The Range, a Major Resource

HOW RANGE FORAGE GROWS

William G. McGinnies, John L. Retzer

TO UNDERSTAND and be able to practice good range management it is necessary to know the more important range plants and to know how they grow. How they grow is determined by the interactions of the climate, soils, plant competition, and grazing.

Range plants, like all other kinds of plants, are living factories that have importation, transportation, manufacturing, and storage facilities. They differ from man-made factories in that the manufactured products become a part of the plants themselves as new growth or seeds. Raw materials water and nutrients are taken in through the roots and transported through the stems to the leaves where, with the aid of sunlight and air, food is manufactured for life and growth. These food products are then moved to all parts of the plant. When the importation of raw materials through the roots runs low, the manufacturing process is slowed down or ceases altogether. Likewise, when leaves are reduced productive capacity is lowered.

The life of range plants, from seed germination to old age and death, may extend over a few weeks, as in annuals, or many years, as among perennials. Most range grasses and some weeds are perennials; a few grasses and many weeds are annuals. Perennial grasses and other plants that live over the unfavorable periods of the year have an advantage over annuals that die each year in that they can build up food reserves in their roots to be used for early growth the following year.

The annual growth cycle of mountain brome, a widely distributed and palatable perennial forage grass of western mountain ranges, has been carefully studied in Utah. Primary herbage growth begins before winter snow disappears and continues until the beginning of flower-stalk development. Active midsummer growth includes flower-stalk development, flowering, and seed production. During this period leaf growth is less active, but after seed ripening there is an important secondary herbage growth. The storage of food reserves and the beginning of bud development for the following year take place at this time. Dormancy begins with the drying up of seed stalks and leaves. There are three periods of root growth which alternate with herbage growth. The first occurs in early spring after melting of the snow, the second following flower-stalk production, and the third near the end of the season. The precise time when these various growth stages take place depends on favorable temperature and varies with elevation and from year to year.

Early spring is a critical period in the growth of established perennial grasses. As the food reserves stored in the roots and lower stems the previous fall are exhausted, further growth depends on plant food produced in the new leaves. Too early grazing of these leaves will weaken the plant; if such grazing occurs for several years, the plant will starve and die.

The most active period in the development of range grasses is from the time the flower stalk forms until the seeds are ripe. Immediately following and up to the time the seed stalks and leaves begin to dry up, the plant assimilates and stores most of its winter food reserves. This is the second critical period in the development of perennial range grasses. Growth then depends on current food production. Because food production depends on the amount of leafage, the greater the leafage the more food can be produced for use and storage.

If a range grass is grazed rather closely more than once during the season, the interval between such grazing must be sufficient for it to recover fully from each cropping.

In general, the life processes of range weeds and shrubs resemble those of grasses, but there are some noteworthy differences. The root systems of range weeds and shrubs usually penetrate more deeply and widely and often store more food than grasses. However, the perennial weeds and shrubs do not produce leaf and stem regrowth as readily as the grasses.

Vigorous range plants need nitrogen, phosphorus, and potassium. A dozen or more other plant nutrients are needed in small quantities. When ranges are in good condition, these essential elements are usually available in adequate quantities. Important amounts are returned to the soil by decaying herbage left after grazing.

Nitrogen is one of the most important and the most limited of plant nutrients. The air contains an abundance of free nitrogen but in this form it is not available to range grasses. By the action of nitrogen-fixing soil bacteria, which grow on the roots of legumes such as clovers and vetches, the free nitrogen is made available to all range plants. Legumes, however, are seldom abundant on dry, western ranges.

Individual plants with similar requirements and responses tend to group themselves into types. Although the members of these types are in competition with each other, they do contribute to the welfare of the group. Each plant species has its particular place on the range and holds that place as long as the combination of factors affecting it is favorable for its continued growth and development. A vigorous grass range owes its existence to the soil stability, fertility, and reasonably favorable soil moisture conditions maintained by the grass cover. If the stand of grasses has deteriorated, growing conditions are less favorable and the vegetative cover will show an invasion of weeds and shrubs.

Scientific range management, based on a knowledge of how range plants grow, aims to sustain optimum range forage and livestock production and to rebuild run-down ranges.

Available soil moisture is the most important factor in western range forage production. To be used by range plants, soil nutrients must be dissolved in water. Over most of the western range area the annual precipitation is under 15 inches, at best a low amount for plant growth. Unless the soil is mellow and porous not all of this is absorbed, but part is lost to plant growth by surface runoff. In humid regions grasses grow tall and closely spaced because they have adequate soil moisture for such growth. Such conditions also are favorable to the growth of turf-forming grasses. The low precipitation of western ranges, combined with high day temperatures, low relative humidity, high evaporation, high winds, and a high proportion of sunshine, causes plants to use the available water more quickly. These factors in turn favor the production of a thinner stand of vegetation than in humid areas and bunchgrasses rather than turf-forming grasses. Also, growth in height is usually not so great.

Perennial bunchgrasses have fibrous, spreading root systems, which on semiarid ranges may be several times greater than above-ground stems and leaves. If bunchgrasses are as abundant as the normal soil moisture will permit, such as on ranges in good condition, these fibrous roots interlace between the tufts under what often appears to be small, bare soil spaces. This spreading root system helps to keep the top layer of soil mellow and porous and facilitates moisture penetration, which in turn makes more moisture available for growth. If the vegetative stand has become thin, exposing greater areas of the soil surface, or if the top few inches of soil have been eroded, leaving a compact subsoil, then part of the precipitation will run off no matter how dry the soil may be underneath. The more moisture that is lost as surface runoff the less opportunity there is for adequate forage production.

THE AUTHORS--William G. McGinnies, the director of the Rocky Mountain Forest and Range Experiment Station, graduated from the University of Arizona, College of Agriculture, in 1922. He received his doctorate in ecology at the University of Chicago.

John L. Retzer is a graduate of the University of Illinois and Iowa State College. He has worked in the Soil Erosion Service (now Soil Conservation Service), the former Bureau of Chemistry and Soils, as a soil surveyor in California, and, during the war, in the Emergency Rubber Project. He is a member of the staff of the Rocky Mountain Forest and Range Experiment Station.