16 THE AMES FORESTER 



to their establishment. The reason these species are not mixed all 

 through the forest is not due to the lack of seeding or even 

 to the germination of the seed on the different slopes. This 

 point is emphasized where these types meet on a ridge. The 

 south slope is seeded with the seed of the species found on the 

 north slope and the seeds of the hemlock, cedar and Douglas 

 fir, as well as the yellow pine, germinate in the spring but the 

 seedlings with only shallow roots cannot resist the period of 

 summer drought and in the fall only yellow pine seedlings re- 

 main. These conditions are repeated year after year and still 

 the type remains the same. It is very noticeable that wherever 

 conditions favorable to establishment have obtained on the south 

 slopes the north slope species are found, sometimes in very un- 

 expected localities. 



When the seedlings or trees of a species are found on any 

 site it shows that that particular site is favorable to the species 

 found there, but it does not prove that any other species would 

 not establish itself there or develop on the site if given a chance. 

 Sometimes it is merely a question of which species happens to 

 get possession of the area after the forest was removed, or which 

 species first had the opportunity of migrating there, while in 

 other instances it is clearly a matter of competition or ability 

 to withstand the conditions of the site involved. 



Species are, in general, rather impartial in regard to soil if 

 we except chemical and physical extremes (abundance of com- 

 mon salt, lime, or of water) and if they have no competitors. 

 In the middle of its distributional area a species makes no selec- 

 tion as to soil, but outside this central position it is forced by 

 other species to exercise a choice. Nearly all species are faculta- 

 tive and their occurrence depends upon competitors. If these 

 be present the one drives back the other and the victorious species 

 is the one which can best utilize the given combination of soil, 

 light, moisture, temperature, etc., during its period of germina- 

 tion and establishment. 



Where forests of one species are sharply delimited from forests 

 of other species it is not a question of inability to thrive at the 

 boundaries but rather a question of what use the species can 

 make of the favorable periods during establishment. In many 

 instances a species will succeed and develop as well or better far 

 beyond its natural range if assisted artificially during its seed- 

 ling period. While these facts have been recognized in re- 

 forestation the failure of nature to accomplish the results has 

 been attributed to factors other than seed viability and vitality. 

 The importance of physical factors must not be minimized in 

 considering limitations of type but the most important factor 

 in local limitations, the seed and its characteristics, has not been 

 considered in the past. 



