Posted by Client Relations on Thu, May 06, 2021 @ 12:44 AM
By: Sylvia McNeill, ISA Board-Certified Master Arborist RM-7117B © 2021 McNeill’s Tree Service
Talk about 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.
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”.
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.
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.
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.
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.
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