WHY THE PRAIRIES ARE TREELESS. 43 



water and air may enter the subsoil. It has been estimated 

 that after a sandy soil in New England is so exhausted that it 

 will produce nothing but red mosses it may be renewed to its 

 pristine vigor and productiveness by the growth of trees on it 

 for thirty years. 



WHY THE PRAIRIES ARE TREELESS. 



This question has been answered in many ways, but often, it 

 would seem, by persons not acquainted with the principles of 

 forestry. It seems that the best way of getting a clear under- 

 standing of this matter is to consider two extremes of tree 

 growth. Eastern JNIinnesota has a rainfall of perhaps 26 to 35 

 inches and a comparatively moist air, and at least during a part 

 of the year is well adapted to the growth of the hardier kinds 

 of trees. Here we find the White Pine, Basswood, Oak, Elm, 

 Poplar and other trees attaining large size. Western Dakota 

 has a very light rainfall, mostly in the spring, and a very high 

 rate of evaporation. Trees can scarcely be made to grow in 

 this section without irrigation, and the low vegetation, the 

 grasses, which require a less amount of water, replace the trees. 

 It is evident that between locations having such extremes of 

 tree growth there must be a place where the trees give way to 

 the lower forms of vegetation. Such a meridianal zone is found 

 in central Minnesota, and though it has probably changed with 

 fluctuating rainfall its general location has remained practically 

 the same for many years. The location of this zone was proba- 

 bly gradually driven eastward, for many years previous to set- 

 tlement, by the practice of the Indians of burning over prairies 

 in order to furnish good pasturage for the buffalo. Of late 

 years, since the prairie fires have been largely prevented, the tree 

 line has moved westward and gained a little on the prairies. 

 When left to itself the western limit of this tree zone would not 

 make very great progress westward, but with man's assistance 

 in cultivation and various other ways it may be extended much 

 farther towards the arid regions than if left to natural conditions. 

 So we find that, while great sections of the interior of this coun- 

 try are treeless on account of lack of water, trees planted on 

 them and properly cared for may often grow thriftily. But trees 

 planted on our prairies always require more care to make them 



