by ARTHUR W. SAMPSON, Professor of Forestry Emeritus, and ARNOLD M. SCHULTZ, Associate Specialist, School of Forestry, University of California
This article represents the first section of a paper on equipment for control of brush and undesirable trees which has been prepared at the request of FAO. The further sections of the paper will appear in succeeding issues of Unasylva. The whole series, when completed, will be made available under a separate cover.
MUCH of the earth's surface is covered with brush. It is difficult to estimate accurately the area which is at present dominated by scrubby vegetation. Many of the major brushlands of the world are climax, that is, they constitute vegetative cover that has changed little in composition under existing conditions of climate, soil, and fire. Other brush associations are subclimax such, for example, as stands that occupy a forest site for a relatively short time after a fire or logging operation, but are suppressed when the forest becomes re-established. Still other brushlands are a result of invasions on lands which may never before have supported brush. By slow incipient migration into sites of good quality and by rapid aggregation, often favored by cultural, climatic, and weather change, such brush stands become the most important from an economic standpoint even though they may be far less abundant than the climax brush areas.
A partial estimate of the world's brushlands may be made from statistics provided by FAO in World Forest Resources (17).1 In this compilation, many of the 190 countries and territories listing their land resources failed to separate brush land from deserts, rock and sand, and swamps and bogs. However, this study points out the prevailing attitude of considering brushland as wasteland with little potential for agricultural production. About three million square miles of brushlands are tabulated - just a little smaller than the area of Australia. This does not include the brushlands of the United States, South America (except Venezuela), U.S.S.R., Australia, and most of North Africa.
1 The figures in parentheses refer to literature cited on pages 28 and 29.
In the United States alone, upwards of 500,000 square miles (129,500,000 ha.) of land are dominated by brush. This includes sagebrush, mesquite, salt desert shrub, chaparral, oak, and pinyon-juniper woodland. Many other square miles of forest, mountains, and grassland are infested with brush although it is not the dominant vegetation. Texas alone has 55 million acres (22,258,500 ha.) of mesquite, some 18 million acres (7,284,600 ha.) of juniper woodland, and over 20 million acres (8,094,000 ha.) of scrub oaks; Arizona and New Mexico each have about 10 million acres (4,047,000 ha.) of mesquite and about as many acres of pinyon-juniper woodland. Sagebrush occupies 96 million acres (38,851,200 ha.) of land between the Rocky Mountains and the Sierra Nevada. In California there are 19 million acres (7,689,300 ha.) of chaparral and woodland and 2.5 million acres (1,011,750 ha.) of commercial forest land that is producing little more than brush. No doubt similar figures could be quoted for other countries.
Far from all of the world's brushy growth is lacking in economic value. In many localities brush serves as important food for livestock. In areas set aside primarily for grazing, the aim should be to maintain a proper balance of brush and grass. Semple (38) estimates that on the savannas fully as many domestic grazing animals and big game obtain their sustenance from browse of shrubs and trees as from grass. Included in this estimate are the shrublands of Africa, the maquis of the Mediterranean and the Near East, the heather moors of northeast Europe, the thornwald of South Africa, and the various brushes and trees of Australia and western United States.
Brush clearing and conversion to grassland, forest, or cropland is going on, though slowly, largely because of lack of suitable machinery, low quality of much of the land, excessive costs compared to low returns, and to insufficient knowledge of the ecology of the brush species involved.
Most of the brush clearing acreage is being done for range improvement. The vast expanses of range lands at the lower and intermediate altitudes lend themselves to massive clearing operations with heavy machinery. Concomitant with the removal of the brushy cover is the restoration of native herbaceous vegetation or the artificial seeding to grasses and legumes. Many of the mechanical devices for clearing brush simultaneously prepare a good seedbed for grasses. Besides providing more forage, other benefits of brush clearing are that the forage becomes more accessible to grazing animals, handling of livestock is easier, fewer lambs and calves are lost by straying, fleeces do not become snagged, seed sources of noxious brush species are restricted, and in many instances accelerated erosion is reduced when a grass cover has been established. Fire and chemicals are easy to use on rangelands because the woody plants are generally susceptible to these killing agents whereas most grasses are not. Large-scale or "broadcast" methods of application are consequently effective.
Brush control in forests is practiced to release crop trees for faster growth, reduce fire hazard, encourage natural reproduction or to facilitate planting and seeding. However, heavy tractor-powered machinery is difficult to use in forests where the terrain is rugged and where desirable trees and seedlings are closely spaced. Smaller equipment is available for such conditions and not infrequently portable hand-tools are the most effective. Fire can be used to advantage under special conditions but with the present state of knowledge concerning its use it is hazardous. Chemicals also are effective but must be applied to individual plants, instead of broadcast foliar sprays, so that the desirable trees will not be affected.
Brush fields on level and fertile soils and on areas close to population centers are being cleared for agriculture, notably for nut and fruit orchards, grain and fibre crops, irrigated pastures and hay meadows. For such intensive use, the clearing job needs to be thorough. All stumps are removed and roots are dug out of the soil as these may later become hazardous obstacles to lightly built farm machinery.
Brushland clearing for cultural developments, such as rights-of-way for roads and power lines, irrigation ditches and drainage systems, buildings, and parks, is carried out with essentially the game kind of equipment but cost-benefit relationships are entirely different than for agricultural brush control work.
For the purposes of this paper no distinction is here made between shrubs and undesirable trees since the same principles apply, the main difference being chiefly a matter of size. Consequently, the terms brush and brushland control, unless otherwise defined, will include shrubs and/or trees. Control of weedy herbaceous plants is excluded from discussion as broad generalizations on this phase can be misleading. Development of equipment to eradicate crop weeds and other herbs has been going on for a long time. There is a vast literature on the subject. New developments of herbicides and spraying equipment, for example, have gained considerable momentum since the second world war, and a complete discussion of them today would be obsolete within a year.
Although the major emphasis in this paper is on equipment, certain non-mechanical procedures are discussed in order to round out the topic of brush control. The roles played by fire, chemicals, grazing management and certain biological factors are treated along with methods of mechanical clearing.
The method of brush control and the equipment to be used depends on the objectives of the control work, the expense that can be put into the operation, and the benefits that may be expected. The choice of method and implement also depends directly upon three physical factors; the kind of vegetation, the topography, and the type of soil.
Objectives of control work
A few examples will serve to show how the objectives of the control work influence the method and equipment used, irrespective of cost, vegetative type, terrain, and other factors. Additional examples are incorporated along with the descriptions of equipment.
Where the brush has little value as browse, it might be desired primarily to provide maximum crops of herbaceous forage for livestock. This involves eradication of the brush cover to favor grass establishment. The best machine to use is one that effectively clears off the brush and at the same time prepares a good seed bed for the grass (Figure 1).
Where it is desired to clear off the brush, the stems and leaves of the plants - the transpiring surface - are severed from the roots, but all roots are intentionally left in the ground to avoid disturbance of the surface soil except to the extent of providing a seedbed for a grass cover. A combination of mashing with a bulldozer to break off the brush tops, and burning to provide an ash seedbed may be used for this purpose, or perhaps any of the various brush chopping machines which leave a heavy layer of mulch on the soil surface may be employed.
The objective might be fire hazard reduction. In well-stocked forest stands brush, dead trees, and slash can be removed with axes, chain saws, or portable circular saws. Small bulldozers (20 h.p.) can be used in the clearings (Figure 2). Frequently decadent brush plants can be pushed out by hand. Combined with prescribed burning during the wet season, the readily combustible material may be reduced, at least in some localities, with little soil disturbance or damage to timber (42). Or the objective might be the release of suppressed trees. In such cases ammate or hormone-type chemicals poured in frills around the base of woody plants, or girdling, and removal later with saw or axe, are effective methods. The first two are best because they prevent sprouting and thereby conserve soil moisture and nutrients.
Improvement of the habitat for game may be accomplished by localized control of the brush. Manipulation of brush covers for browsing animals involves partial clearing and conversion to herbaceous vegetation in certain portions of the range. Frequently repeated removal of the mature lignified top-growth is necessary to insure a constant supply of young, succulent sprouts for browse, and protection from wild-fires or other damage to the cover needed by the animals (7). Wildlife management practices may demand the planting and raising of shrubs where suitable browse is scarce. Bulldozing, burning, discing, reseeding, and selective chemical sprays, alone or in various combinations, have been used limitedly in meeting the objective (Figure 3).
Cost of operation
Intimately related to the specific problem of kind of equipment to use on a particular area is the subject of economics of brush control. This subject can be outlined as a list of questions which a landowner must ask himself when he plans the management of his brush land.
1. Is it physically possible to convert the present vegetation to a more desirable type?
2. What will the land produce when the brush is cleared and how much money has to be put into the operation to come out with significant or profitable returns?
3. What degree of clearing should be made to attain the original objective and to insure economic returns?
4. What other land uses must be considered, and is the brush control work which is necessary for the primary objective complimentary to these other uses?
5. What equipment, labor, time, and money is available for the brush clearing operation?
6. What is the final choice of method, considering the objectives and the characteristics of the area concerned?
Physical possibilities of brush control
It is only logical that the question: "Can it be done?" be asked before: "How much will it cost?" As far as the possibility of brush and tree removal are concerned, however; that question is becoming less important day by day. Modern equipment is approaching the irresistible force while vegetation remains as movable as ever. Whether grass or grain or some more desirable form of the same shrubs can be substituted for the present cover, that is the most important problem at hand. It is conceivable that the present kind of vegetation on a site, whether brush or trees, may be the only kind that can grow there unless a drastic change is made in soil, climate, and plant competition. Research and experience determines whether this is true, and what changes must be made. Over a long period of time, research is cheaper than lay experience because its findings are basic and recorded.
Production, costs and profit on cleared land
The production on grazing land that has undergone brush control is expressed as weight of forage or as grazing carrying capacity, either in absolute terms or as increment over the production attained before brush conversion. The productive capacity affects the assessed value of the cleared as well as the adjoining uncleared lands of the same potential. This concept does not involve the original value of the land or the cost of the clearing operation.
The land manager must know the costs of alternative methods of controlling his brush as well as the returns from alternative crops that can be grown on the cleared land. Together with a knowledge of the present land value and its resultant worth, he plans his clearing operation and subsequent management. If a brush field is located near an area of high population, would it be profitable for the owner to clear it partially for the purpose of growing timber and for recreations Another decision might be whether to allow native herbaceous vegetation to become established on the cleared land - the cheap way - and raise beef cattle or to clear, till, and seed the area to choice pasture plants and raise dairy cattle. Timber and recreation might eventually give high returns but not for many years after planting; beef may not gross as much income as dairy products near a city. Questions like those enumerated cannot be answered until the initial investment is considered.
FIGURE 1. Bulldozer and disc. The bulldozer glade breaks off and mashes the woody stems and the disc chops them up and incorporates the mulch with the soil. The soil is tilled, suitable for seeding. In the photo a 100 horse-power Caterpillar Diesel D-7 tractor is pulling a Towner offset disc. (Caterpillar Tractor Co., Peoria, Illinois)
FIGURE 2. Brush clearing in the forest. A small bulldozer mashing manzanita in preparation for a prescribed burn under second-growth ponderosa pine trees in California. Small dozers, up to 40 horsepower, are powerful enough to do the job, yet can easily be maneuvered between the trees. (University of California)
FIGURE: 3. Brush control for wildlife management. Strips of chamise have been double-disced and seeded to herbaceous plants in the north Coast Range of California. The standing brush provides cover for game; the grasses, fortes, and sprouts in the open strip are used by deer during the winter months and by quail the year round. Narrow bands give the greatest possible "edge" effect. Contouring prevents excessive erosion. (University of California)
FIGURE 4. Relationship between percent of crown cover of mesquite and percent of ground cover of perennial grasses on southern Arizona rangeland. (After Parker and Martin (26))
Degree of brush clearance for economic returns
Cattlemen who have been battling the brush on their ranges feel that every noxious shrub is taking up space, nutrients, and water that ought to be used by more desirable forage plants. This suggests a quantitative, linear relationship between the number of shrubs per unit area of land and the amount of forage produced. Such a relationship is shown in Figure 4 (26). As brush increases in abundance the area occupied by grasses declines.
The graph can be interpreted in another way: when brush is eradicated, the area that can be occupied by grasses increases. Interpolations made from the graph show that where brush has 12 percent crown cover grass density can be,. increased 100 percent (doubled) by removing the brush and where the brush crown cover is 25 percent, potential increase of grass density is 1,000 percent. On the other hand, where only 1 or 2 percent crown cover of brush is present, maximum increases of grass can be no greater than 3 or 8 percent respectively, by removing the few shrubs from the cover. Is this profitable? A control method which reduces the brush cover from 15 percent to only 5 percent, with a potential grass increase from 11 to 22 percent density (or 100 percent increase), may cost $5.00 per acre to effect. On the other hand, a more thorough method which removes all the brush gives 6 percent more grass density (or an increase of 150 percent), but may cost $20.00 acre. Which is the more profitable method'
Admittedly, there are more factors involved than those represented by the simple relationship given above, but the example serves to illustrate the kinds of decisions that must be made by land managers who wish to clear brush.
Another example will show how costs, returns and effectiveness of brush control are interrelated (26). On an area in Arizona, mesquite control by the cheapest known method in 1947 cost $4.04 per acre. This control method was estimated to result in an average increase of beef production of 3.36 pounds per acre per year. When calculated on the basis of 17 cents per pound, which was the 10-year average price between 1941 and 1950, the value of this increased production is 57 cents. If, the original investment of $4.04 is compounded at a 5 percent interest rate, the increased returns obtained from the mesquite control would liquidate the capital outlay in 9 years. Assuming that the control is effective for 25 years, its benefits would be available for 16 years after the investment was paid off. Again, several factors have been disregarded, such as the purchase price of additional cattle to stock the range as the carrying capacity increased, the compensating sale of old cows and bulls, and the effect of the gradual re-encroachment of brush prior to the end of 25 years.
If the mesquite control method were effective for a period less than nine years, it would be a losing proposition. This is why "broadcast" control burning, since it is relatively inexpensive, may be profitable for a range and livestock enterprise, whereas land cleared by a combination of grubbing, root-raking, and burning of brush piles may not return the investment unless intensive agricultural practices are followed.
Multiple uses of land
Although there may be one primary objective for a given brush control operation, usually one or more secondary values of the land are either benefited or harmed. In those brushlands that have been essentially unchanged for decades or even centuries, there is a reasonably stable balance between the vegetation and wildlife, soils, and ground water levels. Tampering with the vegetation upsets this balance but the change may be either favorable or unfavorable to society. It may be favorable for a while and gradually become detrimental or it may in time become more favorable.
The subject of soil and water conservation as it relates to land clearing practices is so important that a separate section of this paper is devoted to it.
The choice of certain methods aims to assure good conservation practice. For instance, crawler-type tractors are ineffective on terrain which has 60-percent slopes or steeper. This, then, automatically precludes disturbance on the steeper and poorer sites when mechanical clearing methods are used. Work done with wheel tractors is still more restrictive to better sites. Broadcast burning, on the other band, does not provide this selectivity; updrafts on steep slopes may increase the effectiveness of the fire if adequate fuel is present, but a suitable vegetal cover may not become established before serious erosion has occurred.
In most cases where brush is partially cleared for range improvement, wildlife habitats are also improved. Burning or mashing of sprouting brush species provides good browse for deer, scattered unburned brush piles are excellent cover for quail, dove or other small game, and reduction of much of the transpiring surface may initiate flow of springs which have dried up as the brush cover became dense. The result is an increase in and better distribution of game. The increase in game on the range may not be worth a specific amount of money per acre but it may be subjectively a determinant of the method used in brush clearing. In some instances an increase of game may even be a nuisance; then methods which tend to discourage game should be chosen, such as complete clearing.
The many secondary benefits of brush clearing not only affect the parcel of land on which the work is done, but also adjacent lands and the community as a whole. Besides an increase in game, increase in usable water, fire hazard reduction, removal of insect and-fungal hosts, and decreased invasion of brush species are other values which do not appear on the landowner's balance sheets but are not so intangible to his associates.. Not all secondary effects are benefits. Erosion, gullies, and silt deposits do not stop at a property line. And it is unfortunate that the eradication of large tracts of certain kinds of brush may eliminate entirely the presence of many birds, mammals, insects and wild flowers which have tremendous inspirational and aesthetic value to a greater number of people than do larger herds of livestock.
Availability of equipment, labor, time and money
An efficient, brush-clearing operation cannot be planned unless proper equipment and labor are available. In the United States during the second world war, an opportunity for profitable brush clearing on rangelands was present because beef prices were abnormally high. However, steel was scarce, implement factories were engaged in making war materials, and labor, when available, was costly. Consequently, little mechanical work was done during those years.
The extensive brush fields of the world are too formidable to tackle with a grub hoe; on areas where heavy machinery cannot be purchased, large-scale land-clearing operations will be deferred, even though labor costs are cheap.
In addition to cost of labor a qualitative aspect must be mentioned. skill of an experienced bulldozer operator - "cat-skinner" - compared to that of a novice, far more than offsets the higher wages that he receives. Wear on machinery is minimized; more brush is cleared per day; costs per acre are lowered; the job of clearing is more uniform; there is less disturbance of soil. Hence, availability of skilled labor is an important factor in the choice of method and its subsequent success.
Frequently the time allowable to complete a job will determine what method is to be used. Chemical sprays, for example, react slowly on certain shrub species. If it is desired to have the land cleared off within a few weeks so that a particular crop such as rice or cotton can be planted on time, the slow chemical method would not be satisfactory. The difference of a year in timing may change a venture from success to failure.
The initial investment in equipment and supplies is frequently the most important determinant of the method used. For example, the brushland plow and the bushwhacker, including the tractor needed for power, are expensive items, and a man with 160 acres (66 ha.) of brush cannot afford to buy them. He is better off to modify his own farm equipment, or to buy portable, second-hand equipment, or perhaps use fire followed by pasturing with sheep or goats. In localities where much brush clearing is done, persons who own equipment may be employed to do the clearing work on a contract basis. Whether it is cheaper to hire a contractor or to buy the equipment and assume maintenance and repair costs depends on the size of the job.
Final choice of method
The foregoing discussion has pointed out some of the considerations in choosing the equipment for brush control. Both qualitative and quantitative factors are involved, hence no single directive can be followed which can help to make the decision. Descriptions of equipment in the following section include notations on such attributes as effectiveness of kill, adaptability to terrain, effect on soil, availability, and approximate costs per acre. These notations are to summarized in a single Table in the Appendix, using various brush types as examples.
Kind of vegetation
The term brush denotes a woody growth form of smaller stature than a tree. Unfortunately, woodiness and size class, while expressing the similarity between the various shrubs of different species, do not indicate their diversity which makes a given brush cover difficult to control. Such characteristics as reproductive habits, depth of rooting, abundance, shapes, and brittleness
FIGURE 5. Grown sprouting of chamise (upper) after fire in the California chaparral. The dead, blackened stems or "snags" arising from each clump of sprouts indicate the former height of the plants. Over half of the shrub species of the chaparral are sprouters. Except where there is intensive browsing, pre-fire densities are re-attained in 5 to & years. (University of California)
FIGURE 5. Scrub oak after fire in the California chaparral. The dead, blackened stems or "snags" arising from each clump of sprouts indicate the former height of the plants. Over half of the shrub species of the chaparral are sprouters. Except where there is intensive browsing, pre-fire densities are re-attained in 5 to 8 years. (University of California)
of stems, and density within the various species necessitate different methods of attack for each brush type. In addition, the size of stems and height of plants are determinants of the kind and size of equipment to use. No generalization of the effectiveness of a given method of eradication can safely be made, except, perhaps, in a stand composed of an individual species. The three most important factors with respect to the brush cover itself are whether the plants crown sprout or not, the size of the stems, and the density the brush stands attain.
Crown sprouting versus non-crown sprouting
Many woody plants sprout from the crown - the junction between root and stem - after the top has been removed or killed (Figure 5). This characteristic is found in many families scattered through the plant kingdom and is diagnostic of neither genus nor species. In North America, two large genera, Arctostaphylos and Ceanothus, have species that do not sprout, some that sprout weakly, and some that are strong sprouters. Throughout its range bitterbrush (Purshia tridentata) is a non-sprouter but in several small localities sprouting forms have been observed.
The habit of crown-sprouting means that mere removal or killing of the top growth does not result in its eradication. Vigorous new shoots may arise from the crown and, since the root system is still intact, the plant soon resumes its former stature and density. Frequently the leaves of sprouts are larger than those on mature branches, hence water use may equal or exceed that of the original plant. The vigor of the sprouts depends on the amount of food stored in the underground organs. This is a clue to the method of control, to remove the tops when the food reserves in the crown and roots are lowest. Frequent removal of the sprout growth, so that more food reserves are translocated to the tops than are manufactured and stored in the roots, gradually reduces the vigor of the plant until it dies. This can be effected with a heavy concentration of browsing animals, such as goats, sheep, or deer, or by repeated burning, contact spraying or discing. Reburning is not always possible; succulent young brush plants do not burn readily unless they are sparsely scattered in dense dry grass fuel. Contact spraying is usually more successful when applied on vigorous pants, so that this method seems to become less effective with each repeated spraying. Bulldozing and discing also have their drawbacks in respect to sprouts because they are not brittle and spring back erect after the machine passes over.
The buds from which the shoots arise may be just at the soil surface or as much as a foot or more under ground (Figure 6). Any method that seeks to kill the plant completely in one operation must be one which removes all buds below the soil line (15). Various types of grubbing or sub-surface cutting equipment are used for this purpose. While time is saved by grubbing, soil disturbance is usually greater than with the "starvation" method described above.
FIGURE 6. Location of dormant buds on stems of two woody plant,. Above, mesquite; right, sprouting juniper. In juniper the buds lie in a thick mass just above the soil surface whereas in mesquite they may be from 6 to 12 inches under ground. (Soil Conservation Service)
FIGURE 6. Location of dormant buds on stems of two woody plant,. Above, mesquite; right, sprouting juniper. In juniper the buds lie in a thick mass just above the soil surface whereas in mesquite they may be from 6 to 12 inches under ground. (Soil Conservation Service)
A somewhat different problem is encountered with the non-sprouting species. For example, the chaparral and maquis of the countries with Mediterranean climate, are comprised of "fire" species. Seeds of such species may be produced year after year but they do not germinate until a fire burns over the stand. It is thought that the heat cracks the seed coat or in some way makes it permeable to water. Scarification, as may be induced by bulldozing and use of certain other kinds of equipment, produces an effect similar to that to fire. This characteristic results in a paradoxical phenomenon: brush removal by fire or mechanical methods may result in brush becoming more dense than the original stand (Figure 7). In such cases it is imperative that follow-up management is practiced; these may be reburning, spraying, heavy browsing of seedlings, or reseeding to grass to provide competition (36).
Character and size of stems
The bigger the plants, the heavier the equipment needs to be to remove them, but there are a few exceptions to the rule. Certain trees are poorly rooted and can be pushed over without great effort. Brashy growth and brittle stems can be broken more easily than tough, springy stems even if the latter are found on smaller plants.
Two considerations to bear in mind when selecting equipment for clearing brush are the weight of the implement and the power needed to operate it. There is no handy table from which can be derived horsepower of tractor and size of disc when the average diameter of brush stems is known. This must come from the judgment of an experienced machine operator. Since the variables of objectives and costs, kind of brush, and character of terrain enter in, any recommendation applies to local areas only; consequently few examples are given in the literature.
In dense stands of California chaparral, where the brush consists almost entirely of small plants under 6 feet (1.8 m.) in height and 3 inches (7.6 cm.) in stem diameter, the most efficient bulldozer is one of 40 or 50 horsepower (6). Where the stand is dominated by medium-sized plants 6 to 12 feet (1.8 to 3.6 m.) or occasionally 15 feet (4.6 m.) in height and 3 to 6 inches (7.6 to 15.2 cm.) in diameter, a bulldozer of 40 to 50 horsepower is about at the lower limit of efficiency for mashing or uprooting. Where many of the plants are larger than 15 feet (4.6 m.) high and 6 inches (7.6 cm.) in diameter, other methods of clearing must be used to supplement bulldozing. The heavier tractors (80 horsepower and over) are powerful enough to mash down plants of this size but maneuverability is limited and much backing and turning is required.
In trees, size is a function of age. Young thin-barked trees are often easily killed by fire or by basal application of chemicals; mature trees are more resistant while old, decadent ones may be easily killed.
Frilling and notching for chemicals and girdling are methods in which a major portion of the cost involves work with an axe. Naturally, the larger trees have greater circumferences and consequently the cost per tree increases with its size.
Density of cover
In some forms of vegetation woody plants are sparsely scattered, such as savannas and woodlands. The trees and shrubs do not tend to aggregate, even after allowing sufficient time for seed dissemination. Supposedly the association of plants is stabilized. However, during periods of stress - drought, excessively wet periods, overgrazing, lack of grazing, lack of naturally occurring fires - the more aggressive, more tolerant, and usually least palatable species invade adjacent areas and thicken their stands. The encroachment and increase of mesquite in southwestern United States illustrates this process.
Mesquite reproduces by seed and it also sprouts from the crown. It is hardy and withstands drought by reason of its root system which may extend 50 feet (15.2 m.) laterally from the stem and 60 feet (18.3 m.) deep. Its growth form is plastic, mature plants varying from single-stemmed trees 50 feet (15 m.) tall to multi-stemmed bushes only 3 feet (0.9 m.) tall. The seed pods are palatable to livestock and game animals but a large proportion of the seed remains viable after passing through the digestive tract. The foliage is seldom browsed closely enough to hamper the growth of the plant. These characteristics of mesquite have allowed it to increase rapidly but insidiously.
The denser stands are difficult to clear. It takes more power and time to run machinery through it, and there is more wear on equipment than in lighter stands. The difficulty of control is lessened when airplane spraying of chemicals is employed. Here complete coverage of the area is imperative, providing uniform distribution, regardless of whether the infestation is heavy or light. For this reason incentive payments by the United States Federal Government to Texas ranchers, amounting to 12 cents per acre for each 1 percent infestation, has been made. Thus assistance for control of each acre of mesquite with a crown cover of 42 percent would be $5.00, which is the maximum allowance. Where the infestation is less than 1 percent, no payment is made. Thus there is a bonus for those land managers who clear their heavily infested range lands of brush, but none for the all-important stopping of initial invasion before the 1 percent density is reached. At this incipient stage of encroachment, the plants can be effectively and cheaply eradicated.
Density of brush is a determinant of method of brush removal in another way. Under the shade and competition of dense brush there is little herbaceous vegetation and consequently not much seed of grasses or other herbs. Therefore the method of brush clearing must be such that artificial seeding is possible and successful.
The character of the terrain is important to brush control from three standpoints: it determines whether the land is accessible to or can be traversed and worked by machinery, it determines the usability of the land as range, cropland or watershed; and it affects the amount of erosion that may take place after brush clearing.
Steepness of slope is the most important aspect of topography in this regard. The critical nature of the gradient, however, is modified by different lengths of slope, erosibility of soil, type of cover, and brush control methods.
Slopes with gradients less than 30 percent (3 feet [0.9 m.] vertical for every 10 feet [3 m.] horizontal) are well suited to grazing and forestry practice. At the lower range - less than 15 to 20 percent - cultivation is usually not hazardous. There is likely to have been little recent geologic erosion from such moderate slopes, consequently the soils are apt to be deep and fertile. A good index to the fertility is the size of the brush or growth rate of the trees.
FIGURE 7. Bush seedlings appearing after a fire in woodland-grass chaparral where many of the shrubs are non-sprouters The seedlings in the foreground are wedgeleaf ceanothus. In the background are yerba santa sprouts and fire-killed interior live oak trees. During the first year after fire, brush seedlings frequently number 250,000 per acre or more but many will die within one or two years. About 500 to 1,000 mature plants constitute a full density stand. (University of California)
Moderate slopes are conducive to almost any method of brush removal. Ground equipment can be used if there are no rock outcrops. Controlled burning can be employed providing enough natural barriers are available to make it safe and convenient. Chemical control, either by air or on the ground, is feasible. Control by grazing management is easy on moderate slopes since the lush grass on fertile soil is responsive to manipulation. Erosion hazard is likely to be minimum following any rational clearing technique.
Where the terrain has a high proportion of steep slopes, great care must be taken in selecting sites where brush control is possible, as well as suitable. Slopes above 50 percent are too steep to work with implements which disturb the soil cover. Crawler tractors have enough power and traction to go up the steepest grades but gullies will follow in the wake of the tracks; on the other hand, if the slope is too steep, the tractor may tip as it travels along the contour. :Brush control methods are limited almost entirely to broadcast burning, chemical spraying by aircraft, or to hand methods.
On such steep slopes there is the problem of establishing a grass or tree cover on thin soils, and if grass can be established, however sparsely, slipping of soil, trampling and sliding of grazing animals, and the loss of fertility is always present. Generally, wildlife and watershed management will be the highest use to be made of such areas and there should be no complete manipulation.
In between these extremes, roughly in the range of 20 to 50 percent slope, is the place where the most cogent decisions need to be made regarding choice of methods for brush control. The land is not as valuable, potential production is not as high, and costs of clearing will be greater here than at lower elevations. At the lower limits of this range, bulldozers, discs, and other implements can be operated efficiently; at the upper limits, controlled burning and chemical spraying are the best - methods. On rugged terrain there occurs the greatest heterogeneity of soil conditions, vegetal cover, precipitation patterns, and land use potentials. Sites must be carefully selected both from the viewpoint of: What can be done? and: What can be done profitably?
Rinds of soil
There are as many different soils as there are combinations of different climates, vegetation types, relief patterns, parent materials and geological histories. Brush and trees occur on a large number of different soils and on many of them these plants are thought of as being undesirable. Consequently, no hard and fast rule can be established in deciding on which soils brush control is feasible. However, several general characteristics of soils are important to keep in mind in deciding whether and how to clear brush.
The thickness of the solum is most important because it determines what the land use potentialities might be after the brush is cleared. Deep soils are desirable for most crops. It is easier to convert brushland to grass or trees when the soils are deep because there is a greater store of nutrients, more water can be held, and plants are more firmly anchored.
The ease with which a soil will erode is a function of its structure and texture. Soils with such profile characteristics as non-structural sandy topsoil with clayey subsoils erode readily. It might be best to leave them alone unless a better vegetative cover can be provided before erosion takes place. A bare soil is apt to erode no matter what structure it has.
Even shallow soils may be fertile. The relatively young soils derived from rocks of recent uplift have high nutrient content. The use potentials of such soils will be high. The fertility factor is related to erodibility: rich soils produce more cover and may provide a crumb structure in the upper horizons. Almost as important as the innate fertility of a soil is its potential fertility. Fertilization of cultivated crops is an old practice, but its possibilities on range lands and forests are being studied. Soils which respond to fertilizers should be given priority to those which do not in a brush control program.
Boulders and rock outcrops on the soil seriously hamper the use of equipment in brush clearing and subsequent management. Machinery must be built more sturdily, yet breakage is common and expected. Certain types of equipment, such as the pipe harrow, described later, have been designed especially for such soils. Where rocks make up a considerable proportion of the ground cover, crop yields are naturally low so that little profit is realized.
(This article is to be continued in subsequent issues of Unasylva).
Annual or Italian rye (Lolium multiforum)
Bitterbrush (Purshia tridentata)
Bur clover (Medicago hispida)
Cereal rye (Secale cereale)
Chemise (Adenostema fasciculatum)
Cheat brome (Bromus mollis)
Common alfalfa (Medicago sativa)
Coyote bush (Baccharus pilularis)
Creosote bush (Larrea tridentata)
Crested wheatgrass (Agropyron cristatum)
Fescues (Festuca spp.)
Hardinggrass (Phalarus tuberosa)
Interior live oak (Quercus wislizenii)
Intermediate wheatgrass (Agropyron intermedium)
Juniper (Juniperus spp.)
Ladak alfalfa (Medicago saliva ladek)
Manzanita (Arctostaphylos spp.)
Mesquite (Prosopis spp.)
Needlegrasses (Stipa spp.)
Ponderosa pine (Pinus ponderosa)
Prairie brome (Bromus catharticus)
Pubes cent wheatgrass (Agropyron trichophorum)
Rose clover (Medicago hirtum)
Sagebrush (Artemisia spp.)
Scrub oak (Quercus dumosa)
Shinnery oak (Quercus spp,)
Smilo (Oryzopsis miliacea)
Smooth brome (Bromus inermis)
Wedgeleaf ceanothus (Ceanothus cuneatus)
Yerba santa (Eriodictyon californica).
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