How Many Trees Does It Take to Protect a Stream?

400 295 Stroud Water Research Center

Scientists Set Buffer Width Minimum Standard

Stroud Center Director Bern Sweeney practicing what he preaches at a tree planting event. Photo: David Arscott

A strip of forest along a stream channel, also called a riparian forest buffer, has been proposed and used for decades as a best management practice to protect streams by filtering out contaminants from agriculture and other land uses before they can enter them.

Their benefits are many, but one benefit has dominated social and political conversations, and that is their role in preventing contaminants from entering streams.

A few years ago, Stroud Water Research Center proposed that riparian forest buffers also play another important role by improving the health of the stream and enabling it to provide more and better ecosystem services for both humans and wildlife — the processing of natural organic matter and pollutants, for example. Thus, a forest buffer provides a first line of defense (keeping sediment and nutrients out) as well as a secondary line of defense (keeping sediment and nutrients from moving downstream) for maintaining clean water in our streams and rivers.

Good News, Bad News

The Stroud Center recently published a case study showing that creating a forest buffer can in fact keep on average 43% of sediment and 27% of nutrients such as nitrogen from entering a stream. This is good news.

The bad news is that 57% and 73%, respectively, of the substances still penetrate the barrier.

However, the Stroud Center has also shown that improved stream health due to a forest buffer can increase the level of in-stream processing of nutrients and organic matter by 2-8 fold. So the case is good for their widespread use.

Stroud Center Director Bern Sweeney explained, “It’s important that we look at forest buffers both as a pollutant barrier and as an enhancer of in-stream function when looking to them to maintain and improve our waterways. What happens in the stream is just as important as what happens on the banks. When we consider both of these benefits, it is clear that a forest buffer provides better protection and more bang for the buck than a buffer filled with grass.”

Riparian buffers protect stream habitat, filter nutrients and sediments, and disperse concentrated runoff.

Consideration and focus on the in-stream side of forest buffers is relatively new. However, as Sweeney points out, “by enabling streams and their ecosystems to work more efficiently, forest buffers can now be considered best management practice for both nonpoint and point source pollution. After all, if a forest buffer enables a stream to better process nitrogen, it does not matter if the molecule of nitrogen that the stream is processing comes from a farm or a wastewater treatment plant.”

Creating a More Functional Ecosystem

Trees along stream banks prevent bank erosion, keep stream channels wide, keep temperature more natural via shading, and promote increased amounts and diversity of food in the form of leaves, algae, and dissolved substances.

All this adds up to a more diverse and functional stream ecosystem. “Trees help streams work like a water treatment facility, and the price is right once we have the trees established — it’s free!” said Sweeney.

In 1992 Sweeney published a paper in which he declared, “The quality of streamside forests” was likely the “single most important factor altered by humans that affects the structure and function, and ultimately water quality, of the streams providing water to coastal embayments.” A bold statement at the time, it’s proving true.

The Stroud Center has long advocated for forest buffers, with staff scientists publishing studies on them in the 1970s and initiating experimental tree plantings as early as 1982. Planting trees is an important tool for the Stroud Center’s Watershed Restoration Group, which assists landowners (especially farmers) in protecting streams.

How Wide is Wide Enough?

Forest buffers provide healthy aquatic habitat for freshwater fish and other aquatic life. Photo: Willy Eldridge

Of course, it makes sense that wider buffers would work better than narrower ones, but exactly how wide has been up for debate. So Sweeney, along with Stroud Center colleague Denis Newbold, recently conducted an extensive review of scientific literature on the topic of forest buffer width.

They looked at how the width of a buffer affects eight different functions: the removal of nitrogen and sediments, stream channel width, channel meandering and bank erosion, temperature, and large woody debris.

They concluded that forest buffers should be at least 30 meters, or nearly 100 feet, wide to adequately protect streams.

“That’s a lot. We know it’s a lot. But this is what the science is saying, and the reward for a wide buffer is huge.”

Sweeney mentioned many other benefits to consider: Studies showing how trees and the open space of buffers increase neighborhood property values, absorb CO2 and release oxygen, recharge groundwater, and provide healthy aquatic habitats for human recreation, freshwater fish, and other aquatic life.

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