FRST 320: ABIOTIC DISTURBANCES – Wind and Fire
Fall 2014
FIRE EFFECTS ON SOILS, WATER AND AIR QUALITY
Objectives:
1. Identify and explain fire effects on the physical and chemical properties of soil
2. Identify and explain fire effects on hydrology and the water cycle
3. Identify and explain fire effects on air quality and methods of smoke management
1. FIRE EFFECTS – SOIL
Soil Heating
- maximum temperature and duration of elevated temperatures due to combustion determine impacts on soil at
different depths
- fuel size, abundance and configuration will determine flaming vs smoldering combustion and heat transfer to
the soil
- soil moisture affects its temperature
- for a given input of heat, increase in temperature of moist soils < dry soils
- water has high specific heat and requires abundant heat energy to warm and evaporate
- soil temperature increases above 100°C only after water has evaporated
- heat conductivity of water > air so thermal diffusivity increases in moist soils and heat energy is
conducted through the soil
Altered Thermal Properties
- following fire, reduced canopy + less insulating organic matter + direct soil radiation into dark soil surface
results in increased daytime temperatures and greater nighttime radiant heat loss
- rooting zone of trees warm more quickly in the spring and cool faster at night with 2 effects:
- extend the growing season by promoting earlier root growth
- when cooling at night, energy release can reduce damage by frosts
Physical Properties of Soil
Consider the biophysical properties of the forest floor organic horizons (leaf litter and duff):
- reduces raindrop impact, prevents reduction of soil porosity, and traps sediment
- organic matter increases field capacity of the soil profile (amount of soil water at saturation)
- organic material is important for water infiltration rates, limits overland flow and surface erosion
- provide habitat for soil fauna
What happens when surface fuels burn?
Soil texture – severe heating (>200°C) at soil surface fuses clay particles into sand-size particles
Soil bulk density – porosity decreases following intense or repeated burns increasing bulk density and decreasing
the field capacity
Fire Effects on the Environment – Page 1 of 3
Hydrophobicity - water repellency of soil caused by coating soil particles with water-repellant organic substances
that include lipids (waxes, fats and oils)
- organics volatilize during heating and concentrate near the soil surface
- condense at different temperatures and spread over several centimeters in the soil profile
- repels water and reduces the infiltration capacity of the soil, especially in coarse-textured soils
- subsurface hydrophobic layers result in restricted infiltration and increased lateral and overland water
flow; surface erosion may result during heavy rainfalls
Chemical Properties of Soil
Nutrient volatilization
- depends on amount of biomass consumed and fire temperature
- “soil nutrient thermometer”
770°C
750°C
550°
375°C
175°C
100°C
inorganic phosphorous
calcium, magnesium and sodium
clays collapse (hygroscopic water lost)
potassium
sulfur
hydrocarbons
nitrogen
water
carbon
- post-fire conditions: organics reduced, nutrients lost depending on temperature, other nutrients altered to a
more mobile form
Acidity (pH)
- soil pH increases due to accumulation of base oxides (CaO, MgO and K2O)
- effects depend on the cation exchange capacity (CEC) of the soil
- if CEC large (basic, well-buffered), pH changes will be short in duration
- if CEC low (acidic, not well-buffered), changes can be pronounced and long-term due to increased
leaching (2-3x rates of unburned soil)
- implications of changes to pH:
- at higher pH, more CEC sites become available but forest floor combustion may negate gains
- soil microbial community changes: bacteria more resistant to heat than fungi
- nutrient availability may decrease, especially metals like Fe, Cu, Zn and Mn
Nutrient changes through time
- strong early growth may occur due to the initial increase of nutrients
- after 7-9 years, productivity decreases possibly due to the effect of long-term nutrient losses
- longer-term growth may be higher due to the release of immobilized nutrients and fixation of N by earlysuccessional species with nitrogen-fixing symbionts
Fire Effects on the Environment – Page 2 of 3
2. FIRE EFFECTS – HYDOROLOGY
Impacts on the Hydrologic Cycle
Decrease in infiltration rates due to hydrophobicity and loss of organic matter
Increase in overland flows and surface erosion
- depending on fire severity, slope and type of ecosystem
Increase in snowpack due to reduced interception from trees
Increase peak flows after precipitation events and snowmelt due to overland flow, reduced field capacity of the
soil and reduced evapotranspiration
Riparian Zones and Streams
Decrease shading of streams and increased water temperature
Increased scouring of stream beds with increased peak flows
Increased siltation results from increased overland flow and erosion
3. FIRE EFFECTS - AIR QUALITY
Smoke = chemicals and particulates
Amount depends on:
- quantity of fuel consumed, size and length of burn
- amount of smoldering, determined by fuel compaction
- fuel moisture content and amount of live foliage burned
- atmospheric conditions (stability and inversions)
Chemical components
- 90% of output is CO2 and H2O = greenhouse gases contributing to global warming
- other carbon: particulates, cabon monoxide (CO), volatile organic matter
Gaseous pollutants (risk to human health, although release levels generally low)
- CO - most prevalent but dissipates quickly due to mixing
- aldehydes - responsible for eye irritation
- polynuclear aromatic hydrocarbons (PAH) – carcinogenic
Particulates
- microscopic solids and liquids suspended in air
- includes acids (e.g. nitrates and sulfates), organic chemicals, metals, soil and dust particles
- most harmful components of smoke for human health
- cause respiratory problems and reduced visibility
- weather can exacerbate (inversions) or reduce (mixing of unstable air) impacts
- small (<2.5 microns) are most harmful and 50-90% of smoke is 0.1-0.3 microns
Smoke Management
Avoidance strategies – avoid smoke intrusions into sensitive areas
- wind direction or atmospheric stability
Dilution – mixing of smoke with air to keep concentrations low
- burning slowly and early in the day to allow dissipation
Emissions reduction – burn less fuel; chipping on-site or removal
Source:
Agee, J.K. 1993. Chapters 5 and 6 in Fire Ecology of Pacific Northwest Forests. Island Press. Washington, DC.
Fire Effects on the Environment – Page 3 of 3