by ARTHUR W. SAMPSON, Professor of Forestry Emeritus,
and ARNOLD M. SCHULTZ, Associate Specialist, School of Forestry, University of California
This article represents the third section of a paper which has been prepared at ¿he request of FAO. Earlier sections appeared in Unasylva Vol. 10, Nos. 1 and 3. The concluding section will appear in Vol. 11, No. 1. The whole series will then be made available under a separate cover.
CONTROLLED burning is the planned application and confinement of fire to a preselected area, usually on "wild lands." The actual burning may be done in a number of ways, varying from prescribed burns in which a narrow range of weather and fuel conditions are needed to accomplish a particular objective, to "convenience fires" in which the only predetermined elements are time and place of burn (31).1
1 The references to literature cited were given in Unasylva Vol. 10, No. 1. with the first part of this paper.
As a rule, fires burn better and kill more plants when temperatures are high, humidity low, and where there is some wind movement. At such times, hazards are great and control is difficult. The objective of controlled burning is to obtain as effective a burn as possible under the safest conditions.
Steps carried out in this method of brush control are locating the limits of the area to be burned, preparing the control lines, preparing the fuel, and conducting the burn.
Locating and preparing control lines
The area to be burned must be bounded by natural or artificially prepared barriers. These serve simultaneously as points from which the fire is ignited and as barriers to its uncontrolled spread. Whenever possible, natural barriers should be used as control lines; they are often udder and more effective, and their only expense involves taking a little more area into the burn or leaving some out. They can be classed as physical (recent burns, streams or roads, rock ledges), topographic (ridge tops, wide ravine bottoms), and fuel-type barriers (sparse vegetation, green grass, mesic brush types on north-facing slopes). The safest control lines are combinations of these, as a road running through green grass near a ridge top (4).
Artificially constructed control lines must take topography and fuel-type into consideration in order to stop surface, crown and spot fires. Surface fires are stopped by interrupting the continuity of the surface fuel. The line need not be wide but it should be bared to the mineral soil. A good rule to follow is to make the line at least as wide as the standing height of the fuel adjacent to it. Thus, in grass the fire line need not be as wide as in tall dense brush. Under conditions favorable for prescribed burning in forests, a narrow line through the needles made with a hand rake is adequate. This means that, in order to prepare lines as cheaply as possible, the boundary of the control burn should be dropped back into grass or cultivated land at the edge of the brush or under trees where there is no heavy debris or standing inrush in the understory.
Crown fires are stopped by clearing a wide space through the brush or tree canopy. The width of this space depends on the amount of heat developed at the edge of the burn, the direction and strength of the wind across the line, and the flammability of the fuel on the outside of the line.
Spot fires are difficult to stop but they can be reduced by avoiding the inclusion near the line of shrubs and trees which explode flaming leaves high into the air, and by locating the control lines where wind movement can be predicted most accurately.
The area to be burned should always lie up-slope from the control line (Figure 40). Lines should never be located part way up the slope or on ridgetops; rather they should be placed immediately over the ridge on the side opposite the burn.
The position of the control line must also be placed where ignition is easy. The ease of ignition, or firing, is inversely related to the size, moisture content, and sparsity of the fuel, and directly with the wind velocity, air temperature and dryness, and steepness of slope. The most important criterion for a good firing line is a light, dry, and continuous fuel for some distance inside the boundary and continuing into the brush to be burned. The firing can be done rapidly; there is little heat next to the line, and a particular segment of the line need not be held for any length of time. Examples of such fuels are dry grass, leaf litter, pine needles, and light brush which has been mashed down with a bulldozer.
Mechanical equipment which can be used to prepare fire lines has already been described. In heavy brush or flammable trees, the bulldozer is used to clear a path wide enough to meet safety specifications. The broken stems are pushed inside the line - never outside. If a considerable length of time is to elapse between the time of preparing the control line and the controlled burn, it is best to move all the bulldozed material far into the brush at intervals along the periphery Punky wood such as wood-rats' nests and rotten logs should also be pushed well inside of the line. Tall snags should be pushed over with a bulldozer as they burn for a long time, throwing sparks for considerable distances. If the brush stand is not too dense, a strip the width of one or two dozer blades can be mashed directly inside the control line to make firing easier.
Where the- control lines are laid out in grassland, a disc is used to provide a barrier of mineral soil. This may be done while the grass is green or when the soil has sufficient moisture for easy tilling. The same effect can be obtained by scraping the grass off with a bulldozer or road grader. Often an adequate line can be made with a shovel or hoe.
In pine needles a rake or McLeod fire tool, which is a combination hoe-and-rake, is a most useful device. Pine needles and duff should be removed to mineral soil and heaped up or spread on the inside (side to be burned) of the line. The same tool can later be used for igniting the fire - by dragging a rake-full of burning needles.
Another means of preparing fire lines combines mechanical or chemical methods with pre-burning. A strip of brush is mashed with a bulldozer or sprayed with a quick-acting desiccant. When the fuel moisture has been sufficiently reduced, it can be ignited with but little danger of burning the green, untreated brush adjacent to the strip (Figure 41). Pre-burning is also used to prepare wide lines running through grass, firing may be done kite in the evening when the humidity is high enough to insure safe burning. Pre-burning consumes the fine, dry fuel, precluding hazard later, but the fire may not be intense enough to kill brush plants.
Igniting and conducting the burn
Setting the fire around the periphery of an area to be burned in the conventional way is a procedure requiring care and patience. Since the entire operation is slow, no speedy or complicated devices are needed for ignition. A few simple methods are described briefly.
Throwing lighted matches into the dry fuel along the edge of the control line is a method which obviously requires no equipment. However, a light McLeod tool is handy and easy to carry; one of these should always be available in case of spot fires or fire over the line.
It can be used to "drag fire" by passing a flaming tuft of fuel slowly through the grass. An old tire casing partially filled with crank case oil aflame can be towed through the dry grass. The drip torch produces the same effect. This is a gallon can containing Diesel oil. It has a wick at the end of a long spout. The wick, saturated with the oil, is lit and when the can ix tilted, oil drips through the wick and is burning as it drops out on the fuel (Figure 42). While these rustic methods are useful in dry grass and pine needles, they are unsatisfactory wherever the fuel structure is not of low stature nor compact, ax ix found in a dense growth of standing brush. Here a portable, pneumatic flame thrower is necessary to create enough heat to ignite the loosely arranged stems (Figure 43). The flame thrower, like the drip torch, burns Diesel fuel. This ix held in the can under high pressure, and may be ejected ax a hot flame 20 feet or more. The oil is ignited as it passes through the nozzle over a burning wick.
Three general plans for firing have been used. The simplest and most common is edge firing (Figure 44, left). Fires are started along control lines and allowed to burn to the center. This procedure is the safest plan, especially in dense brush where there are no escape trails out to the control line. The entire area should be so situated that air movement will be into the burn as firing is done along the periphery. There must be enough dry fuel in a uniform, continuous pattern to carry the fire to all the undesirable plants. The area must be small or compact enough to be burned over during the short period of the day when temperature, humidity, and wind are fairly constant.
In "center" firing the first fires are started in the center of the area and allowed to spread until a large quantity of heat is generated (Figure 44, center). Then other fires are set near the periphery. These are drawn toward the center fire by in-draft of air, creating winds, up to 10 miles (16 km.) per hour from all sides. The center fires themselves are stabilized and rarely burn out toward the edge. This method is limited to small areas of level terrain and free from wind movement.
Strip firing can be used on slopes or where prevailing winds will cause fire to burn in one direction. First a narrow strip is burned by setting a line of fires a little way downslope from the leeward edge of the area (Figure 44, right). When that fire is burned out, another strip is started about 100 feet farther in or down-slope. Each successive strip can safely 'be wider than the last since the firebreak ahead is wider. This method is advantageous where an edge-set fire would be difficult to ignite or if allowed to burn briskly might burn back across the control line.
The foregoing discussion has described conventional ii broadcast "burning methods (Figure 45). Success of such fires is variable. Ordinarily a controlled burn is planned far in advance for a particular date. Control lines have been installed, a crew of men must be on hand (Figure 46), and grazing has been deferred so that there will be a heavier grass fuel to carry the fire. Slight changes in weather, too subtle to warrant calling off the burn for that day, may be effective enough so that only the grass is burned, thus wasting the whole year's efforts. Recent trends have been to mash the brush a year in advance so that the stems and leaves will be dry and compacted near the ground, and grass will have grown up through the dead limbs. Even on a relatively cool day, fires will consume most of this kind of fuel. This combination of mechanical treatment and fire is highly effective in many localities.
It has been found that in order to achieve a clean burn it is not necessary to mash all of the brush. The brush may be bulldozed in any of several different patterns, alternately mashing and leaving unmeshed strips, so that only one-half or one-third of the area is treated, thereby cutting down the cost proportionately.
A fourth firing plan, called "area ignition," may be used to burn brush fields thus prepared after the mashed strips have dried. It involves the distribution of a large number of individual fires over an area in a short space of time (4). The fires are so closely spaced that the heat radiated from one causes the others to burn more intensely, and the fuels in between are dried and heated so that they burn more rapidly. The many flames coalesce into one intense fire-storm whose strong indraft keeps the fire from running. In practice, this is effected by a crew of men (Figure 47), each equipped with a drip torch, quickly lighting fires as they move down the parallel aisles of mashed brush. They must try to keep abreast of each other so as not to endanger any one who may lag. If the crew travels at about 2 or 3 miles per hour (3.2 to 4.8 km/ph.), and the fuel is at the desirable moisture content, the blush will be ignited at the speed needed to make a safe and effective burn. The intensity of the fire can be controlled by increasing or slowing the rate of firing.
The heat generated by area ignition is intense enough to consume the green, standing brush between the mashed strips. The method can most safely be used during spring months when wood is moist and hard to ignite (Figure 48). It Call also be used to burn out wide control lines before the conventional broadcast burn is conducted.
Special equipment for ignition
The use of pyrotechnics in controlled burning is a new field of research. A number of devices have been tested for igniting fuels but as yet there is little to report concerning advantages of one method over another, their comparative effectiveness, or the differential in cost.
The devices include a variety of grenades which can be thrown by hand, shot out from a gun barrel, dropped from pianos, or strategically placed on the ground and fired by electrical circuits or by fast fuse (Figure 49). One method which has been tested in the chaparral of California consists of spacing grenades at intervals of 50 or 100 feet (15 to 30 m.); the grenades are nested in fuel accumulations made by cutting and piling brush and are fired simultaneously. In another method good results are obtained when grenades ale dropped from a slowly moving plane (90 m.p.h. or 144 km/ph.) flying at an altitude of about 500 feet (150 m.). When a continuous compact fuel is on the ground, such as dry grass, small or large and individual or clustered grenades are equally efficient in starting fires.
The advantage of using pyrotechnic devices is that it facilitates burning out wide strips of brush which are practically inaccessible on foot with flame throwers and other pieces of conventional equipment. This may be in areas of rugged terrain or in the center of controlled burns which have been fired along the edge. There is no danger of crews being trapped by fire when the grenades are laid in advance and connected by a long fuse. The simultaneousness of firing accomplishes the same effect as area ignition when the grenades are laid out in a suitable pattern. Hence this method may be used effectively for winter and spring burning to improve game ranges or for construction of wide fire control lines.
Prescribed burning in forests
Prescribed burning has been defined as the planned application and confinement of fire to an area to accomplish a particular objective. The chief objectives in forests are to:
1. reduce fire hazard by removing brush and woody debris;2. prevent the invasion of inferior species in the understory;
3. reduce growth stagnation in stands of dense reproduction;
4. prepare ash seedbeds for natural reproduction of desirable trees;
5. reduce the amount of unnecessary litter and vegetation that intercepts precipitation and renders it unavailable to tree growth and/or ground water supply;
6. reduce the incidence of fungal diseases and insect attacks;
7. improve range land for cattle grazing;
8. improve habitat for game and facilitate its harvest.
No special equipment is needed for such burning. The important requirements are favorable weather and fuel. Although the technique varies from place to place, the following principles are involved. Fires under conditions of low humidity and high temperatures may kill any tree. Wet or cold season burning when the fuel is dry and trees dormant is the safest practice. Many of the broadleaved shrubs and trees are thinbarked and easily killed by light fires; on the other hand many of the most valuable trees for lumber are remarkably resistant to fire; examples are longleaf, slash, and loblolly pine in southeastern United States and ponderosa and sugar pine in western United States. The fire-tolerant species are usually the dominants of sub-climax plant communities, having developed in an environment where fires were natural and frequent in occurrence (43).
The fuel to carry fire to the brush and debris may be grass, as in southeastern United States, or the needles of the forest trees. Fires should be kept low on the ground at all times and special preparation of the fuel structure will be necessary in some cases. Burning uphill may start crown fires. Snags, punky logs, and cat-face trees should be treated with proper caution.
Costs of controlled burning
Costs are variable from place to place depending on wages, availability of equipment and kind of vegetation, and from year to year, depending mainly on the progress in developing techniques that make a suitable compromise between safety and effectiveness of burns. The source of most available data on burning costs is in California, where burning of brushlands under permit was legalized in 1945 and where research and practice have made great strides since then.
Costs of burning under permit has two components: costs to the permitee or land owner and costs to the State or local government (34). Examples of these costs are as follows:
|
Costs to Permittee |
Costs to State |
|
1. Preparation of control lines. |
1. Time required by administrative personnel in technical advice and inspection of control. |
|
2. Other protective measures. |
2. Equipment and expenses for stand-by service to protect adjacent property. |
|
3. Labor, equipment, and food incident to execution of the burn. |
3. Time of stand-by crews. |
Permittee costs not considered are the loss of grass as fuel which could be used as forage, and the consequent rental of supplemental lands or purchase of additional stock feed to compensate for that sacrifice. Subsequent management costs, such as reseeding and fencing, are also not included. The state costs are not a contribution to the permittee, sense that a profit may accrue to his benefit. Rather, they are accounting charges stemming from the administration and regulation of the use of fire in range improvement for the protection of the public interest.
The two most important factors in determining the cost per acre are size of burn and extent of preliminary treatment. In size of burn the largest component of cost is in assuring safety, namely, constructing the control lines. In preliminary treatment the cost is for assuring effectiveness.
Figure 50 shows the relation between size of burn and cost per acre for controlled burns conducted in northern California in 1947 and 1948. For burns 640 acres (260 ha.) or less in size, total costs have been broken down into permittee and state costs. Although not included on the graph, costs per acre for representative burns of larger sizes have the following trends: for areas over 700 acres (283 ha.), costs decrease from $1.20 to about $0.40 until a size between 5,000 to 10,000 acres (2,024 to 4,047 ha.) is reached: beyond that point, costs per acre increase somewhat again. The initial drop in the curve results from the fact that there are certain basic costs which occur no matter how many acres are in the burn. Beyond 440 acres (178 ha.), larger equipment is needed and more elaborate protective measures must be provided, but on a cost per acre basis, the high point is reached at about 700 acres (283 ha.). The burns of big acreage again need larger equipment and more men, and different techniques must be employed to obtain safety.
TABLE 1. - FACTORS AFFECTING COSTS OF CONTROLLED BURNING
|
Brush condition |
Brush density |
Percentage of area to be mashed |
|
Young, brush growing vigorously, few dead twigs and leaves, litter on ground will not carry fire |
Open-brush crowns |
100 |
|
Medium - 50-80 % |
100 |
|
|
Dense - 80 % density |
100-50 |
|
|
Mature brush plants which have attained maximum height, visible dead twigs and branches in crown, litter on ground enough to carry fire under normal burning conditions in summer |
Open |
100 |
|
Medium |
50 |
|
|
Dense |
33 |
|
|
Over-mature plants with crowns beginning to thin, many dead twigs and branches, litter on ground will carry fire whenever it can be ignited |
Open |
100 |
|
Medium |
33 |
|
|
Dense |
25 |
From the information given in Table 1 the approximate relative costs for burning can be ascertained.
The amount of mechanical or chemical treatment needed prior to a burn is decided by the intensity and cleanness of burn desired. Edge firing around an area of standing brush may result in low brush kill percent ages and little consumption of fuel while railing or mashing of the brush before a clean burn can be assured (Figure 51). The proportion of area that may be treated is shown summarily in Figure 52. Experimental area ignition tests in chemise and mixed chaparral types, where the average diameter of main stems is 1 to 4 inches (2.5 to 10 cm.), indicate the following rough guide for brush mashing (14).
FIGURE: 52. Experimental photo for area ignition tests.
(a) 50 percent mashed - strips of mashed and unmashed of equal width
(b) 33 percent - unmashed strip two dozer-widths across;
(c) 25 percent unmashed strip 3 dozer-widths;
(d) 50 percent mashed in gridiron pattern;
(e) 100 percent mashed;
(f) untreated brush. (U.S. Forest Service)
The use of chemicals for brush control is not new. Such household materials as kerosene, table salt and arsenic have long been used to kill plants. Yet no large scale efforts were made to control brush and undesirable trees with chemicals until compounds were developed which were specific for this purpose. Chemical brush killers are now the basis of a great research effort and a large industry. Several factors are responsible for this recent progress: the demand for a safe, sure, and cheap method of eradicating undesirable plants; advanced knowledge of basic physiological activities in plants; and the profit motive of chemical manufacturers.
Advantages of chemical control are that complete kills of plants are obtained more easily than with mechanical grubbing methods, and that ordinarily few hazards are involved. Disadvantages are that much research or trial-and-error is needed to learn of the right chemical, the dosage, and the proper timing of application for each species, and that, after a plant is killed, the entire woody portion remains erect and intact. Moreover, the chemical frequently renders the wood less susceptible to rotting. Because the chemical method kills but does not remove the brush, combinations of either mechanical equipment or fire are generally also employed.
In order to understand why certain methods of application should be used it is important to know the action of the common herbicides. To be effective as a brush killer, a chemical should have the properties of being readily absorbed by plant tissues, being translocated within the plant, and killing the cells or tissues and eventually the plant. Of these, translocation is not an absolute necessity. A brief discussion of the physiological activity of some of the commonly used chemicals follows. Certain chemicals may fall under more than one of the categories of the classification.
Selective or hormone-type herbicides
These are so-called growth-regulating chemicals which are translocatable within the plant and act basically on the enzyme systems. Enzyme systems vary in chemical structure from one plant group to another, so that a particular hormone-like compound reacts specifically to the protoplasm of a certain genus, species, or perhaps a strain. This is the basis of the selectivity. In a heterogeneous brush stand, composed of a dozen or more species, the herbicide will affect the growth rate or respiration rate of some shrubs, but may have minor or no effects on others. This is an advantage only where the undesirable species are susceptible, but where certain components are to be maintained for browse.
Many herbicides of the hormone-type have been formulated but only two are well known - 2,4-D and 2,4,5-T. They have been widely used on brush species, and with varying success. In order to reduce the selective action, the two chemicals are frequently mixed, thus being effective on a greater number of species.
Either 2,4-D or 2,4,5-T can be applied in the ester, amine salt, or acid form. Of these, the esters are most soluble in oil and because of this they are the most readily absorbed by the oily or waxy cuticles which are common to leaves of many brush plants, especially those of the Mediterranean climates. Next to the esters, free acid formulations are absorbed better by leaves than are the water-soluble amine salts. However, once the chemical is through the epidermis, it must be dissolved in the water of the mesophyll in order to enter the phloem and take action in the meristematic cells. Now the amine form functions best, while the esters are least readily translocated. Thus it can be seen how the nature of the cuticle and leaf tissues within a species can determine whether esters or amines of 2,4-D or 2,4,5-T will be the more effective chemical to use.
One of the serious disadvantages of using ester forms of either 2,4-D or 2,4,5-T is the high volatility. Evaporation of the chemical from the sprayed foliage may occur for some time after spraying, and may affect neighboring vegetation. This is not to be confused with the "drift" that occurs during the act of spraying, which may be as bad with acids or amines as with esters.
2,4-D and 2,4,5-T have several advantages over many of the other chemicals to be discussed. These include the following: they are not poisonous to livestock; they are non-corrosive to metal parts of equipment; a relatively small amount of undiluted chemical is needed - usually about 1 to 4 pounds per acre (1.13 to 4.5 kg. per ha.) and they are effective under a wide range of volumes making them equally adapted to airplane, ground-rig, or hand application. Since grasses are not affected, these selective compounds are popularly used for brush control on range lands.
Disadvantages are the close dependence of effectiveness on concentration, growth stage of plant, weather and soil conditions, and the tendency to drift. Either aircraft or ground-rig application may result in a spreading of particles for miles around, culminating in the loss of valuable crops. Aircraft application is bad even on still days due to the turbulence set up by the propeller.
Non-selective contact herbicides
There are many water-soluble chemicals which are translocated but not selective. They can be used either as foliage sprays or applied to cut surfaces. The better known of this group are ammate (ammonium sulphamate) and sodium arsenite, although a number of chlorate and thyocyanate compounds have the same action. Being water soluble, they can be translocated to various organs of the plant when they reach phloem elements. Plant cells which have prolonged contact with the compounds are killed. The effect is about the same on the protoplasm of any plant but rate of translocation, the dosage needed, and time of greatest susceptibility differ enough between species that kills are not uniformly good.
The manner in which the cells are killed varies with the chemical: ammate tends to precipitate the proteins in the protoplasm, arsenic breaks down the structure of the nucleus. whereas most of the salts have a plasmolytic action. In common, these reactions disrupt normal water relations in the cells and tissues. These compounds remain soluble in the plant so they are carried through the vascular system to the various organs, thus killing the entire plant.
When used as foliage sprays, aqueous solutions are applied with machinery as in 2,4-D. Such solutions may also be applied to the cut surfaces of stems; or the crystals themselves may be poured into the frills or notches at the base of trees.
Neither ammate or sodium arsenite are volatile, so they can be used safely in respect to adjacent crops. The arsenite, however, is highly poisonous to animals and should be used with caution. Also, wood from trees poisoned with sodium arsenite should not be used as fuel, because the smoke may be poisonous. Both ammate and the arsenite are extremely corrosive to equipment and in general are more costly to apply than the hormone-type sprays. Ammate has proved effective on a greater number of brush and tree species than either 2,4-D or 2,4,5-T.
Contact herbicides which are not translocated
These include the various oils which are used as herbicides. Most oils have some harmful effects on plants. Oils will spread over the leaves in a film by reason of their low surface tension and penetrate the cuticle. They are not mobile in the vascular systems, yet they penetrate deeply into the plant by diffusion through the lipoid phase of tissues. Oils are soluble in the lipoids of the cell walls. Saturation of the cell walls with oil prevents normal gaseous exchanges, hence the cell is smothered. Besides this lethal effect, oils have a certain amount of toxicity which depends on their content of unsaturated compounds.
Two types of toxicity are recognized: acute, which is a rapid burning of leaves; and chronic, in which the symptoms are chlorosis and are slow in appearing. Chronic toxicity alone is seldom observed among brush plants. Light compounds, such as gasoline, will burn the leaves rapidly but the injury is usually slight because most of the oil evaporates before the tissues can be saturated, unless an excessive volume is used. Acute toxicity is obtained by application of the heavier, aromatic hydrocarbons. Weed oils and diesel oil are examples. The oils most commonly used for brush control are diesel oil, stove oil, kerosene distillate, and weed oil. Diesel oil is low in cost, safe to handle, does not corrode equipment, and induces both acute and chronic toxicity.
The toxicity of oils can be increased by adding small quantities of such fortifying agents as sulphur, pentachlorophenal, or dinitrocresol which rupture the cells after being carried through the cuticle by the oil. Such solutions are used as desiccants, that is, they are used to reduce the moisture content of foliage rapidly so that it can be ignited soon after treatment. In California chaparral, a 1 percent solution of pentachlorophenol in diesel oil, applied at 50 gallons per acre (472 liters per ha.) reduced the water content of standing brush perceptibly in two hours and by 50 percent within a day.
Soil sterilants
These compounds not only kill all plants present on a treated area but prevent any other vegetation from growing for a considerable period of time. Soil sterilization is used to kill and keep out brush along power and fence lines, irrigation ditches, roadsides, and fire control lines. It is not a practical method for range improvement or forest management.
Soil sterilants are applied to the soil "broadcast" over an area or just around the base of the undesirable plants. They are carried into the soil by rain and remain there until either absorbed by plants, leached down by more rain, or broken down structurally by chemical or bacterial action. The chemicals may be absorbed by soil colloids and thus become unavailable to plants. After being taken up in solution by the plants, the cells contacted are broken down structurally.
Examples of permanent soil sterilants are: sodium arsenite, arsenic trioxide, borate compounds, and CMU [3- (parachlorophenyl) -1,1-dimethyl urea]. With several exceptions, these result in complete destruction of all plant growth. Sodium arsenite and the borates will leach out of sandy soil with excessive flooding, and certain plants are resistant to arsenic trioxide.
Examples of temporary sterilants are chloropicrin, carbon disulphide, and methyl bromide. Either 2,4-D, 2,4,5-T, or ammate can be used as a soil sterilant. Since the use of such compounds is not great in large scale brush control work, the relative advantages and disadvantages of these different compounds are not here listed.
The basic requirements of soil sterilizing chemicals are that they must remain in the root zone of the soil in an absorbable (soluble) form, and in sufficient concentration at the time the plant roots are actively absorbing (13).
Diluting agents and additives
Diluting agents or "carriers" make possible a uniform distribution of the highly concentrated chemicals while the additives increase their effectiveness.
Water is the most commonly used diluting agent for foliage sprays chiefly because it is cheap. Diesel oil is used as a carrier for basal sprays, but for foliage sprays it serves both as carrier and in smaller amounts as an additive to increase penetration. When used with translocated herbicides, only small proportions of oil can be used, for otherwise the leaves will burn and the movement of the chemical into the tissues will be retarded.
Additives include sticker-spreaders and emulsifiers. Sticker-spreaders are ionic or neutral wetting agents which improve adherence to the plant surfaces and reduce the surface tension of the carrier. Emulsifiers keep the oily materials in suspension in water while spraying. Aqueous solutions of ammate and amine salts of 2,4-D and 2,4,5-T usually require an emulsifier and sticker-spreader but the ester formulations generally have adequate amounts.
Four basic methods are used to treat trees and shrubs with chemicals: foliage sprays, basal sprays, cut surface applications, and soil saturation. The latter has been discussed under the heading of soil sterilants.
Foliage spray
This method involves complete covering of the leaves of shrubs or trees with finely divided particles of the herbicide. The method is most commonly used in broadcast application where all plants are sprayed, desirable as well as undesirable. For this reason the selective herbicides, which, for example, may not kill grass, are especially conducive to application by sprays.
Chemicals can be sprayed in high or low volume. High volume refers to a greatly diluted form of chemical which thoroughly wets the vegetation. Quantities of 100 gallons per acre (945 liters per ha.) of chemical plus carrier are commonly used for spraying brush with ground-rig power sprayers. Low volume refers to a more highly concentrated form in which quantities applied per acre seldom are more than 10 gallons (38 liters).
Low volume application is done by airplane or helicopter where cost of transporting large amounts of liquid is prohibitive.
Equipment with which spraying is done consists essentially of these items: pump, tank, agitator, boom, nozzle, and a source of power to operate the pump and move the equipment over the brush.
The simplest types of sprayers are the portable compressed-air tank sprayer and the constant-pressure knapsack sprayer. These are pumped and carried by hand. A simple air-displacement pump, mounted inside a 3 to 4 gallon (11.3 to 15 liters) cylindrical tank will provide pressure of about 80 pounds per square inch (5.6 kg. per cm2). This forces the liquid out through the nozzle. The pressure is reduced as the liquid level goes down, and then the tank has to be pumped up again. In the knapsack type, plunger or diaphragm pumps are equipped with air chambers which keep the pressure constant no matter how much liquid is left. Hand pumping is continuous.
The number of variations in power sprayers is almost infinite but it will suffice to diagram a standard model and discuss the essential parts briefly.
Figure 53 is a diagram of a sprayer with a positive displacement pump. It is equipped with a by-pass valve or pressure regulator and a diaphragm type pressure reducing valve. The by-pass valve lowers the pressure to 100 pounds per square inch (7 kg. per cm2) or less, which is adequate for spraying brush. The tank can be back-filled by attaching a suction hose to the refill valve, closing valve "A" and opening valve "B".
The necessary pump capacity is determined by the maximum amount of liquid to be discharged from the nozzles. The following equation can be used to calculate the value (1): rate of feet per boom length gallons travel × mile × in feet × per
Thus, with a boom 25 feet long (7.6 m.), traveling at 5 miles per hour (8 km/ph.), the pump capacity must be 25 gallons (95 liters) per minute if it is desired to apply 100 gallons per acre.
The size of the tank to be used depends on the capacity of the pump, on the volume or concentration of the liquid, and on the proximity to water or other diluting agent supply. Ground rigs can have large tanks - from 300 to 1,000 gallons (1,134 to 3,785 liters) - but the tanks on fixed wing airplanes are seldom over 150 gallons (567 liters) in capacity and for helicopters 60 to 80 gallons (227 to 302 liters). The tank can be filled by using the pump as described above. Metal tanks are easier to keep clean and are less likely to leak than wooden ones, but corrosion is a problem when ammate and arsenic compounds are used. If it is necessary to use dirty water as a diluting agent, strainers should be inserted between the tank and refill valve and also ahead of the nozzles, as shown in Figure 53.
Agitators are necessary for all mixed sprays. Stirring should be constant where soil emulsions and heavy suspensions are used, but only occasional agitation is necessary in the tank when aqueous solutions of 2,4-D, 2,4,5-T, or ammate are used. A mechanical agitator consisting of a series of paddles is mounted on a revolving shaft which runs through the bottom of the sprayer tank. It can be geared to the pump engine. Hydraulic agitation can be installed by recirculating the excess flow of liquid from the pump through a boom-like pipe laid near the bottom of the tank. This works best with a centrifugal pump (Figure 54).
Booms are made of pipes ranging from 3/4 inch to 2 inches (1.9-5 cm.) in diameter. Smaller pipe is unsatisfactory because the resistance to liquid flow lowers pressure at the nozzles during large volume spraying. The larger pipe is stronger and less apt to whip around when projected out over the brush. Also, it is easier to cut holes in larger pipe to insert nozzles. The length of the boom is governed by the size of the tank and the pump. The length needed to cover an area in a given amount of time can be calculated as follows:
To compute the necessary boom length to cover 500 acres (202 ha.) in 10 eight-hour days with a rig which will average 4 miles per hour (6.4 km/ph.), the values are substituted as follows:
Two types of nozzles are used: those giving a flat, fan-shaped spray and those giving a hollow, cone-shaped spray. The flat spray gives the more uniform coverage and a more forceful application. A minimum of 10 to 25 pounds per square inch (0.7-1.8 kg. per cm2) of pressure is needed to make the liquid fan out from the nozzle. A wider fan and finer droplets results with increasing the nozzle pressure. Nozzles which produce the conical spray do not plug as easily and are less likely to fog at low rates of discharge. The width of the fan, spacing of the nozzles, and height the boom determine the uniformity of coverage (Figure 55). Discharge in gallons per acre can be computed of from the following equation:
The value 5,940 is a constant derived from the equation:
For aircraft, the effective swath width is greater than the length of the boom. If the discharge rate of the nozzles is known, gallons per acre can be computed from this equation:
The power needed to run the pump can be calculated when the discharge in gallons per minute is decided upon and the efficiency of the pump is known. The following equation is helpful in finding the approximate power needed to drive a pump:
Pumps vary from 20 to 80 percent efficiency; if a low value is used, the calculations will lead to an engine that will have ample power.
The engines can be separate from the one that drives the rig or the same in which case the belt pulley or power take-off is connected to the pump. Pumps for aircraft sprayers can be powered by the hydraulic motors which operate the flaps or landing gear, wind-driven propellers, accessory drive pads from the plane's engine, or by separate electric motors.
The various types of mountings that can be made either on ground rigs or aircraft are too numerous to discuss here, but some are shown in Figures 56, 57, 58, 59, and 60.
Aircraft spraying of brush foliage has certain advantages over ground spraying. Large acreages can be covered cheaply and rapidly. A considerable portion of the cost of air spraying involves locating a landing strip near the area and getting the plane to it. This cost is the same whether the acreage is large or small, and consequently, the cost per acre for aircraft spraying is high when few acres are being treated. On small areas, ground application is generally cheaper. Aircraft can be used when the ground is wet or covered with snow. It is possible to cover fully, but not necessarily uniformly, the vegetation growing on rugged terrain. Chemicals can be applied in low volume, and for many species this is a decided advantage.
Spraying from helicopter can be done much closer to the ground than by fixed-wing aircraft. On rough terrain, the helicopter is able to follow crooked contour lines, so that constant distance above the ground surface will give more uniform spray distribution. Scattered brush patches or clumps of undesirable trees can be pin-pointed at slow speed from low altitudes, obviating the spread of chemicals over the non-susceptible vegetation between. The downwash from the craft's rotor results in a turbulent action which is especially advantageous in foliage of medium or high density. The spray is forced downward violently; then there is a sharp uplift. This action coats both under and upper sides of leaves, and distributes the chemical throughout the depth of the foliar stratum. Another advantage of the helicopter is the small space needed for take-off and landing; seldom is it impossible to locate a suitable helicopter port somewhere within or adjacent to the brush field being sprayed. However, its pay load is smaller than that of airplanes, thus increasing the cost of operations. The helicopter is not effective in mountains higher than 6,000 feet (1,830 m.) above sea level.
The chief disadvantage in using aircraft to spray brushland is the probability of drift due to air currents at flight altitude. Besides damage to neighboring crops, it results in poor aerial distribution of liquid on the foliage.
Aircraft spraying is hazardous to pilots. Much of the land covered by brush today is characterized by steep slopes and canyons. These are extremely dangerous to fly at altitudes which provide most effective dispersal of chemicals.
Ground rigs are most useful where high volume sprays can be employed, where the soil surface is level and firm enough for wheeled machinery, and where the brush is not too dense to impede travel of the implement. Hand sprayers are useful on smaller tracts, on terrain too rough for wheeled machinery, and in forests where only a small proportion of the plants is to be eradicated.
Basal spray
This method involves the application of chemicals in a concentrated form on the lower portions of the stems of woody plants. The herbicide can be poured out of a can or painted on with a brush. When done in this way, waste is not excessive and the chemical can be used at full strength. When a sprayer is used, nozzles giving a fan-shaped spray are best. There is apt to be waste so some dilution is recommended However, it is desirable to apply enough liquid so there is runoff down the stem into the soil.
Any of the translocated oil-soluble herbicides can be used for basal spraying. They are especially effective when light diesel oil or kerosene is used as carrier to penetrate through the bark to the phloem. Small trees, 3 inches (7.6 cm.) or less in diameter, are readily killed by this method because of their thinner bark. Oils alone are also useful for basal treatment. They are most successful in sprouting species. The oil is allowed to run down the stem to the bud zone. Both the crown and the immediately adjacent soil is saturated; the oil penetrates and kills the meristematic tissues in the buds, thus preventing sprouting (Figure 61).
Cut-surface applications
The application of chemicals on the cut-surfaces of stems kills top growth and prevents sprouting in the notching and frilling methods and prevents crown or stump sprouting in the stump treatments. The techniques involved are relatively simple. Ammate, sodium arsenite, and amine or ester forms of 2,4-D and 2,4,5-T are most frequently utilized in the cut-surface methods.
Notch method (Figure 62). Notches or cups are cut through the bark of shrubs or trees at intervals around the base. A satisfactory notch can be made with two strokes of an axe, the lowermost cut being downward and extending into the sapwood or at least to the cambium so that the phloem vessels are exposed on the upper surface. Since there are no suberized layers to be penetrated, oil solutions are not necessary. Water soluble chemicals of the translocated type, either in solution or as crystals, can be poured into the cup with a can or spoon. The pump-type engineer's oilcan with a long spout works well or some other can-and-spout device can be improvised for this purpose. The number of notches needed depends on the species and diameter of the stem, as does the concentration and absolute quantity of chemical per notch.
Frill method (Figure 63). A frill is essentially a series of contiguous notches around the base of a stem. Frilling need not result in a clean, wide cut through the cambium as does girdling but should provide a convenient receptacle around the circumference of the stem in which to place the chemical. There is little if any lateral movement of water solutions, consequently the notch method kills certain segments of the plant aligned vertically with the notches, whereas those aligned with the interspaces may sprout. This effect is minimized with frills. The frill method is best for larger trees, those 12 inches or more in diameter at the base. Naturally frilling a tree takes more time than to notch it, and more chemical is used, but these disadvantages are compensated by the higher percentage of kill.
Cornell tool (Figure 64). This tool consists of a 5-foot (1.5 m.) length of pipe in two sections. The upper part is 4 feet (1.2 m.) long and is the reservoir for the chemical; the lower foot is fashioned into a cutting spud with a concave edge. Inserted between the two sections is a brass cylinder with a 1/2-inch (125 mm.) bore which serves as a valve-block and dosage meter.
FIGURE 65. Pouring ammate crystals on V-cut stump to prevent sprouting. (U.S. Forest Service)
Two valves, attached to an 1/8-inch (32 mm.) wire stem that passes through the cylinder and then up through the reservoir, are so spaced that when the upper valve is seated, the lower valve is open, and vice versa. In operation, a downward thrust of the cutting tool is followed by a quick pull and push of the stem rod which allows a measured amount of liquid to flow into the cut made in the tree. This model was designed at Cornell University (9). Other "tree injectors" are based on the same principle but have the trip valve operated automatically by the impact of the tool into the tree.
FIGURE 64. Diagram of the Cornell tool. (After Cope and Spaeth (9))
Stump treatments. Stumps of sprouting species can he treated in any one of three ways: by basal spray, by pouring the poison into a notch or frill around the base, or by applying the poison to the top or cut surface. The chemical can be applied with a spoon (Figure 65) or with a paint brush. The solution should be daubed on until it nearly runs off. If the stumps are cut low and square, most of the liquid will take effect. Small stems can be cut to give the stump a V-shaped surface. The groove can be filled with crystals or painted with solution.
Chemicals are most effectively used along with some other method of brush control, such as mechanical treatment, fire, or grazing management. The reason for this is that brush species in mixed stands are not equally susceptible to chemicals and that the dead stems remain standing for some time. Following successful kills by chemicals, railing, cabling, or bulldozing can be used to break up the stems. Fire is frequently used either before or after spraying. Sprouts and brush seedlings are usually more susceptible to hormone chemicals than mature plants of the same species; thus controlled burning followed by foliage spraying is an effective brush control measure. For burning in the spring when the moisture content of the plants is high, the use of quick-acting contact sprays as desiccants, is a worthwhile preparatory measure:
The possibility of using selective foliage sprays in conjunction with intensive browsing has not been fully explored. Hormone-type herbicides which react better when a larger leaf surface is exposed will, therefore, kill those plants in a mixed stand of brush that are least browsed, that is, those least preferred by stock. This selective action favors the desirable browse plants.
Biological vegetal control
Brush clearing by so-called "biological" means is essentially concerned with destruction of the sprouts and seedlings of woody plants after the cover has been burned or worked-over mechanically. Goats, and to a lesser extent deer, are the most effective animal for killing thinned stands, such as interior live oak of California and certain other brush plants (32).
When palatable sprouts begin to appear, after the clearing operation, enough goats should be placed on the area to keep the shoots and seedlings browsed down (Figure 66). The animals are restricted to relatively small areas by fencing. Areas having abundant may require three goats to the acre, and others only two goats during the entire grazing season. As the vegetation gradually changes from brush to grass the acreage per animal is increased to maintain the goats in fairly good condition. The area must be stocked to the full capacity of the browse, usually for three to five years; otherwise only a portion of the sprouts will be destroyed. As a rule goats will show at least a small profit when handled as described. Similar results can be obtained in many instances with sheep and cattle (Figure 67).
On small burned areas - 4 to 5 acres in size - deer will destroy sprouts and seedlings of chemise, oaks, and other palatable woody plants (Figure 68). Larger burns fail to result in clearing because the browsing is not concentrated enough.
Another effective means of controlling brush seedlings is by planting burned areas with aggressive forage plants. Studies have shown that in dense grass brush seedlings failed to extend their roots into zones of available late summer water (36). In contrast, brush seedlings growing where there was no competing vegetation sent abundant roots into the zone of late summer water (Figure 69). In addition, the grass cover stabilizes the soil, supplies forage, and provides fuel if additional clearing by fire is desired.
Biological control of vegetation by diseases and insects is a natural phenomenon that often thins native stands of various noxious plants, including brush species. These depredations, however, seldom result in lasting control. On the other hand, biological control offers great possibilities with introduced noxious plants because their natural predators may be introduced unaccompanied by their own natural diseases or parasites. In Australia, remarkable success was achieved in controlling introduced species of cacti, and more recently in California and Oregon with St.-Johns-wort by beetles that fed exclusively on these plants.
FORTHCOMING MEETINGS, 1957
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21 January to 5 February |
Geneva |
International Consultation on Insulation Board, Hardboard and Particle Board |
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March |
Indonesia |
Asia-Pacific Forestry Commission (4th Session) and Teak Subcommission (2nd Session) |
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22 April to 1 May |
Paris |
International Poplar Commission (9th Session) and Poplar Congress |
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7 to 14 May |
Rome |
European Forestry Commission (9th Session) |
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3 to 19 June |
Moscow |
FAO/ECE Joint Committee on Forest Working Techniques and Training of Forest Workers (2nd Session) |
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9 to 20 July |
Guatemala |
Latin-American Forestry Commission (8th Session) |
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28 September to 10 October |
Iraq |
Near East Forestry Commission (2nd Session) |
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November |
Rome |
FAO Conference (9th Session) |
