Saving Trees During Construction: One Arborist’s Inside View
An arborist shares the behind-the-scenes challenges of determining for homeowners and developers if existing big trees can survive nearby construction. His observations suggest to us ways to strengthen tree preservation-related City codes and practices.
By Kyna Rubin
The incense-cedars before construction. Photos by Ryan Gilpin unless otherwise noted.
In May 2023, the co-owner of a 1909-built house in southeast Portland hired arborist Ryan Gilpin of Nidus Consulting to see if two large incense-cedar trees could be preserved while rebuilding her home’s foundation. Judy Morton had noticed damage to it from inside the basement, and these trees might be part of the cause. If true, a lot of tree roots would have to be cut to accommodate the new foundation, says Gilpin. And that can kill trees. If not right away, then over time.
The larger tree, at 42-inches in diameter at breast height, was only seven feet from the planned excavation trench around the foundation. The smaller, 32-inch-diameter tree stood only five feet from the dig. In theory, says Gilpin, there was no way to preserve the trees because of how close they would be to the construction. He could not guarantee their survival.
Trees of this size and species provide nearby residents a slew of health benefits. Incense-cedars (Calocedrus decurrens) are sweet-smelling, climate-resilient evergreens that have been doing well in our area’s increased heat and droughts. Judy Morton’s daughter and her daughter’s partner live in the house. The family and the neighbors, she says, all loved and benefited from these trees and wanted to protect them. But this was a tricky scenario.
Gilpin is a consulting arborist. His three-year-old company “owns no saws,” as he puts it. Before starting his own business, he spent more than a decade working on hundreds of projects for other companies in Portland and elsewhere. (He still works in cities outside of Portland.) Rather than removing trees, he assesses if and how trees might survive nearby construction. He seeks ways to protect or expand root protection zones. He writes tree reports, and he oversees excavation around trees. In an ideal situation, he gets brought into a project early enough to be able to have input into tweaking a building design to create adequately large root protection zones for the trees that for reasons of code, cost, or desire the client wants to save.
“We don’t know what we’re going to find when we start digging and we don’t know how the tree will respond when we cut the roots that we do find.”
Gilpin has co-authored a guide to best practices for managing trees during construction, an International Society of Arborists publication used worldwide. He’s even co-written a guide to protecting wildlife during tree care. He conducts workshops around the country on these topics. His volunteer work includes leading bird and plant tours for the Columbia Slough Watershed Council.
The Challenge
The people who hire Gilpin often want to know how close they can build to a tree without destroying it. He says that’s an impossible question to answer.
“Trees are complicated, biological organisms that respond in surprising ways a lot of times. We have an imperfect knowledge of their root system,” he says. “We don’t know what we’re going to find when we start digging and we don’t know how the tree will respond when we cut the roots that we do find.”
For example, from a layperson’s perspective, it may seem that saving visibly larger roots rather than smaller ones would be an obvious approach. But it’s not that simple.
“A smaller number of large roots are more important than a small number of small roots,” explains Gilpin. “But ultimately the small roots are doing a lot, so if you remove all of them and leave all the big roots, that’s probably also bad for the tree.”
Some of the literature suggests that one-third of a tree’s roots can be safely removed, a principle that Gilpin doesn’t vouch for. Without knowing exactly where all the roots are, how can you measure a third of them? Precisely pinpointing the entire root system would require lots of digging and harming the tree, he says.
While techniques such as air spading can give arborists more confidence about where the roots are, none of these methods is perfect, in his view. (An air spade uses highly compressed air to expose roots while causing minimal damage.)
How to Proceed
“If more excavation revealed evidence that saving the trees wasn’t possible, the incense-cedars would have to be removed. This would complicate things for all parties. ”
So what are arborists to do? The short answer: the best they can.
The only way to help preserve big trees like the incense-cedars he was dealing with in Portland’s Creston-Kenilworth neighborhood, is to give them a lot of space, he says. That wasn’t available here, especially if the tree roots were found to be already pushing up against the foundation. But were they?
Gilpin spoke to a planner at the Portland Bureau of Development Services (BDS), which manages building permits and implements the zoning code, which, together with the tree code, contains provisions for tree preservation. Judy Morton says that because of the large size of her trees, that bureau’s Field Issuance Remodel Program, through which she was working, required a tree plan be sent to Urban Forestry to review. Eventually Gilpin and the City agreed on how to proceed.
The incense-cedars' proximity to the northwest corner of the homeowners' foundation, pre-construction.
Judy Morton says that Gilpin told her he expected significant root loss. He suggested that she—her daughter’s partner did the work—use a shovel to do some exploratory root trenching. Around the corner of the house closest to the two trees he instructed them to dig a small trench 2 feet deep and about 4 to 6 feet in either direction. They then sent him photos of the roots they were finding. He assessed the pictures, then went back to check things out himself.
That initial exploration, Morton says, yielded three 1-inch roots, only one 3-inch root, and no significantly large roots. Gilpin determined that at this point it was safe enough to bring in the excavator for further probing.
He then worked in tandem with the excavator, making decisions as they went along: cut this root you just hit, leave this one, move this one aside. If below the preliminary dig they found large roots that they’d have to cut to enable the new foundation construction, he would deal with that then.
They cut a few roots but not many, because every step revealed fewer roots than the arborist had expected.
But making the call to move to each further stage is not without risk. If more excavation revealed evidence that saving the trees wasn’t possible, the incense-cedars would have to be removed. This would complicate things for all parties. To remove a private tree greater than 36 inches in diameter would likely cost the homeowner (or, in other cases, the developer) tens of thousands of dollars, Gilpin says, and would require 30 days to notify the neighborhood.
Liking what he saw the deeper they dug, Gilpin felt more reassured. “This is good to go. Let’s try to preserve these trees,” he signaled.
When an Arborist is Brought into the Process Matters for Tree Preservation
Recently the City of Portland has taken steps to streamline the building permitting process to facilitate housing construction. That helps explain why permit applicants and the City, now more than ever, might favor a more straightforward plan when it comes to managing trees in development situations.
The fenced off incense-cedars, in back, and the trench for the new foundation. A worker is standing inside the basement.
Through its tree code (Title 11.60.030C), Portland grants two options for protecting tree roots during construction. The faster, less costly, and more cookie-cutter approach, called the prescriptive path, requires the tree root protection zone (RPZ) to have a 1-foot radius from the trunk center for every 1 inch of tree diameter at breast height, with allowances made for some encroachment. According to Gilpin, an arborist “is basically not needed” for projects taking this route.
BDS does not track how many permit applicants check this first option, says Jill Grenda, but she and her colleagues estimate that some 70 to 80 percent of applicants start with this approach. (More below about why they don’t always end up taking the prescriptive path.)
But, as the code itself states, using this method is not always “practicable.” This was so for the two incense-cedars, because there wasn’t enough space between them and the foundation redo to create a RPZ hewing to the prescriptive path.
Gilpin therefore knew early on that this project would require taking the tree code’s second, more flexible option, called the performance path. This option allows an arborist to take steps such as exploratory root trenching to see if a tree could likely survive in a smaller RPZ. This kind of controlled probing is a way of gathering more information that, he says, would rarely be needed on a project that has enough space to use the prescriptive pathway.
But for the homeowner or developer, this second option can cost more, because it requires more of an arborist’s time, presence, a detailed tree protection plan, plus verification of the tree’s viability prior to the City’s final inspection.
“Nobody on their team or on the City team noticed that [using the more standard formula] was not possible because the tree is five feet away from the construction.”
Coming in Mid-Project
The owner of the incense-cedars hired Gilpin early. But on bigger development projects involving builders and a large team, Gilpin or other consulting arborists may not be contracted until partway through the process. That’s where things can sometimes get more dicey for tree preservation.
He’s been involved in cases where, prior to his arrival, a landscape architect, homeowner, or another arborist decided on their own to follow the easier, standard, prescriptive path. And the City reviewers didn’t catch the problem.
“On their own, BDS planners often don’t understand trees, observes Gilpin. Grenda agrees. ‘We should have arborists reviewing [private-site tree permits]. But that’s not currently who we employ. We employ city planners.’”
“Nobody on their team or on the City team noticed that [using the more standard formula] was not possible because the tree is five feet away from the construction,” he says, describing one scenario. “You have to do the performance path,” because in the construction plan there’s simply not enough space for the standard-size RPZ mandated by the prescriptive path. And only the performance path allows more leeway to use other ways to preserve the tree.
The Bureau of Development Services should flag that error in its permitting review process and notify the project manager who, says Gilpin, would contact an arborist like him for help. One reason that doesn’t always happen, he finds, is that the project doesn’t necessarily go to Urban Forestry staff for review.
The incense-cedars are trees #59 and 60 in the tree protection plan. The dotted lines show the impossibility of using the tree code's faster, cheaper prescriptive path for this project. Courtesy of Judy Morton.
Indeed, says Jill Grenda, a planning and zoning supervisor in BDS’s land use services division, by law, due to a bifurcated set-up, BDS generally is only responsible for what happens with trees on the private-property portion of a development site, and Urban Forestry for trees outside privately owned land, including street trees in the right-of-way. BDS’s land use services, she says, would love to have expert arborists reviewing or at least advising BDS staff on private-site trees, especially when it comes to performance path permits. But the system is structured so that Urban Forestry has to charge BDS for these services. BDS is a fee-run agency that in January laid off a large portion of staff due to a construction slowdown, which resulted in fewer permit fees. It can’t often afford to pay UF arborists to do this, says Grenda.
(Note that, according to Grenda, Urban Forestry staff who inspect permits involving street trees frequently do not approve them once discovering that the tree-preservation option proposed does not match the on-the-ground reality. The applicant then has to revise the permit and convert from the prescriptive path to the performance path.)
“The consequences for everyone and especially for tree preservation can be significant when plans that are impossible to complete get approved.”
On their own, BDS planners often don’t understand trees, observes Gilpin. Grenda agrees. “We should have arborists reviewing [private-site tree permits]. But that’s not currently who we employ. We employ city planners.”
The consequences for everyone and especially for tree preservation, can be significant when plans that are impossible to complete get approved. This error, observes Gilpin, can result in delays, or in trees being damaged or removed that could have been preserved with minor redesign. It can cause design changes that preserve low-value trees, and almost always adds to distrust and animosity among designers, cities, and construction crews.
The main problem, he says, is that the people approving the plans aren’t understanding the cost of this kind of slip-up to the community in terms of removing trees and having fewer nearby trees.
This is what motivates Gilpin to give workshops to landscape architects to help them understand what to look for when reviewing design plans to determine when the one-size-fits-all RPZ size won’t work on the ground, and they’ll need to follow the more adaptable but more arborist-intensive performance path.
“This is a really important piece to understand,” says Gilpin.
The arborist is also working to educate city planners on how to flag unworkable projects. In late April he co-led a workshop on trees in construction sponsored by Clackamas Community College’s Environmental Learning Center. “The prescriptive path is great, but we rarely have the space to do it,” he explained to his audience, which included city planners and urban foresters in the greater Portland area.
Arborists Need to Be Hands-on Through Every Phase of Construction
““Once we’re in the construction phase a lot of bad things can happen,” Gilpin says. The arborist needs to be out there problem-solving and constructing fencing, irrigating, and pruning for clearance.”
Since he’s not always brought into a project early enough to suggest building-design tweaks that would provide bigger RPZs for trees the builder wishes to preserve, Gilpin works with what he’s got. (Occasionally a creative developer who is motivated to preserve big trees when building a new structure will tailor-design a foundation to keep tree roots safe.)
Protective fencing around the trench being dug for the new foundation. The incense-cedars, not visible here, stand off to the right and behind the photographer's view.
He spends time trying to get “every inch of space” he can for the RPZ, visits the site regularly, and maintains a good relationship with the construction crew. On site, he’ll negotiate with them if he observes changing conditions that would adversely impact the tree’s space.
The question about the right time to require arborists to come onto projects is being discussed across the country, Gilpin says. Also, he asks, “Should it be a private arborist like me or a city arborist like those in Portland’s Urban Forestry office?” In his writing he’d like to explore how private consulting arborists and city arborists could better work together and sort out their respective roles.
Gilpin argues that construction projects go best from a tree preservation perspective if an arborist is involved very early in the planning phase, diving into resource evaluation, an on-site tree inventory, and permitting needs. Crucially, he says, an arborist can’t provide clients with the right data they’ll need to design without knowledge of the local code, which stipulates trees’ suitability for preservation. In his workshops with arborists, he stresses the pre-construction phase, because once construction starts, it moves really fast, at times within only a few days.
“Once we’re in the construction phase a lot of bad things can happen,” Gilpin says. The arborist needs to be out there problem-solving and constructing fencing, irrigating, and pruning for clearance. He believes that arborists should play a role in every single phase—planning, design, preconstruction, construction, landscaping, and post-construction.
Like all consultants whose advice is not always heeded, Gilpin can get frustrated when trees that “don’t need to be removed get removed,” he says. “I’m often in the situation where I say, I don’t know that this tree is going to survive this. And they say, OK, we’re doing it [anyway].” He accepts this as part of being a consultant. He is not the final decider or the ultimate authority. The clients, their teams, and the city usually hold that power.
Policy and Other Changes That Could Preserve More Trees
The corner of the tree code that Gilpin deals with suggests to Trees for Life Oregon another way to strengthen Title 11’s ability to preserve big trees. (Read here for other revisions we propose when the City-approved tree code revision takes place.) Instead of offering two options for preserving trees in construction, the code could probably preserve more trees by requiring that all construction applicants take the more adaptable performance path.
“Offering the faster path complicates and likely increases costs when, midway in the process, it’s found not to be feasible on the ground. Then builders have to choose between constructing less housing or removing more trees. ”
The prescriptive path, if properly followed, is quicker and cheaper. The two-track system was intended to provide a more affordable option to building applicants, says BDS’s Jill Grenda. But it appears that offering the faster path complicates and likely increases costs when, midway in the process, it’s found not to be feasible on the ground. Then builders have to choose between constructing less housing or removing more trees. Neither of these is palpable to builders’ bottom line nor to our current housing and climate crises.
At the very least, Portland and other cities should require a full arborist’s report (including tree inventory, tree protection plan, on-site monitoring throughout construction) for all development projects, not only those going the performance route. Gilpin notes that Lake Oswego already does this. Portland requires a progressive inspection process for other aspects of construction, like plumbing and electrical. Why should trees, our key green infrastructure, not receive equal expert attention from start to finish?
“In the quest to facilitate building permits, the City of Portland should be ensuring the integration of housing and canopy goals in tandem, not viewing them as an either/or priority.”
Another change relates to practices. Trees for Life Oregon has always advocated for the Bureau of Development Services to have arborists on staff to inventory development site trees early on and to review plans affecting trees, be they right-of-way or private trees. Minimally, City leaders might devise cost-effective ways of sharing Urban Forestry staff for this purpose. After all, Parks/Urban Forestry is the entity that is responsible for implementing the tree code, and the tree code applies to street trees and private-property trees. This change might well help prevent needless loss of healthy canopy.
In July, the City will implement the new single permitting authority, a concept that City Council approved to streamline the building permit process due to the housing crisis. It is unclear what that will mean for Urban Forestry staff who now review development permits affecting street trees. Will they be moved into BDS? Could this be an opportunity to expand City arborists’ review of BDS tree-preservation permits to include those for private-site trees?
Whatever happens, preserving more big trees, not less, should be the result. Governor Kotek stated in February that we are in a housing and climate crisis, and “the health and well-being of our residents…hinge upon the presence and preservation of trees.”
In the quest to facilitate building permits, the City of Portland should be ensuring the integration of housing and canopy goals in tandem, not viewing them as an either/or priority.
What Happened to the Incense-Cedars?
Beyond the new steps the homeowners added to the front porch stand the two preserved incense-cedars, post-construction.
Some research suggests that seven or eight years after construction is when we can know if a tree has safely weathered impacts from nearby construction. But it’s difficult to say if that’s true, says Gilpin. He, for one, has never monitored a private-property tree for that long a period. The system lacks incentives for arborists or property owners to conduct long-term monitoring, and permission would be needed to get back onto private sites to do such assessments.
Gilpin says that trees that survived construction but lost roots in the process often don’t reveal negative impacts until the next dry season. If a tree lost 30 percent of its water uptake ability owing to root loss, he recommends that, after construction, clients provide it with at least 30 percent more water.
But he has no concerns about negative construction effects on Judy Morton’s incense-cedars. The trees’ owners did everything he told them to do throughout construction, which stretched over the hottest part of the year—June, July, and August. Morton says they covered the roots with burlap and kept them wet and mulched the whole summer. She observed that Gilpin instructed the Bobcat operator to come in from the sides to excavate for the new foundation, to keep heavy equipment as far as possible from the tree roots.
Morton knows there’s no guarantee her trees will last forever. Forces outside of construction such as climate, weather, and drought could adversely affect them in the future. But they remain healthy for now, a year after construction, and, she says, everyone involved did the very best they could do to make this happen.