Gel-Gravel-Soil Mix Gives City Trees Room to Grow

John Roach
for National Geographic News
September 10, 2004

Trees bring both comfort and measurable benefits to urban environments. The problem is that tree roots don't flourish under all that pavement. Now scientists think they may have found the answer.

"The way we lay pavement is antithetical to how we want trees to grow," said Nina Bassuk, director of the Urban Horticultural Institute at Cornell University in Ithaca, New York.

Bassuk and her colleagues have developed a high-tech mixture of gravel, gel, and dirt that they hope will sort out the differences between pavement and trees, allowing humans holed up in high-rises to wake each morning to birdsong drifting from the treetops.

The mixture, known as structural soil, overcomes a conundrum that has vexed urban planners for years: Pavement requires densely compacted soil that can stand up to the weight and abuse of cars and trucks. But tree roots need room to breathe and grow.

The status quo is for developers to plant trees in a pit in the pavement just large enough to get a sapling into the ground. But with nowhere for maturing roots to wander, these trees usually die within 7 to 15 years.

Structural soil, by contrast, provides urban planners with a rock-hard surface on which to pour pavement while still givinh growing tree roots plenty of room to stretch.

"The whole thing is one of engineering of trees into infrastructure so they have the best chance of success from the get-go, not shoehorning them in as an afterthought," said Gregory McPherson, director of the U.S. Forest Service's Center for Urban Forest Research in Davis, California.

Value of Trees

The sound of birdsong drifting up through an apartment window early on a summer morning may be reason enough for many people to cherish trees. But trees are also a measurable benefit to the health and pocketbooks of city dwellers.

The research of McPherson and his colleagues quantifies the value of trees. The plants provide services such as cleaning the air, reducing water pollution, keeping cities cooler in summer and warmer in winter, and slowing the pace of global warming.

"In a general sense any single benefit is not something that will knock your socks off, but when added together they begin to be substantive, and they do have an effect in the aggregate that is worth taking notice of," McPherson said.

Continued on Next Page >>


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