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Preventing forest fires through silviculture

R. Vélez

Ricardo Vélez is chief of the forest fire service of the Spanish Ministry of Agriculture, Fisheries and Food, Madrid.

This article is based on a report prepared for the 13th Session of the AFC/EFC/NEFC Committee on Mediterranean Forestry Questions held in Zaragoza, Spain, in September 1987.

As students, many of us learnt from weighty tomes how to treat a forest stand so that it would produce trees whose trunks were straight, clean, of large diameter and yielding high-quality wood. As our studies progressed, however, we discovered that very few areas could be treated in such a way: most of the land was suffering from deforestation, so that the first thing to do was to create wooded areas; ecological conditions in most of the country, moreover, made it almost impossible to achieve those "straight, clean trunks, etc."; the agents of deforestation had not disappeared, and in fact they sprang into action as soon as a new forest stand seemingly incompatible with their interests (agriculture, cattle raising or town planning) began to take shape; lastly, we had been horn in the Mediterranean region, not Scandinavia, and we needed a silviculture oriented toward protection rather than production. Forest fires are one of the signs that the agents of deforestation are at work, and they are becoming worse from year to year. Protective silviculture, silviculture for the Mediterranean, would have to include norms improving the capacity of forest areas to protect themselves against fire. Some ideas on this question, and a final recommendation, are outlined below.

Considerations on fire resistance

We know that the forest environment is composed of inflammable organic material, and that no fire prevention measures can alter this fact. Nevertheless, a fire is more than just combustion. It consists of flames moving across inflammable matter.

Prevention therefore aims at hindering this movement, thereby blocking the spread of fire.

Two principles should guide the design of preventive measures. The first is the concept of the resistance to fire of specific species. The second is that of fire resistance in forest vegetation as a whole.

Species resist fire in two ways: passively, by means of thick bark which protects the cambium (for example, cork trees) or through the presence of dormant buds that regenerate after the exposed parts are killed by the fire (canary pines, practically all leafy species, and many scrub species); or actively, through intense dissemination after the fire to replace the individuals killed in it (pines, eucalyptus, cistus, etc.). Most species populating Mediterranean ecosystems resist fire in one or other of those ways, or even both, as a result of long-term adaptation to a history of fires. The speed with which they regenerate actively or passively is not, however, the same for all of them. Repeated fires have therefore contributed to natural selection. The more frequent the fires, the less chance woody species have to regenerate, leading to a dominance of fast-growing, herbaceous plants. When, on the other hand, there are no fires for a long time, the land will be invaded by scrub and, later, arboreal species will appear.


Species' fire resistance can also be assessed in terms of their inflammability, in other words the ease with which they burst into flames at a given heat. This depends basically on the moisture content, which varies throughout the vegetative cycle for the living parts, and on the atmospheric humidity as regards dead combustible material, including leaves, branches, and the like. Studies carried out at the Mediterranean Silviculture Centre, INRA, Avignon (France) and at the Fire Laboratory, INIA, Madrid (Spain) have enabled the typically Mediterranean species to be ranked according to inflammability (see Chart).

The degree of resistance to fire and the probability of its spreading is also a consequence of the structure of the vegetation cover. A few examples will illustrate this. A new plantation, in which the soil has been worked, eliminating all the vegetation that might play a role, is resistant to spreading, as long as the soil is kept clean and the young plants are small and kept well apart.

In holm oak scrubland with a lot of undergrowth, fires spread easily. In fully grown woodland in which the trees have thick trunks and dense, above-the-ground foliage which limits the regeneration of the undergrowth, it will be hard for fire to spread.

A fully grown, dense pine wood gives a lot of shade and lacks undergrowth, as a result of which it is hard for fires to spread. On the other hand, an open wood of pines, cork oaks and holm oaks enables a lot of sun to reach the soil, stimulating the growth of a range of heliophile species that form a thick undergrowth in which it is easy for fires to begin and to spread.

It is evident from these examples that resistance or spread is a matter of the horizontal and vertical continuity of inflammable matter. Interruptions in continuity help make it difficult for fires to spread, limit the damage that they cause and make it easier to put them out. Wind is also a factor to be taken into account.

Tall woodland is a more effective windbreak than scrubland, which is more effective than pasture. On ridges, where the wind changes, and along watercourses, which direct it, tree cover may be an important obstacle to fire, since it reduces wind speed.

As regards the controversial subject of terracing new woodland, it cannot be claimed that terraces clearly contribute to either improving or worsening the woodland structure in terms of resistance to fire spread. Initially, terraces increase separation between rows of trees, creating horizontal discontinuity. Nevertheless, in such space heliophiliac plants that create continuity may prosper, and these will favour fire spread. If a fire breaks out, terraces constitute an obstacle, at times an important one, to fire fighters' ease of movement.


From the above considerations, we may formulate certain conclusions regarding species selection. There are certain species of arid zone genera (Atriplex, Tamarix, etc.) which have a high salt content, burn slowly and could be tested in several locations.

However, a definitive solution to fire cannot be found by replacing certain species with others, for practically all species burn under the tough conditions imposed by the Mediterranean summer. Accordingly, if we cannot base vegetation measures on intrinsic resistance, we must try to hamper spreading by creating discontinuities, avoiding very extensive, monospecific surface areas and creating patchworks of different inflammability levels that "disturb" the fire. In particular, wherever there is sufficient humidity, especially watercourses, the opportunity should be seized to plant species that make use of it.

The aim should be to create mosaics of species, by integrating other activities that give rise to discontinuity, such as roads, electricity line fuel breaks, cultivations, and recreational areas. Likewise, in exploiting the wood, an effort should be made to maintain its density, so as to limit undergrowth.

Inflammability of selected Mediterranean tree or shrub species

Species highly inflammable throughout the year

Calluna vulgaris

Erica arborea (heath)

Erica australis (heather)

Erica herbacea (heather)

Erica scoparia (heather)

Phillyrea angustifolia

Pinus halepensis (Aleppo pine)

Quercus ilex (holm oak)

Thymus vulgaris (thyme)

Species high inflammable only in summer

Anthyllis cytisoides

Cistus ladaniferus (cistus)

Genista falcata (furze)

Pinus pinaster (maritime pine)

Quercus suber (cork oak)

Rosmarinus officinalis (rosemary)

Rubus idaeus (red raspberry bush)

Stipa tenacissima (esparto grass)

Ulex parviflorus (furze)

Moderately or slightly inflammable species

Artutus unedo (strawberry)

Cistus albidus (cistus)

Cistus salvifolius (cistus)

Erica multiflora (heather)

Juniperus oxicedrus (juniper)

Olea europaea (wild olive)

Quercus coccifera (kermes oak)

Source: Instituto Nacional de Investigaciones Agrarias, Laboratorio del Fuego, Madrid.

It is also worth keeping hillsides that face into the prevailing winds well covered with high vegetation that acts as a windbreak, while opening fuel breaks on the leeward side, avoiding ridges.

Forested areas should be split up by fuel breaks of up to 200 metres in width, within which discontinuity should be enhanced by means of pruning, ground clearance, plantation differentiation, pathways and, in certain cases, fuel-breaking swaths of bare soil. Such fuel-break areas are always necessary at the edge of woodlands, to separate forest areas from agricultural or urban land.

Methods of reducing combustible matter

Creating both horizontal and vertical discontinuities requires a variety of techniques for the elimination of inflammable matter. These include mechanical and manual clearance, manual pruning, restricted burning, controlled pasturage and the use of phytocides.

In choosing the most appropriate techniques for each case, social, ecological and economic conditions should be borne in mind. For example, in areas of high unemployment, manual clearance is to be preferred. If there is a demand for land on which to raise cattle, controlled pasturage is likely to be a good choice, since it makes for an economic return as well as clearing fuel-break areas.

Prescribed burning is a very economical technique which nevertheless requires specific training. When combined with controlled pasturage, it can be highly recommended. One of its most promising forms is the burning of agricultural and scrubland zones, organized collectively in areas in which the rural population traditionally uses burning as a means to rejuvenate vegetation. Naturally, it requires intensive agricultural extension work in order to create a consensus in favour of using it and thereby rationalizing the use of fire by the rural population.

The use of phytocides should always be highly restricted, in view of the cost and of the difficulty of controlling its effects outside the treatment zone.

Mechanical ground clearance requires the use of machinery that is suitable for the various types of combustible material and terrain. Such machinery might include excavators which allow the energy resources of the inflammable materials to be tapped. However, at present it is questionable whether such exploitation is economical, given its cost in comparison with product prices. It could really only be made economically attractive through subsidies, which would be justified by the necessity of clearing and pruning in order to establish discontinuities in inflammable material.

Conclusions and recommendations

In too many cases heavy investments are made in fire-fighting equipment (planes and fire engines) rather than in prevention through improving woodland self-protection. Concrete measures coordinated among the Mediterranean countries concerned are therefore highly recommended.

A first step would be to spread knowledge and experience of how to modify inflammable materials so as to harmonize and improve the techniques of preventive silviculture. To this end, a team of experts could be assembled to produce a "forest fire prevention handbook". It might deal with the following topics: up-to-date technical knowledge concerning fire prevention silviculture; inflammable forest matter, techniques for altering inflammable forest matter, such as prescribed burning, controlled grazing, clearance and crushing, phytocides and species replacement; preventive silviculture, including the structure of forest concentrations and fuel breaks; and examples of concrete action.

The second step would be to carry out a joint comparative study of methods of conserving fuel-break areas, with assessments of their cost and effectiveness. Inflammable vegetation is manipulated primarily in these so-called fuel breaks. The objectives of such a study would be to assess manual and mechanical ground clearance methods in terms of the cost and length of their inflammable vegetation-reducing effects; to carry out restricted burning trials in the Mediterranean area and compare their results and costs with manual and mechanical clearance methods; to try out, as an alternative to conserving fuel breaks, the introduction of creeper species with a high salt content and low inflammability, of the genus Atriplex and others; and to study their maintenance through controlled grazing.

These measures could be coordinated by the recently created Forest Fire Management Research Network of the AFC/EFC/NEFC Committee on Mediterranean Forestry Questions, "Silva Mediterranea". It is in any case clear that if these efforts are to be effective, they must form an integral part of a wide-ranging, long-term regional cooperation programme.

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