Pre-fire

Water and nutrients that aren’t used by plants and microorganisms flow through (infiltrates) soil, eventually reaching groundwater. This pushes deep groundwater out into the stream, which is cold and usually low in pollution.

 

Fire

Small plants are burned and their root systems quickly die. The heat from the fire turns fatty molecules (lipids) that were in soil organisms into a gas. They move away from the heat source until temperatures are cold enough that they condense. You might have noticed the same thing happen with grease in your kitchen, if you have ever tried to clean the top of your cabinets! If the lipids move down into colder soil, they consolidate there.The depth of the layer where the grease is deposited is determined by how hot the fire is (the heat goes deeper into the soil with hotter fires.

 

Post-fire

There is little vegetation and root systems left to hold soil in place. The fatty layer acts as a water repellent (hydrophobic), keeping water from infiltrating below the boundary. Without plants to use the water and hold it in place, a little bit of rain can make the top of the soil can get too wet and heavy to stay in place. Inches of soil can wash away, flowing over the surface of the hydrophobic layer and straight into streams taking all of the dirt, nutrients, and other chemicals with it.

 

Recovery

Eventually, the organisms in the area will begin to return and do their jobs, beginning with bacteria and other microbes (they can help decompose the hydrophobic layer) and ending with larger trees and large animals. This process is called succession, and can take hundreds of years—sometimes the process starts over again because of another fire or landslide before it is even finished.