The Use of Fire in Silviculture1
Pierre Delabraze and Jean Ch. Valette2
During the last century, the maintenance of
chestnut plantations to make harvest easier, of
pinelands to reduce fire risk and damage, and of
pastureland to get rid of ungrazed grass, was
carried out by means of small winter prescribed
burns. This method was nearly completely given up
as a consequence of the exodus from rural areas.
In forests under the authority of the state and
the departments, prescribed burning was not accepted as a forestry technique. Currently the use
and even the introduction of fire in forest areas
have been banned for fear of fire outbreaks.
Owners are given impairments from October 15 to
April 1.
The decrease in fire danger (outbreaks and
especially spread) is related to the clearing of
underbrush. The four techniques: clearing by
hand or machine, use of weed-killers, use as
pasturelands, and prescribed burning are scientifically compared in order to present a set of
efficient and often complementary methods to the
managers.
To deal with prescribed burnings, the Mediterranean Sylviculture Center of the I.N.R.A., Forest
Research Department, has developed research in
order of urgency established after consultation
with the forest managers. Dealing first with
forecasting of fire danger, the research has led
to study of the silvicultural possibilities of
fire along these lines:
Abstract: In the French mediterranean area the
use of fire, practiced in the past by the farmers,
is now being reactivated by the forest managers.
It relies on the flammabilities of the most important species and on the combustibilities of the
main vegetal associations. The first prescribed
burns demonstrate the sensitivity of forest trees
and the role of dead material and wind. The
results of these tests lead also to a better
evaluation of fire risk and to organization of the
clearings.
- use of data gathered through these investigations, in order to perfect the prescribed burning methods.
FLAMMABILITY OF FOREST SPECIES
Principle of Measurement
The flammability of a vegetal sample is calculated according to the time necessary for appearance of flame when the sample is subjected to a
fierce heat radiation.
Apparatus and Method
Experimental Apparatus
A radiator sends out a flux of about 7 watts
per square centimeter and 3 microns wavelength.
The pilot flame allows the ignition of the air-gas
mixture resulting from the thermic decomposition
of the sample, but it does not play any part in
this decomposition.
−
flammability of the main species of forest
trees, in order to determine the risks of fire
outbreak and to understand their development
−
combustibility of the main forest associations, to define the risks of fire spread
−
combustibility of the forest litter responsible for both the outbreak and spread of fires
1
Presented at the symposium on Dynamics and
Management of Mediterranean-type Ecosystems June
22-26, 1981, San Diego, California.
2
I.N.R.A. Station de Sylviculture Mediterraneenne Avenue A. Vivaldi - 84000 AVIGNON.
Gen. Tech. Rep. PSW-58. Berkeley, CA: Pacific Southwest Forest and Range
Experiment Station, Forest Service, U.S. Department of Agriculture; 1982.
Figure 1--Flammability measurement apparatus.
475
The Fuel (table 1)
Only the flammability of the fine elements of
the main forest species is measured, as these
elements cause the fires.
Experimental Checklist and Significant Data
For each studied species, the monthly periodicity of testing is reduced to 10 days when the
fire danger increases, generally in the summer
time.
Picked according to very precise criteria on
homogeneity, the 200 or 300 grams of green matter
required for a specific test are put into tight
bags under partial vacuum and then placed in a
cooling-box; thus the water-loss is reduced to a
minimum during transport. A specific test is
composed of 100 basic tests made in three running
series. Each 1-gram sample is put on the heating
disk. The time between the moment when the sample
is set in place to the appearance of the first
flame is recorded. The test is positive if the
time is less than 60 seconds.
Then for each specific test these values are
calculated:
−
the percentage of positive tests
− the mean flammation time, averaging the
flammation times of the positive tests, expressed
in seconds
− the mean moisture content, averaging the
moisture contents of the four samplings, expressed
as a percent of the ovendry weight (24 hours at
60ºC).
The close relation between the percentage of
positive tests and the mean flammation time have
made a scale of flammation possible. (table 2)
Results
Species can be classified at regular intervals
according to their flammability. Table 1 makes
clear that the flammability is:
− grossly linked to plant physiologic activity;
very low or close to null at the time of bud
burst, it increases with the time of new tissue
lignification and reaches a maximum at the time of
summer dormancy
− temporarily influenced by rainfall or the air
moisture content; specific reactions take shape-the pluviometry, and more precisely the amount of
water stored in the soil, bring about wide variations in flammability.
Use of the Results
Knowledge of the specific flammabilities leads
to:
476
- defining the fire outbreak risks and above
all their sudden aggravation and their territorial
distribution
- making seasonal maps of fire danger, from the
forest associations maps, which should give a
sufficiently accurate account of the flora composition of the various stories.
Table 1--Specific flammabilities during the fire
danger period
Studied species
Flammability
July
August
September
4 4 4
4 4 4
5 5 5
4 4 4
5 4 4
5 5 5
4 4 4
4 4 4
5 5 5
0
1
2
2
4
4
5
5
5
5
4
5
3
2
4
Calcareous Provence
Tree story
Pinus halepensis
Quercus ilex
Quercus pubescens
Shrub story
Quercus coccifera
Phillyrea angustifolia
Rosmarinus officinalis
Cistus albidus
Thymus vulgaris
Herbaceous story
Brachypodium ramosum
Crystalline Provence
Tree story
Pinus pinaster
Quercus suber
Shrub story
Erica arborea
Erica scoparia
Arbutus unedo
Cistus monspelliensis
Calluna vulgaris
2
4
3
3
3
3
4
4
3
5
3
5
5
4
5
4
5
5
2
4
4
4
3
3
4
3
4
3
2
5
- 4 5
5 5 5
5 5 5
3 3 3
4 5 4
1 2 2
5 4 5
3 3 3
5 5 4
2
4
0
2
3
4
4
2
3
4
4
4
1
2
3
4
4
1
3
4
4
4
3
3
3
4
4
2
3
4
4
4
3
2
3
4
4
1
1
2
3
4
1
0
3
Table 2--Flammability marks levels as a function
of the percent of positive tests and the mean
flammation time
Mean flammation
(time seconds)
Percent of positive tests
98 to 95 to 90 to 85 to 80 to
100
97
94
89
84
80
less
from
from
from
from
more
5
4
3
2
1
1
than
12.5
17.5
22.5
27.5
than
12.5
to 17.5
to 22.5
to 27.5
to 32.5
32.5
4
3
2
1
1
0
3
2
1
1
0
0
2
1
1
0
0
0
1
1
0
0
0
0
The forester in charge of forests threatened by
fires is given the decision elements to rank his
intervention:
- on areas of species which are highly flammable or dangerous owing to sensitive surrounding
plantations
1
0
0
0
0
0
− on bushy and low-branching plants such as
Pinus halepensis, with its dead lower verticils,
covered with dry needles, which come nearly in
contact with the high calorific potential associations of Ulex parviflorus, Quercus coccifera, and
Brachypodium ramosum
− last but not least, when he uses prescribed
burning.
COMBUSTIBILITY OF FOREST ASSOCIATIONS
Principle
First restricted to the low bush associations,
the study of combustibility is based on a replanting of the various vegetal stories, from the
litter to the bush, on a combustibility measurement apparatus, and consists in recording the data
of the combustion.
Test Phases and Data Collected
The vegetation and litter are both gathered
from homogeneous and representative associations
on eight aligned and continuous plots, each 1
square meter. Before each sampling a sketch of
the plant layout on the soil is drawn, along with
a description of the phenologic stages and of
respective measurements and weight. The crop and
all the following processes are preceded by measurements of the air temperature and moisture
content and of the wind speed and bearing. Transport is carried out under cover. The plants are
planted out on the apparatus according to the
sketches. A detailed checklist states the fire
ignition conditions. During the burning, the fire
rate of spread and the flame front characteristics
are recorded. At the end of the test, the unburnt
plants are measured and weighed.
Apparatus and Method
The Combustibility Measurement Apparatus (figures
2 and 3).
The litter and collected vegetation are laid on
identical surfaces on eight trucks, each 1 square
meter by 25 centimeters. The moving walls act as
the surrounding vegetation by protecting the fire
from external agents (wind) and by reflecting the
heat; a top screen simulates the tree canopy. The
variable thread blade fan allows creation of wind
at speeds ranging from 5 to 40 meters per second.
Fuel
The study deals with these main forest associations:
Calcareous Provence
Quercus coccifera
Ulex parviflorus
Rosmarinus officinalis
Quercus ilex
Crystalline Provence
Arbutus unedo
Erica raborea
Cistus monspelliensis
Calluna vulgaris
Figure 3-- Combustibility measurement apparatus
(inside).
Results (table 3)
Figure 2--Combustibility measurement apparatus
(outside).
The litter plays a determining part because of
the surface dead fuel. Fire has great difficulty
in spreading when the litter is missing, even if
the living fuel is at its lowest moisture content.
Only the surface of dead fuel burns when it is
bulky; when it is abundant and light it encourages
the fire spread. The fire rate of spread is
negatively linked with the litter moisture content--hence the propitious consequences of a light
rainfall.
477
The herbaceous story transmits the fire from
the litter to the shrubby story. Its phenologic
stage and its abundance affect fire spread more by
its high flammability than by the released energy.
The graminaceae that dry out in summer are the
more dangerous; some of these turn green again
after the first storms of late summer.
The rain brings about a quick change in combustibility. It increases the moisture content of
the litter and leads to a new start of the vegetative activity. In summer, 50 percent of the tests
in which fire covers the 8 meters of the apparatus
are in samples taken more than 10 days after the
last rain; on the other hand, 85 percent of the
tests whose combustion stops within the first
meters of the apparatus have been on samples taken
less than 10 days after a rain.
Wind speed and rate of fire spread are positively linked to one another. The increase of the
flame length and the width of the flaming area the
flaming front incline on the combustible explain
it.
Fire spread is linked with the plant structure:
for the same biomass, an effective low bulk
density increases combustibility owing to the
large heat-absorbing surface and to the possible
ventilation inside the fuel.
Table 3--Combustibilities of the main forest
associations
Forest Associations
Combustibility
Calcareous Provence
Quercus coccifera
Ulex parviflorus
Rosmarinus officinalis
Quercus ilex
Crystalline Provence
Arbutus unedo
Erica arbora
Cistus monspelliensis
Calluna vulgaris
Good in summer, poor in
autumn
Excellent in summer,
existent in winter
Poor
Similar to Q. coccifera
Poor
Excellent in summer,
Excellent in case of
summer drought
Good in summer
A fuel bed (86 x 58 x 6 centimeters) rests on
scales (15.00 + 1.5 grams). Eight thermocouples
(Nc-Na), one above the other, and a fluxmeter are
connected to recorders. A photographic apparatus
records the combustion.
Studied Species
Use of the Results
The combustion of the low story vegetation is
correctly modelled on the apparatus:
− the flame front specific energies are calculated according to Byram's formula from the vanished combustible mass, its moisture content, fire
rate of spread, and knowledge of the heat release
− the energy radiated by the flame front is
inferred from study of its development and of the
measured temperatures (adaptation of the StephanBlotzmann formula)
− also, the biomasses and the laws of their
development are drawn from these tests.
Material is harvested in autumn from branches
of Pinus nigra Arn. ssp laricio, Pinus pinaster
Soland, Pinus halepensis Mill, Cedrus atlantica
Manetti.
Fuel
It has these characteristics:
− surface/volume ratio of the leaf or the
needle, expressed as
=
2
e
or =
2 (+2)
e
where e = thickness of leaf or needle
− initial mass
DEAD MATERIAL COMBUSTIBILITY
− moisture content, expressed as a percent of
ovendry weight
Principle of Investigation
− dry material initial mass
Currently, dead fuel combustibility is only
studied on leaves or needles picked on trees,
desiccated, and then laid out by hand in regular
layers.
−
bulk density, , in grams per centimeter
Results
Apparatus and Method
Experimental Apparatus (figure 4)
478
Dried needles of Pinus nigra laricio
= 4.7 mm
-1
= 1.8 10
-2
g.cm
-1
All the combustion data of the tests made in
horizontal position are positively related to the
initial ovendry mass from a threshold of steady
rate of spread on 3.6 t/ha. The most accurate
fittings are linear and the correlation coefficients are superior to 0.9.
When the fire moves upwards, from a 20º slope,
the fire spread laws are totally altered. The
fire spreads up the slope very fast, burning only
the upper layer of the dead fuel.
The tests made with fires moving downwards give
results which can be compared with those made on a
flat position, yet the slopes of the regression
lines are lower.
Dried needles of Pinus halepensis
= 6 mm
-1
= 3.3
.10
-2
g.cm
-3
The fire rate of spread threshold is close to
1.8 t/ha. All the data are related to the initial
ovendry mass, yet the slopes of the regression
lines are significantly lower than those of the
two aforementioned pine trees.
Dried needles of Cedrus atlantica
= 33.10
-2
g.cm
-3
Achieving a continuous carpet requires the
equivalent of 5 t/ha. Even for the highest values
the fire does not spread.
Dried needles of Pinus pinaster
HIGH CALORIFIC VALUES, ASH AND MINERAL CONTENTS
= 2.7 mm
-1
= 2.10
-2
g.cm
-3
The characteristics of the combustion of this
dead fuel are akin to those obtained with needles
of Pinus nigra laricio. The slopes of the regression lines are not significantly different whereas
the ordinates at the origin are slightly superior.
The data allow an accurate estimation of the
energy released during the combustion.
The High Calorific Value
Determining this allows us to calculate the
potential energy of forest combustibles and the
maximum heat release. Measured every month according to the classical calorimetry method, the
H.C.V. are comparatively lower during growing time
than after lignifying time. The H.C.V. of Erica
arborea is higher than the values given for most
of the tree species (table 4). The H.C.V. of
mediterranean species are generally superior to
those of more northern species. As for the
building material, no link between the H.C.V. and
the specific flammabilities has been clearly
shown.
Ash and Mineral Contents
Determined every month by decomposition at
450°C in a muffle furnace, the ash contents are
low during growing time and higher after lignifying time. The ash content of the Arbutus unedo
leaves is double that of the ash content of the
Erica arborea leaves, whose Ca content is very
similar to the Ca content of the Ulex parviflorus
shoots (table 4).
The study did not show any clear link between
these contents and the characteristics of
combustion.
THE PRESCRIBED BURNINGS IN FRENCH MEDITERRANEAN
AREA
Widely used in the past, the prescribed burning
method is nowadays only used by a few shepherds in
Corsica and some peasants in the Maures and
Cevennes Mountains.
Figure 4--Diagram of the apparatus for measuring
the dead material combustibility.
479
Table 4--Specific high calorific values, ash and mineral contents
Fuel
Contents
Leaves of Erica arborea
Needles of Pinus halepensis
Shoots of Ulex parviflorus
Leaves of Arbutus unedo
Leaves of Quercus ilex
Leaves of Quercus coccifera
H.C.V.
(kJ.g-1)
Ash
24.0
22.2
20.9
21.0
20.3
20.0
2.42
3.20
2.34
4.15
4.05
4.20
K
Mg
P
(percent ovendry weight)
0.07
0.11
0.08
0.10
0.09
0.07
0.49
0.50
0.62
0.61
0.50
0.53
0.23
0.15
0.13
0.27
0.15
0.14
Ca
0.30
0.63
0.37
0.99
0.96
1.22
Aims
Results (table 6)
The tests on prescribed burnings are intended
to define the rules of use according to the climatic, relief, and soil conditions and to improve
knowledge of the reaction of forest tree species
to the various types of prescribed burning.
These tests being on the whole quite recent,
the results are only provisory.
− large damages to the tree story in tests 3, 8
and 9, which were made at the bottom of the slope
on account of an adverse wind
Plotting Out and Method
Each test area is delimited by a surrounding
cleaned area, gridded with posts to make marking
and measurement easier to determine.
− importance, nature, and qualities of the dead
fuel and of the various vegetation stories
− measurements of the trees where thermosensitive plates are set
−
microclimatic conditions
− aspect of the ground and conditions prevailing there after the test.
− if the wind direction is well established and
if its speed is low and steady
−
The prescribed burns are generally made against
the wind or at counterslope.
Table 5--Scale for obvious fire damages
480
− the high combustibility of Quercus ilex and
of Pinus halepensis foliage and the relative
endurance of needles of Pinus pinaster.
− if the dead fuel or herbaceous layer is
continuous enough to guarantee a steady rate of
fire spread
Later, the restoration and new setting up of
vegetation will be observed with interest
(table 5).
Obvious damages
− satisfactory aspect of tests 4 and 7, ignited
at counterslope, and of tests 1, 2 and 6, which
were made against the wind: the low stories are
well burnt back and trees are spared
Prescribed burning can only be correctly
carried out
− fire ignition conditions, flame front spread,
development of flames on trees, and reactions of
the various species
No damage
Over 2/3 of the initial leaf mass are
green
From 1/3 to 2/3 of the initial leaf mass
are green
Less than 1/3 of the initial leaf mass is
green
Some green leaves
No green leaves
From the first investigations, the following
points can be noted:
if the fuel on the ground is dry enough
− if the low verticils of trees are more than 2
to 4 meters above the low story.
Conclusions
Scale
0
1
2
3
4
5
These first tests show that planted firebreaks
or trimmed bushes of Quercus pubescens covering a
graminaceous grassland can be maintained, that
there are maintenance problems in planted firebreaks where there are no resinaceous species,
owing to a scarce litter, and that there are
limits to the use of prescribed burning in
forested areas as soon as the mattoral (maqui and
garrigue) are fully grown.
PUBLICATIONS OF THE CENTER ON THESES SUBJECTS
Caramelle Ph., Clement A., 1978 - Inflammabilite
et combustibilite de la vegetation mediterraneenne. Flammability and combustibility of
the mediterranean vegetation. (3rd year thesis
E.N.I.T.E.F., July 1978, 158 p.)
Delaveaud, P., 1981 - Le feu, outil sylvicole.
Utilisation pratique des donnees de combustibilite. The fire as a sylvicultural technique;
practical use of combustibility data. (3rd
year thesis E.N.I.T.E.F., July 1981, to be
published.)
Doat, J.; Valette, J. Ch., 1981 - Le pouvoir
calorifique superieur d'especes forestieres
mediterraneennes. High calorific value of
mediterranean forest species. Ann. Sci. For.,
to be published.
Valette, J. Ch.; Clement, A.; Delabraze, P.,
1979 - Inflammabilite d'especes mediterraneennes. Flammability of some mediterranean
species. (Research note 79-3, Mediterranean
Sylviculture Center, 39 p.)
Valette, J. Ch,; Vannier, G., 1981 - Inflammabilite estivale des principales especes
forestieres de Provence calcaire. Summer
flammability of the main forest species of
calcareous Provence. To be published.
481
Table 6 --First tests of prescribed burnings
482