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Fire Effects on Soils: An Introduction

Chad Hoffman

During this weeks lecture we will be discussing the effects of fire on soils, and you will review the WEPP-FUME model. One point I would like to reemphasize is that fire is a dynamical process that varies over time and space. It has shaped plant communities for as long as vegetation and lightning have existed (Pyne 1982), and the recycling of carbon and other plant nutrients depends upon biological decomposition and fire. In regions where decay is constrained by environmental factors fire is the dominate recycling agent of organic matter.

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What is Soil?What is Soil?

• Soil – is the unconsolidated, variable-thickness layer of mineral and organic matter on the Earths surface• physical, chemical and biological processes

functioning on geological parent material • a continuous interaction between the soil system

and the biotic, climatic, and topographic components of the environment

So with this in mind lets think about what soil actually is. Soil is the unconsolidated, variable-thickness layer of mineral and organic matter on the Earths surface that forms the interface between the geosphere and the atmosphere. This layer has formed as a result of physical, chemical and biological processes acting upon the geological parent material and a continuous interaction of these processes with the biotic, climatic and topographic components of the environment.

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What is Soil?What is Soil?

• Parent material – is the starting material from which soil is formed. • Soils inherit their properties from those of the

parent material

• Weathering is the process of breaking down rocks, soils and minerals through direct contact with the atmosphere.

As I just mentioned soils form from some starting material such as consolidate rock or unconsolidated rock deposited by wind and water. This starting material is called the Parent material The parent material consists of specific minerals of different sizes or plant material of different types. A soil inherits the mineral types found within this parent material and over time losses these minerals and gains new minerals. The process of losing mineral and gaining new minerals is called weathering. The rate of soil formation (weathering of the parent material) is dependent upon the climate. Specifically the annual precipitation and temperature as well as the vegetation type, and activity and type of other organisms acting upon the parent material.

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What is Soil?What is Soil?

• Topography – affects the amount of precipitation as well as the temperature of the material. • These factors all

influence the amount of time it will take to develop a specific soil

James Henderson, Gulf South Research Corporation, www.forestryimages.org

Other factors which influence the formation of soil are time and topography. Topography is the slope of the land and the compass direction it faces. These factors will affect the amount of precipitation that enters the parent material or soil and the temperature of the soil. As an example of this think about how temperature, vegetation change as you go from the south side of a mountain to the north side. The change in precipitation and temperature will then influence the kinds and numbers of organisms that live on and in the soil. However an important aspect to consider is that all these processes take time to develop. Time zero is the point at which the soil begins to form from the parent material and it may take thousands or tens of thousands of years for the parent material to be converted to a highly weathered and developed soil.

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What Makes Up the Properties of a Soil?What Makes Up the Properties of a Soil?

• Mineral content• Organic Matter• Air• Water

Thomas D. "Tom" Landis, USDA Forest Service,

www.forestryimages.org

R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company,

www.forestryimages.org

To many people soils are merely the inconspicuous median that is walked upon, used to plant grow trees or a garden creates dust during a windstorm, or that is used to create a fire line. However there many properties which distinguish one soil type from another. For example the minerals, organic matter, air and water all help define the properties of a soil.

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Soil MineralsSoil Minerals

• 2 sources of minerals:• rocks • and organic material

Francis Gwyn Jones, Private, www.forestryimages.org

Linda Haugen, USDA Forest Service, www.forestryimages.org

Soil has two kinds of solid components those made up from the weathering of rocks and those made up from the weathering of organic material. As I mentioned earlier rocks change when exposed to water, air and organisms and change due to weathering. Weathering is the physical separation of the rocks individual mineral particles and their alteration or destruction and re-synthesis to form new minerals. It is this weathering rock that becomes the parent material. Some common soil minerals include silicone, aluminum, iron, calcium, potassium magnesium, clay and many others.

In the two picture I have here you can see two very distinct parent materials and other factors which will effect the development of the soils. Take a few moments and think about how these two soils might differ in their mineral contents and development stages.

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Organic Matter in SoilsOrganic Matter in Soils

• Organic matter is composed of a wide arrange of living and dead biomass which contains a range of plant nutrients.

Roger Lopez-Chaves, Universidad de Costa Rica,

www.forestryimages.org

David J. Moorhead, University of Georgia,

www.forestryimages.org

The organic component of soils are made up of a combination of living and dead organisms which provide a wide arrange of plant nutrients. The living biomass in the soil consists of plant roots, microorganisms, invertebrates, and small and large fauna. The nonliving organic matter is made up of dead bark, large woody debris, litter, duff and finely decomposed humus. Soil organic matter is one of the most important factors in assisting in evaluating the effects of fire on soils. Ecologically soil organic material plays three major roles. First organic matter enhances the structural properties of the soil. Second organic matter maintains and regulates the biogeochemical cycling of nutrients by providing an active medium. Last soil provides a habitat for plant and animal organisms.

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Soil Particle Size and OrganizationSoil Particle Size and Organization

• Soil texture – refers to the particle size• Soil Structure – refers to particle

arrangement• Particles are bounded together into units called

aggregates

Soils contain mineral particles of many different sizes large particles are typically called gravel, followed by sand, silts and clays. The proportion of these size classes in any given soil form it’s texture. Typically textures are defined as coarse (gravelly or sandy), intermediate (loamy) or fine (clayey). The arrangement of the particles defines the soil structure. In most soil types the individual particles become bounded together to form aggregates. The size, form and strength of these aggregates vary and highly influence the behavior soils by affecting the pores or spaces within and between the aggregates.

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Soil PoresSoil Pores

• the pores between the particles are the soils ventilation system, water intake system, water storage and drainage system.

If the mineral particles and organic matter form the Skelton of the soil, than the pores between the particles are the soils ventilation system, water intake system, water storage and drainage system. The size and shapes of the pores will determine the effectiveness of these systems. For example small pores hold water well but larger pores are required for water and air to move freely throughout the soil. The ability of air to move within the soil allows for oxygen and other gases to diffuse through the soil. The soil water is often termed the soil solution and is the solvent in which many reactions occur and which provides dissolved nutrients to plants and microorganisms.

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Soil HorizonsSoil Horizons

• The properties of the soil are typically ordered horizontal with in the soil profile

The variable amounts and combinations of minerals organic matter, air and water produce an astounding number of possibilities in the range of physical, chemical and biological properties that a soil may have. However these properties due occur in some orderly horizontal arrangement within the soil. This arrangement is often referred to as the soil profile. These horizontal divisions are called soil horizons. The uppermost soil horizon is the O horizon and consists mainly of organic material at various stages of decomposition. This layer is typically divided into layers beginning with the litter or L layer. Immediately below this layer is the F layer or fermentation layer. In this layer some plant components are still recognizable. The next layer is the H layer or humus layer. In this layer all plant material is decomposed. The combination of the F and H layer is commonly referred to as the duff, while the L layer is simply the litter layer.

Below the o horizon we begin to see the mineral soil horizons beginning with the a horizon and ending with the r horizon which is the bedrock.

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Fire Effects: Terms and ConceptsFire Effects: Terms and Concepts

• Fire Intensity - used to describe the rate at which a fire produces thermal energy

• Fire Severity – used to describes the ecosystem response to fire• Fire severity depends upon:

• Fuel accumulation• Properties of the fuel• Heat transfer into the soil

The effects of fire on soil properties must be evaluated within the context of a complex organic and inorganic matrix which makes up the soil. When discussing the effects of fire on the soil it is important to differentiate between the terms fire intensity and fire severity. For the purposes of this class we will consider fire intensity as a term used to describe the rate at which a fire produces thermal energy and fire severity to describe the ecosystem response to that thermal energy. The level of fire severity in terms of the soil ecosystem is dependent upon 3 major factors.

First the length of time fuel has been accumulating and the amount of these fuels that are combusted during a fire. Second the properties of the fuels that are burning during the fire. And third the heat transfer into the soil during the combustion of the fuels.

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Soil HeatingSoil Heating

• Heat transfer from the soil downward is dominated by radiation

• Heat transfer through the soil is dominated by convection.

Heat produced during the combustion of aboveground fuels is transferred to the soil surface and downward through the soil by several heat transfer methods. It has been estimated that only about 10 to 15 % of the heat released during combustion is absorbed and transmitted directly downward to the soil. The main heat transfer method contributing to this process is radiation. Heat transfer within the soil is dominated by convection and vaporization and condensation in dry soils and dominated by conduction in a wet soil.

The greatest increase in soil temperature occurs at the or near the soil surface. Most research suggests that with In the first 4 inches or 10 cm of the soil surface temperatures are rarely above ambient temperatures. However the magnitude of the depth and temperatures with in the soil profile will be dependent upon many factors one of which is the residence time and intensity of heating.

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Effects of Temperature on Soil Properties Effects of Temperature on Soil Properties

• Changes to the soil structure due to heating:

• Collapse and increase in the density of the soil

• Reduced soil porosity

• Hydrophobic soils

• Soil erosion

Table Adapted from Neary et al 2005

One of the key aspects in understanding the effects of fire on soils is understanding at which temperatures soil nutrients volatize and irreversible damage occurs to soil properties. These temperatures are often times referred to as temperature thresholds and they have been identified for many physical, chemical and biological properties of soil. Lets concentrate for now on the temperature thresholds for several key physical characteristics of the soil.

The soil structure is an important soil characteristic which has been shown to increase water relations and productivity in wild land soils. Loss in soil structure reduce the amount and size of sol pore space. The major effects of fire on soil structure are on the following:

First the soil structure collapses and we see an increase in the density of the soil. This is due to the loss of organic matter which acts like a glue holding the soil together. This collapse ultimately reduces soil porosity. Next we can get further compaction of the soil by rain fall. In this process surface ash and particles are displaced by the rain fall and the soil pores become partially or totally sealed. This is often referred to as hydrophobic soils. Finally the hydrophobic soils surface will reduce infiltration rates into the soil and produce rapid runoff and hill slope erosion.

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Water Repellant SoilsWater Repellant Soils

Figure from Neary et al. 2005

• Can be found in both fire and non-fire environments

• Can be caused by:

• irreversible drying

• leachates from organic materials

• Hydrophobic microbial byproducts

• intermixing of soil particles and organic matter

The creation of water repellent soils involves a combination of physical and chemical processes. Water repellent soils are produced by soil organic matter and can be found in both fire and non-fire environments. Hydrophobic soils can be caused by several methods including an irreversible drying of the organic matter, coating of mineral soil particles with leachates from organic material, the coating of soil particles with hydrophobic microbial by products or an intermixing of dry soil particles and dry organic matter. However in a fire environment water repellant soils form through the vaporization of organic matter and condensation of hydrophobic substances on mineral soil particles. This production of hydrophobic soils by fire has long been known and was called the tin roof effect until the 1960’s.

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Formation of Fire Induced Hydrophobic SoilsFormation of Fire Induced Hydrophobic Soils

This figure shows the formation of fire induced hydrophobic soils. If you look at the left hand side you can see that water repellent soils generally form near the top of the soil surface in the absence of fire. Particularly this event occurs in the organic layer of the soil when it is highly colonized by fungal mycelium. In the middle column you can see that when fire vaporizes the organic material it is moved downward into the soil layer until it condenses and forms a water repellent layer. The right hand side of the figure is showing the post fire environment where we have developed a wettable layer above a water repellent layer.

The magnitude of fire induced water repellant soils is dependent upon the severity of the fire, the amount of organic matter present, the temperature gradient with in the soil, the texture of the soil, and the water content of the soil.

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Effect of Hydrophobic Soils on ErosionEffect of Hydrophobic Soils on Erosion

• Fire effects water entering the soil in two ways:• Burned soil is unprotected from rain drop impact• Fire may cause a water repellent layer within the

soil

Figure from dartmoor-npa.gov.uk

Fire typically effects water entering the soil in two ways. First the burned soil surface is unprotected from raindrop splash, and second soil heating during a fire may produce a water repellant layer with in the soil profile. The severity of the water repellent layer will decrease with time as it is exposed to moisture, and in many cases does not last more than one year.

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Raindrop SplashRaindrop Splash

Raindrop splash occurs when a raindrop falls on exposed soil. This raindrop displaces the soil and moves the particles off during runoff. However in water repellent layers at the soil surface produce fewer, slower moving droplets which carry the particles a shorter distance. Raindrops may also further seal and compact a hydrophobic layer when they fall which can further reduce the effects of raindrop splash. Once the hydrophobic layer is broken particles again become easily displaced.

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Rill FormationRill Formation

• Stages of rill formation in hydrophobic soils:• Saturation of wettable soil layer• Formation of a fail zone• Free flowing water• Erosion of the water repellent layer• Infiltration into the underlying soil

Rill formation in hydrophobic soils has been well documented and appears to follow several defined stages. First the wettable soil surface layer becomes saturated. The wettable surface layer allows for infiltration until water can neither drain downward or laterally across the affected area. If the water repellent soil layer is on the soil surface runoff begins immediately after rain drops reach the soil surface. Because of the water repellent layer beneath the surface the saturated pores cannot drain and they create an increase in pressure which decreases the shear strength of the soil. The decrease of soil shear strength produces a failure zone located at the boundary of the wettable and water repellent layers. Once failure occurs turbulent flow develops which accelerates erosion and entrains particles from both the ash layer and the water repellent layer. The downward erosion of the water repellent layer continues until it is completely eroded away and water begins to flow into the underlying soil. The final result of this process is the formation of a rill immediately below the water repellent layer.

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Some Notes of ErosionSome Notes of Erosion

• 3 components of erosion:• Detachment• Transport• Deposition

Figures from Neary et al 2005

Erosion is a natural process occurring on landscapes at different rates and scales depending upon geology, topography, vegetation and climate. Natural erosion rates tend to increase as annual precipitation increases peaking in semiarid ecoregionswhen moving from desert to wet forests. Above this point the increased precipitation leads to increased plant and litter cover that decreases the natural erosion rates. However disturbances such as fire the erosion rates tend to increase. In fire altered landscapes the surface conditions after fire are important in determining the erosion rates. The erosion process is based on three separate components that are a function of sediment size and transport medium velocity. The transport vector in erosion can be many things but typically we think of air or water as the primary transporters of material. The table you see here shows different sizes of sediment the detachment velocity and the deposition velocity.

In general as more heating occurs and the litter is lost and hydrophobic soil increase we also see a decrease in infiltration rates and an increase in runoff and erosion rates.

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Types of ErosionTypes of Erosion

• Progressive erosion: sheet, rill and gully• Dry Ravel• Mass Failures

In a general since there are three types of erosion that occur. Progressive erosion, dry ravel and mass failures.

Sheet erosions occur when the slope surfaces erode uniformly, this type of erosion proceeds to rill erosion in which small linear, rectangular channels cut into the surface of a slope. Further development of rills leads to the formation of deep large rectangular to v-shaped gully's. This process is often referred to as progressiveerosion.

Dry ravel is a gravity induced down slope surface movement of soil grains, aggregates and rock material. Dry ravel erosion may be triggered by animal activity, earthquakes, wind, the freezing and thawing of materials and many other physical processes. Fire can greatly increase dry ravel erosion by reducing plant cover and removing obstacles that were preventing the movement of materials.

Erosion caused by mass failures includes slope creep, falls, topples, slides, debris flows and other such complex movements of materials. Most fire related mass failures are debris flows associated with the development of water repellent layers in the soil.

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Fire Induced ErosionFire Induced Erosion

Natural background erosion rates in western forests are reported to be between 0.01 and 2.47 tons/acre/year. Landscape level disturbance activities such as site preparation are estimated at about 6.7 tons per acre/year, agriculture 249.8 tons/acre/year and road construction at 62.4 tons/acre/year. Fire related sediment yields vary greatly depending upon fire frequency, climate, vegetation, and geomorphic factors such as topography, geology and soils. In most cases the majority of erosion after a fire takes place during the first year. In general prescribed fires will have erosion rates between 0.1 tons/acre/year to 6.7 tons/acre/year and wildfires will have erosion rates between 0.1 tons per acre to 164.6 tons/acre/year. The picture you see here shows the amount of material that is possible to be eroded in a wildfire. In this case a 10 acre lake was filled in after the Rattlesnake fire of 1996 on the Coronado National Forest in Arizona.

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Fire Effects on Soil ChemistryFire Effects on Soil Chemistry

• Fire affects individual soil chemical characteristics, chemical reactions, and chemical processes

Table from Neary et al 2005

Fire effects individual chemical characteristics, chemical reaction and chemical processes within the soil. A few of the common soil chemical characteristics that are affected include organic matter, carbon, nitrogen phosphors, sulfur cations, cationexchange capacity, ph, and buffer power, the chemical processes that are generally affected involve nutrient availability and the losses and additions of nutrients to the soil. As you can see in this table the temperature thresholds for soil characteristics varies greatly. As a general rule of thumb the total amounts of chemical elements are never increased by fire, although in some cases will remain the same if the element has a high temperature threshold such as calcium. However it is important to note that fire does change the form of some elements and in many cases makes them more available for plants and other organisms .

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Fire Effects on Soil ChemistryFire Effects on Soil Chemistry

• In non-fire environments nutrient recycling depends upon decomposition

• In fire dominated environments nutrient recycling is dominated by the combustion process

The sustained productivity of natural ecosystems depends upon a regular and consistent cycling of the nutrients that are essential for plant growth. Nutrients are added to the system by precipitation, dryfall, nitrogen fixation and geochemical weathering of rocks. In a non-fire environment nutrient availability is a function of decomposition. However as you are aware the combustion process speeds this up. The magnitude of fire related changes depends largely upon the fire severity. Nutrient losses during and following fires mainly involve chemical processes. The disposition of nutrients during and following fire generally occur in one of five ways. First Direct gaseous volatilization into the atmosphere takes place during the fire, second particulates are lost in smoke, or nutrients remain in the ash deposited on the soil surface fourth substantial losses of nutrients deposited in the surface ash layer can occur during surface runoff and erosion and lastly some of the nutrients remain onsite as part of the incomplete combusted vegetation.

Following fire variable amounts of ash can be left remaining on the soil surface. This ash is either blown away or is leached into the soil by precipitation. Ash concentration are generally present after burning a large concentration of fuels such as a slash pile. The accumulation of think layers of ash residue remain on the soil surface after a fire is referred to as the ash-bed effect. The chemical nature of ash has the following general effects on the soil. It increases the PH, changes the solubility of organic matter and associated minerals in water, and can add available nutrients for microbial populations.

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Some Notes of Fires Affects on Soil BiologySome Notes of Fires Affects on Soil Biology

Soil biological properties involve a wide range of living organisms that inhabit the soil, along with the biologically mediated processes that they regulate. The welfare of these organisms directly affect the short and long term productivity and sustainability of wildland ecosystems. The living organisms within the soil can be classified first as either flora or fauna. Soil flora include algae, cyanobactria, mycorrhiza and plant roots, and soil fauna includes earthworms, insects and protozoa. The soil fauna have further been dived up into micro-, meso, and macrofauna based on body lengths of less than 0.2 mm, .2 to 10.4 mm and greater than 10.4 mm respectively. The table shown in this slide shows the types and sizes of soil organisms that can be affected by fire.

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Some Notes of Fires Affects on Soil BiologySome Notes of Fires Affects on Soil Biology

Fire effects soil organisms both directly and indirectly. The direct effects are short term changes that result when any particular organism is exposed directly to the flames, glowing combustion, hot gases or is trapped in an environment where enough heat is supplied to kill or severely injure the organism. Indirect effects typically involve longer duration changes in the environment that impact the welfare of the organism after the fire. These indirect effects often include affects on plant succession, soil organic matter transformations and micro climate. The temperature thresholds of biological organisms is typically much lower than that of the soil characteristics. You can see from the table shown here that plant roots can only withstand temperatures of around 48 degrees Celsius, while mycorrhizae can withstand temperatures above 90 degrees Celsius.

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A Few Final ThoughtsA Few Final Thoughts

As we end this lesson, I would like you to consider the complexity of the interaction of fire and soil properties, specifically try and think about the physical aspects that would be required to develop physical models to predict these occurrences and think about the measurements and relationships that could be used to develop empirical models to make these predictions.