How Trees Clean Water

The Critical Role Trees Play in Water Quality and Access

Trees and forests play a critical role in capturing rainwater and reducing the risk of natural disasters such as floods and landslides. Their intricate root systems act like filters, removing pollutants and slowing down the absorption of water into the soil. This process helps to prevent erosion and reduce the risk of over-saturation and flooding. Together, these actions make trees a vital element of "green infrastructure" across urban and rural areas. Learn more about the important role that trees play in improving water quality and access. 

How Trees Absorb and Release Water

According to the UN Food and Agriculture Association, a mature evergreen tree can intercept more than 15,000 liters of water every year. So how do they do it? 

Similar to us, trees rely on water to survive, and they have developed a range of uptake strategies. Water that is present in the soil as absorbed via tiny, hairlike roots. Once it has entered the root system, it is pulled up the tree’s trunk, down its branches, and into the leaves. Trees can also absorb airborne moisture directly through their leaves — although a higher percentage of their water intake comes from the soil. 

How does water defy gravity and travel upwards from the roots of a tree, through its woody tissue, and into its leaves? To answer this question, requires understanding the inner workings of trees. Similar to our vascular system, which transports blood throughout our bodies, plants have an internal network of "plumbing", otherwise known as xylem and phloem tissues. These tissues begin in the roots and extend upward through the trunk, and into the branches. From there, they connect to every single leaf. 

So how is water pulled through the system? Transpiration — or, the process of water evaporation through openings in the leaves (stomates). When the water evaporates, it creates negative pressure (like a vacuum) within the leaf. As a result, water is pulled into the leaf from the xylem, replacing what was transpired.

Because xylem is a water column that extends from leaf to root, when transpiration occurs at the top of a tree, the vacuum pressure is felt all the way down at its roots. This causes the roots to pull water in from the soil, and the cycle continues. Pretty incredible, right?

How Trees Prevent Floods

When trees absorb water, it doesn't just contribute to that individual tree's growth and survival. The benefits extend to the surrounding landscape and ecosystem. As trees absorb water through their roots, they filter pollutants, and also slow the flow of rainwater toward creeks, rivers, lakes, and other water bodies.

After periods of heavy rainfall or flash flooding, trees will absorb a significant amount of stormwater via their roots, which reduces floodwater damage to ecosystems and infrastructure. Over time, the storm water is released back into the earth and atmosphere via transpiration.

How Trees Protect Watershed Health

A watershedis an area of land that drains rain water or snow into one location such as a stream, lake or wetland. These ecosystems supply drinking water for surrounding communities — and water for agriculture and manufacturing processes. They also offer opportunities for recreation, and provide habitat to a range of plant and animal species. 

Trees are intertwined with watershed health, and provide a range of water-related ecosystem services, including filtering excess sediment, nutrients, and toxins before they enter waterways, reducing erosion and flooding, and shading streams so they are at optimal temperatures for aquatic wildlife.