Posted by Client Relations on Thu, Jul 15, 2021 @ 7:25 AM
The Problems Urban Street Tree Face
By: Sylvia McNeill, ISA Board-Certified Master Arborist RM-7117B © 2021 McNeill’s Tree Service
Urban street trees have a tough life. Before I start on what could well end up being a rant, let me state: there are no trees “native” to the built environment.
When discussing the built environment, we are referring to anything and everything created by people. It is contrived and manipulated. The complete antithesis to where trees naturally grow. And yet we want, and indeed need, them in our proximity. If you doubt that, simply go someplace where there are no trees or plants whatsoever and then transport yourself to a place with them. If you prefer the area without, believe me, you are in the minority.
Our cities are “built environments”. To our credit, we keep trying to incorporate trees and plants within these areas to provide some kind of respite from the harsh concrete climate. Are we successful? According to some studies, not so much, suggesting the typical street tree mean life expectancy is 19-28 years. However, it is further suggested the tree population half-life is 13-20 years, meaning for every 100 trees planted, only half of those will make it to 13-20 years. (Roman 2014)
With all our modern technology and innovations, we are having a hard time achieving longevity in our street trees within urban areas compared to cities of the past. Here in our area of Montana, some of our cities have street trees that are over 80 years old, some even older. But as they decline and have to be removed, their replacements are having a hard time establishing and surviving. So, what gives?
Modern cities are far different from cities of the past. In the past, streets tended to be wider and unpaved. Rainfall would be absorbed by the entire street area and any tree planted near that street would likely be able to access that water.
Common Reason Trees Struggle in Urban Environments
| Problem | Description |
|---|---|
| Lack of Soil Space | Insufficient space for root expansion due to limited soil volume in urban environments. |
| Soil Compaction | Soil in urban areas is often compacted, restricting water and nutrient absorption to the tree roots. |
| Root Damage | Tree roots can be damaged during construction, heavy construction, pedestrian traffic, impacting stability and health. |
| Pollution | Urban pollution, including air and soil pollutants, can harm street trees. |
| Watering Issues | Overwatering or underwatering can lead to root rot or drought stress. A poor management plan and lack of adequate soil is often the reason trees fail. |
| Disease and Pests | Due to street trees having less then ideal growing conditions, in urban areas trees are susceptible to diseases and pests that can weaken or kill them. |
| Mechanical Damage | Physical damage from vehicles, pedestrians, or vandalism can harm street trees. |
| Poor Pruning Practices | Incorrect pruning can lead to structural issues and reduced health. |
| Inadequate Drainage | Poor drainage can result in waterlogged soil, suffocating tree roots. |
| Infrastructure Conflicts | Trees may face conflicts with underground utilities, leading to root damage or tree removal. |
| Competition with Other Plants | Planting trees too closely to other vegetation can lead to competition for resources. |
| Root Girdling | Circling or girdling roots can constrict the trunk and affect stability and also lead to death due to root strangulation. |
| Climatic Stress | Extreme weather conditions, like drought or heavy rain, can stress street trees. |
| Soil Contamination | Contaminated soil can harm tree roots and affect overall health. |
| Tree Vandalism | Deliberate damage, like graffiti or cutting branches, can harm street trees. |
| Lack of Maintenance | Neglecting regular maintenance, like pruning and soil aeration, can lead to tree problems. |
Streets now are paved where stormwater drains may be designed to bypass roots. One of the many ecosystem benefits touted for trees is their ability to mitigate stormwater runoff…if it is channelled away from their roots, how are they to perform this job?
There are often paved sidewalks on one side of a tree with the parking area and adjacent street on the other side, again all paved. These areas between the sidewalk and street often referred to as boulevards, may be very narrow or very wide. Trees fortunate enough to be planted in wide boulevards have a much better chance of survival. The caveat here is “if the quality of soil is sufficient to their needs”.
A lovely Gleditsia triacanthos ‘Sunburst’ – Sunburst honeylocust planted in a wide boulevard
In situations where trees are planted between a and a carpark. Solutions such as Stratavault soil cells are ideal in ensuring trees get large enough space for their tree roots to grow effectively and avoid damaging nearly infrastructure.
Infographic on how stratavault grows and protects street trees
The quality of soil in boulevards can vary from not bad to really awful. With the construction of surrounding infrastructure and housing, all manners of debris may be buried in this area. This may include concrete, building debris, contaminants – basically, anything that needed disposed of during construction could have been tossed here. It may be rocky, it may be silty. It may be seriously compacted, which restricts root growth and water drainage.
Related: How To Avoid Tree Roots From Drowning
Trees planted in the narrower boulevards are subject to conflicts with the sidewalk and streets as their roots grow. As sidewalks require repair, roots are often cut to the detriment of water uptake or even stability of the tree itself.
Roots were severely cut back to enable sidewalk repair. Tree declined to a point where removal was mandated.
Tree Protection
Sometimes trees are completely surrounded by concrete or grates may be used to distance the trunk from the concrete. Unfortunately, if the grates aren’t maintained, they can also damage the tree.
Related: Tree Grates & Cities: What To Know
Photo on the left example of poor tree grate maintenance. Citygreen’s Duraplate tree grate (on the right) incorporates specifically designed features for easy maintenance.
Another challenge experienced by city street trees is the pruning required to keep them above vehicle and pedestrian traffic. Merchants do not like their signage blocked and it is important to keep trees from blocking clear view triangles on street corners to ensure public safety.
Please remember, the trees did not plant themselves. Very few are volunteers in these areas. It is up to the people who plant them to select a species that is hardy enough to have a reasonable chance of survival in the site selected, that the site is actually suitable for a tree at all (some sites simply should not be planted), to ensure the tree has the potential structure to minimize pruning needs (which is a factor not only in the health of the tree but the expense for management), to ensure planting is done correctly and that aftercare is prearranged and committed. These last two conditions are the most common reasons why trees do not survive the establishment period for any area, let alone for the challenges faced by street tree selections.
Additional challenges include damage by vehicles, vandalism, and extreme weather conditions enhanced by the infrastructure around them. I often think trees survive in spite of us, not because of us.
These photos show the opposite sides of the same tree taken on the same day.
We need to work on being better stewards because we need trees. There is no need to quantify “ecosystem benefits”. All you have to do is remember without plants, we (humans) would not exist.
References: Roman, Lara A. (2014) How Many Trees Are Enough? Tree Death and the Urban Canopy. Scenario 04: Building the Urban Forest