After the Forest Fire: Evergreen Needles Prevent Soil Erosion

Once a raging forest fire is quelled, the next worry is erosion of the landscape. With vegetation destroyed, rain easily washes away the soil, causing large flows of debris and landslides. Erosion endangers sources of drinking water, streams, and roads.

In an unprecedented study, Chris Pannkuk and Peter Robichaud show that scorched evergreen needles can play a key role in preventing erosion after a forest fire. They found that ponderosa pine needles were effective in reducing erosion caused by water running over the soil, and Douglas fir needles were effective in reducing erosion caused by rain hitting and splashing into soil. Their report will appear in Water Resources Research, published by the American Geophysical Union.

These findings can help post-fire rehabilitation teams decide where to apply treatments to reduce erosion. “If you see brown needles in the trees,” Robichaud said, “then let’s take advantage of Mother Nature and not add any treatments to that area of the forest.”

Wildfires usually burn in mosaics, with patches of low, moderate, and high severity. In areas of low or moderate severity, needles from partially burned conifer trees fall to the ground within a few months after the fire. Robichaud noticed that needle cover seemed to reduce erosion on forest soils after a fire. Since no one had formally studied this effect, he and Pannkuk used an artificial rain laboratory at the U.S. Department of Agriculture’s Forestry Science Laboratory in Moscow, Idaho, to see how much burnt needles could reduce erosion.

They filled four-meter by one-meter [13-foot by 3-foot] boxes, set at a 22-degree slope, with soil taken from burnt forests. After covering the soil with various amounts of scorched ponderosa pine and Douglas fir needles, they applied artificial rain for 25 minutes at an intensity that would simulate 34 millimeters [1.3 inches] of rain per hour. During each test, they also introduced a stream of water at the top of the box to simulate overland water flow.

The researchers collected and analyzed run-off soil and water from the boxes. They found that a 50 percent ground cover of Douglas fir needles reduced water flow erosion by 20 percent and rain-induced erosion by 80 percent. A 50 percent ground cover of ponderosa pine needles reduced water flow erosion by 40 percent and rain-induced erosion by 60 percent.

Robichaud, who has been studying and modeling erosion after prescribed and wildfire for 13 years, directs several treatment effectiveness projects in California, Colorado, Idaho, Montana, Nevada, and Washington. Pannkuk, who worked with Robichaud as a post-doctorate on this project, is currently a natural resources consultant.