES has ramped up the cypress trees it plants because its medium size
is useful in sites without enough space for other conifers like Douglas-fir or
ponderosa pine. Other top BES performers:
▪ coast live oak (Quercus agrifolia), from California
▪ Gambel oak, a scrubby, deciduous oak from the Southwest
▪ Natchez crape myrtle and Tuscarora crape myrtle, both summer-flowering
▪ silverleaf oak (Quercus hypoleucoides)
▪ Vanderwolf pine, a small to medium cultivar as a stand-in where larger conifers won’t fit
▪ Chinese pistache (Pistacia chinensis), a common, fall-color street tree for smaller planting strips
▪ strawberry tree (Arbutus unedo), a small evergreen; and Tuscarora crape myrtle.
BES is hoping this summer to do longer term monitoring, of 8- to 10-year-old trees to evaluate their performance.
Besides Chinese oak and silverleaf oak, mentioned above, FoT’s Andrew Land also finds Quercus virginiana very durable, as are broadleaf evergreens like magnolia cultivars ‘Edith Brogue’ and Moonglow. He goes for ginkgo. And he likes the wildlife-friendly Pacific madrone despite some of its challenges, but says this touchy native species will fail if planted at 1.5-inch caliper. He is hoping soon to procure some smaller, 5-gallon stock.
Crape myrtles are a dynamite option, says Land, for people who have small yards but want a tough tree with pretty flowers, fall color, and appealing bark. As Portland rushes to increase density, he’s finding fewer people with yards large enough for the big trees that would provide them with the most health and environmental benefits. But as he sees it, a small tree is better than no tree.
Land also values honey locusts (Gledistia) and bald cypress (Taxodium), a deciduous conifer that FoT has been planting for over a decade.
For its part, Urban Forestry’s parks, street tree, and yard tree giveaway programs have started planting hardy, drought-tolerant trees from southern Oregon, California, and the Mediterranean such as incense cedar, interior live oak, cork oak, and Chinese pistache. Tony Mecum says they monitor most of these young trees, some of which are new to the bureau’s planting palette, to see how they’re doing so the bureau is not over relying on them.
As for trees that haven’t fared so well here? BES has found poor survival rates for Japanese hornbeam (Carpinus japonica), Summer Sprite linden, and the natives cascara and Pacific madrone. Madrone, says Matt Krueger, is finicky, doesn’t like to be transplanted, and doesn’t take well to human intervention. His team loves the tree and still plants it, but only in isolated areas, using smaller caliper stock and placing a few together in case not all survive.
FoT’s Andrew Land has found cork oak (Quercus suber) doing well except for its tendency to lose branches during ice and wind events, a problem that, importantly, can be avoided with young tree pruning. Absent pruning, ice and wind can damage some other climate-resilient species as well, he says.
It’s important to keep in mind, reminds Jim Gersbach, that many once popular and widely planted deciduous street trees such as birch and pear fare even worse than cork oak does in storms.