ROTATION AND DISTRIBUTION.
It has been observed by foresters that there is a tendency in any soil which has long been occupied by a certain kind of timber, to produce, after the trees have been felled, a very different kind, if it be left to its spontaneous action. The laws affecting such rotations have been very well ascertained, and a careful investigation of the subject would undoubtedly reveal many curious facts not yet known. If the stumps of the trees, consisting of oak, ash, maple, and some other deciduous kinds, remain after the wood is felled, they will throw up suckers, and the succeeding timber will be an inferior growth of the original wood. But if the stumps and roots of the trees should be entirely removed, it would be more difficult to determine what would be the character of the next spontaneous growth. It would probably be planted by the kinds that prevail in the neighboring forests, and it would depend on the character of the soil whether the hard or soft-wood trees would finally predominate.
There is an important chemical agency at work, that originally determines the distribution of forests, and afterwards their rotation. The hard-wood trees require more potash and a deeper soil than the coniferous and soft-wood trees. Hence they are found chiefly on alluvial plains and the lower slopes of mountains, where the soil is deep and abounds in all valuable ingredients for the support of vegetation. Pines and firs, on the contrary, though frequently discovered of an immense size on alluvial soils, are generally crowded out of such grounds by the superior vigor of the hard-wood trees; and they can only maintain their supremacy on barren and sandy levels, and the thin soils of mountain declivities, too meagre to support the growth of timber of superior kinds. But a wood must stand a great many years, several centuries perhaps, after its spontaneous restoration, before this order of nature could be fully established. We must observe the spontaneous growth and distribution of herbaceous plants in different soils to ascertain these laws, which are the same in a field as in a forest.
When any growth of hard wood has been felled and the whole removed from the ground, the soil, having been exhausted of its potash, cannot support a new and vigorous growth of the same kind of timber. The succession will consist of a meagre growth of the same species from seeds already planted there; but the white birch and poplar, especially the large American aspen, usually predominate in clearings in this part of the country. When a pine wood is felled, it is succeeded by an inferior growth of conifers, and a species of dwarf or scrub oak. Seldom, indeed, after any kind of wood has been cut down and carried away from the spot, can the exhausted soil support another that is not inferior in quality or species. Though an oak wood may be succeeded by pines, a pine wood will not be succeeded by oaks or any other hard timber, unless the trees were burned and their ashes restored to the soil. Hence we may account for the fact that poplars, white birches, and wild-cherry-trees, occupy a larger proportion of the ground that is now covered with wood than they did a century ago, in all parts of the country.
I have already alluded to the well-known fact, that the generic character of the timber, in the distribution of the primitive forest, in any country, is determined in great measure by the geological character of the soil. On sandy plains in the primitive forest, the white birch, the poplar, the aspen, and the pitch pine were abundant, as they are now on similar soils. The preference of the red maple for wet and miry soils is well known; while hard maple, oak, beech, and hickory do not prosper except in strong alluvial tracts. A heavy growth of hard timber indicates a superior soil; pine indicates an inferior one, if it has been left to the spontaneous action of nature. In the primitive forest we were sure of finding such relations of soil and species. They are not so invariable since the operations of agriculture have interrupted the true method of nature.
When a wood has been burned, the process of renewal, when left to nature, is much more tardy than if it had been felled, since it can now be restored only by a regular series of vegetable species, which must precede it, according to certain inevitable laws. The soil, however, being improved and fertilized by the ashes of the burnt timber, is in a chemical condition to support a luxuriant forest as soon as in the course of nature it can be planted there. Trees will not immediately come up from this burnt ground as in a clearing; and if they should appear, they would mostly perish from the want of protection. In the order of nature herbaceous plants are the first to occupy the soil, and these are followed by a uniform succession of different species. There is an epilobium, or willow herb, with elegant spikes of purple flowers, conspicuous in our meadows in August, which is one of the earliest occupants of burnt ground, hence called fireweed in Maine and Nova Scotia. The downy appendage to its seeds causes it to be planted there by the winds immediately after the burning. The trillium appears also in great abundance upon the blackened surface of the ground in all wet places. Plants like the ginseng, the erythronium, and the like, whose bulbs or tubers lie buried deep in the mould, escape destruction, and come up anew. These, along with several compound plants with downy seeds, and a few ferns and equisetums, are the first occupants of burnt lands.
But the plants mentioned above have no tendency to foster the growth of young trees. They are, however, succeeded by the thistles and thorny plants, which are nature’s preparation of any tract, once entirely stripped of vegetation, as a nursery for the seedlings. All the phenomena of nature’s rotation are but the necessary giving place of rapid-growing and short-lived plants to others which are perennial and more capable of maintaining their ground after being once planted. Thorns and thistles soon appear on burnt lands, and protect the young trees as they spring up, both from the winds and the browsing of animals. Thus many an oak has been nursed in a cradle of thorns and brambles, and many a lime-tree growing in a bower of eglantine has been protected by its thorns from the browsing of the goat.
We very early discover a variety of those woody plants that bear an edible fruit, which is eaten by birds and scattered by them over the land, including many species of bramble. The fruit-bearing shrubs always precede the fruit-bearing trees; but the burnt land is first occupied by those kinds that bear a stone-fruit. Hence great numbers of cherry-trees and wild-plum-trees are found there, as the natural successors of the wild gooseberry and bramble-bushes. These are soon mixed with poplars, limes, and other trees with volatile seeds. But oaks, hickories, and the nut-bearing trees must wait to be planted by squirrels and field-mice and some species of birds. The nut-bearers, therefore, will be the last to appear in a burnt region, for the little quadrupeds that feed upon their fruit will not frequent this spot until it is well covered with shrubbery and other vegetation. If the soil be adapted to the growth of heavy timber, the superior kinds, like the oak, the beech, and the hard maple, will gradually starve out the inferior species, and in the course of time predominate over the whole surface.
When I consider all these relations between plants and animals, I feel assured, if the latter were destroyed that plant their seeds, many species would perish and disappear from the face of the earth. Nature has provided, in all cases, against the destruction of plants, by endowing the animals that consume their fruits with certain habits that tend to perpetuate and preserve them. In this way they make amends for the vast quantities they consume. After the squirrels and jays have hoarded nuts for future use, they do not find all their stores; and they sow by these accidents more seeds than could have been planted by other accidental means, if no living creature fed upon them. Animals are not more dependent on the fruit of these trees for their subsistence, than the trees are upon them for the continuance of their species. And it is pleasant to note that, while plants depend on insects for the fertilization of their flowers, they are equally indebted to a higher order of animals for planting their seeds. The wasteful habits of animals are an important means for promoting this end. The fruit of the oak, the hickory, and the chestnut will soon decay if it lies on the surface of the ground, exposed to alternate dryness and moisture, and lose its power of germination. Only those nuts which are buried under the surface are in a condition to germinate. Many a hickory has grown from a nut deposited in the burrow of a squirrel; and it is not an extravagant supposition that whole forests of oaks and hickories may have been planted in this manner.
These facts are too much neglected in our studies of nature. A knowledge of them, and a consideration of their bearings in the economy of nature, might have saved many a once fertile country from being converted into a barren waste, and may serve yet to restore such regions to their former happy condition. But these little facts are not of sufficient magnitude to excite our admiration, and they involve a certain process of reasoning that is not agreeable to common minds, or even to the more cultivated, which have been confined chiefly to technology. The few facts to which I have alluded in this essay are such as lie at the vestibule of a vast temple that has not yet been entered. I am not ready to say that no single species of the animal creation may not be destroyed without derangement of the method of nature; for thousands have, in the course of time, become extinct by the spontaneous action of natural agents. But there is reason to believe that, if any species should be destroyed by artificial means, certain evils of grievous magnitude might follow their destruction.
The frugivorous birds are the victims of constant persecution from the proprietors of fruit gardens. Their persecutors do not consider that their feeding habits have preserved the trees and shrubs that bear fruit from utter annihilation. They are the agents of nature for distributing vegetables of all kinds that bear a pulpy fruit in places entirely inaccessible to their seeds by any other means. Notwithstanding the strong digestive organs of birds, which are capable of dissolving some of the hardest substances, the stony seeds of almost all kinds of pulpy fruit pass through them undigested. By this providence of nature the whole earth is planted with fruit-bearing trees, shrubs, and herbaceous plants, while without it these would ultimately become extinct. This may seem an unwarrantable assertion. It is admitted that birds alone could distribute the seeds of this kind of plants upon the tops of mountains and certain inaccessible declivities, which, without their agency, must be entirely destitute of this description of vegetation. But these inaccessible places are no more dependent on the birds than the plains and the valleys. The difference in the two cases is simply that the one is apparent, like a simple proposition in geometry, and the other requires a course of philosophical reasoning to be perfectly understood.