The Earth as Modified by Human Action

by George P. Marsh

There must, I think, be some error or exaggeration in these figures. Perhaps instead of cubic feet we should read square feet. Two hundred millions of cubic feet of timber would require more than half the entire tonnage of England for its transportation.

I suppose the quantities in the following estimates, from a carefully prepared article in the St. Louis Republican, must be understood as meaning square or superficial feet, board measure, allowing a thickness of one inch:

"The lumber trade of Michigan, Wisconsin, and Minnesota, for the year 1869, shows the amount cut as being 2,029,372,255 feet for the State of Michigan, and 317,400,000 feet for the State of Minnesota, and 964,600,000 feet for the State of Wisconsin. This includes the lake shore and the whole State of Wisconsin, which heretofore has been difficult to get a report from. The total amount cut in these States was 3,311,372,255 feet, and that to obtain this quantity there have been shipped 883,032 acres, or 1,380 square miles of pine have been removed. It is calculated that 4,000,000 acres of land still remain unstripped in Michigan, which will yield 15,000,000,000 feet of lumber; while 3,000,000 acres arc still standing in Wisconsin, which will yield 11,250,000,000 feet, and that which remains in Minnesota, taking the estimate of a few years since of that which was surveyed and unexplored, after deducting the amount cut the past few years, we find 3,630,000 acres to be the proper estimate of trees now standing which will yield 32,362,500,000 feet of lumber. This makes a total of 15,630,000 acres of pine lands, which remain standing in the above States, that will yield 58,612,500,000 feet of lumber, and it is thought that fifteen or twenty years will be required to cut and send to market the trees now standing."

See also Bryant, Forest Trees, chap. iv.]

Effects of Forest Fires.

The operations of the lumberman involve other dangers to the woods besides the loss of the trees felled by him. The narrow clearings around his shanties form openings which let in the wind, and thus sometimes occasion the overthrow of thousands of trees, the fall of which dams up small streams, and creates bogs by the spreading of the waters, while the decaying trunks facilitate the multiplication of the insects which breed in dead wood and are, some of them, injurious to living trees. The escape and spread of camp-fires, however, is the most devastating of all the causes of destruction that find their origin in the operations of the lumberman. The proportion of trees fit for industrial uses is small in all primitive woods. Only these fall before the forester's axe, but the fire destroys, almost indiscriminately, every age and every species of tree. [Footnote: Trees differ in their power of resisting the action of forest fires. Different woods vary greatly in combustibility, and even when the bark is scarcely scorched, trees are, partly in consequence of physiological character, and partly from the greater or less depth at which their roots habitually lie below the surface, differently affected by running fires. The white pine, Pinus strobus, as it is the most valuable, is also perhaps the most delicate tree of the American forest, while its congener, the Northern pitch-pine, Pinus rigida, is less injured by fire than any other tree of that country. I have heard experienced lumbermen maintain that the growth of this pine was even accelerated by a fire brisk enough to destroy all other trees, and I have myself seen it still flourishing after a conflagration which had left not a green leaf but its own in the wood, and actually throwing out fresh foliage, when the old had been quite burnt off and the bark almost converted into charcoal. The wood of the pitch-pine is of comparatively little value for the joiner, but it is useful for very many purposes. Its rapidity of growth in even poor soils, its hardihood, and its abundant yield of resinous products, entitle it to much more consideration, as a plantation tree, than it has hitherto received in Europe or America.]

While, then, without fatal injury to the younger growths, the native forest will bear several "cuttings over" in a generation—for the increasing value of lumber brings into use, every four or five years, a quality of timber which had been before rejected as unmarketable—a fire may render the declivity of a mountain unproductive for a century. [Footnote: Between sixty and seventy years ago, a steep mountain with which I am familiar, composed of metamorphic rock, and at that time covered with a thick coating of soil and a dense primeval forest, was accidentally burnt over. The fire took place in a very dry season, the slope of the mountain was too rapid to retain much water, and the conflagration was of an extraordinarily fierce character, consuming the wood almost entirely, burning the leaves and combustible portion of the mould, and in the many places cracking and disintegrating the rock beneath. The rains of the following autumn carried off much of the remaining soil, and the mountain-side was nearly bare of wood for two or three years afterwards. At length a new crop of trees sprang up and grew vigorously, and the mountain is now thickly covered again. But the depth of mould and earth is too small to allow the trees to reach maturity. When they attain to the diameter of about six inches, they uniformly die, and this they will no doubt continue to do until the decay of leaves and wood on the surface, and the decomposition of the subjacent rock, shall have formed, perhaps hundreds of years hence, a stratum of soil thick enough to support a full-grown forest. Under favorable conditions, however, as in the case of the fire of Miramichi, a burnt forest renews itself rapidly and permanently.]

Aside from the destruction of the trees and the laying bare of the soil, and consequently the freer admission of sun, rain, and air to the ground, the fire of itself exerts an important influence on its texture and condition. It cracks and sometimes even pulverizes the rocks and stones upon and near the surface; [Footnote: In the burning over of a hill-forest in the Lower Engadine, in September, 1865, the fire was intense as to shatter and calcine the rocks on the slope, and their fragments were precipitated into the valley below.—Ricista Firrestate del Regna d'Italia, Ottobro, 1865, 1865, p. 474.] it consumes a portion of the half-decayed vegetable mould which served to hold its mineral particles together and to retain the water of precipitation, and thus loosens, pulverizes, and dries the earth; it destroys reptiles, insects, and worms, with their eggs, and the seeds of trees and of smaller plants; it supplies, in the ashes which it deposits on the surface, important elements for the growth of a new forest clothing, as well as of the usual objects of agricultural industry; and by the changes thus produced, it fits the ground for the reception of a vegetation different in character from that which had spontaneously covered it. These new conditions help to explain the natural succession of forest crops, so generally observed in all woods cleared by fire and then abandoned. There is no doubt, however, that other influences contribute to the same result, because effects more or less analogous follow when the trees are destroyed by other causes, as by high winds, by the woodman's axe, and even by natural decay. [Footnote: The remarkable mounds and other earthworks constructed in the valley of the Ohio and elsewhere in the territory of the United States, by a people apparently more advanced in the culture than the modern Indian, were overgrown with a dense clothing of forest when first discovered by the whites. But though the ground where they were erected must have been occupied by a large population for a considerable leagth of time, and therefore entirely cleared, the trees which grew upon the ancient fortresses and the adjacent lands were not distinguishable in species, or even in dimensions and character of growth, from the neighboring forests, where the soil seemed never to have been disturbed. This apparent exception to the law of change of crop in natured forest growth was ingeniously explained by General Harrison's suggestion, that the lapse of time since the era of the mound-builders was so great as to have embraced several successive generations of trees, and occasioned, by their rotation, a return to the original vegetation.

The succesive changes in the spontaneous growth of the forest, as proved by the character of the wood found in bogs, are such as to have suggested the theory of a considerable change of climate during the human period. But strobus, as it is the most valuable, is also perhaps the most delicate tree of the American forest, while its congener, the Northern pitch-pine, Pinus rigida, is less injured by fire than any other tree of that country. I have heard experienced lumbermen maintain that the growth of this pine was even accelerated by a fire brisk enough to destroy all other trees, and I have myself seen it still flourishing after a conflagration which had left not a green leaf but its own in the wood, and actually throwing out fresh foliage, when the old had been quite burnt off and the bark almost converted into charcoal. The wood of the pitch-pine is of comparatively little value for the joiner, but it is useful for very many purposes. Its rapidity of growth in even poor soils, its hardihood, and its abundant yield of resinous products, entitle it to much more consideration, as a plantation tree, than it has hitherto received in Europe or America.] without fatal injury to the younger growths, the native forest will hear several "cuttings over" in a generation—for the increasing value of lumber brings into use, every four or five years, a quality of timber which had been before rejected as unmarketable—a fire may render the declivity of a mountain unproductive for a century. [Footnote: Between sixty and seventy years ago, a steep mountain with which I am familiar, composed of metamorphic rock, and at that time covered with a thick coating of soil and a dense primeval forest, was accidentally burnt over. The fire took place in a very dry season, the slope of the mountain was too rapid to retain much water, and the conflagration was of an extraordinarily fierce character, consuming the wood almost entirely, burning the leaves and combustible portion of the mould, and in many places cracking and disintegrating the rock beneath. The rains of the following autumn carried off much of the remaining soil, and the mountain-side was nearly bare of wood for two or three years afterwards. At length a new crop of trees sprang up and grew vigorously, and the mountain is now thickly covered again. But the depth of mould and earth is too small to allow the trees to reach maturity. When they attain to the diameter of about six inches, they uniformly die, and this they will no doubt continue to do until the decay of leaves and wood on the surface, and the decomposition of the subjacent rock, shall have formed, perhaps hundreds of years hence, a stratum of soil thick enough to support a full-grown forest. Under favorable conditions, however, as in the case of the fire of Miramichi, a burnt forest renews itself rapidly and permanently.]

Aside from the destruction of the trees and the laying bare of the soil, and consequently the freer admission of sun, rain, and air to the ground, the fire of itself exerts an important influence on its texture and condition. It cracks and sometimes even pulverizes the rocks and stones upon and near the surface; [Footnote: In the burning over of a hill-forest in the Lower Engadine, in September, 1865, the fire was so intense as to shatter and calcine the rocks on the slope, and their fragments were precipitated into the valley below.—Rivista Forestale del Regno d'Italia, Ottobre, 1865, p. 474.] it consumes a portion of the half-decayed vegetable mould which served to hold its mineral particles together and to retain the water of precipitation, and thus loosens, pulverizes, and dries the earth; it destroys reptiles, insects, and worms, with their eggs, and the seeds of trees and of smaller plants; it supplies, in the ashes which it deposits on the surface, important elements for the growth of a new forest clothing, as well as of the usual objects of agricultural industry; and by the changes thus produced, it fits the ground for the reception of a vegetation different in character from that which had spontaneously covered it. These new conditions help to explain the natural succession of forest crops, so generally observed in all woods cleared by fire and then abandoned. There is no doubt, however, that other influences contribute to the same result, because effects more or less analogous follow when the trees are destroyed by other causes, as by high winds, by the woodman's axe, and even by natural decay. [Footnote: The remarkable mounds and other earthworks constructed in the valley of the Ohio and elsewhere in the territory of the United States, by a people apparently more advanced in culture than the modern Indian, were overgrown with a dense clothing of forest when first discovered by the whites. But though the ground where they were erected must have been occupied by a large population for a considerable length of time, and therefore entirely cleared, the trees which grew upon the ancient fortresses and the adjacent lands were not distinguishable in species, or even in dimensions and character of growth, from the neighboring forests, where the soil seemed never to have been disturbed. This apparent exception to the law of change of crop in natural forest growth was ingeniously explained by General Harrison's suggestion, that the lapse of time since the era of the mound-builders were so great as to have embraced several successive generations of trees, and occasioned, by their rotation, a return to the original vegetation.