The Use of Fire in Silviculture1

1 The Use of Fire in Silviculture 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. 2 Pierre Delabraze and Jean Ch. Valette It relies on the flammabilities of the most impor- tant 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. During the last century, the maintenance of - use of data gathered through these investi- chestnut plantations to make harvest easier, of gations, in order to perfect the prescribed burn- pinelands to reduce fire risk and damage, and of ing methods. pastureland to get rid of ungrazed grass, was carried out by means of small winter prescribed burns. This method was nearly completely given up FLAMMABILITY OF FOREST SPECIES as a consequence of the exodus from rural areas. In forests under the authority of the state and Principle of Measurement the departments, prescribed burning was not ac- cepted as a forestry technique. Currently the use The flammability of a vegetal sample is calcu- and even the introduction of fire in forest areas lated according to the time necessary for appear- have been banned for fear of fire outbreaks. ance of flame when the sample is subjected to a Owners are given impairments from October 15 to fierce heat radiation. April 1. The decrease in fire danger (outbreaks and Apparatus and Method especially spread) is related to the clearing of underbrush. The four techniques: clearing by Experimental Apparatus hand or machine, use of weed-killers, use as pasturelands, and prescribed burning are scien- A radiator sends out a flux of about 7 watts tifically compared in order to present a set of per square centimeter and 3 microns wavelength. efficient and often complementary methods to the The pilot flame allows the ignition of the air-gas managers. mixture resulting from the thermic decomposition of the sample, but it does not play any part in To deal with prescribed burnings, the Mediter- this decomposition. ranean 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: − 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 respon- sible for both the outbreak and spread of fires 1Presented at the symposium on Dynamics and Management of Mediterranean-type Ecosystems June 22-26, 1981, San Diego, California. 2I.N.R.A. Station de Sylviculture Mediter- raneenne Avenue A. Vivaldi - 84000 AVIGNON. Figure 1--Flammability measurement apparatus. Gen. Tech. Rep. PSW-58. Berkeley, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; 1982. 475 The Fuel (table 1) - defining the fire outbreak risks and above all their sudden aggravation and their territorial Only the flammability of the fine elements of distribution the main forest species is measured, as these elements cause the fires. - making seasonal maps of fire danger, from the forest associations maps, which should give a Experimental Checklist and Significant Data sufficiently accurate account of the flora compo- sition of the various stories. For each studied species, the monthly perio- dicity of testing is reduced to 10 days when the fire danger increases, generally in the summer Table 1--Specific flammabilities during the fire time. danger period Picked according to very precise criteria on Studied species Flammability homogeneity, the 200 or 300 grams of green matter July August September required for a specific test are put into tight bags under partial vacuum and then placed in a Calcareous Provence cooling-box; thus the water-loss is reduced to a Tree story minimum during transport. A specific test is Pinus halepensis 4 4 4 4 4 4 4 4 4 composed of 100 basic tests made in three running Quercus ilex 4 4 4 5 4 4 4 4 4 series. Each 1-gram sample is put on the heating Quercus pubescens 5 5 5 5 5 5 5 5 5 disk. The time between the moment when the sample Shrub story is set in place to the appearance of the first Quercus coccifera flame is recorded. The test is positive if the 0 2 3 4 3 4 4 4 3 Phillyrea angustifolia 1 4 4 5 5 5 5 4 4 time is less than 60 seconds. Rosmarinus officinalis 2 3 4 5 5 5 3 3 3 Cistus albidus 2 3 3 5 4 2 2 3 2 Then for each specific test these values are Thymus vulgaris 4 3 5 5 5 4 4 4 5 calculated: Herbaceous story Brachypodium ramosum − the percentage of positive tests - 4 5 5 5 5 5 5 5 Crystalline Provence − the mean flammation time, averaging the Tree story flammation times of the positive tests, expressed Pinus pinaster in seconds 3 3 3 1 2 2 3 3 3 Quercus suber 4 5 4 5 4 5 5 5 4 − the mean moisture content, averaging the Shrub story Erica arborea moisture contents of the four samplings, expressed 2 4 4 4 4 4 4 4 3 as a percent of the ovendry weight (24 hours at Erica scoparia 4 4 4 4 4 4 4 4 4 60ºC). Arbutus unedo 0 1 3 2 2 3 1 1 1 Cistus monspelliensis 2 3 3 3 3 2 2 1 0 The close relation between the percentage of Calluna vulgaris 3 4 3 4 4 3 3 2 3 positive tests and the mean flammation time have made a scale of flammation possible. (table 2) Table 2--Flammability marks levels as a function of the percent of positive tests and the mean Results flammation time Species can be classified at regular intervals according to their flammability. Table 1 makes Mean flammation Percent of positive tests clear that the flammability is: (time seconds) 98 to 95 to 90 to 85 to 80 to 100 97 94 89 84 80 − grossly linked to plant physiologic activity; very low or close to null at the time of bud less than 12.5 5 4 3 2 1 1 burst, it increases with the time of new tissue from 12.5 to 17.5 4 3 2 1 1 0 lignification and reaches a maximum at the time of from 17.5 to 22.5 3 2 1 1 0 0 summer dormancy from 22.5 to 27.5 2 1 1 0 0 0 from 27.5 to 32.5 1 1 0 0 0 0 − temporarily influenced by rainfall or the air more than 32.5 1 0 0 0 0 0 moisture content; specific reactions take shape-- the pluviometry, and more precisely the amount of water stored in the soil, bring about wide varia- The forester in charge of forests threatened by tions in flammability. fires is given the decision elements to rank his intervention: Use of the Results - on areas of species which are highly flam- mable or dangerous owing to sensitive surrounding Knowledge of the specific flammabilities leads plantations to: 476 − on bushy and low-branching plants such as Test Phases and Data Collected Pinus halepensis, with its dead lower verticils, covered with dry needles, which come nearly in The vegetation and litter are both gathered contact with the high calorific potential associa- from homogeneous and representative associations tions of Ulex parviflorus, Quercus coccifera, and on eight aligned and continuous plots, each 1 Brachypodium ramosum square meter. Before each sampling a sketch of the plant layout on the soil is drawn, along with − last but not least, when he uses prescribed a description of the phenologic stages and of burning. respective measurements and weight. The crop and all the following processes are preceded by measurements of the air temperature and moisture COMBUSTIBILITY OF FOREST ASSOCIATIONS content and of the wind speed and bearing. Trans- port is carried out under cover. The plants are Principle planted out on the apparatus according to the sketches. A detailed checklist states the fire First restricted to the low bush associations, ignition conditions. During the burning, the fire the study of combustibility is based on a re- rate of spread and the flame front characteristics planting of the various vegetal stories, from the are recorded. At the end of the test, the unburnt litter to the bush, on a combustibility measure- plants are measured and weighed. ment apparatus, and consists in recording the data of the combustion. 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 Crystalline Provence Quercus coccifera Arbutus unedo Ulex parviflorus Erica raborea Rosmarinus officinalis Cistus monspelliensis Quercus ilex Calluna vulgaris Figure 3-- Combustibility measurement apparatus (inside).

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