With autumn right around the corner, trees are getting ready for one of nature’s most anticipated events, where seas of green change to colorful hues of reds, oranges, and yellows. This time of the year, also known as leaf peeping season, attracts visitors who are eager to watch forests undergo this critical seasonal transformation. But these colors aren’t just for show; they’re part of how trees prepare for winter, protecting themselves by conserving water.
Throughout the summer, tree leaves remain green and release water back into the air, but during the winter, this would leave trees at risk of moisture loss. So, trees cope by putting on a fiery show, perhaps a warning, of the cold season to come, where water conservation is especially important.
Autumn is clearly a colorful reminder of the relationship between trees and water. We often admire trees for the shade they provide, the homes they create for wildlife, the fresh air they give us, and their stunning autumn beauty. But did you know that trees also help ensure the water we drink is clean and safe?
Each individual tree, from its leaves, trunk, and roots, all play a role in filtering water. Whether it’s a single tree or an entire forest, these natural wonders are essential in managing our water resources. They keep our rivers and drinking water clean, and they are responsible for making sure there’s enough water for everyone. So just how do forests provide us with these benefits?
Forests increase infiltration.
When it rains, rainwater (often referred to as stormwater in urban environments) is both filtered and stored in the trees, plants, and in forest soil, during a process called infiltration. It is why forests are referred to as “sponges,” because water can infiltrate into the physical structure of forest, just like it does in a sponge.
Forests promote evapotranspiration.
When water moves from the land to the atmosphere (through trees, plants, and other pathways), this process is called evapotranspiration. It is the combination of both evaporation and transpiration. For example, trees use water for growth and photosynthesis, and the leaves of trees transpire (put) water back into the atmosphere. During the summer, large trees can transpire up to 100 gallons of water per day, with some wetland species, such as bald cypress, pumping up to 800 gallons per day (Penn State).
These essential connections mean that forests play a big role in shaping our water resources. For example, forests protect us from extreme weather events like flooding due to their water storage capabilities. Their ability to store water also regulates the flow and quantity of water in our rivers, streams, and aquifers. Without forests, we lose these crucial benefits, making our water supply less reliable and more vulnerable to sudden changes.
Forests promote better water quality in our rivers, lakes, and aquifers, and lower water treatment costs.
Years of scientific research and observation show that when our watersheds are covered with healthy, intact forests, the water we drink is cleaner and safer. (Caldwell et al., 2022, Shah et al., 2022). This is because forests act as natural filters, limiting the need for harmful chemicals like fertilizers and pesticides. The roots of trees play a crucial role by holding the soil in place, preventing erosion, and stopping sediment from polluting our water sources (Lopes et al., 2019).
Trees also absorb water through their root systems and purify it through a process called phytoremediation, where they remove pollutants from the water and soil and store them in their leaves.
This natural purification process means cleaner water in our rivers, lakes, and aquifers. And, as a result, the water that reaches us is cleaner, reducing the need for costly treatment. In other words, trees help lower the costs of providing safe, clean drinking water, benefiting both our wallets and the environment.
What’s more, cleaner water requires less treatment, which is good news for both the environment and our wallets. American Forests reports that the more forest cover we have in a watershed, the lower the cost of water treatment. So, by protecting our forests, we’re not just preserving nature—we’re also investing in our own health and the health of our communities.
Forests purify water, protect us from flooding, and recharge our aquifers.
In the summer, when tree canopies are at their peak, the leaves are the first line of defense in slowing down and intercepting water, which reduces flooding. Remarkably, a single large deciduous tree can hold a thousand gallons of rainwater each year in its canopy alone (Penn State).
Tree roots also play a vital role by allowing more water to soak into the ground, which decreases runoff and helps replenish underground water supplies. Research shows that forest soil can absorb nearly 10 inches of rain, while nearby lawns can only absorb about 4 inches (Penn State). Trees truly are nature’s rainwater managers!
In a forest, every layer— from the towering canopy to the lush understory to the rich soil beneath—works in harmony to manage the flow of water. The trees catch and slow down rainfall, the undergrowth holds onto moisture, and the soil filters and stores water. Water infiltration into the soil is essential for groundwater supplies to be recharged. As water moves further down into soil, it trickles down into porous rocks and fills up our aquifers. So, by protecting forests and increasing water infiltration, we are ensuring that our aquifers are recharging and providing us with a sustained water supply.
The hidden power of forests: It’s all in the soil.
Forest soil is like a sponge made up of layers of organic material, tree roots, and plants, creating a vibrant, living system. This special structure allows forest soils to absorb and hold much more water than other types of soil, like those found on farms.
Forests also naturally protect the soil from washing away. When trees are full of leaves, their canopies act like umbrellas, shielding the soil from rain and almost entirely preventing erosion. Even after the leaves fall, the tree roots keep the soil firmly in place. By stopping erosion, forests help keep the soil rich, which makes it even better at soaking up water. This natural process is key to keeping our water clean and our ecosystems thriving (USFWS).
Properly stewarding our forests is key to healthy water resources.
While forests generally benefit our water resources, there are exceptions that highlight the importance of careful forest management. For example, clearcutting—harvesting large areas of trees—can lead to increased sediment and erosion in waterways (Penn State). Using pesticides and chemicals in forest management can also pollute our water. Additionally, planted forests, where trees are all grown at the same time, may demand more water compared to natural forests due to what’s known as the “plantation effect” (USDA). Unmanaged or overly dense forests can also lead to higher water use, reducing groundwater recharge rates (USDA).
To truly maximize the benefits forests provide, thoughtful management that considers water resources is crucial. Preserving and managing forests wisely ensures clean waterways and stable water supplies for everyone.
Current funding opportunities and calls to action for forest preservation:
- Save Our Urban Forests (Indiana Forest Alliance) – Indiana Forest Alliance members and concerned Indianapolis residents are organizing across the city and asking for $6 Million in funding to be placed in the 2025 municipal budget to purchase urban forests. This money would be utilized to buy privately owned forests that would then be placed into the city’s park and greenway system.
- Protect Our Climate Forests (Earth Justice) – The U.S. Forest Service has proposed a policy that could reduce the logging and destruction of old-growth trees in national forests, but it needs to be dramatically strengthened before it is finalized. The Forest Service also needs to issue strong protections for mature trees, which are our future old-growth forests and exist in much greater numbers than old-growth.We need the administration to commit to strengthening the proposed protections for old growth in national forests and also protect mature trees from logging. There’s a comment period open now until September 20th that needs your input.
- Rooting Resilience Program (Water Network) – Funding provided by the Inflation Reduction Act and the USDA Forest Service, Urban and Community Forestry Program, will provide funding and technical assistance to community-based projects that build resilience and capacity for the planning and implementation of forestry projects.