Trees as Stormwater Controls
We live in the eastern deciduous forest of North America. Clearing forested land to establish communities has been the story of this country since Europeans first arrived on its shores. Over the years as we have cut down more and more trees and paved over more acres of land to develop homes, factories, businesses and highways, we have also inadvertently created a huge problem. There is no place for rainwater to go.
Nature’s plan is for rainfall to soak into the soil onto which it falls, where it sustains the complex forest ecosystem that, in turn, supports wildlife and, ultimately, human life. When we interrupt that cycle by replacing forests and meadows with surfaces impervious to water, such as concrete, asphalt, roofs, and compacted soil, stormwater has no choice but to collect on these surfaces and flow to lower ground, gaining destructive power as it travels. The water sluices over the ground rather than soaking into it. As it seeks a lower grade it erodes soil, washes pollutants into local creeks which then eventually flow into and pollute drinking water reservoirs, rivers, and bays. It also destroys streambanks, causing flooding. The valuable water is not available to recharge aquifers, keep our streams flowing steadily year-round, or nourish our plants. Rather than being a gift, rainfall becomes a danger.
Only in relatively recent times have government officials at all levels required that stormwater control measures be part of any new development plan in a community. Various technologies and methods for reducing the dangers and destruction of uncontrolled stormwater flow have been created for this purpose. They are referred to as Best Management Practices or BMPs. Properly designed and installed, they are successful in varying degrees in managing intense rainfall so that the water soaks into the ground in the area where it falls. Some examples are: retention basins, detention basins, rain gardens, cisterns, swales, rooftop gardens and pervious paving.
Often overlooked in the effort to devise effective BMPs for development projects is the simplest, least expensive, most environmentally desirable, most attractive, most reliable, longest lasting, and the oldest one of all—trees! Before they were all cut down, trees did the job of controlling stormwater runoff. In the fifteen years, between 1990 and 2005, the five-county Philadelphia area lost 5 million trees. It is no coincidence that the removal/destruction of the tree canopy in our region correlates with the rising tide of destructive stormwater here.
How Trees Trap Rainfall
A deciduous tree is a veritable water machine. It absorbs up to 14 times more rainwater than a turfgrass lawn. It helps control flooding and runoff in your yard aboveground, at ground level and below ground.
A healthy foliage canopy blocks and softens rainfall, delaying the onset of peak flow. Individual leaves intercept the water, absorbing some, then retaining some of it on their surfaces where it eventually evaporates (evapo-transpiration). So does the bark of limbs and trunk. The rest of the rainfall, its impact reduced, then drips down on the soil more slowly. Research indicates that 100 mature trees can intercept about 100,000 gallons of rainfall per year in their crown. A single mature, deciduous tree intercepts from 500 to 700 hundred gallons of water from rainfall each year. Imagine how just a few trees on your property would mitigate heavy rainfall.
Tree roots near the soil surface increase the capacity and rate of rainfall infiltration into the soil, reducing surface flow. They also hold soil in place as raindrops fall, discouraging erosion. Spongy decaying leaves or mulch on the soil over the roots beneath the tree canopy further cushion the fall of rainwater from the canopy onto the soil, preventing compaction and keeping the soil absorbent. The mulch layer encourages “surface ponding,” temporarily retaining the rainwater, which allows some evaporation as the water slowly soaks through the natural filter of leaves into the soil beneath.
As tree roots decay, they leave behind channels that encourage infiltration of the rainwater deeper into the soil for storage, so it is available for uptake by other trees and plants in the area.
Trees are a solution
Let us not underestimate how helpful and effective trees are as BMPs. While they are not the only stormwater management tool, they are particularly effective and appropriate for residential settings. By urging homeowners and requiring developers to restore tree populations in towns and cities, by regulating tree removal and by planting public trees communities can reestablish the protective leaf canopy and go a long way toward retaining our precious fresh water, and healthy soil. We also enjoy the bonus of a lovely community.
By Liz Ball, Marple Tree Commission