430 REPORT OF THE COMMISSIONERS OF 



CLIMATIC LIMIT OF FOREST GROWTH UNDER NATURAL 



CONDITIONS. 



In his Report on the Forests of North America for the tenth census, Professor 

 Charles S. Sargent discusses, in his introductory chapter, the reasons why there are no 

 forests upon the prairies of Minnesota, Wisconsin, Iowa, Illinois and Missouri, taking 

 the ground that other influences than insufficient rainfall have prevented the general 

 growth of trees in these prairie regions. The rainfall, Professor Sargent says, is 

 sufficient to insure a heavy growth of forest here. The soil of the prairie is not 

 unsuited to tree growth, as is proven by a vigorous and rapid growth when trees are 

 planted, nor is it want of sufficient heat or equally distributed moisture which has 

 checked the growth of forest over these prairies. Professor Sargent expresses the 

 opinion that the forests of the Atlantic region once extended continuously as far west 

 at least as the ninety-fifth meridian, although he is unable to cite decisive evidence 

 that this is true. It is Professor Sargent's opinion that we must trace the destruction 

 of the forest over this area to accidental causes. Among others he mentions forest 

 fires and the force of the wind, which would make the spread of forest 

 growth slow and difficult. 



Professor Sargent says the assumption that these eastern prairies may have once 

 been covered by forests is strengthened by the fact that since they have been devoted 

 'to agriculture and the annual burning stopped, trees which were formerly confined to 

 the river bottoms have, in many cases, gradually spread to the uplands. In many 

 places small prairies just within the edge of the forest have entirely disappeared 

 within the memory of persons still living. In western Texas the mesquit, forced by 

 annual burning to grow almost entirely below the surface of the ground, is now, that 

 forest fires are less commonly destructive, spreading over what was formerly a 

 treeless prairie. 



There is a popular view that forests cease to be abundant when rainfall becomes 

 less than thirty-two inches per annum and practically disappear when the rainfall 

 becomes as low as twenty-six inches. In many places less than twenty inches of rain- 

 fall is apparently accompanied by an exclusively pastoral country and with less than 

 fourteen inches rainfall vegetation at many places in the West practically disappears. 

 Facts of this character have been cited to indicate that forests do not thrive with less 

 moisture than other vegetation, but such facts, when examined, are susceptible of 

 another interpretation. We must take into account very many conditions besides that 

 of the rainfall. Indeed, when we study the question broadly we find that in many 

 places in the West there are forest growths with comparatively very slight rainfalls. 

 On this point we may cite experience in California. 



