Soil Grass Conservation

ENVIRONMENT OF NATURAL GRASSLAND

G. P. Barnes

THE FOUR great plant formations of the land surface of the earth are forest, grassland, desert shrub, and tundra. Grasslands are believed to include about a fourth of the area occupied by these types of vegetation and about a fifth of the land surface of the globe.

These four formations reflect chiefly climate. Generally speaking, humid lands are woodlands and dry lands are desert shrub or waste; the grasslands lie between these climatic extremes in zones of intermediate moisture supply. There are many important exceptions to this generalization : Lack of moisture does not explain the marshlands, and some kinds of trees grow even in desert climates, like the Joshua tree of the Mohave Desert. Actually, grasslands will stand greater extremes of environment than forest not only greater aridity but greater cold and will grow in wetter places.

Grass is usually first to invade shallow water, and the marshlands thus created may exist for centuries until the accumulating organic remains of the grasses and the sediment lodging in them create an environment where trees can live. Grassland extends into the frigid zones above the timber line on high mountains, forming alpine meadows. Grass will stand soils with more soluble salts than forest, even though the mangrove tree can grow with its roots in sea water at high tide.

Near the transition zones between the major plant formations, soils may offset the effect of climate and tip the scales in favor of forest, grass, or desert shrub, although the climate might lead us to expect a different formation. In some places we find natural grassland not because forest would not grow there but because the forest has not had time enough or opportunity to invade and occupy them. In Manitoba and Saskatchewan and on Kodiak Island, for example, forest has advanced considerably into grassland during the past century.

Once the land is covered with vigorous turf grasses, it is hard for forest to gain an entry; the tiny tree seedling must compete with the grass for moisture and light. Conversely, grass advances with great difficulty into dense forest, even though it might grow perfectly well were the forest cleared away. Quite often a plant formation dominates in the transition zones until some accident or slight change in environment gives the competing formation opportunity to invade.

The grassland formation tends to dominate when the upper layers of the soil are moist during a considerable part of the year but the deeper layers are too dry for such deep-rooted plants as trees. The soil moisture, therefore, rather than total rainfall, governs the distribution of grasslands. Deep sandy soils that allow moisture to penetrate deeply may support trees in a region that is generally grassland, while adjoining fine-textured soils that allow water to run off or evaporate from the surface before it can penetrate will be grass-covered.

Three broad kinds of grassed lands are recognized: The short-grass land, sometimes called steppe; the tall-grass land, or prairie; and savanna. Each has many variations. The first and third make up the great bulk of the earth's grasslands.

Short-grass lands characterize a large part of the subhumid lands of the Temperate Zone. The largest and best known are the Great Plains of North America and the steppes of Eurasia. The rainfall of the short-grass lands keeps the upper soil layers moist during the warm season in most years, sufficient to support the shallow-rooted short grasses.

The rainfall is not enough to moisten the soil down to the ground water; hence, a permanent dry layer lies beneath the moist layer. The depth of the moist surface layer varies with the rainfall and the soil. Near the humid border of the short-grass land, several years of above-normal rainfall may encourage tall grasses and convert the short-grass land to tall. Sandy soils that permit water to penetrate and moisten a deeper layer of soil will often bear tall grasses in a short-grass region. The western part of the Nebraska sand hills is a good example.

Rainfall in the short-grass lands fluctuates enormously from year to year. Years occur when the rainfall is no greater than might be expected in the desert, and so do years when it equals that in the humid forested regions. Evaporation is rapid in the warm season because of much wind and high temperatures. Farming must be adjusted to these conditions. Land is kept fallow in alternate years to accumulate enough soil moisture for good crops.

Some of the feed produced in the moist years must be stored for use in dry ones to come. Incomes must likewise be husbanded. The soils of the short-grass areas tend to be fertile, but the rainfall in most years is not enough to allow the full potentialities of the fertile soils to be realized. Fertilizers, as a rule, do not produce substantial increases in crop yield because moisture rather than fertility usually limits production. Limestone is seldom needed since the short-grass lands exist under climates where lime accumulates in the soil, instead of leaching out.

The four great plant formations of the earth's land surface are forest, grassland, desert shrub, and tundra. Grasslands are believed to include about one-fourth of the area occupied by these types of vegetation and about one-fifth of the earth's land surface. These four formations reflect chiefly climate. As a rule, humid lands are woodlands and dry lands are desert shrub or waste; the grasslands lie between these climatic extremes in zones of intermediate moisture supply. There are, however, many important exceptions to this generalization.