292 THE PLANT WORLD. 



CERTAIN RELATIONS OF RAINFALL AND TEM- 

 PERATURE TO TREE GROWTH. 



Under the above heading Mr. Henry Gannett, in the Bulletin 

 of the American Geographical Society for July, 1906, states that 

 temperature and rainfall are the two factors which determine 

 whether trees can grow, and, if so, what species. Other factors, 

 such as seasonal temperatures, seasonal rainfall, atmospheric 

 humidity, wind and slope exposure, angle of slope, and texture 

 and depth of soil, are secondary factors which have a modifying 

 effect. 



The paper " is designed to show how much and what informa- 

 tion our present knowledge of the climatic elements develops con- 

 cerning tree growth in the western United States." The records 

 are derived from a little over four hundred stations having a rain- 

 fall record of more than five years. " The distribution of these 

 stations is not, however, by any means what could be desired, the 

 great majority of them being situated in towns and cities, and 

 therefore in low and open or non-timbered country ; and very few 

 of them are high up in the mountains, so that in those parts of 

 the West in which the timber is confined to the mountains the 

 timbered regions are not well represented." 



The author's results are tabulated. " The table relating to 

 yellow pine shows that the greatest number of stations is found 

 where the temperature is between 50 and 55 degrees, while the 

 entire range of the species, as indicated by the stations, is from 

 40 to 65 degrees." 



" Nearly all the stations in the red-fir region are found between 

 50 and 55 degrees. Indeed, outside of this group the stations are 

 but scattering." 



" In the red-wood region all the stations are between 50 and 

 60 degrees." 



From the results of this study and those published in a former 

 paper on " The Timber Line," in the Atn. Jour. Sci., 1882, p. 275, 

 the author concludes that the timber line has a mean annual tem- 

 perature of approximately 30 degrees, so that the location of this 

 isotherm is a simple matter. 



