Prescribed Burning in the California
Mediterranean Ecosystem1
Lisle R. Green2
Wildfires that burn over thousands of acres of
mature chaparral occur primarily when winds are
gusting 30 to 80 mi/h (50 to 130 km/h), relative
humidity and dead fuel moisture are around 5
percent, and air temperatures are near 100° F
(38° C). Such fires are disastrous for soils,
vegetation, wildlife, structures, and sometimes
human life. The widespread use of prescribed
burning is frequently suggested as the only practical way to reduce the intensity and extent of
wildfire acreage burned, and the resultant damage.
Prescribed burning is the scientific application of fire to wildland fuels under conditions of
weather, fuel moisture, soil moisture, and other
factors that allow the fire to be confined to a
predetermined area, while at the same time accomplishing certain planned objectives (FordRobertson 1971). For Mediterranean ecosystem
burning, these objectives usually include wildfire
hazard reduction and wildlife habitat improvement,
but may include others. For example, the National
Park Service uses prescribed fire as a tool to
reintroduce fire as a force in naturally functioning ecosystems (Parsons 1977). If the desired
prescribed burning objective is clearly expressed,
a burning prescription to accomplish the objective
can be written.
Many fuel, weather, and topographic factors
affect fire behavior and must be considered in
planning for prescribed burning. Time of day and
season are important as they interact with the
other factors. Several of the primary determinants of fire behavior can be considered well
ahead of the burn date. These "prefire" determinants will be considered as a group in this paper.
Other factors that must be considered or determined immediately before and during the burn will
then be discussed. These factors all come
Abstract: Prescribed burning is feasible for
reducing conflagration costs. Prescription elements to consider before the burn are dead-tolive-fuel ratio, fuel volume, live fuel moisture,
chemical content, terrain, and season. Just
before and during the burn, 3-day weather forecasts, windspeed and direction, dead fuel moisture, relative humidity, and air temperature are
important. A chaparral stand with 30 to 40 percent dead fuel might be burned during the winter
with 60 to 75 percent live fuel moisture, 8 to 12
km/h (5 to 8 mi/h) of wind, 6 to 10 percent dead
fuel moisture, 23 to 32 percent relative humidity, and air temperature of 10° to 22° C.
together in the prescription. Writing the prescription is the task that has been the greatest
worry to prescribed burning planners and bosses,
most of whom feel more comfortable with suppression procedures. The guidelines presented here
should help.
Although this paper is based primarily on
experience in California, the principles dealt
with are the same elsewhere. Once objectives are
well defined, the guidelines provided can be used
in any Mediterranean ecosystem. This paper summarizes a recent report, "Burning by Prescription
in Chaparral" (Green 1981) which provides a more
detailed discussion of prescribed burning,
including topics not covered here, and a more
complete review of the literature.
PRESCRIPTION ELEMENTS TO CONSIDER BEFORE THE BURN
DATE
Fuel Volume
Fuel volume, or loading, is expressed as
pounds or tons of fuel on an area of land. The
term commonly covers total biomass, but no fire
burns all biomass, except perhaps in grassland.
The part of the biomass actually consumable by
the fire is the available, or burnable, fuel.
Range Scientist, Pacific Southwest Forest and
Range Experiment Station, Forest Service, U.S.
Department of Agriculture, Riverside, Calif.
Total biomass in annual grassland is typically
1/3 to 1 ton/acre (0.7 to 2.2 t/ha) and when dry,
essentially all will burn. Soft chaparral
(Paysen and others 1980), sagebrush (Artemisia),
or light chamise (Adenostoma fasciculatum H. &
A.) biomass varies from 3 to 10 tons/acre (7 to
22 t/ha), and 70 to 85 percent is consumed by a
hot fire. Biomass of dense chaparral dominated
by chamise is typically around 15 to 25 tons/acre
(33 to 56 t/ha), about two-thirds of which burns.
Only about 50 percent of brush dominated by large
shrubs--those 6 ft (1.8 m) or more in height with
basal stems 2 to 5 inches (5 to 13 cm) diameter-is consumed. Such brush has a biomass of 30 to
45 tons/acre (67 to 100 t/ha). Remaining, following any prescribed burn, are the branches
larger than 1/4 to 1/2 inch (0.6 to 1.3 cm) (fig.
1). Even in hot wildfire, green branches are
seldom burned to diameters greater than 1/2 inch
464
Gen. Tech. Rep. PSW-58. Berkeley, CA: Pacific Southwest Forest and Range
Experiment Station, Forest Service, U.S. Department of Agriculture; 1982.
1
Presented at the Symposium on Dynamics and
Management of Mediterranean-type Ecosystems, June
22-26, 1981, San Diego, California.
2
years, or if the mature brush is sparse, a stand
of annual grasses and forbs may have developed and
their residue often determines the fire behavior.
Brush stands in which annual grass and forbs
intermingle with soft chaparral species, such as
bush buckwheat (Eriogonum fasciculatum Benth.),
sage (Salvia spp.), and California (coastal)
sagebrush (Artemisia californica Less.), occur
frequently at low elevations or intermingle with
chaparral communities on south slopes; these, like
grassland, have a high dead-to-live ratio and can
burn fiercely at almost any age.
Stands of chamise on south slopes accumulate
dead fuel faster than do chaparral communities on
northerly or easterly exposures, especially if
chamise is associated with soft chaparral species
(fig. 2). Such south slope brush can be burned
by prescription at an earlier age--perhaps at 15
to 20 years--than the chaparral of northerly
exposures.
Figure 1--During a hot chaparral fire, all dead
fuel is consumed, but most green branches larger
than 1/4 inch diameter remain. Unburned green
fuel may amount to half or more of the chaparral
biomass on north exposures.
(1.3 cm). Actual quantities of burnable fuel are
becoming more important as greater restrictions
are placed on the quantities of burn residues that
can be added to the atmosphere during a prescribed
burn.
Stands of chaparral dominated by such species
as scrub oak (Quercus dumosa Nutt.), ceanothus,
chamise, manzanita (Arctostaphylos spp.), toyon
(Heteromeles arbutifolia M. Roem.), and mountain
mahogany (Cercocarpus betuloides Nutt.) are more
resistant to fire than the various chaparral/
grass-forb associations, especially on northerly
exposures. About 5 to 8 years following clearing
of such brush, the new canopy closes and forbs
and grasses are then quite effectively suppressed
(Bentley and others 1966). The chaparral stand
will likely contain no more than 10 percent dead
fuel--skeletons left from previous fires and the
remains of deerweed (Lotus scoparius [Nutt.]
Ottley) or other semishrubby vegetation. With
only 10 to 20 percent of dry fuel, a chaparral
In southern California, knowledgeable foresters
established that 2 tons of fuel per acre (4.5
t/ha), dry weight, was the maximum that should be
allowed on fuelbreaks (Pacific Southwest Forest
and Range Exp. Stn. 1963), because the heat from
that fuel volume could usually be tolerated by
firefighters. Nearly all chaparral fuelbeds contain 8 tons/acre (18 t/ha) or more of available
fuel and can be expected to burn as moderate- to
3
high-intensity fires, if they burn at all. An
objective of prescribed burning for hazard reduction may thus be to reduce the burnable fuel down
to acceptable levels of around 2 tons/acre (4.5
t/ha), sometimes in a series of prescribed burns.
The Dead-to-Live-Fuel Ratio
Fire behavior in brushfields is determined to a
great extent by the amount of dead twigs and
branches present in the brush canopies, and by the
amount of cured herbaceous residue in the understory. If the mature brush has been removed by
fire or equipment during the previous several
3
Fire intensity is the rate of energy or heat
release per unit of time and length of fire front.
Figure 2--Soft chaparral frequently has a high
dead-to-live ratio and burns at a younger age
than chaparral. Dominant species in this picture
are white sage, California sagebrush, and
chamise.
465
stand is quite resistant to prescribed fire, and
burning attempts in such stands are nearly always
futile.
As the proportion of dead fuel in a brushfield
reaches 25 to 35 percent, the stand is susceptible to prescribed burning. This proportion may be
reached when the chaparral on southerly exposures is around 20 to 25 years old. Chaparral on
northerly or easterly exposures is probably older
before it accumulates enough dead fuel to be
burned successfully. Chaparral 40 to 80 years
old is seldom more than 40 to 50 percent dead.
Soft chaparral stands may be two-thirds dead, if
the abundant litter is considered, a fact which
explains in part why there are many more wildfires in the soft chaparral type than in the
chaparral, and why the soft chaparral communities
can be burned by prescription at an earlier age.
The proportion of dead fuel in chaparral communities can be estimated roughly from the age of
the brush, can be estimated in the field, or can
be cut on plots and weighed. The latter method is
extremely time consuming and is usually a research
activity. Mature chaparral tends to have roughly
1 percent of dead twigs, branches, or plants for
each year since the brush canopies closed. Thus,
20-year-old chaparral is likely to be 10 to 15
percent dead, while 40- or 50-year-old chaparral
may be estimated to be 35 or 40 percent dead.
This is a very rough rule of thumb, but it can be
of considerable value to fuel managers.
Since the proportion of dead fuel is so important in prescribed burning, it should be checked
by field observation. Dead material is generally
obscured from above by the green canopy, so an
observer must get under the canopy cover at
several locations within a proposed burn. The
proportion of main branches that are dead should
be counted on several plants, then the small dead
twigs attached to live branches evaluated. Keeping in mind that most of the weight is in the
larger branches, an observer can approximate the
proportion of dead fuel.
Live Fuel Moisture
Live or green fuel moisture is the moisture
content of living twigs to 1/8 inch (3 mm) diameter and attached leaves, expressed as a percentage of dry weight, unless some other size class is
specified. This definition was agreed on by
California agencies concerned with wildland fire,
and has been in use for two decades.
The live fuel moisture has been recognized as
important in prescribed burning (or wildfire
danger) by some fuels management workers, but
ignored by others. Fuel moisture content of
living fuel is usually so high that the fuel will
not burn unless dried by an outside heat source.
Heat released from dry fuel as it burns must dry
out the live twigs so that they will burn and add
energy to the fire if the burn is to be success-
466
ful. Most forest fuels, when ovendry, have a heat
value near 8500 Btu/pound (3860/kg). If the fuel
moisture content is 80 percent, the effective heat
value is cut in half to about 4200 Btu (1930/kg)
(Countryman 1977). The greater the live fuel
moisture percentage, the more dead fuel must burn
to drive off the water.
Live chaparral fuel moistures are typically
high during the spring, 130 to 200 percent; they
decline through the summer and reach a minimum of
50 to 80 percent in September or October. With
several inches of rain during the fall, there is
some recovery of live fuel moisture. Otherwise,
it may remain low until spring.
If green fuel moisture is greater than 85
percent, prescribed burning is seldom successful
unless there is a very high proportion of dead
fuel or unless the brush is crushed or sprayed to
reduce the moisture content. Green fuel moistures
less than 60 percent in old brush stands indicate
hazardous conditions and burning should be
avoided or special precautions taken. A green
fuel moisture range of 60 to 75 percent is
usually about right for burning standing mature
chaparral.
Procedures for measuring green fuel moisture
have been published (Countryman and Dean 1979),
and general trends of green chaparral fuel moisture throughout California are published each 2
weeks by the Southwest Region, U.S. Forest
Service, during the spring, summer, and fall.
Green fuel moisture of chamise or other abundant
species to be burned should be determined 3 or 4
weeks before a projected prescribed burn date,
and again 1 or 2 weeks before the burn. This
allows for adjustment of other prescription
elements if the green fuel moisture is high or
low.
Chemical Content
The chemical content of shrubs is generally
ignored during prescribed burning, but perhaps
should not be. One class of chemicals--the ether
extractives--make up a substantial part of the
dry weight of many flammable species, from about
8 percent of pine needles (Rothermel 1976) to 15
to 18 percent of California sagebrush and the
shrubby Salvias (Montgomery 1976) (fig. 3). The
extractive content is highest during the fall and
lowest during the spring (Philpot 1969). Extractives are readily volatilized by heat and frequently burn fiercely several feet above the
shrubs.
If an area to be burned contains considerable
soft chaparral, and the chaparral species bigberry manzanita (Arctostaphylos glauca Lindl.)
and chamise, it can be expected to burn hotter
than an area dominated by such chaparral species
as toyon, laural sumac (Rhus laurina Nutt.),
ceanothus, scrub oak, and mountain mahogany--just
because of the high chemical content.
Growth starts in the spring during periods when
there is available soil moisture, and daytime
temperatures are above 40° F (5° C) (Bentley and
Talbot 1951). Moisture content of shrubs increases rapidly during the spring, and the risk of
escape is less than at any other time of year.
This is an excellent time to burn crushed brush,
brush piles, old stands of south slope vegetation,
or other concentrations of dead woody fuel. It is
not the best time for good consumption of green
brush during broadcast burning.
Terrain Considerations
Prescribed burning in chaparral is always in or
near rough topography that affects burning decisions in many ways.
Figure 3--Oils, fats, terpenes, and other chemicals are volatilized from flammable brush by heat
from fire, and these products then contribute to
the intensity of the fire. Soft chaparral contains more of these products than most chaparral
species.
Slope has an effect on fire similar to windspeed, and the steeper the slope, the greater the
uphill rate of fire spread. During daytime hours,
air movement is normally upslope and this reinforces the slope effect, thus ensuring rapid
spread of fire up to the ridgetop. Because this
is so, prescribed burns are generally ignited on
the highest ridges that form burn boundaries, and
a fireline is burned into the wind and downslope
from the ridgetop. Prescribed burn bosses must
also be aware of downcanyon air movement that
begins shortly after sunset, or sometimes earlier
on shaded north or east exposures.
Gusty, turbulent windflows occur at canyon
intersections or where canyons change direction.
Eddies are created where wind crosses a ridgetop,
and windspeeds are higher through saddles than at
adjacent higher elevations along the ridge.
A second class of chemicals, the mineral elements, have an opposite effect from the ether
extractives and tend to make vegetation less
flammable (Philpot 1970, Shafizadeh 1968). Phosphorus has been more effective than other elements
for reduction of flaming combustion.
Seasonal Considerations
The season for prescribed burning can be anytime that burning can be accomplished within the
prescribed limits of weather, fuel, and manpower,
and when burn objectives can be accomplished.
Late summer and early fall contain the fewest burn
days because of weather extremes and extremely dry
fuel.
The early winter months--October, November, and
December--contain days suitable for burning in
California. Days are short, nights are cool, and
there has been little recovery in the moisture
content of green brush. This is a good time for
hot burns with maximum consumption of brush.
Midwinter also presents some good burn opportunities. Soon after rainstorms, 1-hour timelag
fuels, those less than 1/4 inch (6 mm) diameter,
can be burned, and after several dry days, small
brush fuels. Excessive quantities of available
fuel can be burned in stages during this season.
The most severe fire microclimates are on
southerly or southwesterly exposures, and fire
danger increases from northern to southern exposures. Chamise, a flammable species, frequently
dominates on southerly exposures and may be
burned at times without firelines if less flammable species with higher fuel moistures grow on
adjoining northern exposures.
PRESCRIPTION ELEMENTS TO CONSIDER AT BURN TIME
Fire intensity and rate of spread are directly
affected by several factors which must be determined shortly before the fire is to be ignited,
and during the prescribed burn. These include
windspeed, dead fuel moisture, relative humidity,
and air temperature. If these are within the
prescription range, ignition and firing can
proceed. If one or more is not within range,
unless some trade-off can be made with another
prescription element, the burn must be postponed.
Windspeed and Direction
Prediction of windspeed and direction is our
greatest problem in local weather forecasting,
especially in mountainous terrain. Wind, more
467
than any other factor, is responsible for erratic
fire behavior, for prescribed burn escapes, and
for large wildfires. Some wind, except on steep
slopes, however, is needed to move fire through
chaparral during prescribed burns.
Windspeed is measured by the U.S. Weather
Service and by Fire Danger Rating stations at a
standard 20 ft (6.1 m) above open ground or vegetation. However, wind velocity measurements
taken on prescribed burns usually approximate the
"midflame" windspeed zone for chaparral. Winds
at midflame height are usually about half the
velocities at 20 ft (6.1 m) for fuels such as
grass and brush. Windspeed as given in this
paper should be considered to be midflame windspeed.
The maximum safe windspeed for prescribed
burning in chaparral is generally considered to
be 10 mi/h (16 km/h). Gusting above this windspeed will occur, and if these gusts reach 15 to
20 mi/h (24 to 32 km/h) during the burn, control
problems will surely arise. Windspeeds of 4 to 8
mi/h (6.4 to 12.8 km/h) are about right for
prescribed burning in chaparral.
In other vegetation types, higher windspeeds
have been recommended. Winds of 8 to 15 mi/h
(12.8 to 24 km/h) nave been suggested for level
terrain in Texas where grass carried the fire
into and through brush (Wright and Bunting 1976)
and in juniper (Martin 1978; Pase and Granfelt
1977; Northwest Region, Forest Serv. 1973).
Wind direction and changes in wind direction
may be as important as windspeed to the prescribed
burn operation. Usually, a prevailing wind pattern can be identified before the burn, and firelines and ignition patterns are planned with this,
and the terrain, in mind. Winds tend to change
direction and vary in velocity as the airstream
flows around and over ridges and through saddles,
and otherwise adapts to the topography. Near the
ocean, sea breezes may disrupt the wind pattern,
or create their own pattern. During warm daylight
hours, the wind movement is typically upslope. At
night, after air near the ground has cooled, it
flows downslope. Santa Ana winds can override
this pattern (Schroeder and Buck 1970).
Dead Fuel Moisture
The moisture content is the most important
factor determining whether or not fuels will
ignite and burn. A fuel moisture content of about
25 percent of the dry weight of the fuel is the
approximate value above which fuels will not burn
(Rothermel 1972). The precise value depends on
the type of fuel, the fuel loading and arrangement, size of firebrand, windspeed, and perhaps
other factors. Fuels generally do not burn vigorously if the fuel moisture content is above 15
percent, unless fanned by strong winds or on steep
slope.
468
As the moisture content of wildland fuels
decreases below 15 percent, the flammability
increases rapidly. The fire spread rate is estimated to double as moisture content drops from 15
to 10 percent, and to triple when it drops from 10
to 5 percent (U.S. Dep. Agric., Forest Serv.
1975). Thus, fuel moisture content changes below
10 percent can markedly affect fire behavior, and
the prescribed burn boss should be very aware of
this. Fuel moisture contents of 5 percent or less
encourage spotting and excessive spread rates.
Moisture contents of 6 to 10 percent are frequently good for prescribed burning, but if the
proportion of dead fuel is greater than 40 percent
or if burning is done under tree canopies, higher
fuel moisture contents--10 to 15 percent--are
needed to keep the intensity and spread rate
within bounds.
The moisture content of dead fuels 1/4 to 1
inch (0.6 to 2.5 cm) diameter can be determined
accurately by laboratory techniques (Countryman
and Dean 1979), but is frequently estimated in the
field through the use of "fuel moisture sticks."
These are 1/2 inch (1.27 cm) ponderosa pine dowels
mounted on two hardwood pins and weighing 100
grams, moisture free. Any weight in excess of
this is an estimation of the moisture content in
percent.
Relative Humidity
Moisture in the atmosphere--the humidity--is
important in prescribed burning because of its
effect on moisture content of fine dead fuels.
Relative humidity is the amount of moisture in
the air at a given temperature and air pressure
compared to the amount that it would hold if
saturated. A low relative humidity, 10 or 20
percent, indicates a great capacity for the
atmosphere to take up moisture and dry out fuels.
Dry fuels will absorb moisture when the percent
relative humidity is high, until they reach about
20 percent moisture. At that level, fuels are
difficult to ignite and burn.
Relative humidity can be quickly and accurately measured by a sling psychrometer. However, a word of caution: The psychrometer should
be fanned or twirled until there is no further
decrease in the wet bulb temperature reading
before it and the dry bulb readings are recorded.
A common error is reading it too soon.
Atmospheric pressure has enough effect on
relative humidity readings that charts or slide
rules designed for low elevations should not be
used at higher elevations. Errors of several
percent in relative humidity readings can easily
result. Charts are available for various elevations that assure accurate relative humidity
measurements.
Experience has shown that to burn standing,
untreated chaparral 25 to 40 or more years old,
and with about one-third of the fuel dead, rela-
to convection and spotting becomes more of a
problem, particularly as air temperature rises
above 80° F (26° C).
tive humidities of 25 to 35 percent are about
right. If the relative humidity is above 40
percent, and particularly if it has recently been
higher, fire will not spread without strong wind
or steep slope. If chaparral is 40 to 60 percent
dead, as after spraying, relative humidities of
35 to 60 percent will be needed to keep the fire
intensity within bounds. If the proportion of
dead fuel is only 20 percent, relative humidities
of 15 to 18 percent, and winds near the upper
prescription limits, will be needed.
High air temperatures contribute to crown
scorch, and are desirable if the objective is to
kill trees. For cleaning up the forest floor
with minimum damage to crowns, 55° to 70° F (13°
to 21° C) is about right.
Time of Day
Air Temperature
The safest time of day for prescribed burning
is generally from midday to midafternoon, providing prescription requirements are met. As
burning is extended into late afternoon, temperatures tend to decrease and relative humidity to
increase, and control problems are less.
Air temperature has little direct effect on
fire, but considerable indirect effect. When air
temperature rises, relative humidity decreases,
evaporation proceeds more rapidly, fine fuels
become drier, and less heat energy is required to
cause a loss in fuel moisture. Air movement due
Table 1--Prescription elements for burning chaparral.
Fire intensity
Factors affecting fire intensity
Low
Medium
High
Prefire consideration
Total biomass, tons/acre
Available fuel, tons/acre
3 to 10
31 to 45
6 to 10
10+
Dead fuel, pct. of available
20 to 30
31 to 40
41+
Live fuel moisture, percent
90 to 76
75 to 60
59 to 45
Low
Medium
High
Spring
Winter and
early spring
Summer, fall,
early winter
0 to 19
20 to 40
41 to 70
N, NE
E, SE, NW, W
S, SW
0 to 4
5 to 8
9 to 12
Chemical content
Season
Slope, percent
Aspect
3 to 6
11 to 30
Burn date consideration
Windspeed, mi/h
Dead fuel (fuel stick) moisture
percent when chaparral is:
20 to 30 percent dead
12 to 9
31 to 45 percent dead
18 to 12
8 to 6
5 to 3
11 to 7
6 to 5
46 to 65 percent dead
20 to 15
14 to 9
66 to 100 percent dead
30 to 19
18 to 11
10 to 8
8 to 6
Desired relative humidity
percent when fuel is:
20 to 30 percent dead
35 to 26
25 to 18
17 to 15
31 to 45 percent dead
45 to 36
35 to 24
23 to 18
46 to 65 percent dead
60 to 41
40 to 31
30 to 25
66 to 100 percent dead
Desired air temperature, °F
Time of day
75 to 41
40 to 36
35 to 20
20 to 59
60 to 80
81 to 95
Late morning or
late afternoon
Midday to
midafternoon
Early
morning
469
Sometimes, prescribed burns are conducted as
early in the morning as fuels will burn so as to
complete the burn or to burn out a safe line
before the heat of the day. Such burns are facilitated by low nighttime relative humidities
(Philpot 1965). In some localities near the coast,
the movement of maritime air determines the time
of day for burning.
THE PRESCRIPTION
Information in the previous discussions can be
summarized and made more convenient for use by
listing the recommendations in a table. This has
been done for chaparral in table 1, and for
burning under oak or pine tree canopies in table
2.
Before a burning prescription can be developed, the piece of brushland under consideration
must be evaluated to determine the prescribed
burning prospects. If most elements for prefire
consideration fall in the medium intensity range
(table 1), the area is probably right for burning. An approximate date can be selected and
local weather monitored as the date approaches.
If most elements are in the low intensity range,
consideration should be given to (1) delaying the
burn for a few years, (2) applying desiccants or
crushing treatments to dry out the brush, or (3)
compensating by burning when windspeed, dead fuel
moisture, relative humidity, and air temperature
are in the high intensity range during the late
fall or winter.
If the elements for prefire consideration are
mostly in the high intensity range of table 1,
some cautions are in order. If the total biomass
and available fuel fall under "high" intensity, if
the dead fuel comprises 40 to 60 percent of all
that will burn, and if the burn is on a steep
southerly exposure, consideration should be given
to burning during late winter or early spring when
the green fuel moisture has risen and when the
extractable chemicals are not at a peak. Also,
burning when dead fuel moisture is greater than 10
percent, when relative humidity less than 30
percent is not expected, when maximum air temperatures will not be higher than 60° or 70° F (15° to
21° C), and when windspeeds are 0 to 5 mi/h (0 to
8 km/h) will contribute to safe burning.
Brush grows among oak trees at all elevations,
and among coniferous trees at the higher elevations. If the canopies are open, as mature pine
trees may be, there will be brush and tree reproduction under the crowns, and perhaps stairstepped into them. If tree branches are dense
and brush the ground, as oak branches frequently
do, brush under the canopies may be low in volume
and mostly dead, but brush from outside will
surround the trees and finger into the canopies.
In the mixed-conifer forest, prescribed burning can be accomplished readily when there is a
good needle fall, where there is bearclover
(Chamaebatia foliolosa Benth.), or in openings
where herbaceous plants form a continuous cover.
Flame heights can usually be kept to 3 ft (1 m)
or less, and out of tree canopies.
Burning under and around oak canopies without
damaging crowns is usually difficult (Green
1980). Hand pruning of lower branches may be
needed. Bulldozers can sometimes be used to push
brush away from trees, or a dozer can crush brush
for burning during late winter or spring. The
actual burning must be done with a low-intensity
fire (table 2).
Table 2--Prescription elements for burning under pine or oak tree canopies.
Fire intensity
Factors affecting fire intensity
Fuel that will burn, tons/acre
Medium
High
1 to 2
3 to 5
Proportion of fuel that is dead, pct.
15 to 25
25 to 30
Dead fuel moisture, percent
18 to 12
11 to 7
Live fuel moisture
85 to 76
75 to 60
59 to 50
Relative humidity, percent
60 to 41
40 to 31
30 to 25
Windspeed, mi/h
0 to 2
3 to 4
Air temperature
20 to 39
40 to 70
Time of day
Fuel arrangement
470
Low
Morning
Late morning or
late afternoon
Crushed or cut
Beneath tree
canopies
6+
31+
6 to 5
5 to 10
71 to 85
Midday to
midafternoon
Brush extends
up into tree
canopy
LITERATURE CITED
Bentley, Jay R.; Green, Lisle R.; Evanko, A. B.
Principles and techniques in converting chaparral to stable grassland in California. Proceedings of the X International Grassland Congress; 1966; Helsinki, Finland: X Int. Grassi.
Congr. Sec. 4, Paper 14; 1966; 55-59.
Bentley, J. R.; Talbot M. W. Efficient use of
annual plants on cattle ranges in the California
foothills. Washington, D.C.: U.S. Dep. Agric.:
1951; Circ. 870. 52 p.
Countryman, Clive M. Heat and wildland fire--part
1. The nature of heat. Berkeley, Calif.: Pacific
Southwest Forest and Range Exp. Stn., Forest
Serv., U.S. Dep. Agric.; 1977. 11 p.
Countryman, Clive M.; Dean, William A. Measuring
moisture content in living chaparral: a field
user's manual. Berkeley, Calif.: Pacific Southwest Forest and Range Exp. Stn., Forest Serv.,
U.S. Dep. Agric.: 1979; Gen. Tech. Rep. PSW-36.
27 p.
Ford-Robertson, F. C. (ed.). Terminology of forest
science, technology practice and products.
Washington, D.C.: Soc. Amer. For.: 1971; Multilingual Forestry Terminology Ser. 1. 349 p.
Green, Lisle R. Prescribed burning in California
oak management. Plumb, Timothy R., tech. coord.
Proceedings of the symposium on the ecology,
management, and utilization of California oaks,
June 26-28, 1979, Claremont, California.
Berkeley, Calif.: Pacific Southwest Forest and
Range Exp. Stn., Forest Serv., U.S. Dep. Agric.
Gen. Tech. Rep. PSW-44; 1980; 136-142.
Green, Lisle R. Burning by prescription in chaparral. Berkeley, Calif.: Pacific Southwest
Forest and Range Exp. Stn., Forest Serv., U.S.
Dep. Agric.: 1981; Gen. Tech. Rep. PSW-51. 36 p.
Martin, Robert E. Fire manipulation and effects in
western juniper (Juniperus occidentalis Hook.).
Proceedings of the Western juniper ecology and
management workshop, January 1977, Bend, Oregon.
Portland, Oreg.: Pacific Northwest Forest and
Range Exp. Stn., Forest Serv., U.S. Dep. Agric.
Gen. Tech. Rep. PNW-74; 1978; 121-136.
Montgomery, Kenneth Reid. Ether extractives and
flammability of Mediterranean-type shrubs.
Pomona, Calif.: Calif. State Polytechnic Univ.;
1976. 38 p. Dissertation.
Northwest Region, Forest Service. Crooked River
national grasslands begins prescribed burning
program. Fuel Manage. Notes 1(7): 1-2. 1973.
Pacific Southwest Forest and Range Experiment
Station, Forest Service, U.S. Department of
Agriculture. Guidelines for fuel-breaks in
southern California. Fuel-Break Rep. 9.
Berkeley, Calif.; 1963; 25 p.
Parsons, David J. Preservation in fire-type ecosystems. Proceedings of the symposium on the
environmental consequences of fire and fuel
management in Mediterranean ecosystems, August
1-5, 1977, Palo Alto, California. Washington,
D.C.: Forest Serv., U.S. Dep. Agric. USDA
Forest Serv. Gen. Tech. Rep. WO-3; 1977;
172-182.
Pase, Charles P.; Granfelt, Carl Eric, tech.
coords. Use of fire on Arizona rangelands. Fort
Collins, Colo.: Rocky Mountain Forest and Range
Exp. Stn.; Arizona Interagency Range Committee:
1977; Publ. 4. 15 p.
Paysen, Timothy E.; Derby, Jeanine A.; Black,
Hugh, Jr.; Bleich, Vernon C.; Mincks, John W. A
vegetation classification system applied to
southern California. Berkeley, Calif.: Pacific
Southwest Forest and Range Exp. Stn., Forest
Serv., U.S. Dep. Agric.: 1980; Gen. Tech. Rep.
PSW-45. 33 p.
Philpot, Charles W. Diurnal fluctuation in moisture content of ponderosa pine and whiteleaf
manzanita leaves. Berkeley, Calif.: Pacific
Southwest Forest and Range Exp. Stn., Forest
Serv., U.S. Dep. Agric.: 1965; Res. Note
PSW-67. 7 p.
Philpot, C. W. Seasonal changes in heat content
and ether extractive content of chamise. Ogden,
Utah: Intermountain Forest and Range Exp. Stn.,
Forest Serv., U.S. Dep. Agric.: 1969; Res.
Paper INT-61. 10 p.
Philpot, C. W. Influence of mineral content on
the pyrolysis of plant materials. For. Sci.
16(4):461-471; 1970 April.
Rothermel, Richard C. A mathematical model for
predicting fire spread in wildland fuels.
Ogden, Utah: Intermountain Forest and Range
Exp. Stn., Forest Serv., U.S. Dep. Agric.:
1972; Res. Paper INT-115. 40 p.
Rothermel, R. C. Forest fires and the chemistry
of forest fuels. In: Thermal uses and properties of carbohydrates and lignins. New York,
San Francisco: Academic Press; 1976: 245-259.
Schroeder, Mark J.; Buck, Charles C. Fire
weather--a guide for application of meteorological information to forest fire control
operations. Washington, D.C.: U.S. Dep. Agric.:
1970; Agric. Handb. 360. 229 p.
Shafizadeh, F. Pyrolysis and combustion of cellulosic materials. Adv. Carbohyd. Chem. 23:419474; 1968.
U.S. Department of Agriculture, Forest Service.
Fireline handbook. Washington, D.C.; 1975.
Wright, Henry A.; Bunting, Stephen C. Prescribed
burning in the Rio Grande plains. Noxious brush
and weed control. In: Research highlights, Vol.
7. Lubbock, Texas: Texas Technol. Univ.; 1976:
42.
471