City trees have a hard life. Polluted, pruned, and squeezed into tiny spaces, but urban forests need to thrive as much for our health as their own. Not that we treat them that way. Most cities stick trees in too little soil, give them minimal attention and hope for the best--usually stunted growth and an untimely death.
A firm called DeepRoot, essentially an architecture firm for the urban forest, is advocating "high performance urban forestry." It has designed an underground structure called Silva Cell a tiered network of pillars holding up platforms under the pavement, for urban tree roots to grow deeper into the ground instead of breaking up the asphalt and sidewalk. The approach prevents soil compression, allowing trees to mature and grow deep, with healthy roots in loose soil, while better filtering rainwater. The test results are dramatic: larger, fuller trees that create a forest and pay back their investment in less than two decades, just a fraction of their expected lifespan.
Spreading oaks and maples are pleasant, of course, but all of this makes sense to urban planners and cash-strapped municipalities when they better understand the economics. The conventional approach to urban trees--"planting trees in 4 by 4 cutouts and replacing them every 13 years"--is backwards, argues DeepRoot’s Leda Marritz. "We already know we can do better than that," she writes.
Rather than investing billions of dollars in “gray” infrastructure such as sewers and pipes that lose value (and deteriorate) immediately, we can plant trees that serve similar functions yet appreciate as soon as they are in the ground while supplying ever more benefits as they enter their fifth--or even eighth--decade.
"They require us to plan a little differently, yet the potential payback is enormous," writes Marritz. DeepRoot first needs to convince cities to recalculate their investment in urban development. It has already met some success incorporating its designs into major projects in Canada and elsewhere. Its selling point is treating trees as utilities that "perform on-site stormwater management, enhance home and property values, traffic safety, public health, and wildlife habitat (among other things)."
A few cities have already started rethinking their zoning and development policies to make big trees a part of their landscape (see a list here). Although that takes a little extra space, DeepRoot estimates trees planted in 1,000 cubic feet of soil (enough to mature to full size) can pay off their investment after about 20 years with substantial economic benefits continuing for many decades beyond that.
It doesn’t take much for that math to add up. In Washington, D.C., the nonprofit group CaseyTrees estimates the capital’s 1.9 million trees provide more than $10 million in annual carbon, air quality, stormwater, energy, and property value benefits.