When you look a little closer, though, you will find that planting trees is an essential part of ensuring clean and abundant water for our city. So what are the connections between trees and Nashville’s waterways?
Planting trees is one of the best things you can do to enhance water quality, especially in urban areas. Nashville’s urban tree canopy provides a bounty of benefits to the Cumberland River and its tributaries. We’ve compiled a list of the ways that our trees help improve the health of our waterways — read them below to see for yourself!
Slow Down Rain
Imagine standing in an intense rainfall in a front yard without any trees. Now picture how different that same rainfall might feel if you were standing under a large oak tree. The tree’s canopy acts a bit like a big umbrella, allowing rain to drop down to the ground more slowly as the water hits the leaves and branches.
Simply by being present, a tree’s structure slows down rain as it falls. This reduces the risk of water rushing down too quickly in extreme rainfall events, which can lead to flash flooding, ponding, and soil erosion (more on that later on). A gentler rainfall allows water to infiltrate the soil at a slower rate, making the impacts of these heavy rains less severe.
As the climate continues to change, Middle Tennessee is experiencing more extreme rainfall events. Warmer temperatures lead to more evaporation, which leads to more rain (see graphic below). In our changing climate, trees serve as a line of defense to mitigate severe weather events.
Soak Up Water Through Roots
Trees also help prevent flooding by soaking up water through their roots. (When roots slowly soak up water throughout the growing season, this increases potential groundwater storage during future storms.) Water is a key ingredient in photosynthesis, the process that keeps a tree alive and growing. As a tree grows larger, the water quality benefits it provides also grows.
Additionally, trees soak up much more water than they actually need for photosynthesis. The vast majority of the water that a tree soaks up – up to 95%! – travels up through the roots all the way to the leaves, where the water is slowly released in a process called evapotranspiration (evaporation + transpiration).
Side note: Ever thought about how strange it is that water travels upwards in a tree, from roots to leaves?! Do trees defy gravity? Not quite. This phenomenon is possible due to several combined effects: how trees create osmotic pressure in their xylem (aka a tree’s “pipes”) and the unique characteristics of water molecules themselves. Learn more in this fun, scientific video: “How Trees Bend the Laws of Physics.”
Remove Pollution from Stormwater Runoff
Not only do trees slow down rain and soak it up, they also filter stormwater runoff. “Stormwater runoff” isn’t just a fancy way of saying “rain”. Rain becomes stormwater runoff when it hits an impervious surface it can’t infiltrate, like concrete and hardscapes. In the process of moving along these impervious surfaces to find a place to soak in or drain, the water picks up pollutants and becomes polluted stormwater runoff.
Trees filter these pollutants through phytoremediation, a process that is described in this USDA article:
“Through… phytoremediation, green plants are used to remove, degrade, or stabilize pollutants and contaminants, such as toxic metals, from soil or groundwater. The practice of using trees as waste cleanup tools has been around for many decades and its early promise as a low-cost alternative to other cleanup methods has borne out.”
Because of this water quality benefit, partners in the Root Nashville campaign love to plant trees near impervious surfaces (along streets, driveways, and edging parking lots) where stormwater runoff ends up infiltrating. When we plant trees in these locations, we help prevent pollutants from entering our waterways and our ecosystems.
Improve Soil Health
Almost all of a tree’s roots are located in the top foot and a half of soil; this means that a tree’s root structure is much wider than it is deep. This structure helps trees keep upright, and allows roots to get the oxygen they need near the surface.
Trees prevent soil erosion by slowing the rate that rain hits the ground, as previously mentioned, and also through their root systems. All this root mass near the surface of the ground helps keep the soil in place, with the roots acting like anchors that prevent soil from being washed away.
And there’s more that trees do for soil health! Tree roots also aerate the soil, add healthy nutrients, and support microorganisms.
Provide Shade for Waterways
The temperature of a stream is critical for its overall function. Trees along a stream shade its water in the summer, preventing it from getting too hot. This natural temperature regulation protects many species of aquatic life that can only thrive within specific temperature ranges. Without the regulating effect of trees’ shade, warmer water also increases the odds of unhealthy algal blooms and speeds up chemical reactions that alter oxygen levels and negatively affect aquatic ecosystems.
To provide temperature regulation in creeks, the Cumberland River Compact and other partners in the Root Nashville campaign plant trees in riparian buffers along streambanks. Along with cooling the waterways, planting trees in riparian buffer areas also helps prevent erosion and restore degraded streambanks!
Nashville’s fresh water
The connection between water quality and tree canopy cover is easy to see in Nashville. The EPA’s How’s My Waterway tool shows whether a stream is healthy or unhealthy, with red indicating impaired or polluted waterways.
The Middle Cumberland Watershed is shown in the snapshot below. Notice how the blue lines (healthy waterways) are located in the northwest part of the watershed, overlapping with the most green, where there is the highest density of tree canopy.
Of course, trees aren’t the only determinant of stream health. Other causes of stream impairments, some of which are unrelated to tree canopy cover, exist throughout the Middle Cumberland Watershed. Check out the Cumberland River Compact’s Problems and Solutions page to learn more about water quality issues throughout the Cumberland River basin and to address them.
As you can see, though, trees – and all of their incredible qualities – have the potential to make a big difference in stream health.
We love Nashville’s urban trees because of all the benefits they provide for water quality. But that’s just one category of benefits! Trees sequester carbon, reduce temperatures, provide cleaner air, and so much more. Our tree canopy keeps our environment (including our waterways) and our community healthy.
Do you want to protect the creek running through your backyard? Prevent ponding after it rains? Check out the Neighborhood Planting Captain program to bring free trees to your own yard, and your neighbors’ yards, too.
Additional sources: Review of the Available Literature and Data on the Runoff and Pollutant Removal Capabilities of Urban Trees; Urban Forest Systems and Green Stormwater Infrastructure; Forest Cover, Impervious-Surface Area, and the Mitigation of Stormwater Impacts