FACTORS INFLUENCING REPRODUCTION 853 



them may have been stronger light than at present. Indications of this 

 were found in the fact that some of these cases of fir reproduction 

 occur in stands which were formerly more open than they now are ; 

 also, small fir reproduction is abundant in young stands with a full but 

 not very heavy canopy. That light is a factor would be in accordance 

 with Zon's conclusion.-" Another factor may be decreasing moisture, 

 due to the interception of precipitation by the crowns of the spruce 

 trees, and to a fuller use of the humous water by the increasing mat of 

 spruce roots. Whatever the factors are, it seems evident that fir re- 

 produces within a more or less sharply marked range of environmental 

 conditions. 



Each species reproduces only within a certain range of factors. This 

 range is probably a specific characteristic of each tree, possibly of each 

 plant, and appears to be different even for trees growing together in 

 the same association.-^ Determination of this range for even a few 

 of our more important trees would be a valuable contribution to science. 



'"Zon, R. : "Balsam fir." U. S. Dept. Agr. Bull. 55, 1914. 



" Shreve, Forrest : "The vegetation of a desert mountain range as conditioned 

 by climatic factors." Carnegie Inst, of Washington, Publ. 217, 1915. Has 

 recently shown that different plants growing together in the same association 

 have different life requirements. 



