How what’s overhead affects what’s underfoot
The Trees All Waved Their Giant Arms
by Rikki Hall
Knox County has problems. Our air is too dirty to meet federal standards, and our sewers back up during rainstorms and disgorge their foul contents into neighborhoods and parks. Trees are waving and whispering in the wind, “We can help.” If you listen closely, they spare no details. One breezy afternoon, an excited leaf sputtered to me, “An acre woodlot can hold 3,700 cubic feet of stormwater.” I had heard the same thing from the U.S. Department of Agriculture, so I believed it.
Stormwater is a big issue for Knox County and governments across the nation, as decades of sprawling growth have filled roads and sewers to capacity and beyond. Here, the stormwater problem is exacerbated by aging sewer pipes. Cracks and holes lead to trouble not when they let sewage out, but when rainwater gets in. A system designed to handle occasional flushes and bouts of dishwashing can be overwhelmed by a constant influx of rainwater. When that happens, pipes back up, and low-lying storm drains become filthy fountains.
Sewage overflows notwithstanding, the main stormwater problem is ordinary flooding. Development can make flooding worse by proliferation of impervious surfaces: parking lots, buildings, anything that prevents the soil underneath from soaking up water. Inside the city limits, 27 percent of the ground is impervious.
This is where the trees come in. Trees increase the water-absorbing capacity of land by holding soil with their roots and creating it as their leaves decompose. They drink water too. Not just pervious, trees retain water, making them a valuable tool in flood prevention.
In the mountains, where clear, clean water is wonderfully abundant, soils accumulated over thousands of years hold almost as much rainwater as nature can drop. Both filter and buffer, ancient soils slowly release water into seeps and springs, so even when it hasn’t rained in a month, Appalachian rivers still plunge happily valleyward. Water is the stuff of life, and evolution drives forests to fill all the life-giving capacity available; thus our old, wet mountains abound with salamanders, darters, mayflies, crayfish, waterthrushes, herons and the trees that love them.
Biological richness does not pay the bills, at least not directly. Nonetheless, you can measure how much water a one-acre woodlot can hold and compare the cost of building culverts and detention ponds with the same capacity. Knox County’s trees are worth over a billion dollars in stormwater control, according to a 2002 study by American Forests. Trees also clean the air. They consume carbon dioxide and emit oxygen, and along the way they remove over two million pounds of pollutants from the air in Knox County alone, preventing $43 million in medical expenses and missed work yearly.
As county officials ponder how to bring our region back into compliance with federal air and water standards, figures like those should get their attention. Vehicle inspections and lowered speed limits are not popular with voters. Enforcement of pollution laws is in the hands of state and federal agencies. Within their domain is an easy sell that would net real results: a tree protection ordinance.
A tree ordinance needs no enforcement authority to be effective. The city’s tree ordinance, passed almost a half-century ago, requires keeping six trees per acre when clearing land and leaving eight growing per acre after development. Violations are $50, reasonable exemptions granted. The limits are so trivial as to be self-enforcing. Not intended to be punitive, the city tree ordinance created an advisory Tree Board to help city crews, developers and neighborhoods make good forestry decisions.
Four decades later, we can precisely compute the watershed management value of land use and site planning decisions. We can estimate the rise in health care costs and flood control if Knox County’s impervious surfaces double in 30 years and grant incentives accordingly for pervious design. We can make smart forestry and stormwater decisions worthwhile for developers. County engineers already have the tools.
It is possible to write a tree ordinance better than Knoxville’s, but even if the county merely adopted the city Tree Board as its own, it would realize air pollution and flooding reductions for the cost of coffee and donuts at the monthly meetings. There is money to save and sickness and destruction to prevent not by hugging our trees, but by noticing trees have always kept us safe in their giant arms.
Rikki Hall is managing editor and publisher of Hellbender Press , a non-profit environmental education journal.