Pawnee Woman.
When wandering over the wide Iowa and Nebraska plains, many problems present themselves to the mind with regard to their formation and existing condition. Of these, none are so perplexing to the farmers as those which relate to the absence of trees. Several theories have been mooted and many conclusions have been adopted, and thought to be rational, but as a rule they cannot be accepted as being altogether satisfactory. One of the most general opinions, is that the treeless state of the land has been caused by the destructive effects of fire. Another opinion is that which attributes the greatest counteracting influence to the nature of the soil.
One of the most careful investigators into this subject was the accomplished geologist Mr. J. W. Foster, whom I met at Chicago, and who was at that time President of the American Association for the Advancement of Science. He told me what had been the results of his work, and accompanied me to parts of the outlying Illinois prairies, to examine the character of the earth near the surface. He had, a short time previously, carried out a series of experiments relating to the physical geography of the Mississippi valley, and had given his attention to the composition of the upper formation of the prairies on both sides of the river.
He had arrived at the conclusion that the absence of trees upon them was not caused by fires or by the character of the ground, but was the consequence of the conditions of rainfall, temperature, climate, and exposure, all acting in a direction opposed to that natural order of things in which trees would be inclined to flourish.
On the way south from Mankato, I observed that the banks of the Des Moines river were thickly timbered, and that on the borders of the various rivulets which had cut their way through the ground to a considerable depth, there was usually a flourishing growth of trees, chiefly consisting of oak, black walnut, basswood, and maple varieties. In all these instances the trees were sheltered, but on the exposed prairie immediately adjacent the ground was bare, and without a vestige of any signs of trees or shrubs. In that particular region it seems therefore probable, that the exposure to gales of wind sweeping over the plateau, may be the principal cause of trees not being able to take root and live.
There are, however, other prairies equally bare of timber which are not so exposed. Upon those situated near the water shed or dividing ridge of the tributaries of the Missouri and Mississippi, it may be the quantities of water lying permanently within a few feet below the surface which prevent any growth taking place. No single theory seems to explain the facts, but from my own observation, I am inclined to think that the chief deterring influence is the nature of the soil.[40]
There is an unexplained problem respecting the growth of trees in certain parts of North America, which has received much local notice. My attention was directed to the subject when I was in the neighbourhood of Lake Simcoe in Upper Canada. An Englishman, who had established a homestead there and made clearings in the forests, told me he had observed that after the old trees had been felled, new shrubs and timber of an entirely different character grew up in their place. There was nothing in the nature of the surrounding woods which, to his mind, could account for the change, as there were no trees of the class that had taken root existing in that part of the country.
A similar alteration in the order of succession of forest growths was reported by the geologists employed by the Government in conducting the early surveys of Michigan. They stated that large tracts of land, originally covered with pines, had been succeeded by a second growth consisting of white birch, aspen, pine and hazel. In this case the primeval forest had been destroyed by high winds and afterwards burnt. It was supposed that where clearings had been made by fire, changes of the above nature occurred, but no suggestions were given as to the manner in which these new and strange growths established themselves.
Near Ishpeming, I saw that in all cases where forest openings had been made by the action of fire, luxuriant masses of dense raspberry bushes occupied the land; and it was said that where the woods were cut down by the axe bushes of another class, bearing different berries, sprang up and flourished.
In other parts of the North-west, variations in the succession of forest trees have been seen to occur; and usually there seems to be some connection between the type of the new plants and the methods by which the clearings have been made. As a general rule there are very few birds to carry seeds or pips from distant regions and therefore it is possible that in places where the ground has never for many centuries been previously disturbed and where the thick forest has been for the first time removed, and sunlight and fresh atmospheric conditions are admitted upon the land, new circumstances arise which are favourable to the development of dormant life.[41]