122 



SCIENCE 



[N. S. Vol. XLI. No. 1047 



seed trees that may be on the bum is very- 

 limited in restocking the ground. This dis- 

 covery enabled the service to change mate- 

 rially the present methods of cutting in the 

 white pine and Douglas fir forests, to the 

 mutual advantage of the government and 

 of the logging operators. 



I shall give briefly a few other illustra- 

 tions of the life of the forest which stamp 

 it as a distinct plant society. 



The first social phenomenon in a stand 

 of trees is the differentiation of individuals 

 of the same age on the basis of differences 

 in height, crown development and growth, 

 the result of the struggle for light and 

 nourishment between the members of the 

 stand. A forest at maturity contains 

 scarcely 5 per cent, of all the trees that have 

 started life there. Yet the death of the 95 

 per cent, is a necessary condition to the 

 development of the others. The process of 

 differentiation into dominant and sup- 

 pressed trees takes place particularly in 

 youth and gradually slows down toward 

 maturity. Thus, in some natural pine 

 forests, during the age between 20 to 80 

 years, over 4,000 trees on an acre die; 

 whereas at the age between 80 and 100 only 

 300 trees die. "With some trees this natural 

 dying out with age proceeds faster than 

 with others. Thus in pine, birch, aspen, 

 and all other species which demand a great 

 deal of light, the death rate is enormous. 

 With spruce, beech, fir, and species which 

 are satisfied with less light, this process 

 is less energetic. The growing demand for 

 space with age by individual trees in a 

 spruce forest may be expressed in the fol- 

 lowing figures : 



Sq. Ft. 



At 20 years of age 4 



At 40 years of age 34 



At 60 years of age 70 



At 80 years of age 110 



At 100 years of age 150 



If we take the space required by a pine 

 at the age between 40 and 50 years as 100 ; 



then for spruce at the same age it will be 

 87; for beech 79; and for fir 63. This 

 process of differentiation is universal in- 

 forests everywhere. 



Another peculiarity that marks a tree 

 community is the difference in seed produc- 

 tion of trees which occupy different posi- 

 tions in the stand. Thus if the trees in a 

 forest are divided into five classes ac- 

 cording to their height and crown devel- 

 opment, and if the seed production of 

 the most dominant class is designated 

 as 100, the seed production for trees 

 of the second class will be 88 ; for the third 

 class, 33; for the fourth class only .5 per 

 cent., while the trees of the fifth class will 

 not produce a single seed, although the age 

 of all these trees may be practically the 

 same. The same struggle for existence, 

 therefore, which produced the dominant 

 and suppressed trees works toward a natu- 

 ral selection, since only those which have 

 conquered in the struggle for existence, and 

 are endowed with the greatest individual 

 energy of growth, reproduce themselves. 



In a forest there is altogether a differ- 

 ent climate, a different soil and a different 

 ground cover than outside of it. A forest 

 cover does not allow aU the precipitation 

 that falls over it to reach the ground. Part 

 of the precipitation remains on the crowns 

 and is later evaporated back into the air. 

 Another part, through openings in the 

 cover, reaches the ground, while a third 

 part runs down along the trunks to the 

 base of the tree. Many and exact measure- 

 ments have demonstrated that a forest cover 

 intercepts from 15 to 80 per cent, of precipi- 

 tation, according to the species of trees, 

 density of the stand, age of the forest, and 

 other factors. Thus pine forests of the 

 north intercept only about 20 per cent., 

 spruce about 40 per cent., and fir nearly 60 

 per cent, of the total precipitation that falls 

 in the open. The amount that runs off 

 along the trunks in some species is very 



