Botany: The Science of Plant Life

by Norman Taylor

The transition from the tropical rain forest to our own woodlands is one of the most interesting, as it is certainly the most gradual in nature. Lack of space prevents our stopping to note those strategic points along this pathway from a hot, steaming forest to the cool shade of our open woods, where traveling, in at least a virgin forest, may be done easily on horseback. As we come northward, and if we could travel continuously through the forest, we should lose first the epiphytes, then most of the lianes, and finally all the condition of vegetation crowding into every inch of space suitable for it. While trees in our virgin forests are as thick as they can be, the forest floor is open and on it grow only a few herbs that will stand lack of sunshine.

But the really great difference is the long, unfavorable season in temperate regions where the forest must drop all its leaves, after, in at least our own Eastern States, the most gorgeous foliage coloring of any forest in the world. The winter months when the woody vegetation is practically fully exposed to the elements, are particularly severe in their effects. Leaf-fall, which is such a common sight as to arouse scarcely any interest, is the only device by which the great bulk of our forest trees survive, and only in the southern part of the region are there found such woody plants as the mountain laurel, rhododendron, American holly, and a few others which are evergreen but not cone-bearing ones, and are the only reminders of the truly evergreen forests of the tropics. The winter winds farther north and in the central treeless part of the United States prove too much for many kinds of trees, for instance, all the oaks and sassafras, none of which go very far north.

There is sufficient rainfall to produce forests much farther north than they are found, but lower temperatures prevent trees from growing just as too little water stops their growth altogether. Toward the northern limits, or upon high mountains, the upper limits of the forest, we get the best idea of how persistent woody vegetation is in the general forest area of the eastern United States. Stunted, wind-swept and weather-beaten trees are often found only a couple of feet high and over sixty years old. Sometimes they will be flattened out on the ground or on bare rock, making great patches of bushy growth quite unlike their lofty relations in the lowlands. The growth rate for such plants is so slow that their annual rings are all but obscured. With such persistence in the production of these elfin forests, high up on mountains under the most unfavorable conditions, it is little wonder that below this are trackless woods, and that the northeastern United States has one of the finest developments of the temperate or summer forest in the world.

Nor are all our woods of this general type made up of the same species, for everyone knows about the endless spruce and fir forests of the north, exclusively evergreen, and in the summer nearly always moist. This spruce belt stretches practically across the continent, where, in the West, other and our most gigantic evergreens, replace the eastern spruce and fir. A little farther south is the region of the white pine now nearly unknown as a virgin forest type, as its great value led to early and ruthless cutting. The white pine region is generally the area from New England southward along the Alleghenies and westward to Illinois. But the most characteristic of the temperate forest types is our summer forest, so called from its general lack of evergreens and its beautiful green foliage of summer and its bare branches in winter. Beech and birch and maple, in different proportions according to local conditions, predominate in such woods. These hard-wooded trees, with many others that are scattered through them, have been among the most valuable of all the natural plant products of our country and their destruction has been upon such a scale that only in a few places may the virgin forest be seen at the present time. Where it does occur we find the forest floor often with nothing growing on it except a mass of spring flowers which are half matured before the leaves of the forest canopy close out nearly all the light and much water and put them to rest until another year. The great preponderance of spring-flowering herbs in Eastern North America is due to their early warming up before the foliage of the trees cuts off their light. And in some virgin forests of this sort, particularly where there is a large mixture of oak, the writer has seen hundreds of square rods without undergrowth or herbaceous vegetation of any kind. Such places, very rare indeed at the present time due to senseless and wicked cutting, are rather dark, perfectly open to view for hundreds of feet ahead, and dotted only with the huge trunks of the trees that characterize this climax type of the temperate or summer forest.

The absence of direct sunlight and interception of much rain under the forest canopy has other effects besides stopping the growth of herbs and shrubs which are common enough along the edges, or where openings are made by the fall of old trees. It prevents the germination and growth of nearly all the seeds falling from such trees, and in a really virgin forest of this sort, almost no seedlings will be found. Upon cleared or open land thousands of saplings will cover much of the ground, but nearly all these will die off due to crowding, and leave as the climax only enough trees to close over the forest canopy.

Forests may be found in all stages of succession from those just beginning the process to those final forest monarchs which, having won out in the race, are, until one of them falls, often slow to perpetuate the type. For, as often as not, a new growth will spring up once a very large tree falls, and a very different kind of growth from the climax forest. At once a host of species, that one might almost say had been waiting for the tragedy of the monarch’s fall, will rush in and convert the opening into a nondescript brush patch, out of which will rise another tree that means business. As it grows to maturity, it kills off these smaller triflers one by one, until, when the canopy is finally closed, all of them will have disappeared, or, as often happens, retreated to other parts of the forest, where they wait for another chance. This succession of different kinds of growth in a temperate forest is so well known that in England they have for centuries practiced it, for commercial or pleasure purposes. In their oak-hazel copses they cut the trees enough to partly open the canopy, which permits a dense growth of hazel bushes and other plants. Every twelve or fifteen years the latter are cut down for various purposes, and will gradually spring up again to renew the dense growth. The spacing of the trees is sufficient for them to branch freely and yet not close the canopy enough to kill off the hazel. The trees are cut off a few at a time, not oftener than one or two hundred years in any one spot. By this procedure the owner gets a regular crop of hazel once in twelve or fifteen years, occasional big trees, and on many places a cover for pheasants. Under the hazel there is a regular progression of herbs, very plentiful just after the bushes are cut, and decreasing almost to nothing when the end of the growth period of the bushes is near.

The English oak-hazel copse, now much less grown than formerly, and the general lack of undergrowth in our own virgin forests, are both responses of the forest to light and other factors that are related to an open or closed canopy. As we stated a page or two back, it is not impossible, it is even frequent in some parts of the country, for a forest to produce, by its own growth, conditions inimical to its perpetuation. Where the casual falling of a forest giant is the only opportunity which that forest offers to perpetuate its type, it may well be said to be a climax forest, incapable of further development. But in some such woods a curious provision of nature insures an invasion of the gloomy forests by trees less light-demanding than the dominant ones. And often these trees that can get along with less light will capture considerable parts of forests that light-demanding trees could never conquer.

If the soil in which these temperate forests happen to grow is sandy or otherwise poor in plant food, the broad-leaved trees that make our woods such a delight in summer are replaced almost universally by pines. Along the sandy stretches of the coastal plain from Long Island, New York, to the Gulf, there are immense tracts of these pine forests, different species often being locally dominant, such as the pitch pine in the pine barrens of New Jersey and the long-leaf pine farther south. Almost throughout the world there is this monopolization of the poorer and drier forest sites by pines, which maintain the forest plant society in regions where the broad-leaved rapidly transpiring trees could not grow.

No account of forests, however brief, can omit some mention of the greatest agency for their protection in North America, the United States Forest Service. With corporation and individual cutting and attendant fire hazard upon a scale almost beyond belief in its ruthless disregard of our chief natural plant product, the Government soon found that Federal ownership or control of forests was the only policy that would maintain even a partially adequate timber supply. National forests, set aside either for pleasure or profit, now total more than the area of France or than all the New England and most of the Middle Atlantic States. These huge tracts, in every part of the country where forests are found, are well managed, properly planted, and most important of all, constantly guarded against fire. Forest fires not only destroyed over $25,000,000 worth of timber annually, but leaving devastation behind them, depleted the water supply in many parts of the country. Nothing but forests will hold the rainfall, to release it slowly through a thousand rivulets and springs that are the source of countless rivers. With the forest cut or burned off these streams are dry most of the summer and raging torrents for a few weeks in the spring, washing out all the priceless accumulation of the ages which the forest has conserved for its own and our benefit. While the reservation of these great national forests has worked individual hardship, experience for many years back in India and Germany shows Federal ownership or control the only wise policy.

Forest covering, whether temperate or tropical, depends for its occurrence all over the world upon an adequate rainfall. As we have seen in the tropics, this may be so great that coupled with the heat it produces a wealth of vegetation beyond the powers of description. Where it is less and the country cooler, the forests are of a different type, but even there the forest covering is, without interference, practically complete. Where, as in parts of Chile, southeastern Australia, and of Japan, there is a heavy rainfall but cool climate, there is a so-called temperate rain forest. Such forests are cold, drab, wet woods of peculiar aspect and extreme interest. For in them grow trees sometimes related to our own, but, due to the special conditions, producing a forest landscape quite unlike anything in America. It would seem as if we might almost plot the distribution of forests in our own country with a weather map showing rainfall, and such is actually the case. When the rainfall becomes less than will maintain forest growth it stops, often very abruptly. Generally speaking, the region west of the Mississippi, and some just east of it, westward to the mountains, is entirely devoid of forest, except in the river valleys. The forests give place to an entirely different type of vegetation—the prairie or grassland.