Getting to the bottom of soil pipes

We go for hikes in the woods to spend time with nature. We look around to appreciate the mammals, birds, trees, grasses, and flowers of the forest. All without a second thought of the soil underneath the dead leaves and pine needles.

We’re so busy admiring what we see at eye level that we usually don’t think much of the ground we walk on. But luckily someone does.

researcher near excavated forest soil

Scientists at the University of Idaho study what are called soil pipes: horizontal tunnels, or “macropores,” in soil. The presence of soil pipes can affect how water drains or flows through the soil and play a role in soil erosion and hillslope stability.

The researchers’ recent study, published in the Soil Science Society of America Journal, examines how these soil pipes are formed and change over time. They found that the decay of underground roots is the largest factor in their formation. Fire, such as wildfires or prescribed burns, can burn stumps and large surface roots, connecting those underground soil pipes to the surface.

Soil pipes were found by excavating three plots that were approximately 20 by 20 feet on a forested hillslope in the University of Idaho Experimental Forest. The site had undergone a prescribed burn in 2010. The mean diameter of the pipes was 1.7 inches, but they can be as large as 12.8 inches. The soil pipes can also connect and branch with each other. Seven branched soil pipes were found on the three plots, with the longest extending a little over 16 feet.

forest after a prescribed burn

The study area has a restrictive horizon soil layer about three feet below the surface. This layer impedes water from flowing further down into the soil. These forested hillslopes play an important role in how water moves through the surrounding ecosystem, said Ian Leslie, the project’s graduate research assistant who has since graduated and is now manager of the university’s Soil and Water Resource Lab.

“Part of what these forested hillslopes do — for not just these forests but the whole region — is that they retain water,” he said. “The unique properties of this soil and the restrictive horizon cause it to have a perched water table – one that sits higher than the rest of the region’s water table in the aquifer – much of the year and retain moisture.”

Robert Heinse, an assistant professor of soil physics, said the presence of soil pipes can cause this retained water to drain faster than normal, potentially causing flooding during the wet season and empty streams in the dry season. This quick drainage through soil pipes can also facilitate erosion and harm the stability of a hillslope.

soil pipe openings

“Current models used by hydrologists to study water flow don’t take soil pipes into account,” Heinse said. “They look at many other components, but a mechanistic model for these soil pipes doesn’t exist even though we are finding out that they matter a great deal.”

The forests of Idaho and similar regions have several characteristics that make them important sites for studying soil pipes. First, large parts of forests are routinely clear cut, leaving stumps and roots behind to decay. Also, wildfires occur and prescribed burns are a common management technique. Fires, coupled with large amounts of evenly decaying tree stumps and roots, can lead to soil piping networks, such as those seen in the study. On these hillslopes, the pipes may last several decades.

Attributes such as those found in the study site leave the forested areas susceptible to the negative effects of soil pipes. The researchers recognize the need for timber harvesting and prescribed burns, but they believe soil pipes must be considered when it comes to the consequences of certain land management techniques on the surrounding ecosystem.

“What really makes this [soil piping] work interesting is that it links everything together,” Heinse said. “It sheds light on how trees interact with soils, how forest fires affect the system, what the consequences are for the hydrology, how the hydrology feeds back into how much moisture is retained in the soils, and finally how that affects the forest. And it's this little tiny piece that really allows you to put these all together.”

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