by R. S. CAMPBELL, Chief, Division of Range Management Research, Southern Forest Experiment Station, United States Forest Service, New Orleans, Louisiana
IN MANY parts of the world, burning, through centuries of use, has become a common practice to aid hunting, to improve grazing and to clear land. Whether caused by lightning or by man, fire has long been associated with several major plant types. As Shanz has said:
"Much of the earth's surface now in grassland would without fire surely pass to forests, many coniferous forests would pass to hardwoods, and much of the higher and better parts of the Mediterranean type would pass to forest".
In some areas, fire is regarded as undesirable, particularly in forest and watershed management. It is the purpose of this paper 1 to summarize the effects of fire under major climatic and vegetation conditions, and to list the salient points to be considered in the development of policies for controlling fire and using it in the management of forest range.
1
Originally prepared as a working paper for the FAO Technical Meeting on Forest Grazing at Rome, Italy 29 March to 3 April, 1954. See Unasylva, Vol VIII, No. 2, p. 81.
The temperate humid zones include the great areas of coniferous forests in the northern hemisphere and the broadleaved or mixed broadleaved-coniferous forests found to some extent on all continents.
The general trend in many temperate broadleaved forests is toward separation of forest from grazing land, especially in western Europe and northeastern United States: in these areas the question of fire as related to grazing is not serious.
The opposite extreme is offered by the Coastal Plain pine belt of the southeastern United States. In this area, the controlled use of fire has reached a high degree of development for forestry, frequently in conjunction with range grazing and wildfire. This situation wild be described in some detail because it illustrates many features applicable elsewhere.
Coastal Plain pine region, southeastern U.S.A.
The soils of the southeastern coastal plain are mostly red and yellow podzolized lateritic types that have developed under the native pine and pine-hardwood forests. It is estimated that 90 percent of the virgin longleaf pine (Pinus palustris) forest burns over at least once every three or four years, mostly with surface fires.
The chemical composition of surface soils in the longleaf area seems to be slightly improved by frequent burning. Potash, calcium and other minerals are returned to the soil in readily available form. Repeated fires result in small but significant increases in soil nitrogen. The increase in nitrogen is attributed mainly to the decay of grass roots on burned areas that would otherwise grow up to brush (which has fewer roots than grass).
The physical properties of surface soils in the longleaf area are unfavorably affected by frequent burning, since burned-over soil is up to five times as hard and absorbs water less rapidly than soil on unburned areas. The organic debris and top 2 inches (5 centimeters) of mineral soil on unburned plots contain far more micro-fauna than similar layers on burned areas.
As to vegetation, annual winter burning of uncontrolled intensity has been found to retard the growth of longleaf pine saplings by about one-fifth in diameter and one-fourth in height. Longleaf seedling survival during a 10-year period was 43 percent on unburned ungrazed plots, and less than 5 percent on annually-burned plots. However, scrubby hardwoods encroached on unburned areas, while annual burning controlled this growth. On unburned tracts the brush and accumulated dead vegetation and pine needles smothered the herbs and grasses, while on areas burned annually, bluestem grasses (Andropogon spp.) and native legumes grew abundantly.
Wildfires, and even controlled annual burning, are obviously too damaging to the forest growth and the site. However, the trend under complete fire exclusion is toward a mixed pine-hardwood-brush association considered undesirable by foresters, range managers, and wildlife managers alike. Some degree of burning seems desirable in the silviculture of longleaf pine. As a matter of policy, prescribed burning is used primarily for silvicultural purposes and only secondarily to improve grazing.
Specific purposes for which fire may be used in the long leaf - slash pine forests are listed by Conarro as follows: preparing seedbeds or planting sites for pine, controlling insects and rodents, eliminating plant diseases and unwanted brush, reducing the fuel that wildfires might feed on, improving grazing, and maintaining favorable conditions for naval stores operations. Harper reports that burning the vegetative cover of the soil by surface fire increased the gum yields of longleaf pines about 4 percent during the succeeding year, while fires that killed needles decreased yield as much as 19 percent. Controlled or prescribed burning is also used to provide food and cover for upland game. Bickford and Curry have worked out detailed instructions for analyzing the need for using fire, and for treating individual burning units.
On ranges where the grasses become dry and innutritious when mature, burning may be used to remove the old growth so that cattle can graze on succulent green grass high in nutrients. In the bluestem type of Mississippi, Louisiana, and east Texas, frequent burning may not be necessary if grazing is close enough to prevent accumulation of excessive grass "rough". In the wiregrass (Aristida spp.) type of the southeastern United States prescribed burning about every third year appears essential if the range is to be grazed effectively.
In spring and early summer cattle spend most of their grazing time on freshly burned areas if any are available. Rates of stocking during early seasons should, therefore, be based primarily on the acreage of burned upland. From year to year it is also important to adjust cattle numbers to any reductions in forage that may occur as low density timber stands become more dense.
Mixed hardwood-conifer and fir forests
In many temperate rain forests the relation of fire to forests and grazing is quite different from that described for the southern pine area in the United States. In New Zealand, for example, when fires occur on the margins of the mixed broadleaved-conifer forests, "the scorched trees die and bracken and scrub encroach, creating vulnerable salients from which succeeding fires sweep further in". In North Island, where very steep slopes have been converted to improved pastures, fairly large areas have been overrun by brush, which cannot be controlled by burning. Similarly, in the Douglas fir (Pseudotsuga menziesii) region of the northwest United States successive fires result in dense stands of bracken (Pteridium aguilinum), creating a vicious cycle that tends to perpetuate the fern patches, and preventing the establishment of either forage plants or a new forest. There are indications, however, that carefully controlled burning may have a place in management of grazed ponderosa pine (Pinus ponderosa) forests of the Rocky Mountains. Its function would be to reduce wildfire hazard and possibly to thin dense stands of reproduction.
The drier areas, such as the Mediterranean type (sclerophyll brushland) and desert scrub, have important needs for grazing, water, and domestic fuel. Foresters have a direct interest in developing fire policy for such areas.
The Mediterranean type is one of the most widely distributed plant communities, yet is relatively small in total area. Shantz states:
"Throughout the world, this sclerophyll brushland is regarded by plant geographers as a fire-induced type... The same physiological and ecological requirements are found in this type whether it be in South Europe, North Africa, Asia Minor, Southern Australia, South Africa, Central Chile, or in California."
The winter rains produce a brush cover that becomes very inflammable during the dry summer season. Shantz thinks that broadcast burning is the main reason that a large area of California is now occupied by nearly useless chemise brush (Adenostoma fasciculatum). In Greece, repeated brush fires have caused entire loss of the soil on large areas.
Sampson summarized the forestry, grazing, watershed, and wildlife values involved in the California chaparral type. He concluded that haphazard burning should be supplanted by controlled burning restricted essentially to the better chaparral lands, and complemented with management methods that favor succession and stability of the herbaceous vegetation. He recommends:
1. consideration of expected benefits in relation to costs;
2. selecting the specific area to be burned and establishing adequate firebreaks;
3. burning late in the autumn if fire risk is high;
4. an adequate, experienced crew.
Desert scrub
On semi-desert ranges, dense stands of brush often reduce the value of the land for grazing. Pechanec and Stewart pointed out that wildfire in the sagebrush (Artemisia spp.) type of the intermountain region, in the western United States is often disastrous to range, livestock, and other property. Areas burned haphazardly may grow up to even denser stands of sagebrush, or the perennial grasses may be replaced by annual cheatgrass brome. However, "increased usability and greater grazing capacity of the range without damage to adjacent property are dependable results of planned burning."
The principal requisites for burning on grazed sagebrush range are that:
1. fires are controlled;2. soils are firm and slopes less that 30 percent;
3. sagebrush is dense and forms more than one-half the plant cover;
4. fire-resistant perennial grasses and fortes form more than 20 percent of the plant cover (if this condition cannot be met, burns should be made only where reseeding is practicable);
5. grazing can be properly managed after burning.
A further study showed that organic matter, nitrogen, and moisture equivalent are significantly reduced, but only temporarily, in the top half-inch (1.2 centimeters) of soil on heavily burned areas. Planned burning appears to benefit rhizomatous grasses and fortes and shrubs with strong sprouting habit, but severely injures non-sprouting shrubs, suffrutescent fortes, and some of the finer bunchgrasses.
A closely related condition is found in southern Argentina. Here, Davies noted the value of periodic burning with flame throwers to control the shrubby growth, which may make up as much as 88 percent of the Patagonian scrub and desert vegetation. He suggests selective burning to prevent damage to palatable shrubs.
This classification includes the tropical rain forest with abundant rainfall at all seasons as well as the humid tropical forests with dry winters. Many such areas around the world have a long history of burning, especially in connection with shifting cultivation. In fact, the practice of shifting cultivation and the attendant risk of burning and brush fires can easily destroy the tropical forest, particularly on steep slopes or in regions with a long dry season.
For Cambodia, Allouard lists burning for pasturage as a primary cause of fires in tropical forests. He recommends a program of land classification and protection, education, and research to promote forestry and meet the needs of the populace for grazing. A similar problem and program is proposed for forests in Venezuela. Aubréville expresses concern over the disappearance of the high tropical forests of central Africa Since the social progress of the native populations is linked with the development of their agriculture, he suggests restrictions on indiscriminate burning and land clearing. He advocates a long-term program on an international scale to study problems of African soils and forests, to develop new agricultural methods adapted to local needs, and to promote policies of forest protection and management.
A different situation is found in the southern highlands of Tanganyika, where the original rain forest is reported to have given way to open grassland through woodcutting and annual grass-burning over probably two or three centuries. This mountain grassland seems to be an unusually stable type, not easily affected by close grazing or burning. Burning during the dry season, possibly every second year, appears to maintain the tallest, densest grass growth and to suppress unwanted brush and weeds.
The gradual development of policy on controlled burning and grazing in India up to 1925 is described in detail by Stebbing. An early policy of complete protection gradually gave way to a recognition of the possible place of early burning in silviculture, first with teak, and later with chin
Banerji calls attention to the general fodder scarcity for the huge cattle population of India, with the inescapable overgrazing of many areas, damage to trees through too severe lopping for cattle feed, and the frequent over-use of fire. He reports that the present trend on reserve forests is initial closure to grazing and protection from fire to allow better grasses to reappear, followed by mowing and light grazing, then by rotational and deferred grazing. Controlled burning for grazing apparently has a place on selected areas. Banerji urges research and co-ordination of range management activities through national policy.
Bharucha and White emphasize the importance of grazing and fire in ecological succession and conclude that:
"the future agricultural development of India depends primarily upon proper integration of new systems of animal husbandry with the development of potential grazing and fodder resources".
Problems of grazing and burning in the tropical dry climates are similar in many phases to those in tropical humid climates. The main areas involved are the tropical savannas, scrub forests, and desert scrub of Africa, Central and South America, southern Asia and northern Australia. According to Roseveare, burning in South America may be for destruction of old herbage to provide fresh young growth, destruction of woodland and undergrowth to provide grazing or arable land, and destruction of pests and parasites. There are conflicting views on forest range burning and research is needed to assess the facts.
Beard states that all the tropical American savannas may be swept regularly by fires, but that the present vegetation is fire-resistant. He concludes that the herbaceous vegetation does not depend upon fire for its maintenance, and that the savanna is an edaphic climax usually on senile land forms subject to alternate waterlogging and desiccation. McCorkle, speaking of savanna burning in British Guiana, concludes that unrestricted burning should be checked, that occasional burning may have some merit in simplifying grazing use, but that correction will be difficult because the practice is deeply rooted in the life of the native Indians.
In Africa, there is agreement on the general trend toward impoverishment of forest and soil but disagreement as to the causative agent. Aubréville condemns fire for its effects on timber production and living conditions, and for its possible ultimate effect on precipitation. Jeffreys quotes native authorities on the need for and practice of burning, and attributes the degradation of plant cover and soil to exploitative cultivation rather than fire. Semple concludes that:
"clearly burning of grassland cannot be abolished or condemned as wholly destructive. If practiced with discrimination, it can be of great value in supplying livestock with more palatable feed".
He suggests that, where economically feasible, mowing be employed instead of burning.
The beneficial use of fire in tropical grass and grass-shrub types has reached a high degree of development in the veld of South Africa. Scott points out that basic principles of veld management must rest on a knowledge of the ecology of the particular veld type and on the seasonal development of the grasses. Management differs for sweet, mixed, and sour veld types, depending on the species present, their palatability to livestock at various stages of development, the encroachment of undesirable brush, and the response to time and frequency of burning. There is need to extend the principles of veld management to the extensive Bantu areas, but difficulties are shifting cultivation, ownership of livestock as a measure of wealth, communal ownership of land, poor distribution of water, and lack of fencing.
Livestock grazing measurably reduces the fire hazard by removing and breaking up potential fuel and by establishing trails through the forest. For example, Lemon found that moderate cattle grazing removed 44 percent of the herbage on an experimental range in southern Georgia, United States.
Grazing may also be employed for fire protection on fertilized and seeded firebreaks through the forest. Peevy reports that fertilized and seeded areas were grazed closely each year during a four-year experiment in central Louisiana, United States.
In grazing forest range as an aid in fire protection it is important to avoid localized overgrazing, with consequent damage to tree reproduction and soil compaction from trampling. Customary methods used to get even distribution of grazing are fencing, herding, water development, and salting on lightly grazed areas.
Means of obtaining more even distribution of grazing and methods of establishing and maintaining grazed firebreaks are promising fields of research that should lead to better integration of livestock with fire protection and forest management generally.
In most parts of the world, the problem of fire in relation to forest grazing is closely interwoven with other problems of land use and of social and economic betterment. Climate, plant cover and its reaction to fire, population, resources, livestock numbers, agriculture and social customs are so variable that the fire problem must be attacked country by country and area by area.
All countries and peoples have a common starting point: the haphazard or too frequent use of fire is condemned universally by those who have studied the subject carefully. The weight of evidence under all climatic conditions is that excessive use of fire, especially when accompanied by overgrazing or shifting cultivation, degrades both plant cover and soil. Accompanying ills are reduced productivity of forests and ranges. These detrimental effects, when carried to an extreme in densely populated areas, result in progressively inadequate nutrition of both livestock and humans, and ultimately in rural slums.
In many parts of the world, fire appears to have a definite place in forest and range management. The problem is to eliminate haphazard burning and to apply prescribed, controlled burning where it is needed.
The fire problem must be considered as only one phase of the whole complex effort to improve management and productivity of renewable natural resources. Policy on the use of fire in forest range management involves the:
1. need to curtail fires started in unwise attempt to improve forage conditions;2. possibility that grazing will aid in protecting forest range against destructive wildfire;
3. use of prescribed burning to improve forage and grazing conditions, consistent with timber values;
4. use of prescribed burning as a tool in silviculture and timber management;
5. integration of fire policy for grazing with policy and administration of other natural resources;
The development and application of a fire policy requires the following measures:
1. Surveys should be made to determine the extent and frequency of burning and the reasons for it.2. Land use must be planned to set up the needs and potentialities for arable land, timber products, watershed protection, and forage for both wildlife and domestic animals. Estimates must be made of the livestock required for work, meat, and fiber, and the feed requirements of such animals in the form of range forage, tree loppings, improved pastures, fodder, and feed crops. The land must be classified to meet these needs and possible modifications of agri-social custom and organization should be explored.
3. Research should be undertacken to work out the applied ecology of each broad plant type; the effects of fire on the vegetation and site; conditions under which fire should be excluded; need, time, frequency, and techniques of prescribed burning; and management after burning.
4. Demonstration areas and education should inform the land users of needed improvements in practice.
5. Action programs could be put into operation to protect against wildfire and to regulate the practice of controlled burning in improved land use.
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