36 ELEMENTARY FORESTRY. 



and in the twigs and branches that are left on the ground 

 when the tree is cut down. The roots deepen the soil and by 

 their decay furnish plant food to the soil and leave channels 

 through which 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 un- 

 derstanding of this matter is to consider two extremes of tree 

 growth. Eastern Minnesota 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 ti'ees. 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 

 meridional 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 probably gradually driven 

 eastward for many years previous to settlement by the prac- 

 tice of the Indians of burning over prairies in order to fur- 

 nish 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 con- 

 ditions. So we find that while great sections of the interior 



