growing the citified tree

by Neil Caudle

Greenville’s tree-shaded Main Street has helped revitalize the city’s downtown businesses. But not every urban tree finds a home amid asphalt and concrete. Photo by Neil Caudle.

The road to hell, a tree might say (if trees could talk), is paved with good intentions. With all the best intentions, we pave our roads and walks and parking lots, and plug in some saplings, and wonder why they die, overheated and parched.

“It’s always intrigued me,” Paul Russell says, “how we take a tree from a nursery in this really happy condition, with healthy roots, and we dig it up and put it on a truck, and we move it to an urban environment with different living conditions, and we expect it to thrive and do well. It just isn’t normal.”

Russell is a landscape architect, so he thinks of urban trees as elements in a pleasing, enduring design. Even in a city—especially in a city—people like the deep, cooling shade and rustling leaves of big trees. Think of downtown Greenville, for instance, where in summer the canopies arch over Main Street, gratifying passersby with shelter from the heat and glare. Even in winter, graceful branches sculpt the space and make it more alive.

Those big, healthy trees also gratify the merchants, who have seen their businesses grow right along with the trees. People like trees, so trees are good for business.

But dead or stunted trees do not gratify anything, except perhaps an infestation of fungi or bugs. So Russell wondered, how do we make a tree healthy and happy, when it’s planted in a city or suburb? How do we know what kind of pavement to use around it, and how near that pavement should come to the base of the trunk?

To answer those questions, Russell needed an engineer, and he happened to know one. Brad Putman is a civil engineer who studies various paving materials and how to make them stronger, more durable, and more environmentally friendly. He is also a friend of Russell’s. Their kids trick-or-treat together every Halloween. So the two hatched a plan. They would study this problem together. They would figure out how to measure, in a systematic way, the relationships between pavements and urban trees.

“We’re starting out with a control-type study looking at baseline,” Putman says. “We want to know how the pavement behaves first. How does the water get into the soil, and what is its distribution? And then, when the tree gets in there, how does that change the game? How does the tree behave?”

There’s more to making a home for a tree than digging a hole. This illustration, by Paul Russell, shows a method for constructing a tree’s environment to stack the odds in favor of survival. Porous cobble pavers allow air and water into the soil. Irrigation supplies water as needed. A slot drain prevents stormwater from flooding and drowning the tree. And the specially prepared planting soil rests between porous layers of stone for drainage below and weight-bearing structure above. Russell and Putman will test and refine this model based on their research.

Recon from the root zone

Students passing through the courtyard between Lee Hall and its gleaming new addition, Lee III, stop to puzzle over a row of wood-framed boxes resting on a bed of gravel. The boxes, eight feet square and four feet tall, don’t seem very massive, at first glance, but each one, full of soil capped with a layer of pavement, will weigh about 20,000 pounds.

The researchers and their students plan to install sensors at multiple locations in each box, to track moisture and temperature, using technology developed for the Intelligent River project. For that project, a high-tech effort to document and monitor the health of the Savannah River watershed, Putman worked on a green-infrastructure retrofit in Aiken, South Carolina. As the retrofit’s pavements expert, he helped design and monitor the porous pavements the team installed downtown, with special attention to protecting some venerable old magnolias. After monitoring the pavements for several years, Putman and the Intelligent River team confirmed that the retrofits had improved storm-water drainage throughout the city, reducing the potential for sudden jolts of polluted storm water that damaged the environment downstream.

“These porous pavements also have the ability to filter out a lot of the contaminants,” Putman says, “so the water is not just running into the curb and gutter and picking up all of this stuff along the way, and then dumping into the storm sewer and going into the nearest water body.”

The idea for his research with Russell is to test various types of paving materials, collect baseline data about their performance and their effect on the soils below, and then to plant the trees.

“We’re going to use Acer buergerianum, the Trident maple,” Russell says. This species of maple has a reputation for tolerating urban environments, and its form will complement the architecture of Lee III, Russell says.

By adjusting the experimental variables—pavement type and its distance from the trunk, soil composition, water, and nutrients, among others—the team will narrow down the options to those that work best and test them in a real-world setting, perhaps in the parking lot of the South Carolina Botanical Garden.

Russell and Putman planted the trees in late March and will track the effects of various paving materials on their health. Photo by Paul Russell.

Not a black-or-white issue

When it comes to choosing a tree-friendly pavement, conventional wisdom doesn’t always get it right. Anyone who’s crossed a road or sidewalk barefoot on a hot summer day knows that white concrete reflects more heat than black asphalt. So if asphalt burns our feet, it must be hotter and worse for the trees, right?

No, not necessarily, Putman says. We have to think about the tree’s entire environment, above and below ground, not just the surface of the pavement.

“Everybody always says asphalt is bad for the heat-island effect, but really it just has to do with the pavement, whether it be concrete or asphalt,” Putman says. Research in Phoenix, Arizona, and elsewhere has shown that urban heating has more to do with the thickness and density of the pavement, and therefore its thermal mass.

“When you go to an urban area that has more of these very dense materials, it’s going to soak up the heat, and it’s going to have a much tougher time dissipating that heat at night,” Putman explains. “For example, it’s going to be hot here in Clemson just as it is in Columbia during the middle of the day, but the real difference is nighttime, when it’s going to cool off quicker here at Clemson as opposed to Columbia, where you’ve got all this thermal mass that’s trying to give up its heat but can’t do it effectively.”

Putman has found that certain kinds of porous pavement, including porous asphalts, may contain 20 percent of their volume as voids. Through these voids, air and water can move, and the pavement has less thermal mass to hold heat.

Modern pavement technologies now offer numerous options, and Putman and Russell will test several of the most promising: porous asphalt, pervious concrete, and permeable pavers, which are pavers with wide joints. The permeability is important not only because it allows air and rainwater into the soil but because it tends to moderate the temperature. Several feet deep in the soil, temperature remains almost constant, so if air can flow through the soil, natural geothermal heating and cooling will tend to moderate temperatures in the tree’s root zone, keeping it cooler in summer and warmer in winter, Putman says.

Environment by design

A successful urban tree installation is not as simple as just choosing a permeable pavement and sticking a tree in a hole, Russell and Putman say. The soil may itself be the number-one factor in a tree’s health and survival, and not all soils are created equal. In many urban settings, poor, compacted soils will have to be replaced with specially formulated soils engineered to drain, circulate air, maintain good structure, and provide sufficient nutrients. Sometimes, this is a difficult sell, Russell says, for budget-conscious clients.

“The biggest challenge I have is trying to convince someone to spend hundreds of thousands of dollars on soil,” he says. “People don’t get that.”

The landscape itself must be carefully designed and constructed to manage water infiltration and maintain the tree’s environment. Poor installations or flaws in the paving material itself can defeat permeability and suffocate the tree, Putman says. “If you’ve got a lot of sediment-laden runoff flowing across these parking lots with permeable pavements, you’re going to clog them up.”

But Putman is optimistic that standards for pavement mixes and construction details are improving. He and Russell plan to develop, from the results of their research, guidelines for pavements and urban-tree plantings that would help landscape designers and their clients select the right system for each location.

“We’d like to come up with a design idea or design guidelines, apply those and verify those, and then get them out to practitioners to use later on,” Putman says.

The task is especially relevant, these days, because the trees in some of our most beloved urban streetscapes are declining. Trees don’t live forever, and replacing a row of majestic but fragile old centenarians isn’t easy—politically, aesthetically, or otherwise.

One recent example, Russell says, is the loss of giant tulip poplars at Biltmore Estate near Asheville, North Carolina. The poplars were part of an elegant landscape planned by Frederick Law Olmstead, who also designed some of the nation’s most famous urban settings, including New York’s Central Park. “Olmstead planted those tulip poplars at Biltmore in eighteen ninety-three and said that the trees would decline in a hundred years and would have to be replaced,” Russell says. “He was right. And it cost several million dollars to do that.”

Russell and Putman are hoping their work will help communities design and install urban plantings that look good and improve the environment, whether the job is to replace old plantings or establish new ones. The team also hopes that their students will learn what they have learned: that a designer and an engineer can work together, despite their different perspectives.

“I’ve always wanted to engage our students a little bit more across colleges,” Putman says, “because engineering students are always going to be dealing with—”

Russell laughs. “Dealing with?”

Working with architects and landscape architects. You hear the stereotypes of architects or landscape architects—”

“What are they?” Russell says, feigning indignant surprise.

“Oh, you know, those architects, they want this or that, and it’s just not practical. You hear that a lot. And then you hear the architects say, ‘Those engineers, they just don’t want to be creative.’ So it will be interesting to engage our students together, and use this research as a learning tool.”

So far, the strategy of teaming landscape designers with engineers seems to work just fine for Russell and Putman. But it isn’t always easy.

“This is kind of pulling Paul out of his comfort zone and pulling me out of my comfort zone,” Putman says, “and we’re kind of meeting somewhere in the middle here, where we’re both learning from each other.”

Brad Putman (left) and Paul Russell are using large planters like the one shown here to study the influence of various paving materials on the health of urban trees. The planters will also serve as a test bed for studying the rhizosphere, a thin zone of soil, in contact with the roots, where microorganisms feed and protect the tree. Landscape architecture students designed the planters to accommodate research as well as outdoor gatherings at Lee Hall. Concepts from the research are finding their way into Clemson’s entry into the Solar Decathlon Design Competition, whose winners will be announced this fall in Irvine, California. The School of Architecture’s entry, called IndigoPine, will include “an integrated planting strategy” for the project’s outdoor spaces, Paul Russell says. Photo by Ashley N. Jones.

Paul Russell is an assistant professor of landscape architecture in the College of Architecture, Arts, and Humanities. Bradley Putman is an associate professor of civil engineering in the College of Engineering and Science.

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