Ignition patterns & prescribed fire behavior in southern pine stands / by Ragnar W. Johansen

GEORGIA FOREST RESEARCH PAPER

N
72
Oct., 1987
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Ignition Patterns
& Prescribed Fire Behavior

in Southern Pine Stands Received

V.

By: Ragnar W. Johansen

MAY 3 1988

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DOCUMENTS UGA LIBRARIES
RESEARCH DIVISION

GEORGIA FORESTRY COMMISSION

About The Author
Ragnar W. Johansen is a Research Forester,
now retired, Southern Forest Fire Laboratory,
Southeastern Forest Experiment Station,
USDA Forest Service, Macon, Georgia.
ACKNOWLEDGMENT
Funding for this was partly provided by the Georgia Forestry Commission.

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ABSTRACT: As an aid to forest managers who use or contemplate using aerial ignition techniques

in their prescribed burning programs, a study was designed to evaluate the magnitude of the dif-

ferences that could occur depending on whether lines of fire were used (ignited by a helitorch) or a

spot-fire technique was used (ignited by aerial ignition devices). Six experimental fires of a

simultaneously ignited backfire, flank fires, and headfires of line origin and spot origin were

observed, and flame spread distances were recorded at 1 -minute intervals to time of burnout. Of

special note were the differences in rate of spread of headfires from line and spot origin during early

development. Headfires of line origin traveled at rates that varied from 1 .5 to 5.9 times faster than

those of spot origin. Line fires would, therefore, develop higher fire intensities than spot fires.

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Ignition Patterns
& Prescribed Fire

Behavior

&." in Southern Pine Stands
By: Ragnar W. Johansen

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INTRODUCTION
Aerial ignition continues to be looked upon with favor in the South as a means of prescribed burning the understories of pine stands. An advantageous aspect of the technique is the speed with which areas can be ignited and burned out, which thus allows for better utilization of favorable burning weather (Johansen 1 984a). The rapid burnout may also prove to be the technique's undoing if unwanted fire intensities develop too often and cause excessive damage to the overstory crop trees (Johansen 1984b).
Only two basic types of ignition from aircraft are currently being used: lines of fire by means of a flying drip torch or
helitorch (Muraro 1 976, Lowman 1 982), and spot fires (or point source) by means of an aerial ignition device (A.I.D.). For

fuel, the helitorch uses regular gas mixed with Alumagel, a
dry metallic stearate available from WITO Chemical Corp.,
Houston, Texas (Stevens 1 985). The A.I.D. system has a dispenser that injects ethylene glycol into plastic spheres (similar to ping-pong balls) containing potassium permanganate, which are than released (Laitand Muraro 1 979). The originating fire behavior that stems from these two types of ignition can be quite different (Rothermel 1 984).
A field study was designed to measure the extent of the dif-
ferences that would occur when line-fire and spot-fire ignitions were made in palmetto-gallberry type fuels
under 18- to 25-year-old plantations of slash pine (Pinus elliottii Engelm. var elliottii) under differing weather and fuel conditions. All burning took place on the Dixon
Memorial State Forest in Ware County, Georgia.

1/2 ch
T"

BACKFIRE HEADFIRE

3 1/2 ch

WIND DIRECTION
FLANK? FIRES LEFT RIGHT

2 ch

I
1/2 ch
T

SPOT FIRE

4 ch

2 ch

2 ch

4 ch

Figure 1 .-Layout of single experimental burn for simultaneous ignition and burning of a headfire, backfire, flank fires, and spot fire.

PROCEDURES
Six experimental fires involved simultaneous ignitions in
X each of three 4- 4-chain (1 .6 acre) blocks (Figure 1 ). One
block contained a headfire and backfire ignited from a single drip torch. The second block had two directional flank fires ignited by a single drip torch line from downwind to upwind. The third block had a single spot fire ignited with matches
when the other two lines were initiated, which allowed flames to develop downwind, upwind, and crosswind. The two 4-chain drip-torch lines of fire were completely ignited within 45 seconds (fast walk).
The experimental fires were completed in a 2-year period under varying environmental conditions. For each set of the three-block fires, six people were used to

observe the following fire movements: line head, line back, right flank, left flank, spot head, and spot back. Each person marked the fire's progress at 1 -minute intervals in the center region of the burn. Lids from 1 -gallon paint cans were used to mark the interval location of fire advance. Following burnout, the distance of each lid from fire origin was measured with a tape and recorded according to time elapsed since ignition. Linear rate of fire spread was then calculated from these
data.
Several 1/4-milacre litter-vegetation samples were collected from the first four sets of burning plots before ignition to estimate available fuel consumed during the fires. Fuel amounts, based on duration of accumulation or "age of rough," were estimated on the last two plot sets. Only areas with litter roughs between 1 and 3 years of age were chosen

Table 1 .Environmental conditions associations with ignition patterns of six experimental fires.

Exp. burn
(No.)

Burn date

1

1/30/84

2 1/31/84

3 1/31/84

4 1/22/85

5 1/22/85

6 1/29/86

Ignition
time
Hours
1545 1130 1450 1300 1530 1145

Fire weather observations

Ambient temp.

Relative
humidity

Windspeed 3

Mean available
fine fuel

Mean Surface
litter
moisture content"

o

F

Percent

mph

Tons/Acre

66

53

2.8

47

46

2.0

55

43

2.3

43

34

2.0

46

32

2.5

60

63

3.0

3.3
3.7
4.3
3.0 C
4.5 C
4.0

Percent
32 34 30 24 23 29

Measured at a 5.0-foot height in the stand. Each value based on nine samples. 'Estimate based on stand density and age of rough.

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for the study so that all understory fuel accumulations were relatively light. Three upper-surface litter samples were collected from each plot immediately before ignition to determine moisture content. Temperature, relative humidity, windspeed at the 5-foot level in the stand, and prevailing
wind direction were also measured. A recording ane-
mometer was set up Vi chain upwind of the center plot and run during the course of the burns conducted in 1 984 to measure windspeed and wind direction; only a hand-held Dwyer anemometer was used immediately before ignition to measure windspeed for the last three burns. Table 1 gives a summary of fire weather elements and fuel characteristics for
each set of fires.
RESULTS
Of the measured differences in fire behavior (Table 2), some were predictable-such as the lowest rates of spread occurring with backfires. The flank fires were difficult to keep maintained as such. If the line of fire from which the flanks developed was not set perfectly into the wind, one flank would be moving as a headfire most of the time while the other would move primarily as a backfire. Minor wind shifts would cause the same problem. Under high fuel moisture conditions the flank that was backing would frequently go out. The differences in fire behavior exhibited between headfires of line origin and those of spot origin were of special
interest. To give some perspective on the disparities between them, two figures depict the forward spread distance plotted over elapsed time: The headfire of spot origin with the greatest spread rate is compared with the associated line
headfire (Figure 2), and the headfire of line origin that had the greatest rate of spread is compared with the spot fire burning
at the same time (Figure 3). In a run of approximately 250
feet for all six sets of fires, the headfires of line origin traveled at rates that varied from 1 .5 to 5.9 times faster than those
originating from spots (Table 2). The exception, 31 times faster, was a special case. In no instance did a spot fire advance faster than a line fire.
CONCLUSIONS
Results from this study indicate that under any weather conditions where fire spread can be sustained, a headfire of line origin will always advance in its early stages at a rate con-
280

siderably faster than would be the case had the fire been of spot origin. This means that in the early stages of fire develc, ment, the fireline intensity (Byram 1 959) at the head of eacn of a group of multiple spot fires along a given length of line will be considerably less than anywhere along a line headfire of the same line length under the same conditions of fuel and weather. Therefore, the rate of heat output per unit of time for a given area will be greatly reduced by spot-fire use; this, in turn, will considerably reduce the height level at which needles would be scorched in a stand (Van Wagner 1 973). This does not preclude the use of line fire (by means of a helitorch) to underburn a stand of pine to reduce fuel buildup. However, it does suggest that more care must be exercised in prescribing fuel and weather conditions to prevent excessive heat buildup that could unduly damage a forest stand.
This information can be extrapolated to a comparison of the helitorch with Alumagel and the A.I.D. system. The
current technique is to pump Alumagel from the torch in a
thin stream that breaks up into individual "globs" of burning gel as it falls to the ground. The rate of breakup and size of globs is dependent on pumping rate, aircraft speed, drop height, and gel viscosity. To date, it is impossible to regulate the spacing between globs due to the difficulty of regulating gel viscosity. Viscosity is strongly affected by age of the
Alumagel powder, how the powder is stored, additives in the gasoline used, temperature of gasoline when mixed, mixing
procedure, and the time allowed for gelling to take place. Because of all these uncertainties, Alumagel is usually dispensed in a steady stream with ignited globs falling to the
forest floor at spacings from 5 to 30 feet apart. Technically
speaking, this could be called spot firing; but it is actually line firing because these closely spaced spots quickly merge to form a line of fire.
Use of the A.I.D system will permit the forest manager to continue underburning pine stands after the helitorch usershould stop. In all cases, however, the keyto the successful use of any aerial ignition techniques is to know when to halt operations. Constant vigilance of the usually increasing fire intensity development as surface fuel moisture decreases from morning to mid-afternoon will allow the burn manager to stop stringing fire when heat begins to reach critical levels in the tree crowns. Burning under stands that are 30 feet tall will require earlier shutdown than burning under 80-foot stands.
250 /

-- LINE ORIGIN
SPOT ORIGIN

5

10

15 20 25

ELAPSED TIME (minutes)

Figure 2. Spread rate of headfires of spot origin and line origin for experimental burn No. 1, 1/30/84 at 1545 hours.

LINE ORIGIN
-- SPOT ORIGIN

10

20

30

40

ELAPSED TIME (minutes)

Figure 3. -Spread rate of headfires of spot origin and line origin for
experimental burn No. 5, 1/22/85 at 1 530 hours.

UNIVERSITY OF
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Literature Cited
Byram, G. M. 1959. Combustion of forest fuels. P. 1 -89 In Forest Fire: Control and Use; K. P. Davis, Ed. McGraw-Hill
Book Co., New York.
Johansen, R. W.1984a. Aerial ignition for speed and control with prescribed burnings. Forest Farmer 43(3):12-14. Johansen, R. W. 1984b. Prescribed burning with spot fires in the Georgia coastal plain. Georgia Forest Research Paper 49.
Georgia Forestry Commission, Macon 6 p. Lait, G. R. and S. J. Muraro. 1979. The PFRC aerial ignition system. Mark II. Canadian Forest Service Information Report
BC-X-167 (rev.). 27 p. Lowman, B. 1 982. The flying drip torch and fireline explosives. P. 2 1 -23. In Proc. of a symposium, Site preparation and fuels
management on steep terrain. D. M. Baumgartner, Ed. Feb. 15-17, 1982. Washington State Univ. Coop. Extension. Pull-
man, WA
Muraro, S. 1976. Improved ignition systems. P. 22-26 In Proc. Western Forest Fire Commission Annual Meeting Western J. & Forest Conservation Association, Portland, OR.
Rothermel, R. C. 1984. Fire behavior considerations of aerial ignition. P. 143-158 In Proc. Workshop: Prescribed Fire by Aerial Ignition. Intermountain Fire Council, Missoula, MT.
Stevens, G. E. 1 98 5. Aerial ignition-flying drip torch. P. 95-99 In Prescribed Fire and Smoke Management in the South: Con-
ference Proc. USDA Forest Service, Southeastern Forest Experiment Station, Asheville, NC.
Van Wagner, C. E. 1973. Height of crown scorch in forest fire. Canadian Journal of Forest Research 3:373-378.

GEORGIA
FORESTRY,
John W. Mixon, Director
Fred Allen, Chief Of Research
/c. $1,950
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