Botany: The Science of Plant Life

by Norman Taylor

THE PRAIRIE OR GRASSLANDS

In nearly every region in the world there is an absence of forests and a replacement of them by grasslands, where the rainfall is less than about twenty-five inches a year, and where the winter winds, often far below the freezing point, are hostile to trees. The distribution of the rain mostly through the growing season also makes a condition peculiarly unfavorable for trees during the winter. Someone has said that the nations have fought since the days of the Romans for the belt of grassland which these climatic conditions have produced all round the world, and it is certainly true that these naturally grass-covered areas have produced the cereals of the world, all of which, except rice, grow to best advantage in such regions.

With of course different species of grass and quite different associated herbs, these grasslands are now found in our own prairies, the steppes of Russia, the plains of Hungary, the pampas of southern South America, the grasslands of Australia, the veld in Natal and in many other but mostly less extensive developments. Some of the grassland regions are warm, but without more rainfall than characterizes such areas the greater heat does not produce a forest. Usually, but not always, these grasslands are not found near the coast, where, as in America, the rainfall is double that of the plains and produces the forests that clothe the Atlantic and Pacific sections of the country. From the east westward there is a gradual decrease in the rainfall, until from about the Mississippi to the mountains it falls below the point where trees can compete with the prairie.

Another characteristic of prairies that once they have started tends to keep out trees is their almost annual firing. Tree seedlings cannot survive this, and we know that the Indians fired huge tracts of prairie every year, not to mention fires started by lightning which may set fire to grasslands and actually does set fire to forests every year.

The prairies in the United States—perhaps the most extensive in the world—are characterized chiefly by several grasses, buffalo grass (Buchloë dactyloides), gama grass (Bouletoua oligostachya), and several prairie grasses, such as Sporobolus asperifolius, Koeleria cristata, and some others. Among these, depending on the soil, are hundreds of prairie flowers which, during different parts of a single season, give quite different aspects to the region. Both the grasses and their associated herbs are well protected against too violent transpiration which their exposure to nearly continuous sunshine, high summer heat, and very considerable winds makes particularly active. In many places where the country is rolling, the lower and moister sites, besides developing more luxuriant growth of prairie plants, permits low shrubs, and in river bottoms even trees to flourish. But climatic conditions of small rainfall, high winds, and bitter winters make anything like a forest development out of the question.

In some regions, both in America and central Europe, a rainfall that is high enough to permit trees to exist and low enough to favor at the same time a grassland formation has resulted in the parklike landscape that creates the most beautiful scenic features of the regions where it occurs. In such places there are irregular patches of forest and grassland, and the struggle for supremacy as between the different types depends, not upon the general climatic conditions to which they both respond, but to local conditions of available water supply and soil conditions and often upon fires. Naturally such regions are places of intense strife for dominance, and in them some remarkable collections of plants have been found.

One of the most interesting of these struggles between grassland and woody vegetation in a region climatically able to produce both, is in Natal. Large sections of that country are grasslands or veld, as the people there call it. Scattered through the veld are various species of acacia trees, locally called thorn, with feathery compound leaves. These do not shut out enough light to prevent the development of grasses directly under their shade, yet the annual firing of the veld prevents, except accidentally, the production of the acacias. But the seeds of this tree, whether from long usage to this burning or not, are actually hastened in their germination by the firing, and it is a common practice in that country to roast or partly boil the seeds of the tree to hasten germination. The presence of the climatically favorable environment for both trees and grassland results in the latter being the dominant type of vegetation over large tracts of the country, largely because fires destroy tree seedlings, and yet the tree seeds, by a quite extraordinary fitness for their peculiar environment, offer a measure of insurance against the total destruction of woody vegetation by the grassland.

The pampas of the Argentine have been vividly described by P. G. Lorentz, who, in writing of the drier parts of it, says: “Viewed from a distance, these grasses seem to form a close grassy covering, and the pampa presents the appearance of extensive grassy tracts whose coloring varies with the seasons: coal black in the spring, when the old grass has been burned; bright green, the color of the mature grass; finally—at flowering time—when the silvery white spikes overtop the grass, over wide tracts it seems like a rolling, waving sea of liquid silver.

“After the Gramineæ (grasses), the family of plants that is represented in the pampas by the greatest number of individuals is that of Compositæ (daisy family); usually twiggy undershrubs with inconspicuous flowers, a bright yellow Solidago (golden-rod) alone gleams out from among the others.”

Here, as in the other grasslands of the world, if a local water supply above the general requirements of the grass exists, there is always a small element of woody plants, low, thick-leaved shrubs usually, but where water is more plentiful, trees, as in the region in Natal, already mentioned, and in many other parts of the world, notably parts of Australia, China, Brazil, and many sections of the western part of the United States.