Urban Forester's Column: Grow Big Trees

February 6, 2024

city trees

Trees in our neighborhoods provide us with many benefits. They cool us to make spaces livable on hot days, they reduce dust and noise and stormwater so that parks, schools, and other shared spaces can coexist with our built environment. They add beauty, color, and a sense of place as they calm our anxiety and improve our mental health.

First of all, thanks trees!

Next, I’d like to argue that it is essential to consider certain guidelines when planning a planting project in order to maximize the benefits of trees.

Guideline 1. Larger trees provide larger benefits.

A larger tree casts a larger shadow. This shadow is the tree’s main mechanism for offering its cooling effects. A tree filters air and reduces noise by way of its leaves. The larger the tree, the more leaves it has to offer these benefits. Stormwater is slowed down by being captured on the tree’s surface on its way to the ground surface. This delays its journey to the waterways so that less water is available to worsen flooding. Also trees soak up excess water by their roots and then evaporate it from their leaf surfaces, causing both cooling and stormwater benefits, and both of these benefits are greater with more leaves, and more roots respectively.

Guideline 2.  Healthy trees provide greater benefits than struggling trees.

A tree’s growth is typically limited by certain factors determined by its location. In a sunny dry location for instance, the limiting factor for the tree is water rather than sunlight. Whereas in a moist dark environment, the tree may have plenty of water, but its growth is limited by sunlight. Considering how to meet the tree’s needs based on its planting site is critical to healthy trees. For one thing the more we can provide healthy conditions for trees, the larger they will be, so that brings us back to Guideline 1. But beyond the size of the tree, a tree’s health is an essential factor in the way it functions in a community. A healthier tree needs less maintenance! A healthier tree has a denser canopy that lasts longer into the fall providing longer lasting and deeper shade. A healthier tree is less likely to pose risks due to breakage. And healthier trees will last longer, thus the need for replanting is delayed, giving a community savings in effort and resources. So at the time of planting, the more we can do to provide ample soil volume and sunlight and water for trees, the better.

Guideline 3. No matter the tree, its benefits are hyper-concentrated around its location.

Even the largest tree in the world only shades the area in its immediate vicinity. The shadow begins at the stump and stretches away from the sun out to a distance determined by the tree’s height. A particle of dust needs to come into contact with a leaf in order to be trapped and effectively filtered out of the air. A soundwave needs to pass through a hedgerow in order to be dampened, being on the street side of that same hedgerow offers no benefit in terms of sound.

This hyper-localized nature of tree benefits is the heart and soul of how we need to look at tree equity (the concept that tree canopy is a privilege that is unevenly shared). Trees in a city do not reduce the heat of the entire city, they reduce the heat only in their shade. And yet, the way we quantify our urban forestry goals, is largely determined by the numbers. “We planted 25 trees!” Can mean that a very targeted community in need received a significant amount of shade and other benefits that they badly needed, or it could mean that virtually none of these benefits were provided to anyone, depending on where exactly in the city they were planted. If we plant trees along the edge of a forest, or on the wrong side of a soccer field, or on the north side of a parking lot. We are investing the same amount of resources in hiring the contractor and buying the mulch and the trees from a nursery, but without the return on that investment.


Tips to Follow Guideline 1:

In order to plant the largest possible tree in a location, first assess the planting site. How much space is there above ground? Are there important sight lines such as to signs or street lights that need to be considered? Are there power or telecom lines above? Where are the nearby buildings, and other trees? If a large tree in this location began to decline, would the removal process be fairly straightforward? With these things in mind, look up some of your favorite trees in a trusted book or online resource (like our Tree Selection Tool) to assess if the tree’s mature size both height and width will fit in the space nicely.

Tips to Follow Guideline 2:

Do the work to improve the below ground characteristics of the planting site. Remove pavement so that the tree can have ample water and air. Replace compacted soil with soil that has been amended with biochar, compost, or other strategies to ensure soil structure can be maintained. Look for opportunities to get water to the tree passively, such as curb cuts, or directing water from rain gutters to the tree. Protect the tree from human uses that exist in the area using metal grates and guards, surrounding benches, curbs with curb cuts, or other innovative designs.

Tips to Follow Guideline 3:

Try weighing the needs of the under-resourced over the needs of the privileged. I look forward to a lifetime of evolving thought on the concept of privilege and equity, but for now I think that income is essentially the right way to look at it. Affluence gives us the privilege of options. We can shop, dine, and live where we want to. We can even donate money or organize groups of people to plant trees in the places we want to see trees grow. But when we have low income, our options are extremely limited. We can’t afford the initial investment of a heat pump, so instead we spend a lot more in the long run operating air conditioners, for example. We can’t afford the initial investment of an electric car, so we spend a lot more in the long run on transportation as we continue filling up at gas stations. In forestry terms, we can’t hire our own arborist to advise us and plant trees to our personal interests, so we are left in the care of our local government foresters, planners, workers, and volunteers. We need to be thinking of these lower income constituents while we plant trees for the public good, because they will benefit most from the trees in their immediate vicinity. Not in their city, but in their front yard. Not a weak tree under the powerlines, but a huge oak with good soil, centered in front of their south-facing windows.

There is another benefit of trees to consider for urban foresters, and that is the goodwill and community building that come from a job well done. A successful tree planting project that delivers on its promises will win over more supporters than a less successful project.

If you are reading this, then your interest in urban forestry is probably a little idealistic. The payoff for this work is the dream of a better community in which an overstory of lush green is the shelter under which a vital, civil, equitable culture can grow and thrive. In order to realize that dream, it bears remembering: Grow big trees. Grow healthy Trees. And Grow trees where they are most needed.


Author: Adam McCullough, VT UCF's Urban Forester