CHOICE OF SPECIES IN ARTIFICIAL REGENERATION 21 



6. THE SITE FACTORS 



The site factors which have a more or less direct bearing upon 

 forest vegetation are so numerous and so intimately related and 

 complex that precise methods of study are very difficult. In ac- 

 cordance with their nature they relate to the atmosphere, to the 

 soil, and to plant and animal life. 1 



The atmospheric factors which influence forest vegetation are 

 temperature, light, humidity, precipitation, and, to lesser extent, 

 wind, lightning, and atmospheric impurities. The soil factors are 

 water content, soil composition, soil temperature, soil gases, and 

 indirectly altitude, slope, exposure, and surface. The life factors 

 refer to the plants and animals in every environment which react 

 upon the forest vegetation. There is no habitat which escapes 

 the influence of these life factors. The trees in every forest are 

 influenced more or less by the various forms of animal life which 

 come in contact with them and by various parasitic plants. The 

 quality or yield capacity of any site is represented by the com- 

 bined influence of all the site factors. 



Since the results obtained from seeding and planting as expressed 

 in vigor of growth and yield are largely dependent upon the com- 

 bined effect of these factors, they are of fundamental importance 

 in the selection of species. No species should be selected for exten- 

 sive use until the closeness of its correlation with these factors as ex- 

 hibited in the particular locality is definitely settled by either direct or 

 indirect means. 



7. THE EFFECT OF ATMOSPHERIC FACTORS ON THE CHOICE 



OF SPECIES 



The heat relations of a tree cannot be determined by direct ob- 

 servation. Thus, plants from tropical regions may resemble in 

 form and habit plants from arctic regions. Although we cannot 

 explain the reason, we know from experience that every form of 

 forest vegetation can live only at temperatures between two ex- 

 tremes which are at variable distances apart for each species. 

 When seed is distributed beyond these points, it does not grow 

 and develop so as to form a permanent part of the vegetation. 

 For this reason each species has its northern and southern range 



1 Clements, F. E.: Research methods in ecology, p. 18. Lincoln, Neb., 

 1905. 



