As Lindsay Abrams noted recently in The Atlantic, our ability to quantify the ways in which trees improve our physical and mental health - by providing oxygen, cleaning our air of pollutants, and in more nuanced ways - is steadily increasing. Despite this recognition, writes Leda Marritz, "The USDA Forest Service recently determined that U.S. cities are losing around 4 million trees annually — this at a time when average urban canopy cover in North America is still lower in most places than what is recommended by American Forests."
Why is it that cities continue to struggle to protect and grow their tree canopies? According to Marritz, creative director of DeepRoot Green Infrastructure and an ISA-certified arborist, at least part of the problem lies in the challenge of providing the right type and volume of soil to support tree growth. She points to examples from Toronto, Denver, Emeryville, and West Virginia, of ambitious policies that require street trees to receive a certain minimum amount of soil in order to support healthy urban forests.
"None of these policies is perfect," says Marritz. "There are tweaks, of varying sizes, that I would make to all of them. Still, they represent a critically important step to creating lasting, healthy, thriving urban canopy cover in cities."
"It takes more than a soil volume minimum to cultivate a thriving, mature urban canopy. Good tree stock, soil quality, adequate water and regular maintenance cannot be undervalued. But we’ll continue to lose canopy cover at an alarming rate if we don’t change the way we plant trees in cities," she concludes. "We know what this means, and we simply need to do it. It starts with soil."