Fire Behavior and Prescribed Fire

Heat Fuel Oxygen Fire Triangle

Three Principal Environmental Elements Affecting Wildland Fire Behavior

Fuels Fuel Type Fuel Moisture Size and Shape Fuel Loading Horizontal Continuity Vertical Arrangement

Fuel Moisture The amount of water in a fuel expressed as a percentage of the oven-dry weight of that fuel

Fuel Types Nonburnable Fuel Type Models (NB) Grass Fuel Type Models (GR) Grass-Shrub Fuel Type Models (GS) Shrub Fuel Type Models (SH) Timber-Understory Fuel Type Models (TU) Timber Litter Fuel Type Models (TL) Slash-Blowdown Fuel Type Models (SB)

Fuel moisture time lags: (Time it takes for a fuel to lose 63% of its moisture) Time lagFuel diameter 1-hour (fine fuels) <¼ inch (twigs, dead grass, leaves, needles) 10-hour ¼-1 inch (twigs, small branches, cones) 100-hour 1-3 inch (branches, tops) 1000-hour >3 inch (large branches, tops, logs) Fuel Size

Fuel Loading The quantity of fuels in an area Generally expressed in tons per acre

Horizontal Continuity: Uniform vs Patchy Vertical Arrangement Ground Surface Aerial

All combustible materials lying beneath the surface including deep duff, roots, rotten buried logs, and other organic material Usually called a “PEAT FIRE” Ground Fuels

All materials lying on or immediately above the ground including needles or leaves, grass, downed logs, stumps, large limbs and low shrubs. Surface Fuels

All green and dead materials located in the upper forest canopy including tree branches and crowns, snags, moss, and high shrubs. Aerial Fuels

Weather Temperature Wind Speed and Direction – Increases supply of oxygen – – Influences spread direction and spotting – – Dries fuels –

Weather Relative Humidity (RH) – Precipitation –

Topography Aspect Slope Position of Fire Shape of Country Elevation – Relates to curing of fuels, precipitation, length of fire season, etc. Aspect Slope Position of Fire Shape of Country Elevation – Relates to curing of fuels, precipitation, length of fire season, etc.

Aspect

Steep Slopes Cause Rapid Fire Spread

Slope Affects Fire Behavior Preheating Draft Faster Ignition and Spread Burning Material Rolling Downslope

Position of Fire on Slope

Box Canyon & Chimney Effect

Radiant Heat Across Narrow Canyon

Spotting Across Narrow Canyon

Mountains Cause Channeling of Wind

Elevation

Characteristics of Fire Behavior Fire Intensity: Heat release per unit time (BTUs) Fire intensity affected by –

Flame Length: The distance measured from the average flame tip to the middle of the flaming zone at the base of the flame

Rate of Spread (ROS): The distance a fire travels during a given period of time

Rate of spread = distance/time Burned Area Increased fire intensity Windspeed Steepness of slope Primary factors affecting rate of spread?

Slope Reversal

Running Types of Fire Behavior – spreading quickly Creeping – spreading slowly with low flames Smoldering– burns without flames; barely spreading Spotting– sparks/embers carried by wind or combustion column or moved by gravity Spot fires– new ignition points Fire brand– a piece of burning material

Torching Types of Extreme Fire Behavior – surface fire moves into crowns of individual trees Crowning – spreads from tree crown to tree crown (dependent, active, or independent) Flareup – sudden acceleration of fire spread or intensity (short duration, for portion of fire) Blowup– dramatic change in the behavior of the whole fire (rapid transition to a severe fire) Fire Whirls– vortex (gas mass with rotational motion)

Fire Effects

Prescribed Fire

Prescribe Fire Plan Burn prescription written Pre-burn site treatments completed Equipment designated & ready Personnel identified & trained Approvals & permits in hand Authorities & interested parties identified & notified* Latest forecasts checked* * Day before & day of burn

Ignition Devices Hand-held/ground-based – Drip torch – Propane torch – Fire fuse (flare) – Flame thrower

Drip torch fuel: 3:1 or 3:2 Diesel/gasoline

Ignition Devices Aerial –

Helitorch

Delayed aerial ignition device

Ignition Devices Lightning

Ignition Techniques Backfire (backing fire) – – Narrow burning zone – – Low smoke output

Backing Fire

Headfire (heading fire) – – Long flame lengths – – Fast rate of spread –

Strip-heading Fire Distance between strips controls intensity

Spotfire – – Useful in shifting winds – – What you get using a DAID

Spotfire Distance between spots controls intensity

Flankfire (flanking fire) – – Fire spreads at right angle to wind – – Requires careful crew coordination –

Flanking Fire

Headfire Flankfire Backfire Wind

Smoke Management

Principles of smoke management Have clear, defensible objectives Comply with local pollution regulations Notify local fire & law enforcement officials, nearby residents, & adjacent landowners Obtain the best available weather forecasts Don’t burn under highly stable conditions Burn during midday; avoid night burns Use caution near, upwind, or up-drainage of smoke sensitive areas

Principles of smoke management (continued) Use test fire to estimate smoke output & behavior Use backing fires if feasible Burn in small blocks if dispersion marginal Do not burn when fuel moisture high Don’t burn organic soils Mop-up along roads first Have an emergency plan!

Prescribed Fire in the Central Hardwood Region

The historic fire regime was thought to play a role in the maintenance of oak dominated forest prior to European settlement Prescribed fire has been suggested as a tool for regenerating oak Potential benefits included

Prescribed fires in late spring and summer are most lethal to oak competitors. – Prescribed Fire in the Central Hardwood Region

Generally, prescribed fire is best used in combination with reductions in overstory stocking (e.g. shelterwood, midstory removal) to release advance oak reproduction Repeated burning most effective at increase oak competitiveness Prescribed Fire in the Central Hardwood Region

The use of prescribed fire in oak forests has increased over the last four decades Results on oak regeneration has been highly variable

Prescribe Fire in the Central Hardwood Region Site Preparation Burn Fire can create conditions suitable for oak establishment by reducing litter layers and understory competition – Do not burn if acorn crop has just fallen or if new oak seedlings from recent crop are needed to regenerate the stand

Prescribe Fire in the Central Hardwood Region Site Preparation Burn Burning can be done in dormant or growing seasons High-intensity fires (flame lengths > 2 ft) in late spring decreases dense understory shaded more quickly Multiple fires over several years are commonly necessary to reduce dense understories to a level that improves oak seedling survival and growth

Prescribed Fire in the Central Hardwood Region Release Burn Burn used to free competitive oak reproduction from competition – Burning done after midstory removal, first removal cut of shelterwood or after final removal cut – Fire should occur after released oaks develop a more robust root system – Typically,

Prescribed Fire in the Central Hardwood Region Release Burn Moderate to high-intensity fires (flame lengths > 2 ft) to ensure topkill of understory layer Done in mid to late spring (April to May)