Informative: How Soil is Restored After Wildfire

In the webinar “Science You Can Use: Dirt Goes Downhill” from the US Department of Agriculture Forest Service, Pete Robichaud from the Rocky Mountain Research Station talked about the mitigation of soil erosion after wildfires.

He discussed the Burned Area Emergency Response (BAER) to assist post-fire assessment teams do the following:

1. Determine the spatial extent of the fire

2. Verify the information in the field

3. Create a burn severity map

4. Evaluate the assets at risk

5. Develop a watershed response

6. Implement treatments & monitor

They use pre & post fire satellite images to validate the burn severity map in the field while evaluating the following:

1. Ground cover – Are all the pine needles burnt? Unburned pine needles reduce erosion.

2. Ash color & depth – Determine heat of the fire

3. Roots – If fine roots are consumed, soil is at risk

4. Water repellency – Determined using the water drop test

5. Soil structure – Are aggregates broken down?  Is it powdery?

The value of the assets at risk is then determined.  This includes physical assets at the base of the watershed (communities), reservoirs (drinking water) and habitats.

Robichaud confirmed that mulch is the most effective treatment for erosion mitigation.   There are four options in order of least to most expensive, agricultural straw, wood shreds, wood straws and hydromulch.  While agricultural straw is the least expensive, it may contain non-native weeds and is easily windblown in 18-20mph winds.  It also decomposes quickly utilizing high quantities of nitrogen compromising the availability of nitrogen for pine seedlings.  Wood however, is heavier and takes longer to decompose, making nitrogen more available to new forest growth.  To manage costs it is suggested to use wood mulch on peaks and straw mulch on less wind-driven slopes.  

Adding wood mulch to the surface of the forest floor does not increase the risk of wildfire because of its relative volume to ground fuels in an unburned forest.  Wood mulch is applied at 3-4 tons per acre versus ground fuels that may accumulate 10-35 tons per acre.  If the mulch were to ignite, there is not enough fuel to propel the fire to the wind-driven canopy of the forest.

A study by the USDA showed that a burned forest floor covered in wood mulch was returned to its original state within 10 years.