For city trees, the root of the matter is the soil
By Leda Marritz
We already know how wonderful and important the presence of an urban forest. Trees are good for our health, they reduce crime in neighborhoods, they promote calmer traffic and they improve home and business values.
But the overall urban tree canopy is in heavy decline, with serious environmental and community consequences.
That may seem like a simple problem to solve: plant more trees. Well, yes and no. Those trees will only be successful if as much thought is put into the soil as the trees.
Although you may not realize it, soil is a crucial component to sustainable development in an urban environment. Soil is the lifeblood of the plants and trees that maintain our connection to the natural world – a connection that even city-dwellers yearn for. This is why trees tend to be such an emotional topic for people, and why they have such a profound effect on how communities function.
Urban environments are harsh growing conditions for trees: pollution, vandalism and inconsistent or absent care all contribute to an urban tree’s decline. But by far the biggest predictor of the success or failure of a city tree is the amount of uncompacted, biologically active soil that it can access. Soil is the stuff that trees live on, and without adequate amounts they stand little chance of reaching maturity. It is why the average life of a street tree is only 13 years.
An abundance of good-quality soil is not a natural outcome of development and city living. In fact, the opposite is true. Soil underneath sidewalks, roads and parking lots is heavily compacted to provide sufficient structural support for surface paving, and is sometimes contaminated with heavy metals, oils, and other types of pollution. Heavily compacted soil is often necessary for engineering and construction safety, but it damages its biological integrity and, by eliminating the small gaps in the soil where tree roots actually grow, it compromises its effectiveness as a source of nutrition.
Herein lies one of the major drawbacks of otherwise terrific and popular “million tree” planting initiatives that have sprouted in cities like Los Angeles and New York. Ambitious tree-planting goals are extremely important but they also require creating a truly tree-friendly environment in which they can thrive.
A typical street tree may have access to only the soil in the opening where it’s planted or if it’s lucky, soil from a nearby park or lawn where its roots can make their way for nourishment. Sometimes this means as little as a tenth as much soil as it requires to thrive and reach maturity.
Exasperated with the hassle and expense of continually replacing dead or dying trees, some cities are introducing soil volume mandates for new tree plantings. Technologies like suspended sidewalks and a stronger focus on tree-friendly site design modifications make providing uncompacted soil for trees a possibility even in ultra-urban areas.
Toronto, Charlotte, NC and Alexandria, Va., are among the North American cities that have implemented minimum soil volume requirements and higher standards for the urban forest. By doing so, they’ve laid the groundwork for solving many of the problems for urban trees. Toronto’s goals are perhaps the most ambitious: setting targets of more than 1,000 cubic feet per tree, or roughly 10 times the soil volume that most trees in most cities currently receive.
Our landscapes, perhaps especially in urban areas, are reflections of our values and priorities as a society. If you think of a city as an organism, the state of its trees indicate the well-being of the entire unit. In areas where trees are never planted or don’t thrive, the surrounding community suffers – through reduced home and business values, poorer air quality, psychological stress or higher crime rates. By this measure, treelessness is not just a regrettable characteristic of a city street. It is an epidemic. If cities are serious about growing forest-quality trees, they have to rethink their methods for planting them.