Understanding our Trees
An arborist’s response to the damage from the ice and wind storm in Portland, January 2024
Reading news coverage in the aftermath of the storm has been heartbreaking; heartbreaking to read the stories of people’s homes being damaged, families without power for days on end, and, in some cases, loss of life. Yet, as an arborist and an educator, it’s also heartbreaking to see such an intensely negative focus on trees.
Since January’s storm, trees have been made out to be villains, menacing figures just waiting for their moment to crush you (or your cat). There’s an air of inevitability in the reporting, a ‘told you so’ mentality that trees—big trees, specifically, tall trees—are hazardous, so many ticking timebombs camouflaged in leaves that should be removed before they surely fall.
But with over 4 million trees growing in parks, yards, and along streets in the City of Portland alone, we must remember how many did not fall and see that the vast majority—over 99.9%—did what trees do best: be trees. The benefits of a healthy tree canopy around you are far reaching, individually impactful, and measurable. In addition to their positive effects on stormwater, pollution, and shade, trees have significant positive effects on our mental health and wellbeing. These should be understood and weighed against any potential risk.
I’d like to explore this side of the issue in hopes of alleviating the spreading fear, to set the record straight about how trees build themselves, what affects the risk of them falling, and what we can do about it.
Trees are naturally long-lived, large-growing organisms that develop over time using responsive growth. Trees do not arbitrarily grow upwards, putting on rings hoping they’ll be strong enough as they age and get taller. They build self-optimized structures, piece by piece, year by year, responding to the forces of wind and gravity.
As trees experience these wind loads, they bend and sway, dampening the loads across their stems, branches, and roots as one interconnected system. Though it can appear precarious, this bending and swaying is precisely how a tree knows where to add new wood to strengthen itself to withstand the conditions it faces. As years go by, a healthy tree will grow into a thoroughly adapted, stable form.
Trees strengthen themselves similarly to how we strengthen ourselves—when we go to the gym or the track, we push our muscles, we stress them, and our bodies respond by building muscle to strengthen weaker areas. Trees respond by building wood, proportionally adding more wood to areas that experienced more stress.
This yearly call-and-response trees share with their environment is an ongoing adaptation of their architecture to their specific circumstances. For example, a tree, say our friend the Douglas-fir, grown in the open without any other trees nearby will be well-adapted physically to take on all the forces around it; it’s slowly built itself to do just that over its whole life.
Alternatively, a tree grown within a grove or forest will grow based on the other trees or structures around it. Those exposed to wind on the edge of the grove will take the brunt of the force and so buffer the interior trees. Not being subjected to the same forces, the interior trees don’t develop the same structure, but instead adapt to their protected conditions.
This same yearly, responsive growth extends belowground, as well. Counter to the common conception that tree roots mirror the canopy above, growing downwards and outwards, they in fact grow primarily out and away. Splitting and interweaving into the soil and any moisture-filled cracks in shallow bedrock, roots get their strength by dispersing the loads from the stem above far and wide through a vast network with a massive surface area.
All trees grow this way. It is a myth that Douglas-firs have abnormally shallow root systems and they inherently grow in a way that is ‘meant to be in a forest’. Trees, whether oaks, maples, birches, pines, or firs, grow based on their conditions whether that’s an open front yard or a closed canopy forest.
Other factors, such as decay in the roots or stem, may influence a tree’s stability. However, these can be assessed, predicted, and mitigated. It’s a healthy tree, able to grow and adapt over time to its surroundings, that is best suited to withstanding extreme weather events.
News coverage since the storm has been flush with terms like ‘hazard tree’ and ‘danger tree’ and calls for removal. A tree that perhaps had been growing in a neighborhood for decades was suddenly being called a hazard simply for being a mature tree near a structure, despite its record of stability and resilience to past weather events.
If we were to follow this line of thinking, every tree within striking distance of homes or powerlines, along with things like tall buildings or bridges, would be considered hazardous because they have the potential to fail. Their likelihood of actually failing is not taken into consideration, even if it’s quite low. Risk is not assessed by only taking the potential of failure into consideration, and we should not remove trees on this basis alone. There’s more to the picture.
It is equally problematic to assume that because one tree failed all other similar trees have the same risk of failure. Just as bridges or buildings, we should not remove all trees simply because we’ve seen that others have fallen. The circumstances of each situation are unique. Rather, we need to assess risk by looking at the likelihood of it failing and the likelihood of that failure resulting in damage.
This is where a qualified arborist comes in. Like plumbers, dentists, or engineers, arborists are specialists in a specialized field. Your trees’ condition, like your teeth, should be assessed by an expert with the appropriate experience, education, and background. Further, activities conducted around a tree can have severe impacts on its health, structure, and stability. Seeking the advice of a qualified arborist can help you predict and mitigate issues before a storm or avoid impacting your tree in a way that could make it more prone to failure.
Then there are the practices to avoid as we know they make trees more vulnerable to failure. These include sudden changes to the canopy conditions like the removal of neighboring trees, removal of significant portions of canopy for view or clearance pruning (called lions-tailing), or the snake-oil practices of “thinning” and topping which are structurally harmful but unfortunately still recommended. Below ground, the severing or killing of roots during excavation or significant alteration to the soil conditions through compaction or installation of grass sod and irrigation can also have negative consequences.
The best way to grow a healthy, structurally sound tree is to let it grow. It’s important to check in with an arborist every few years at first to make sure it doesn’t develop a preventable defect, but, by and large, letting a tree grow and adapt is all that needs to be done. If you live around already mature trees, discuss your concerns or objectives with a qualified arborist to develop a plan to maintain them and mitigate any undue risks.
It’s important to know that arborists, homeowners, and regulators alike want high-risk trees to be removed before they fall. Likewise, most do not want trees removed for no good reason; a large, healthy, sound tree provides far greater service alive than it does by being removed when it poses little actual risk of falling.
Encourage your cities to fund the proactive management of street trees to provide the most benefit for the longest time with the lowest risk. Encourage developers to retain trees, especially along neighboring properties, to minimize the negative impacts of removal on the remaining trees and properties around them. As communities, we need to invest in the care of the forests around us and be involved with their planning and management.
Trees are not inherently dangerous. We understand how they grow, live, and die, and we understand the vast benefits and services they provide our communities. We also understand how to assess the risk they may pose and how to mitigate it. Yet, as our climate changes, we still have much to learn about the impact extreme weather events have on our trees. While we learn, we should apply what we already know, and remember that our trees are some of our greatest assets and they are worth keeping around.
A Heritage American elm (Ulmus americana) in the ice storm of February, 2021
