POSSIBILITIES OF SHELTERBELT PLANTING IN THE PLAINS REGION

CONTENTS

The region
Amelioration of conditions a pressing need
Feasibility of planting demonstrated
    Where shelterbelt planting is most advisable
    The shelterbelt zone
Effects of shelterbelts
Establishment of shelterbelts
    Orientation
    Frequency
    Kinds of belts
    Species
    Design and composition of shelterbelts
    Ground preparation, planting, and future care
    Protection of shelterbelts
Land ownership
Recommendations

THE REGION

The Great Plains region comprises about 30 percent of the continental United States. From a line through central Minnesota and Iowa, along the eastern boundary of Kansas, and southward along the ninety-fifth meridian to the Gulf of Mexico, it extends westward to the Rocky Mountains. This area is grassland except along stream courses where there is a natural growth of trees. The region is divided throughout its length from north to south into two units of almost equal size, the tall-grass prairie to the east, and the short-grass plains to the west.

Climatically it is a transition zone between the humid region to the east and the semiarid region to the west, embracing all gradations between the two. Broadly speaking, however, it is characterized by low annual precipitation occurring mostly during the summer, frequent droughts, great range in temperature extremes, low humidities, and almost constant winds of comparatively high velocities. The land surface varies from level to rolling. The typical soils of the region are the Chernozem and chestnut-colored soils, with considerable areas of dune sand in certain localities.

More specifically the climatic conditions become less favorable for plant growth from east to west. Although precipitation increases from north to south, evaporation also increases. Average daily relative humidities vary from 50 to 75 percent, showing a more or less regular decrease from east to west, and being lowest in the southwest and highest in the southeast. The evaporation from a free-water surface ranges approximately from 29 to 56 inches for the 6 warm months, April to September. The annual mean temperature ranges quite regularly from about 70° F. in lower Texas to 36° at the Canadian border. Average mean temperatures for January vary from -10° in the northeastern part of the region (the coldest area of the United States) to 45° in the south. The mean July maximum temperatures range from 78° in the north to over 100° near the month of the Rio Grande. Within any given season, temperature changes are sharp.

The average wind velocities in the region are 10 to 12 miles per hour, the highest average, 14 miles per hour, occurring on the High Plains of Texas. Hot, desiccating winds are of frequent occurrence in summer. In the winter, cold winds and blizzards are common. In spring the region is often visited by dust storms.

Agriculture becomes more hazardous from east to west and changes from grain production to grazing. The frequency of droughts of 4 months' duration for the last 40-year period ranges from 18 to 20 in the east to 40 in the extreme southwest. The average annual precipitation varies at the north from 21 inches in the east to 14 inches in the west and at the south from 36 inches in the east to 16 inches in the west.

AMELIORATION OF CONDITIONS A PRESSING NEED

As settlement increased and advanced farther west, and as increasingly larger areas of the original sod were broken by the plow, the recurrence of droughts and dust storms resulted in increasing economic losses and human discomfort and suffering. The drought of the last few years clearly brought into focus the need for a coordinated attack on the stern forces of nature and for a planned use of the Plains region if it is to continue as the granary of the United States. Without such united, intelligent effort, there will be further inevitable decline in the physical and economic conditions of life within the region. Coordinated effort must involve land adjustment, control of grazing, diversification of agriculture, water conservation and the building of ponds and reservoirs, shelterbelt and other forms of tree planting, strip cropping terracing, and the development of new varieties of cereals and soil-binding grasses.

This report deals with only one aspect of the broad program, namely, tree planting.

FEASIBILITY OF PLANTING DEMONSTRATED

Although occasional plans have been advanced in the past for water conservation and some other improvements, one measure of amelioration, tree planting, has received attention for a long time. Private initiative, stimulated by Government aid through the Timber Culture Act of 1873, Kincaid distribution as provided in the Agricultural Appropriation Act of 1911, the establishment of the plains field stations in 1913, the Clarke-McNary Act of 1924, and various State tree-bounty laws, accounted for the planting of about 700,000 acres of trees in the Plains region, chiefly in the form of timber claims or as windbreaks. Many of these plantations received little or no care and have since disappeared; many others stand today as witnesses to the foresight of tree-planting pioneers. Although trees do not naturally grow in the grassland except along watercourses, groves may be and have been established and maintained with the aid of man. With irrigation, tree growth is of course possible throughout the entire prairie-plains region. Experience, however, demonstrates that even without irrigation tree planting is feasible over much of the Plains area if proper precautions are taken, and it demonstrates further that such planting may contribute to the improvement of living and working conditions. That same experience also demonstrates that the sporadic efforts of individuals in the States must be systematically correlated and concentrated before any region-wide effects can be expected.

WHERE SHELTERBELT PLANTING IS MOST ADVISABLE

The grassland region for the most part does not lend itself to the planting of solid bodies of forest. The low annual precipitation occurring mostly during the summer may be sufficient for annual crops, but perennial trees and shrubs must depend for their existence on a supply of subsoil moisture which, in the fine-textured soils of the Plains, is often lacking. For this reason only along the "breaks" and ravines, or on sandy soils, where there is a supply of subsoil moisture available to the roots of the trees, can solid bodies of forest be established. Over most of the Plains, tree plantings, to provide protection against wind and storm without impoverishing the subsoil-moisture supply, should take the form of strips, known as "shelterbelts", around fields, farmsteads, and schoolhouses. Such strips do not impose a heavy drain on subsoil moisture. By proper cultivation of the soil on either side, and by the shelterbelt's own ability to accumulate drifted snow to melt in the spring, an actual increase of available moisture is provided for the trees' growth and maintenance.

Shelterbelt planting is advisable, in general, wherever climatic conditions do not forbid tree growth and where agriculture is sufficiently developed and needs the protective influence of such windbreaks. In the light of past experience, shelterbelt planting is advisable on a large scale east of a line marked by 16 inches of annual precipitation in the north and 22 inches in the southern portion of the Plains region. West of this line extensive planting of shelterbelts is considered somewhat hazardous and will require more careful selection of site and species, although there are places in eastern Colorado, Wyoming, and Montana, where shelterbelts are growing well.

In the more easterly part of the prairie-plains region, conditions for tree growth are more favorable; agriculture has been long established, many groves have already been planted by farmers, and this condition will undoubtedly continue in the future. Between the western climatic limit which is generally unfavorable to tree growth, and the eastern portion of the prairie-plains where planting is easy and already well developed, there is a relatively narrow north-south belt within which agriculture, although now beset by many handicaps, holds out the promise of becoming thoroughly established if protective tree planting and other ameliorative measures are given their proper function on the land. This, on the whole, is a zone of fertile prairie soil and is an important part of the granary of the United States. Shelterbelts are fitted directly into its pattern of land use, and their planting should be encouraged. It is within this particular territory that the shelterbelt zone, with which this report is largely concerned, has been marked out for development—because here the soil, the rainfall, and other natural conditions point to the success of the undertaking, while the conditions of agriculture mark it as urgently needed.

FIGURE 1.—Shelterbelts break the monotony of the plains and lend attraction to the farm home: A, A bleak, unprotected farm group (F148315): B, Trees lend a homelike, sheltered aspect to this farm (F44010). (See text, p. 8).

THE SHELTERBELT ZONE

On the basis of the accumulated experience of past shelterbelt planters, further supplemented by surveys carried out during 1934 and 1935 which furnished a correlated region-wide knowledge of the climate, soils, native vegetation, and results of past planting—the present shelterbelt zone (fig. 2) in which such planting is to be encouraged has been delimited. This zone is 100 miles wide and, roughly, 1,150 miles long and is confined to the transition zone between the tall-grass prairie and short-grass plains. As shown on the map, the axis of this zone roughly follows the ninety-ninth meridian, touching Devils Lake in North Dakota, Mitchell in South Dakota, Lexington in Nebraska, Kinsley in Kansas, and Mangum in Oklahoma. Its western limit is generally within the precipitation boundary marked by 16 inches of precipitation annually in the north and 22 inches, to allow for greater evaporation, in the south.

FIGURE 2.—Location of the shelterbelt zone, with factors limiting its westward extension. (click on image for a PDF version)

Within the boundaries of the zone thus designated there are areas where the character of the soil may, as it actually does on limited areas, either preclude forest planting altogether or dictate variations in the manner of planting. Within the zone there are some 114,700 square miles of land. This can be classified, according to soil and suitability for shelterbelt planting, as follows:

66,400 square miles of fine-textured soils—so-called "hard land." Generally, it is good agricultural land, but not all of it is suitable for forest planting. It may be subdivided thus:

36,700 square miles of uplands; shelterbelt planting difficult.

24,900 square miles, principally in the eastern part of the belt, well suited to shelterbelt planting.

4,800 square miles of claypan land and alkali basins, unsuited to tree growth.

29,900 square miles, consisting mostly of sandy loams, which is good agricultural land and all favorable for shelterbelt planting.

800 square miles of coarse, gravelly soils, relatively unfavorable for tree growth.

13,000 square miles of "breaks" or rough land. Of this:

5,000 square miles are favorable for shelterbelt planting.

8,000 square miles are difficult to plant.

4,600 square miles of sand hills, well suited for forest planting in solid blocks.

Of the total area, some 56 percent lends itself to shelterbelt planting, about 39 percent is difficult to plant, and 5 percent is entirely unfit for planting. It is evident, therefore, that there can be no continuous parallel forest strips, but each, planting must be adapted to the soil conditions of the individual farm or farms which it is to protect, and must be oriented according to the damaging winds prevailing in each locality.

The sand hills, which present the most favorable conditions from the standpoint of soil moisture and on some of which the ground water is within reach of trees, lend themselves best to planting in solid forest blocks. A good illustration of such planting is found in the sand hills of Nebraska, where the Forest Service has planted successfully some 20 square miles. About 410 square miles have recently been authorized for purchase by the Federal Government in North Dakota for this type of planting.

Planting in the breaks and gullies will be largely for the purpose of conserving water and checking soil erosion. It will usually be confined to the slopes and active gullies and will follow narrow ravines.

On the "hard land", planting will be mostly in the form of windbreaks around farmsteads and schools.

EFFECTS OF SHELTERBELTS

The need for the protection of farmsteads, orchards, and fields in the Plains region against winds is almost universally recognized.

To shelterbelts there have been attributed popularly such effects as decreasing the range of temperature fluctuations, attracting more rainfall, increasing the retention of snow and providing a more uniform distribution of it, reducing wind velocities and blowing of soil, reducing evaporation, increasing soil moisture, raising the water table, increasing agricultural yields, and, from the human standpoint, providing a more livable and beautiful environment. Quite definite scientific proof has been furnished as to some of these effects; others are more difficult of evaluation.

The actual effects of shelterbelts on crop yields are, so far as they have been measured, largely favorable. Further tests and experiments are needed for the better scientific guidance of the project as it develops. It will be advisable to measure crop yields on experimental fields protected by shelterbelts of various densities, composition, and orientation, under both wet and dry climatic cycles, and to compare these results with those obtained under similar conditions on fields which have been unprotected by shelterbelts. Most observations made in this country have been confined to shelterbelts already established, of variable composition, density, and extent, and on fields planted to a single crop in one place and another crop in an other place.

Long records from foreign experiment stations clearly indicate that the effect of shelterbelts may be to increase grain-crop yields by as much as 25 percent both in grain and straw. In the United States the evidence is generally favorable to the shelterbelt, but, considering that most of our shelterbelts are defective in their density, orientation, and extent, further observations and experiments must be conducted before a proper rating can be made of the degree of benefit that should be expected.

The effect of windbreaks varies with the occurrence of wet and dry years. During the wet years their effect, being a means of protection against damage by drought, is at the minimum. During moderately dry years this effect is at its maximum; within reasonable limits the beneficial effect increases with the increase in drought. Under extreme conditions of prolonged drought, however, shelterbelts may fail completely to save the crops, and themselves may be decimated. It is for this reason that the public planting effort must proceed with the best advisement of science and experience, and with due regard to regional and local conditions. Essential factors of survival have been too often neglected in the past.

Meteorologists and foresters agree¹ that the physical conditions of air and earth, which cannot be altered appreciably by human agencies, basically control the climates of the various regions of the world. Extensive climatic controls are changed only through the slow process of nature operating leisurely through many centuries. While the planting of trees will not change climatic conditions as a whole, it will alleviate or modify many unfavorable features of existing conditions, principally through the diminution of the surface velocity of the wind by the successive forested strips. Also the trees undoubtedly would help in some situations to conserve soil moisture to some extent, at least, by retarding run-off and evaporation and lessening erosion.

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¹In response to numerous inquiries, this statement as to climatic effects is made jointly by the Weather Bureau and the Forest Service.

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Man, in many ways, has modified the effects of climate and has alleviated some of its unfavorable aspects. For example, the planting of windbreaks in California to protect orchards from the damaging effects of desiccating winds and the use of literally millions of orchard heaters for the protection of fruit from frost damage have not changed California's climate in the least, but they have made some localities of the State safer places in which to grow fruit. Many other examples of this kind might be cited. It must be remembered that such modifications of existing conditions are the primary results sought, rather than an appreciable change in the climate as measured by general conditions of temperature, sunshine, or rainfall.

The whole question, therefore, of the effect of shelterbelt planting on the climate of large regions is largely of academic interest. More to the point are its local benefits. The effect of shelterbelts on retarding or modifying the movement of surface wind is definitely recognized. This effect has been proved experimentally to extend to a distance equal approximately to 20 times the height of the trees on the leeward side, and for a shorter distance—not definitely established—on the windward side.

The retardation of the surface wind movement produces a chain of physical effects. Evaporation from the land immediately adjoining the shelterbelt is reduced. The transpiration from the vegetation is lessened. The drifting of snow is checked, and the blowing of the soil is retarded.

A dense shelterbelt about the farmstead accomplishes three definite benefits: Protection of the home against cold winds with a measurable saving of fuel, protection of stock to an extent definitely appreciable, and prevention of snow drifting within the area sheltered.

The protection of gardens and orchards, which is generally accomplished by surrounding them on at least three sides with trees, frequently permits the growing of fruits and vegetables which otherwise would never mature in the dry Plains region.

From the human standpoint, shelterbelt planting. by adding beauty to the landscape, breaking up the monotony of the Plains, and satisfying the craving for growing trees in a treeless region, has an immeasurable value in happiness and contentment (fig. 1); from a more commercial point of view, it adds considerably to property values.

The mechanical protection afforded by windbreaks is perhaps more definitely effective in preventing wind erosion locally than in any other way. To accomplish this, shelterbelts must be either reasonably close together or supplemented by strip cropping, or other methods of direct soil protection, to be effective. They cannot be expected to prevent dust storms, which may originate hundreds of miles away in areas too dry for tree growth.

Although shelterbelts are planted chiefly for local protection of farmstead and soil against wind and storm, they have provided and will continue to furnish considerable quantities of fence posts and fuel wood. Such products are of particular value in the Plains region.

The effect of shelterbelts on wildlife cannot be ignored. In addition to providing a haven from the elements and natural enemies, many tree species also furnish food in the form of seeds and fruits.

In addition to shelterbelts around fields and farmsteads, the planting of large blocks of trees on sand hills and to a lesser extent along breaks of streams and gullies will produce some very considerable benefits. They will protect watersheds, produce a valuable wood supply, and provide very important recreational and wildlife values.

How soon will the benefits of shelterbelts begin to be felt? From past experience it can safely be said that at the end of 5 years the shelterbelts will have reached sufficient height to have some degree of effectiveness in retarding surface wind movement and obstructing drifting snow and soil.

ESTABLISHMENT OF SHELTERBELTS

It must be realized that, even when the proper soil types and tree species are available, shelterbelts can not be established successfully in the zone without painstaking care and considerable cost.

ORIENTATION

In general, the east-west orientation of shelterbelts is most effective for protection against summer winds but will not provide complete protection during other seasons or even against the quartering winds which frequently occur during the summer. Because of the variability in direction of damaging winds, both east-west and north-south orientations should be used. A shelterbelt on two sides of each field over a large area would amount, practically, to protection on all sides.

FREQUENCY

To be fully effective, shelterbelts should be established at intervals not greater than one-quarter mile. The present distance of 1 mile is only a beginning, which must be supplemented later by private or State plantings.

KINDS OF BELTS

Three general kinds of planting are considered:

1. The typical field shelterbelt, which is to make up the bulk of the planting, under the Plains shelterbelt project, should be a planted strip 8 rods wide. The area to be acquired and fenced should be 10 rods wide, so as to provide room for later cultivation and the spread of the roots and crowns, embracing 20 acres in each mile of strip. Under some circumstances, narrower belts, down to single rows, may be used. It is estimated that 1,282,120 acres, all on favorable soils, should be planted to field shelterbelts.

2. On more difficult soils, where the establishment of trees is likely to be more expensive, yet where the need is great, the plan should be to aid farmers in establishing windbreaks around the farmstead through cooperative agreements. For this purpose 897,880 acres should be planted.

3. There are approximately 4,099,000 acres of land within the shelterbelt zone which should be placed under forest and range management to prevent erosion and to protect critical areas from cultivation and overgrazing. Some 400,000 acres of this area should be planted to more or less solid blocks of trees.

Except after very careful planning and with the cooperation of the State highway departments, shelterbelts will not for the present be planted along highways because of possible interference with highway purposes. But, in view of the growing demand for roadside beautification, such planting should be undertaken at once on an experimental scale and the details worked out.

SPECIES

The choice of species will depend upon such factors as adaptability to climate and soil, silvical value, economic value, and susceptibility to damage by pests.

The following species have been found to be of most wide-spread usefulness, each over a large area within the zone: Green ash, hackberry, cottonwood, American elm, Chinese elm, tamarisk, Russian-olive, caragana, honeylocust, Osage-orange, eastern red cedar, and Rocky Mountain red cedar. Complete information on species suitable for planting will be found in a later section (p. 17).

The collection of seed of desirable species should be made only from suitable native trees or others of known climatic adaptation. Species introduced for experimental purposes should not be used extensively until their suitability for shelterbelt planting has been proved. North-south limitations should be observed in the movement of seed from one locality to another, and localized collection of seed and planting of stock should be stressed.

Planting stock will have to be produced both under contract in private nurseries within the shelterbelt States and in Government nurseries already present or to be established within the area. Good, hardy stock grown with the minimum feasible amounts of water and artificial care must be produced. For the hard wood species, which will make up approximately 85 percent of the planting, stock 12 to 24 inches in height, 1 to 2 years old, is most satisfactory. Coniferous stock will be smaller and should be transplant stock 2 to 5 years old.

DESIGN AND COMPOSITION OF SHELTERBELTS

The typical field shelterbelt should be rooflike in cross section. In the center, the peak of the roof, there should be tall trees flanked successively by shorter trees, conifers, and tall shrubs, with low shrubs forming the eaves of the roof on the outside. This type of belt makes for the greatest impenetrability to the wind for the longest period of time, has a fine appearance, and provides valuable food and cover for wildlife. A considerable mixture of species is preferred because of different qualities of usefulness: Rate of growth, form of crown, longevity, and reduction of insect and disease hazards.

Close spacing of the trees within the shelterbelts should be preferred. The advantages of close spacing are: (1) Allowance for inevitable early losses; (2) early closure of crowns, thus shutting out weed growth and ending the period of cultivation; (3) mutual protection of the trees and (4) creation of the densest possible wind barrier. Close spacing implies the making of thinnings as needed and does not mean a crowded stand of trees at maturity.

GROUND PREPARATION, PLANTING, AND FUTURE CARE

For successful shelterbelt establishment it is mandatory that the ground be prepared at least 1 year in advance of planting. This process ordinarily builds up a sufficient reserve of soil moisture to establish the newly planted trees, and with careful cultivation during each of the first 4 or 5 years after planting should see them well on their way toward vigorous growth. After this period, conservation of moisture is to be effected mainly through the accumulation of litter, and this is one of the most important reasons for having the plantations as dense as possible. Cultivation should be done frequently the first year, when there is no protection for the soil, and should be diminished gradually until it is given up after about the fifth year, when it becomes impossible to put machinery between the trees.

Careful methods of planting, giving good root distribution, and providing for good root development are necessary. For this purpose the youngest possible hardy stock is desirable. Planting, almost without exception, should be done in the spring, since the winters on the plains are generally too open and fall conditions too dry to permit of fall planting.

PROTECTION OF SHELTERBELTS

It is absolutely necessary that shelterbelts be fenced to protect them from livestock. Grazing and trampling by stock is one of the most serious causes of the dying out of windbreak plantations. The maintenance of these fences over most of the region will be one of the largest items in the future care of the plantations.

Jack rabbits and other rodents also cause much damage which may be minimized by snaring, poisoning, hunting, the use of repellents, and other recognized measures of control.

General precautions against the spread of insects and diseases in nursery stock and planted trees should be taken. As far as possible, tree species immune to serious pests should be used in planting, and possible losses should be guarded against further by the use of stock of local origin planted on sites to which it is best adapted. Special methods of control will have to be resorted to in case of serious infestations by insects.

The danger of fire loss is not as great as in most forest regions. There are possibilities of damage, however, from spreading stubble fires and these will have to be prevented as far as possible by keeping the fences free of accumulating debris, by cultivation of the surrounding strips, and by educating the public against uncontrolled fires.

LAND OWNERSHIP

Considering the long-time nature of the project, highly stabilized control of the land dedicated to the shelterbelts is essential. Such control can be maintained in the following ways:

1. Ownership in fee simple by the Government. This is the most highly stabilized form of control. Such control may be acquired through either purchase or donation. Certain reservations, in most cases involving mineral rights, may be made as the need is indicated by local conditions.

2. Grant or purchase of perpetual easements.

3. Leases.

4. Cooperative agreements, by which the owner, in consideration of the benefits from shelterbelt planting, agrees to a plan of cooperation with the Government that will satisfactorily assure the future of the shelterbelt project on land retained by the pres ent owner.

It will be necessary to use each of these methods of land control as circumstances dictate.

RECOMMENDATIONS

The general effect of shelterbelts is not the creation of more rainfall over the area covered by tree growth, but the more economic use and conservation of the available rainfall. Shelterbelt planting is not a cure-all of unfavorable climatic conditions in the Plains region. It should be considered as part of a much broader program of water conservation, soil-erosion control, terracing, strip cropping, and other measures tending to hold moisture and to conserve it in the soil.

Although past experience and recent studies have revealed certain desirable procedures for shelterbelt establishment, nevertheless it is essential that the plan should not be rigid. It must be kept sufficiently flexible to adapt itself to such changes as new experience and local sentiment indicate as desirable.

Further investigations, particularly into the effects of shelterbelts on crop yields and physical factors, should be carried on. For this purpose a Great Plains forest experiment station should be established to study the forest problems of the region as a whole. Shelterbelt studies can be made best on experimental areas comprising several sections of land more or less uniform as to topography and soil conditions. On such areas there should be established shelterbelts of different widths, densities, orientations, and compositions, and their effects upon physical factors and crops studied under controlled conditions. Since the development of such experimental shelterbelts, even though established immediately, will require several years to produce measurable effects, it is recommended that immediate investigations be undertaken to study the effect of existing shelterbelts, supplemented by artificial windbreaks or barriers to provide a variety of conditions.

Shelterbelts once established will not be perpetual, but will need care and renewal if they are to serve the purposes for which they were intended. Provision should be made, therefore, to insure the permanence of shelterbelt planting, and to encourage it on suitable sites to both the east and the west of the present shelterbelt zone. This can be accomplished best if the Plains shelterbelt project be expanded into a regional forestry enterprise similar to those now in existence in other parts of the country. In other words, the Great Plains should be organized as a distinct forest region. The regional forest activities then should be coordinated with the other forestry activities of the shelterbelt States. Private and State planting could be encouraged and all forest activities in the region developed in harmony toward definite objectives.