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Extensive studies of the natural plant cover, especially where the root habits have been taken into account, have shown that the native vegetation indicates rather definitely the environmental conditions of an area and, in this way, throws much light upon the manner in which cultivated crops are likely to develop when they replace the native plants. It should be kept clearly in mind, however, that the general adaptation of cultivated plants to a region is controlled by a complex set of conditions by no means thoroughly understood. Nevertheless, the use of native vegetation for indicating possibilities of growth has proved very valuable in those areas where it has been most fully studied. Native plants have the same general requirements as regards heat, light, water, and nutrients as have cultivated crops. Since their growth is controlled by physiological conditions similar to those that hasten or retard the growth of crops, a study of the natural vegetation throws much light upon the possibilities of an area for crop production. Moreover, root habit has been found to correlate rather closely with water content or other soil conditions. Hence, when both root habit and aboveground development of natural vegetation are properly interpreted, they afford a more or less definite indicator of how roots and shoots of cultivated crops are likely to develop, and what may reasonably be expected in the way of average yields.
The natural vegetation, for many centuries, has been sorted out by climate as well as by soil in the process of development. The various species of plants have usually inhabited a given region so long that they are now quite definitely distributed with relation to the environmental complex, species well adapted to a given environment now occurring in abundance. Thus, the growth of the native vegetation becomes a measure of the effects of all the conditions which are favorable or unfavorable for plant production.
It is of more than passing interest that the cereal crops, viz., corn, spring and winter wheat, oats, barley, sorghum, and millet, all of which are grasses, have their center of greatest production in that portion of the United States originally covered by grassland. In fact, some are grown almost entirely in this region, and other crops such as alfalfa and flax, which are similar in growth habits to wild legumes, wild flaxes, etc., growing among the grasses, also have their greatest acreage in the grassland. Likewise, the greatest areas of fruit production, including such tree fruits as apples, peaches, and pears, and such bush fruits as blackberries, currants, and raspberries, are in those portions of the United States formerly occupied by native species of similar habit, i.e., forest trees and shrubs. Since practically no studies have been made on the root habits of native species in other crop-producing regions, our discussion must be limited to the prairie-plains region.
The great grasslands, extending from the forests of the eastern states to the Rocky Mountains on the west, constitute approximately one-third of the entire range of our country and include the most valuable area of agricultural land. The corn belt lies largely within this area, in Illinois, Iowa, eastern Nebraska, Kansas, and Missouri. Cotton is produced without fertilizers on the prairies of Texas and Oklahoma. Winter wheat is a most important crop in the west central portion of the prairie area. Spring wheat production is confined largely to the prairies of North Dakota, South Dakota, and Minnesota. Barley is produced in large quantities on the prairies of North Dakota, South Dakota, Minnesota, and Iowa, and oats on the prairies of Illinois, Iowa, Minnesota, and the eastern portions of North Dakota, South Dakota, and Nebraska, and also in the prairie sections of Missouri, Oklahoma, and Texas. Flax culture is most important on the prairies of North Dakota, South Dakota, and Minnesota. 185 Although the center of production of timothy and clover hay lies farther eastward, yet large amounts are produced in the eastern portion of the prairie area. Great quantities of wild prairie hay ,are also produced in these grasslands. Alfalfa grown without irrigation is confined largely to the prairies of Kansas, Nebraska, and Oklahoma. About nine-tenths of the sorghums raised for either grain or forage are grown on the grasslands of Kansas, Oklahoma, Texas, Colorado, and New Mexico. 59 Millet is grown I extensively for hay on the prairies from the Dakotas to Texas. In the drier portions of the grasslands, a large acreage of sugar beets, potatoes, and other crops are grown under irrigation.
The root behavior of these cultivated plants, which have been studied most extensively in an east and west belt through the center of the grasslands, 221, 225 may be interpreted best in the light of the behavior of their wild relatives and predecessors whose growth and decay through centuries have had so much influence in making the prairie soils so rich, so excellent in structure, and so retentive of moisture. Moreover, the native grassland species are themselves very important forage and grazing crops.
The great grassland area extending across the Mississippi Valley from the forests of the east to the foothills of the Rockies is not characterized by a uniform vegetation throughout. The tall-grass prairies of the eastern portion are distinctly different from the short-grass plains of the West and Southwest, and between these two regions is a broad belt of mixed grassland where tall and short grasses intermingle. 43 The chief causes of these differences in grassland vegetation are the differences in the quantities of soil moisture supplied by the rainfall and the length of time during which soil moisture is available. Decreased relative humidity westward is also an important factor. Differences in soil structure, resulting from differences in climate and vegetation throughout its development, are also pronounced. These factors, which have so largely determined the type of grassland, exert striking influences on the development of both root and shoot, and influence also crop growth and yield. This influence is shown to such a marked degree that root development of many crops has been thoroughly studied in each of these three great grassland communities which occur in Nebraska, Kansas, and Colorado.
This community constitutes the grasslands of Minnesota, Iowa, Illinois, Indiana, and Missouri, as well as approximately the eastern one-third of the Dakotas, Nebraska, and Kansas, and large areas in Oklahoma. The grasses are from 1.5 to over 5 feet tall and are rooted to an equal or greater depth in the dark-colored, deep, rich, moist soils. They form a rank growth, mostly of the sod type, due to extensive rhizome development, and usually continue growth throughout the entire summer (Fig. 29). This is possible because of the presence of abundant soil moisture. Even in the drier portions, the soil is usually moist to a depth of several feet, and moisture is continuous to the ground water in the wetter areas. The surface soil may be dried out each year, and drought may occur in late summer and fall, but the subsoil into which the deeper roots of the vegetation penetrate is permanently moist (See Table 2, Chapter I). Such conditions should promote the development of numerous deeply rooted species and, in fact, prairie vegetation is characterized by this type of root habit.
Fig. 29.--Upland tall-grass prairie in eastern Nebraska. Porcupine grass (Slipa spartea) is the most conspicuous species.
Among the very numerous grass species that occur, some of the most important ones are the bluestems or beard grasses, tall panic grass, tall marsh grass, wild rye, porcupine grasses, and June grass. The deep root habits of cultivated plants in this region might well be predicted from a study of those of the native grasses.
The Bluestem Grasses (Andropogon).--A fair conception of relative heights and root depths of the three common andropogons, which often furnish the bulk of wild prairie hay, may be gained by an examination of Fig. 30.
Fig. 30.--Three species of bluestem or beard grasses. Left, big bluestem (Andropogon furcatus); center, little bluestem (A. scoparius); right, Indian grass (A. nutans). Photographs taken in different scales which are shown in feet at the side.
Big bluestem (Andropogon furcatus) is the tallest and most deeply rooted, sometimes reaching depths of over 9 feet, although in hard clay subsoils, the roots are often 2 to 4 feet shorter. The roots of little bluestem (Andropogon scoparius) vary from 3 to 5.5 feet in depth, being very similar to those of Indian grass (Andropogon nutans). In none of these species do the roots spread much over a foot on all sides of the plant, but they are so numerous and densely branched that they thoroughly occupy the soil and form a dense sod. That the roots of big bluestem are deeper but very much coarser than those of little bluestem is very significant. Throughout the prairie, the former grows best in lowlands or areas of higher rainfall, but the latter tends to dominate higher and drier areas. The much finer, better branched, and more widely spreading roots of little bluestem permit it to extend much farther westward into areas of lower precipitation than the more coarsely rooted, taller species. In drier soils, it abandons the sod habit and grows in bunches, not enough water being present to supply a continuous plant cover. A similar response may be noted in fields of cultivated cereals, the uniform growth in the drill rows in moist prairie soils giving way to irregular clumps of grain in the drier mixed-prairie and short-grass plains areas westward.
Tall Panic Grass (Panicum virgatum).--This grass grows abundantly in many situations throughout the prairie and, like big bluestem, prefers deep, moist soil. Its roots are also very coarse. They penetrate nearly vertically downward to depths of 8 to 9 feet, spreading but little near the surface. In comparison with most grasses, the branching is very poor. But, as among most species of both native and crop plants, the branching habit is greatly modified by variations in soil conditions. When excavated from dry soil, branches are exceedingly numerous and, with the great depth of penetration, enable the plant to occupy many cubic feet of soil (Fig. 31).
Fig. 31.--Roots of tall panic grass (Panicum virgatum), from a block of sod transplanted into. dry soil.
Tall Marsh Grass (Spartina michauxiana).--Tall marsh grass, like various other species which thrive in low, moist soils and furnish an abundant yield of forage, has very coarse, rather poorly branched, and very deep roots. These thick roots, often 3 to 4 millimeters in diameter, spread very little but penetrate vertically downward to distances of 8 to 13 feet, frequently extending into water-logged soil. In fact, this grass has the coarsest roots of any examined; coarse roots are common to several of the species which thrive best in moist situations (Fig. 32).
Fig. 32.--Rhizome and roots of tall marsh grass (Spartina michauxiana).
The rapidity of development of the roots of native grasses is exceeded only by those of cultivated crops. Growth is slower under the exceedingly keen competition for light, water, and nutrients in undisturbed grassland, but in disturbed areas where the sod formed by roots and rhizomes is broken, development is very rapid. For example, under the latter condition, tall marsh grass and panic grass may reach depths of 4 feet at the age of 3 months, a growth rate of over half an inch a day. This rate of growth is not uncommon among native grasses. Moreover, as among cereal crops, tillering begins early. When only 4 to 5 weeks old, tillers frequently begin to appear and, simultaneously with these, a sufficient number of adventitious roots to supply the new shoots with water and nutrients. Thus, many similarities may be found between native and cultivated grasses (Fig. 33).
Fig. 33.--A seedling grass (Sporobolus asper) 44 days old, showing tillering and beginnings of a secondary root system.
Wild Rye (Elymus canadensis).--This grass is widely distributed over the prairie. Like all the preceding species, it is a late summer or autumnal bloomer. Its excellent root system furnishes an abundance of water throughout the entire growing season. The roots, while fairly coarse, spread 1.5 to 2 feet on all sides of the plant and are abundantly supplied with fine laterals, the density of these varying with soil character. Depth of penetration varies from 2 to over 3 feet.
Porcupine Grass (Stipa spartea) and June Grass (Koeleria cristata).-- These species often occur in drier soils than do most of the preceding grasses. This habit of growth in drier soils, together with an earlier seeding habit, may be correlated with the root systemts which are much less extensive than those of most other prairie grasses. Porcupine grass seldom exceeds 2 to 3 feet in depth, and June grass extends to depths of only 1.5 to 2 feet. The root habit of June grass is shown in Fig. 34, where the greater extent of roots over tops, as well as the great mass of rootlets, may be seen. The lateral spread of porcupine grass is even greater, but the roots are scarcely so well supplied with branches.
Fig. 34.--June grass (Koeleria cristata). One of the few shallow-rooted prairie grasses.
The root habits of the few grasses thus briefly described are representative of many others (including the grass-like rushes and sedges) which, with these, make up the major part of the grassland flora. A conception of the root habits in tall-grass prairie would be incomplete, however, without brief mention of the root relations of some of the abundant legumes, composites, mints, and roses so conspicuous in the prairie flora. During spring and early summer especially, as well as later in the fall, so abundant are the non-grassy species that the prairies are veritable flower gardens.
Root Habits of Non-grassy Species.--Wild alfalfa (Psoralea floribunda), frequently so abundant that for a time it obscures the grasses, has a very deeply penetrating root system, as does its cultivated relatives. The strong taproot is sometimes an inch in diameter. It gives off many large, widely spreading branches, especially in the deeper soil, many of which extend to depths of 5 to 7 feet. The main root, seldom branching in the surface foot and absorbing but little in the surface 2 feet of soil, penetrates rather vertically downward to distances of 8 to 9 feet or more (Fig, 35).
Fig. 35.--Root system of wild alfalfa (Psoralea floribunda). The taproot had decayed. Scale in feet.
This root habit is not greatly unlike that of the wild licorice (Glycyrrhiza lepidota), which often branches more profusely and penetrates even deeper, frequently from 10 to 14 feet. The ground plum (Astragalus crassicarpus), likewise, has a large taproot with many strong widely spreading branches which penetrate many feet into the subsoil. A depth of 6 to 8 feet is not unusual. Like the preceding, false indigo (Baptisia bracteata) has little provision for absorption in the surface foot or two of soil. Its rather woody roots spread widely and, although never branching profusely, absorb water and nutrients at great depths, even to 7 or 8 feet. Prairie shoestring (Amorpha canescens), another very common prairie legume, is even more deeply rooted. Relatively little absorption occurs in the 2 to 4 feet of surface soil. Bluestems and other grasses frequently grow vigorously between the spreading Amorpha roots where they doubtless suffer little competition. The chief supply of water and probably of mineral nutrients also is obtained below the surface soil layers, the roots sometimes reaching depths of 13 to 16.5 feet. The roots may spread 4 to 5 feet from the base of the plant. Just as cultivated leguminous crops enrich the soil through the work of nodule-forming bacteria, so, too, these legumes and many others of similar habit, furnished with tubercles throughout their extent, are in a large measure responsible for the high productivity of prairie soils.
Many composites have root habits similar to those of the legumes. That of a blazing star (Liatris punctata) is shown in Fig. 36.
Fig. 36.--A blazing star (Liatris punctata).
On other specimens, small laterals, although scarce to a depth of 6 to 9 feet, were fairly abundant below this level, which indicates the great depth of most active absorption. False boneset (Kuhnia glutinosa), another late summer or autumnal bloomer, is quite as deeply rooted as the blazing star and, like it, is not provided for absorption near the, soil surface. The rosinweed (Silphium laciniatum) has a fleshy taproot 1 to 2 inches thick. It descends rather vertically to distances of 9 to 14 feet (Fig. 37).
Fig. 37.--A portion of the root system of rosinweed (Silphium laciniatum). It usually penetrates to depths of 9 to 14 feet.
Relatively few large branches occur, but these may run horizontally 3 to 4 feet before turning downward. As a whole, this deep-seated root system is poorly branched. Purple coneflower (Brauneria pallida) has a thick, fleshy, poorly branched and rather vertically descending taproot which penetrates to depths of 5 to 8 feet. The root habits of the many-flowered aster (Aster multiflorus) and the Missouri goldenrod (Solidago missouriensis) are very much alike (Fig. 38).
Fig. 38.--Root system of the many-flowered aster (Aster multiflorus).
Both propagate by rhizomes from which many of the cord-like fibrous roots arise. These branch throughout their course and, with the finer roots in the surface soil, fit the plants for absorption throughout the 8 feet or more of their downward course. Other goldenrods and. asters are very similar in their general root habit, absorbing at all levels from the surface to depths of 5 to 11 feet. This is true, likewise, of certain native sunflowers.
This development of extensive root systems is not confined to the grasses, legumes, and composites. Many other prairie species show equally well-developed absorbing organs. Pitcher's sage (Salvia pitcheri), one of the numerous prairie mints, frequently gives rise to a dozen tough, rather woody roots from a single inch of its rhizome. These may run obliquely to horizontal distances of 2 feet from the base of the plant before turning downward, but others grow almost straight downward through distances of 5 to 9 feet. As in many other dicotyledons, the extensive roots are only moderately well branched (Fig. 39).
Fig. 39.--Root system of Pitcher's sage (Salvia pitcheri).
The prairie rose (Rosa arkansana) possesses a large, woody taproot which penetrates even deeper than that of any of the preceding species. Depths of 15 to over 21 feet have been determined. It usually pursues an almost vertically downward course and is poorly supplied with major branches some of which may have a horizontal spread of 4 feet. Many fine branchlets occur, however, and frequently, the larger ones break up into groups of long slender rootlets well fitted for absorption.
Of 43 species selected as typically representative of the tall-grass prairie flora, only 14 per cent absorb almost entirely in the surface 2 feet of soil; 21 per cent have roots extending well below 2 feet but seldom beyond 5 feet; but 65 per cent have roots that reach depths quite below 5 feet, a maximum penetration of 8 to 12 feet being common.
Conditions Indicated for Crop Growth.--The presence of a continuous cover of tall, deeply rooted grasses indicates conditions favorable for the production of cultivated plants of similar habit, a fact fully substantiated by the excellent yields of wheat, oats, and corn. The continued growth of these grasses throughout the season, with the late period of flowering and seed production among most of them, indicates a long favorable growing season uninterrupted by a deficiency of soil moisture. The abundance of water in soil and subsoil is further attested by the presence of so many other herbs, many of which extend much deeper than the grasses and absorb the water that percolates downward through the surface soil. There is water enough for both grasses and legumes, as well as composites, etc. The possibility of so many plants growing in a given area, often 200 to 250 individuals or groups of individuals in a single square yard, is due largely to the fact that the roots absorb at different soil levels, the tops making their development at various heights and at different seasons of the year. Thus, June grass, wild alfalfa, and little bluestem thrive in the same square foot, each absorbing at a different level, each producing seed at a different height and at a different time, really three crops in one area. Following this example, under intensive agricultural conditions, two cultivated crops might well be grown in the same field at the same time. Native plants show clearly that this is feasible. The common practice of sowing a mixture of grasses or grasses and other plants in pastures gives not only a greater production and variety of forage but more continuous grazing and possibly a better balanced food ration.
The deeply rooted species have favorably modified the subsoil to great depths, enriching it with nitrogen, adding humus by root decay, as well as making it more porous, and as a result of absorption, vast stores of nutrients have been brought from the deeper soils and, upon the death of the tops, deposited in the surface soil. Thus the tall-grass prairie furnishes the most productive region for agriculture.
This community extends over areas in western Nebraska, and includes much of the western half of Kansas, eastern Colorado, western Oklahoma, northwestern Texas, and northern New Mexico. 43 According to Shantz, 184 it also occupies extensive areas in eastern Wyoming and Montana. Outlying detached areas also occur south and west of the Rocky Mountains. While the tall-grass prairie may be likened to a luxuriant meadow, the short-grass plains simulate a closely grazed pasture (Fig. 40).
Fig. 40.--Typical view of short-grass plains in eastern Colorado. Over 90 per cent of the vegetation consists of buffalo grass and blue grama grass.
The grasses are truly short; the leafy stems are usually only 4 to 8 inches tall, although the flower-stalks may be 12 to 18 inches high. Absorption regularly takes place in the 16 to 24 inches of surface soil, below which dry subsoil occurs. The grasses form a low mat or sod due to extensive propagation by rhizomes and stolons. In the drier portions, much soil surface is exposed, but under more favorable moisture conditions, the sod mats are more nearly continuous. Because of deficiency of soil moisture and severe summer drought, the vegetation matures early, seeds ripening within 30 to 60 days after the inception of growth. The grasses "cure" on the ground but may resume growth upon the advent of opportune showers. Precipitation is so limited that the soil is seldom moist below a depth of 2 feet (See Table 2, Chapter I). Water penetrates slowly, owing in part to the high water-retaining power of the surface layers of fine sandy loam soils and also to the vigorous absorption by the short grasses. The small amount of moisture stored during the non-growing season in the foot or two of surface soil, together with the rainfall of spring and early summer, may enable growth to continue until early in July, when usually all the soil moisture is exhausted. As a consequence, deeply rooted tall grasses and other herbs are practically excluded, and the typical short-grass cover is very uniform and monotonous as a result. During unusually dry years, even short grasses may fail to flower, but during exceptionally wet ones, growth may continue almost without interruption. The continued penetration of water to only 16 to 24 inches has resulted in a concentration of the leached salts which form a carbonate layer varying from 8 to 24 inches in thickness and sometimes occurring at depths of only 8 inches. Below the hardpan occurs a dry subsoil. 224 Hindering water penetration by vigorous absorption, the native vegetation has exerted a profound effect upon soil structure and soil profile in the short-grass plains. 132
When the natural vegetation is destroyed by cultivation, the depth of moisture penetration is increased even if the land is continuously cropped, and with alternate years of cropping, this is even greater. In the former case, water seldom penetrates below 2 to 3 feet, and under the latter practice, only rarely to 5 or 6. 184 Thus, owing to low precipitation, usually less than 20 inches, high run-off, and great evaporation, the deeper subsoil is constantly dry. Under conditions of cropping, root activity is confined, as among native species, to the surface layers in which the available moisture supply is exhausted almost annually.
The important grasses of the plains are few in number and quite similar in habit. Chief among them are the blue and hairy grama grasses, buffalo grass, Muhlenberg's ring grass, and wire grass.
Blue Grama Grass (Bouteloua gracilis).--Blue grama grass ranks with buffalo grass as one of the most important forage plants of the short-grass plains. It is well adapted to a region of low rainfall. The aboveground parts are not extensive, and the fibrous roots are exceedingly fine and spread widely in the surface soil, often to distances of 12 to 18 inches (Fig. 41). They are so numerous and so exceedingly well furnished with fine laterals that every cubic inch of the surface 2 feet of soil is filled with these highly developed absorbing organs. It is of interest to note that many roots extend well into the carbonate layer, some to depths of 4 feet, indicating that, during unusually favorable years, water penetrates to this depth. This is further shown by the presence of the leached carbonates, the bottom of the layer sometimes being 4 feet below the soil surface.
Fig. 41.--Widely spreading and relatively shallow root system of blue grama grass (Bouteloua gracilis).
Hairy Grama (Bouteloua hirsuta).--This is a species of very wide range and diverse habit, forming a sod in the more favorable situations but often occurring in isolated clumps. It is even more drought resistant than blue grama, reaching its best development on stable sandy or sandy loam soils. The rhizome habit is conspicuous. The roots spread 1 to 1.5 feet or more just below the soil surface, although this habit varies with soil type and moisture conditions. Roots are abundant, exceedingly Well branched with delicate laterals, and fill the soil more or less completely to depths of at least 2 to 3.5 feet (Fig. 42)
Fig. 42.--Three-months-old hairy grama grass (Bouteloua hirsuta). Like the blue grama, it is well adapted to dry soil.
Buffalo Grass (Bulbilis dactyloides).--Buffalo grass, like grama, furnishes excellent forage both summer and winter, the short carpet of leafy stems, interwoven with stolons, curing on the ground. In root habit, it is very similar to grama grass and equally well adapted to dry soils. The abundant, tough, wiry roots, often arising in groups of 3 to 10, not only fill the soil below the sod mat but usually spread 12 to 18 inches on all sides. Many are so shallowly placed that they may absorb when only an inch or two of surface soil is moist. Others penetrate directly downwards, filling the first 2 to 3 feet of soil with great masses of finely branched roots. The deeper, rather constantly dry soil is also occupied to a certain extent although absorption must occur here only during years of very exceptional rainfall. For example, in the buffalo-grama-grass range at Akron, Colo., the soil below 2 feet was moist only once in 9 years, while during several years, no available moisture was recorded in the second foot. 184 Very similar results have been obtained at Burlington, Colo.
Muhlenberg's Ring Grass (Muhlenbergia gracillima.).--This grass is less important than the preceding. It has a mat-like growth, with very short stems and leaves which curl during drought like those of buffalo and grama grass. Great clusters of fine, much-branched roots, many spreading widely near the soil surface, completely fill the soil to the carbonate layer, frequently at a depth of about 2 feet, while a few penetrate into it.
Wire Grass (Aristida purpurea).--Where the soil is somewhat lighter, owing to a greater proportion of sand, water penetration is greater and wire grass occurs in the short-grass mat. This species quite overtops the short grasses, being 8 to 16 inches high. Its roots are not distributed so near the soil surface, nor are they so fine as those of the short grasses (Fig. 43). Where wire grass occurs in any abundance, as in old roads, abandoned fields, or somewhat sandy soils, it clearly indicates areas where water penetration is deeper. In the somewhat sandy wire-grass areas, roots of cultivated crops may develop more normally than in pure short-grass land. Here, drought occurs later in summer. Even in such areas, however, the plants pass into a drought-rest condition usually during late July when soil water is exhausted.
Fig. 43.--Wire grass (Aristida purpurea) from the short-grass plains.
Root Habits of Non-grassy Species.--Since the preceding grasses, with others of similar habits, annually absorb nearly all of the available water, relatively few other herbs are present. Thus, the monotony of the short-grass cover, especially in the drier portions of the short-grass plains, is scarcely interrupted. However, numerous shallow-rooted annuals, various cacti, and certain legumes, etc., occur more or less sparingly.
The cacti are especially well fitted for growth in this dry region. Except for a few anchorage roots, the whole root system is confined to the eight inches of surface soil. The root habit of the Comanche cactus (Opuntia camanchica) is representative of many species. A single plant may have over 20 roots which run in the surface soil, usually at a depth of about 1 inch and seldom deeper than 3 inches, to distances varying from 6 inches to 6 feet. They are usually branched repeatedly from their origin to their extremities with both large and small branches which ramify in All directions and thus furnish an enormous absorbing surface (Figs. 44 and 45). They benefit even from the water furnished by light showers, competing vigorously with the short grasses. Where overgrazing occurs, they greatly increase in number.
Fig. 44.--Top view of surface roots of Comanche cactus (Opuntia camanchica). Scale in square feet.
Fig. 45.--Anchorage roots of cactus shown in Fig. 44.
Ground plum (Astragalus crassicarpus), loco weed (Aragallus lamberhi), and wild alfalfa (Psoralea tenuiflora) are representative legumes. The first two require a relatively short growing season to mature seed. All possess fairly deep taproots (Fig. 46). They occur only scatteringly where the short grasses deplete the soil of moisture, their presence usually indicating a slight depression where water runs in, or a soil disturbance such as is occasioned by rodent burrows. These burrows are very numerous throughout the short-grass plains and afford an entrance for water to the subsoil. An abundance of legumes, such as wild alfalfa, indicates, as does wire grass, a soil of looser texture and, consequently, greater water penetration. These legumes depend scarcely at all upon the first foot for moisture but. branch widely, if not profusely, in the second to fifth foot of soil, many roots penetrating even deeper.
Fig. 46.--A loco weed (Aragallus lambertii) common on the short-grass plains. Its depth of penetration is characteristic of many legumes.
Conditions Indicated for Crop Growth.--The short-grass plains is the last frontier of agriculture in North America, and the problem of land utilization is an important one. The agricultural significance of the vegetation of the short-grass plains is very distinct from that of the tall-grass prairie. 9 It is a region of dry farming, grazing, and crop production under irrigation. Short grasses characterize areas where each year, all available moisture is used by the plants, the supply often being exhausted early in the summer. The presence of a carbonate layer at a depth of 1 to 2 feet indicates the usual depth to which water penetrates and delimits the area in which absorption of water and nutrients usually occurs. The low stature of the plants is correlated with drought. Tall grasses with leaves exposed on elongated stems are not so well fitted to withstand dessiccation, nor are their roots so well adapted to absorb moisture from the surface few inches of soil. Roots fitted for surface absorption are an essential adaptation of plains grasses. In fact, when either buffalo or grama grass grows in moist soil, this root character almost entirely disappears.
Cultivated plants too, when grown here, must adapt themselves to these conditions, although the removal of the sod and the maintaining of a cloddy mulch permits somewhat greater water penetration. Their response is similar to that of the native plants, i.e., low stature and shallow but widely spreading root systems. These are much more profusely branched than normally, many roots occurring just beneath the surface of the soil. Early maturing crops like winter wheat, although of uncertain but sometimes heavy yield, do best. Like the rapidly maturing plains grasses, they may ripen seed before the soil moisture is exhausted. Many small grains and short-season corn are widely grown, and the sorghums are well represented in the southern part of the area.
The high productivity of soil and subsoil is clearly shown where irrigation water is applied. In the moistened soil, abundant crops of alfalfa, sugar beets, and other deeply rooted plants, under the otherwise favorable climate, produce excellent yields. 104 Owing to the uncertain distribution of precipitation, however, crop production without irrigation is always hazardous. Much of the area should always remain unbroken range land. Crops that develop late, such as maize, are best adapted to wire-grass land or other areas where deeper soil moisture is indicated by the abundance of the deeply rooted wild alfalfa and other legumes. Crop roots may here develop more normally and, during favorable years, like those of the native species, secure enough moisture to escape drying out. Increased sandiness of the soil up to a certain limit is very favorable, as the fertility still remains quite high. Such areas, most extensive on the eastern border of the shortgrass plains, are at once demarked by the occurrence of deeply rooted tall grasses which thrive on the water penetrating through the surface mats of short-grass roots. Here, the vegetation merges into mixed prairie, and possibilities for crop production are greatly increased.
This grassland community covers a wide area between the tallgrass prairie on the east and the short-grass plains on the west. It occupies considerable portions of eastern and central Montana, central and western North and South Dakota, eastern Wyoming, and the central parts of Nebraska and Kansas. 43 It extends farthest west on sandy soils which permit the water to penetrate readily, frequently covering isolated sandy areas surrounded by short grasses. Pure short-grass cover extends farthest east on heavy clay soils into which water penetrates slowly, a condition unfavorable for deeply rooted tall grasses. Mixed prairie is limited on the east by increased soil moisture sufficient to support a continuous growth of tall-grass vegetation which shades out the under story of short grasses. 45
The most significant difference from tall-grass prairie is the almost universal presence of one or more short grasses or sedges as a lower layer under the taller prairie species. The tall grasses are thus intimately mixed with the shorter ones. In lowlands of greater water content, the tall grasses dominate; on dry hilltops, they may almost completely give way to short grasses. But over the area as a whole, the medium water relations limit the growth of the tall grasses which frequently resort to the bunch habit, short grasses occupying the interspaces (Fig. 47). On the western and southern borders, soil moisture is usually entirely exhausted by midsummer and the subsoil is always dry. Here, the carbonate layer varies from 20 to 30 inches in depth. During July, the plants pass into a drought-rest condition from which they may be revived by occasional showers. But where the deeper-rooted, later-blooming andropogons form the upper story, this occurs less frequently, and through periods of wetter years, the subsoil may be permanently moist. However, no moisture is lost to the vegetation by percolating beyond the root depth. This is shown by the presence, even on well-drained silty or clayey soils, of a layer of carbonate accumulations at a depth of 5 to 6 feet, rainfall not affording enough moisture to percolate through and carry it away. With increasing water penetration, the carbonate layer becomes deeper and in the tall- grass prairie entirely disappears.
Fig. 47.--Mixed prairie in north central Kansas. Short grasses (buffalo and grama grasses) in the foreground intermixed with or forming a layer under the bluestems and other tall grasses over the rest of the area.
Many species of both adjacent grassland communities are represented. Among the most important tall grasses are wheat grasses, needle grasses, June grass, little bluestem, and wire grass. The short-grass layer is made up largely of buffalo grass, blue and hairy grama, and certain grass-like sedges.
Western Wheat Grass (Agropyron smithii).--This species is well fitted to compete with the short grasses and occurs over extensive areas. Its excellent rhizomes and abundant roots produce a dense sod. If at all abundant, it is an indicator of deep soil moisture, but in thin stands and as dwarfed individuals, it occurs in rather dry situations. Height growth is closely related to the water supply. During years of drought, it makes little or no growth, and the whole area may appear to be dominated by short grasses. But during years of normal or more than normal rainfall, the developing wheat grass has the appearance of thinly planted fields of waving grain and yields much hay, the short grasses. forming a mat-like cover over the ground beneath. The numerous roots spread but little in moist soils but rather extensively in drier ones. Many penetrate vertically downward, branching profusely (Fig. 48). The depth of penetration varies with the presence of subsoil moisture from 4.5 to over 8 feet. It is of great interest that the behavior of the roots of wheat, in these respects, is very similar to that of wheat grass.
Fig. 48.--Roots and rhizomes of western wheat grass (Agropyron smithii).
Needle Grass (Stipa comata).--Needle grass is widely distributed throughout the mixed-prairie region. It is a tall grass with the bunch-forming habit. Many of the exceedingly numerous fibrous roots run at various oblique angles, some of them having a lateral spread of more than 18 inches from the base of the plant. Others penetrate more or less vertically downward, reaching depths, which vary with subsoil moisture, of 3.5 to 5 feet. Beginning at the very surface of the soil, the roots. are clothed with relatively short but well-branched laterals, great masses of absorbing rootlets occurring in the surface 2 to 3 feet of soil (Fig. 49). Thus, western needle grass is provided with a much finer, more branched, and more widely spreading root, system than its sister species of the tallgrass prairie. It is therefore more successful in competition with short grasses.
Fig. 49.--Roots of needle grass (Stipa comata), nearly 5 feet in length.
Little Bluestem (Andropogon scoparius).--This species is a dominant in tall-grass prairie and is also an important mixed-prairie species. Its wide range can be largely explained by a consideration of its excellent and plastic root system, its methods of propagation, and its sod- and bunch-forming habits. Its development in the moist subsoil of tall-grass prairie has been described previously. In the drier mixed prairie, surface branching and lateral spread are pronounced. Here, some roots spread laterally nearly parallel with the soil surface but more often somewhat obliquely downward to distances of 12 to 15 inches before turning directly downwards. These, with the more or less vertically descending roots, form a dense sod, and the soil is filled with finely branched rootlets to depths varying from 3.5 to 4.5 feet. Many of the branches are over 2.5 feet long and are rebranched to the third and fourth order. Profuse branching occurs to the root ends which in dry soil form veritable mats of rootlets, some extending to depths of 5 to 6 feet.
Wire Grass (Aristida purpurea).--The roots of wire grass (Fig. 43) likewise vary under different conditions of environment. In mixed prairie they are usually deeper (about 4 feet) than those growing as isolated clumps in the hard soils of the short-grass plains where the dwarfed plants absorb mostly, if not entirely, in the 2 to 3 feet of surface soil. Where the soil contains more sand, lateral surface spreading and branching are more pronounced.
Short Grasses and Sedges.--As regards the short-grass layer, all of its constituent species absorb at greater depths in mixed prairie than is normally possible in the short-grass plains. Buffalo grass produces great mats of roots to depths of 3.5 to 5 feet, while roots are frequently found in the sixth and seventh foot of soil (Fig.50). As subsoil moisture becomes more abundant, the lateral spread in the shallower soil layers becomes less and less pronounced. This is true, also, of the hairy and blue grama grasses. In areas with dry subsoil, a surface lateral spread of 1 to 2 feet on all sides of the clump is not uncommon, but this portion of the root system fails to develop where water is more plentiful in the deeper soil. Blue grama is not so deeply rooted as buffalo grass but may absorb to depths of over 4 feet when competing with wheat grass or other deeply rooted tall grasses.
Fig. 50.--Typical root development of buffalo grass (Bulbilis daciyloides) in mixed prairie.
Sod-forming species of Carex are similar to short grasses in aboveground development and in root habit, and sometimes play a rôle of almost equal importance. They are important range species, since they furnish considerable forage early in the spring before many of the grasses have resumed growth. "Nigger wool" (Carexfilifolia), for example, has tough, black, wiry roots which bind the surface soil so firmly that new roads through the grassland are very rough for a long time until the root clumps are penetrated. The roots seldom descend vertically but run obliquely away from, as well as under, the plant, forming a great tangled mat to a depth of 1.5 feet. Some of the longer roots spread 2.5 feet in the surface 6 inches of soil. They are often abundant to 4 feet in depth, and some, ending in brush-like branches, terminate in the fifth foot of soil.
Root Habits of Non-grassy Species.--Since competing tall and short grasses reduce the soil moisture of mixed prairie more completely than does the vegetation of tall-grass prairie, other plants are usually less abundant. This is particularly true in the drier portions of the mixed-prairie area. However, the more drought-resisting, non-grassy herbs of tall-grass prairie, as well as species from the short-grass plains, are quite common, even if less abundant. Practically all, except the short-rooted June grass, cacti, and certain short-lived annuals, absorb below the second foot of soil. The rather abundant legumes (loco weeds, ground plums, prairie clovers, etc.) are characteristically deeprooted, usually depending little upon absorption in the surface foot (Fig. 51).
Fig. 51.--A portion of the root system of psoralea (Psoralea tenuiflora). It is not adapted to absorb from the surface soil.
The same is true of the composites (goldenrods, asters, sages, brownweed) as well as such conspicuous species as yucca, milk pink, and prickly poppy. Working depths of 5 to 9 feet are frequent (Fig. 52).
Fig. 52.--Brownweed (Gutierrezia sarothrae). Like buffalo and grama grasses, it is well adapted to secure water when light showers wet only the surface soil.
Root Habits of Sandhill Plants.--In very sandy soil, little or no precipitation is lost in run-off, water penetrates readily to great depths and a dry sandy mulch effectively retards evaporation. Here occur the characteristically deep-rooted, sandhill grasses and accompanying herbs.
Blow-out Grass (Redfieldia flexuosa).--This species roots at depths of 5 to 7 feet. The tough, wiry, much-branched rhizomes, sometimes 20 to 40 feet or more in length, extend in all directions, and with the multitudes of roots originating from them, which likewise run in every direction, some spreading laterally 4 to 5 feet, form a tangle which is exceedingly efficient in preventing soil blowing (Fig. 53).
Fig. 53.--Blow-out grass (Redfieldia flexuosa). Note the great extent of rhizomes and roots compared to the sparse, partlyburried, aboveground parts.
Sand Reed Grass (Calamovilfa longifolia).--Sand reed grass has strong rhizomes, sometimes intermixed with roots in shifting soils to depths of 2.5 feet. Usually the roots penetrate to depths of 6 to 10 feet. Multitudes of short but repeatedly branched laterals fit it remarkably well for absorption throughout its entire extent, the surface soil being occupied by many widely spreading rootlets.
Sandhill Bluestem (Andropogon hallii).--This bluestem has a wonderfully branched and extensive root system which thoroughly ramifies the sandy soil from near the surface to depths of 7 to 10 feet (Fig. 54). Thus, the fairly dry but deep, wellaerated soil of low nutrient content is conducive to deep root development, wide lateral spread, and very profuse branching.
Fig. 54.--Root system of sandhill bluestem (Andropogon hallii).
Little Bluestem (Andropogon scoparius).--Along with the sandhill bluestem and numerous other species of more stable sandy loams, bunches of little bluestem may reach a diameter of 1.5 to 2 feet. The roots spread widely on all sides of the clump, some to distances of over 3 feet in the surface 6 inches of soil Some of the deeper oblique roots send up more or less vertical branches which end near the soil surface. While many of the roots run out obliquely at all angles between the vertical and horizontal and thus furnish an excellent surface-absorbing system, others penetrate straight downward. These, with the oblique roots, which often turn downward and reach a depth of several feet, provide for, absorption in the deeper soil. Branching is very profuse throughout, the root ends usually being well supplied with branches. This species absorbs to depths of 6 to 8 feet.
Other Sandhill Species.--Various sandhill sages absorb to depths of 8 to 10 feet, and legumes and pentstemons to soil levels almost or quite as deep. Many composites are likewise deeply rooted, and the bush morning-glory, roses, and certain other species grow even deeper (Fig. 55).
Fig. 55.--A portion of the underground parts of a sandhill legume (Psoralea lanceolata). Note the abundance of tubercles at a depth of 7 feet.
Of 45 mixed-prairie species characteristic of sandhills, only 9 per cent have roots confined to the surface foot or two of soil; 18 per cent have few or no roots which carry on absorption in this area, and 73 per cent of the species are supplied with an absorbing system of such a character as to get water and solutes from both the shallower and deeper soil layers.
Only 11 per cent of the mixed-prairie species on non-sandy lands are shallow- rooted, 28 per cent have little or no provision for surface absorption, while 61 per cent are fairly deeply rooted and well adapted to absorb water when the surface soil only is moist. Smaller absorbing rootlets are somewhat better developed in sandy soil, but excellent development of surface-absorbing rootlets is marked in both groups.
Conditions Indicated for Crop Growth.--Since the silt and fine sandy loam soils of tall-grass prairie, short-grass plains, and mixed prairie are all very rich, differences in root habit must be due to other factors than nutrients. Soil temperatures are very similar in the three communities, and soil aeration excellent in all. Therefore, the most important and determining factor in root variation is water content of soil. The presence of short grasses, with their root systems excellently distributed for surface absorption, together with the marked development of the shallower portion of the root system of many of the taller ones, points at once to their dependence upon moisture afforded the surface soils by light showers. The great masses of finely branched roots of both tall and short grasses occurring in the deeper soils indicate available water content at these levels also. But the absence of a continuous cover of tall grasses shows at once a periodic deficiency in the water supply. The less abundant the late maturing tall grasses (under conditions undisturbed by grazing or otherwise) and the more abundant those of a shorter growing season, the greater is the probability of midsummer soil water exhaustion and crop failure.
It is through the mixed prairie that the highly productive farm lands of the tall- grass prairie give way to the less productive ranch lands of the short-grass plains. Corn growing becomes less important, listing becomes a common farm practice, and the relative acreage of wheat is greatly increased. Timothy and clover give way to wild grasses and alfalfa; the carrying capacity of pastures gradually decreases. 9 Water content of soil is the controlling factor. Here, crops root at intermediate depths, varying locally with seasonal distribution of moisture, but relying less largely upon absorption from the surface foot than in the short-grass plains and more upon moisture in the deeper soil.
In soil that is intermixed with sand, which is not so abundant as greatly to reduce productivity, crops, like native plants, may root more deeply and produce a fair yield even during years of moderate drought. During normal and exceptionally favorable years, larger yields are obtained on the silt loam and clayey soils.
It is of great interest that the grassland of southeastern Washington and northern Oregon occupied areas now yielding a notably high production of the smaller cereals, particularly spring and winter wheat. This is a region of moderate winter and low summer precipitation. The silt loam soils are usually deep and have a high water-retaining capacity. In early summer, the surface soil layers lose all their available water, a fact indicated by the early maturing of certain shallow-rooted grasses. As the season advances, drought occurs in the deeper soil, the subsoil usually being quite thoroughly depleted of its moisture. 219 Wheat bunch grass (Agropyron spicatum), the most important species, is a sod former in the better watered areas but resorts to the bunch habit over much of its range. It matures early, dries out in July, and renews growth upon the advent of the autumn rains. Little provision is made for absorption in the surface soil, the root system penetrating to about 4 feet. Many of the other species absorb in the first 4 to 6 feet of soil; a few penetrate deeper. 216 Nearly all mature by midsummer, late-blooming grasses being noticeably absent. Thus, the root habits as well as the aboveground behavior of the vegetation, and especially as regards the dominant wheat grass, indicate a set of environmental conditions which, by actual farm practice, has been shown to be very favorable to the growth of wheat.
As already pointed out, practically no studies have been made on the root habits of native species in other crop-producing regions. For example, we are almost totally ignorant of root behavior in our eastern forests. But enough work has been done on shrub-land species, such as coralberry (Symphoricarpos symphoricarpos), sumac (Rhus glabra), and hazel (Corylus americana), which fringe these forests, to show that they, like the species of tall-grass prairie, are very deeply rooted and occupy soils of good water content and usually of high productivity.
The native plant cover integrates the environmental complex of a region and, when interpreted in terms of the behavior of its dominant species, indicates in a general way how crops are likely to. develop when grown in the area. Where water content of soil is the chief limiting factor to growth, a knowledge of root extent and the length of the growing period are important factors. The luxuriance and diversity of the vegetation are also good indicators of crop possibilities. The deep-rooting habit, long-growing season, variety, and luxuriance of tall-grass prairie species all indicate favorable moisture conditions in soil and subsoil throughout the entire summer. It is here that many. cultivated plants make their largest yields. Likewise, the shallow-rooted plants of the less diversified and relatively scanty vegetation of the short-grass plains, together with their shorter period of growth, indicate clearly less favorable or even hazardous conditions for crop production. The development of both native vegetation and crops is intermediate in the broad intervening area of mixed prairie. The indicator significance of the root habits of native plants is thus of much practical value.