FOREST INFLUENCES. 39 



may enter the subsoil. It has been estimated that after a 

 sandy soil in New England is so exhausted that it will pro- 

 duce nothing but red mosses, it may be renewed to its pris- 

 tine 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 understanding of this matter is to consider two ex- 

 tremes 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 evapo- 

 ration. 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 loca- 

 tion of this zone was probably gradually driven eastward, 

 for many years previous to settlement, 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 



