How Wildfire Affects the Forest Soil

The Riverside Fire of 2020 has recently restarted outside of Estacada.

Wildfires in 2020 caused extensive damage to local forests and forest soils.

Did you know that wildfire can affect forest soils? The effect of fire on trees, shrubs, and herbaceous (non-woody) plants are easily visible. It is not so easy, however, to see the effect wildfire has on the forest soil. There is more under your feet than what you see with the naked eye. Whole communities of organisms live in the soil contributing to the health of the forest itself.

Wildfire Damages Soil Organisms

Wildfire can be lethal to organisms living in the soil. The negative impacts on soil structure and soil microbes come from the heat of the fire. When a fire burns rapidly through a forest, most of the energy released by the flames is not transmitted downward. Damage to the soil happens when an area is exposed to prolonged fire and soil temperatures become elevated. This can happen when a log or large root burns slowly over time. In this case, the heat generated in the soil can cause the organisms in the soil to die.

Soil organisms, including microbes, are necessary for healthy forest soils.

Soil organisms, including microbes, are necessary for healthy forest soils. (USDA-NRCS)

Impacts on soil organisms can vary widely in a small area. There may be patches of unburned areas where many species survive. In lightly burned areas, where the leaves and surface litter are charred, soil organisms may be unaffected. However, when the leaves and duff are completely consumed there can be significant heat-related destruction of soil organisms.

Beyond the direct effect of heat, the microbial community is affected in the short and long-term by the destruction of organic matter, altered soil temperatures, changes in soil moisture levels, shifts in above-ground vegetation, and rates of organic matter accumulation.

Prolonged burning kills soil organisms (Oregon Dept. of Forestry)

Prolonged burning kills soil organisms (Oregon Dept. of Forestry)

Many factors play into the recovery of the soil microbial community. It may be difficult to pinpoint management recommendations for post-fire restoration. The productivity of soil after wildfires also depends on the condition of the soil before the blaze. Soils that were nutrient-rich before the fire may not be significantly impacted. Soils that were low in nutrients before the fires, however, may show some change in productivity.

Wildfires Can Increase Potential for Erosion

Another wildfire impact on soil is associated with erosion. It is not surprising that this erosion is, in part, dependent on the weather in the year following the fire. Generally, forest soils are porous and open on the surface due to the accumulation of organic matter (leaves, needles, etc.). Forest soils are normally high in organic matter underneath, so water infiltration is high.

After a fire, erosion potential may increase due to two factors. The first is changes in soil structure resulting in reduced water infiltration. The second is the loss of cover that protects soil from the impact of raindrops, which can lead to splash erosion.

Increased erosion potential due to reduced water infiltration is also dependent on the level of burn. A severe fire that exposes the soil surface, consumes the organic matter in the top layer of the soil, and causes the soil structure to collapse, will reduce porosity and water infiltration. Additionally, ash and charcoal residue can clog pores and reduce water infiltration.

Splash erosion happens when the lack of vegetation and plant litter exposes the soil to the power of the raindrop. If a raindrop strikes soil directly, without any plant cover to slow it down, the raindrop can splash soil particles up to three feet away. Loosened soil may then travel farther as water that is unable to soak into the soil travels overland.

The power of a raindrop. (USDA-NRCS)

Erosion Hurts Native Wildlife and Drinking Water Quality

Soil particles that leave the forest and end up in streams have nutrients attached that cause water quality issues downstream. Additionally, this soil is deposited on the streambed gravel that then becomes clogged. This affects the habitat of stream macroinvertebrates that are a source of food for fish. The deposited sediment can also smother fish eggs.

Save Your Soil With These Resources

Your soil is precious and no one wants to see it wash downstream. If you need help to keep soil from leaving your property, contact your local stewardship forester, OSU extension forester, or conservation specialist at the Clackamas Soil and Water Conservation District (503-210-6000) or Natural Resources Conservation Service office (503-210-6032). These experts can help determine which of many practices may help, as not all practices are appropriate for all sites. You may also check out the erosion control factsheets on the Oregon Natural Resources Conservation Service webpage entitled After the Fire: Resources for Recovery.

Be on the Lookout for Invasive Weeds!

One word of warning! After the fire and spring rains, be ready to control invasive weeds. Bare soil is an open invitation for invasive weed infestations. You may see weed populations cropping up where you do not want them to become established. If you need help identifying or learning how to control invasive weeds, contact our WeedWise Department or call 503-210-6000.

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