by Will Gray environmental scientist
Wildfire can be devastating when it comes into contact with developed areas, but in the untamed wilderness, fire is a natural phase of life. Many plants have incorporated fire into their life cycles. The Lodgepole pine will hold its seed sealed in resin until fire kills the tree and melts the resin, releasing the seed to grow in the soil now cleared of underbrush. Many species of trees can sprout new shoots from underground root nodules that are protected from fire above. Wildfires also have a significant effect on the abiotic aspects of the environment. During particularly intense fires, combustion of vegetative materials creates a gas that penetrates into the soil profile. As the ground cools after the fire, this gas condenses into a waxy solid that creates a water-resistant layer beneath the soil surface.
Rain shortly after wildfires may seem like a welcome reprieve, but instead can cause a myriad of problems. Immediately following an intense fire, water will not penetrate the top layer of soil. Additionally, plant litter that would normally help soak up excess moisture has been removed. These factors cause rapid runoff from even small rain events that can cause flash flooding. Even if it has been weeks or months following the wildfire, significant rainfall can pose a different danger. With the hydrophobic layer a few inches below the soil surface and the loss of anchor roots in the upper portion of the soil profile, erosion and landslides become a major concern.
The hydrophobic layer only forms from fires of high intensity. This intensity is usually achieved when a dense layer of underbrush is present. In the western United States, the fire cycle occurs frequently, and underbrush is generally lacking to fuel fires enough to create this effect. In the eastern United States, however, fires occur much more rarely, and sufficient fuel is often present to create truly intense fires.