10 JULIUS VALENTINE HOFMANN 



As one looks at the peaceful forest one should remember that under- 

 neath the calm serenity of the scene there is a bitter struggle, a relentless 

 internecine warfare between the trees already established there and those 

 that are striving to enter from without. 



Soil temperature, soil moisture, aeration, and light are among the 

 ecological factors which determine the establishment of a forest and deter- 

 mine the types within a forest. A wide variation of any one of these 

 factors on different sites does not mean that the varying factor is the one 

 which determines the type. Other factors, varying less, but approaching 

 nearer to the limit of favorable conditions would have a greater influence 

 on the germination of the seed or the establishment of the seedling. All 

 the factors must be taken into consideration, and also the limits of each 

 under which the seedlings will grow. While the moisture in the soil in 

 two different localities may be equal, the soil texture may have a decided 

 influence on the availability of the moisture for plants; that is, there would 

 be a decided difference in the wilting coefficient. The fact that the surface 

 soil often dries out, while at a depth of about six inches moisture may be 

 present on protected slopes and absent on exposed slopes, gives decided 

 advantage to seedlings with deep roots formed early in their development. 

 For this reason, yellow pine has an advantage over hemlock and its asso- 

 ciates in the forests of western Montana and Idaho. For the same reason, 

 Douglas fir is able to establish itself on the drier slopes of the Cascades, 

 while hemlock and cedar fail. A south slope covered with yellow pine or 

 Douglas fir, and a north slope covered with hemlock, white pine, cedar, and 

 other species, does not mean that each of these species is in its optimum 

 habitat. It is rather a question of competition between species and of 

 establishment. Yellow pine would produce excellent forests on some of 

 the slopes occupied by other species if it could establish itself there. It is, 

 however, crowded out by the large number of seedlings of the other species. 

 On the other hand, the hemlock and cedar do very well under the conditions 

 of the south slopes wherever they can get sufficient moisture to establish 

 themselves. The reason these species are not in mixture all through the 

 forest is not due to a lack of seed or even to the germination of seeds on 

 the different slopes. An example of this nature was noted by the writer 

 where two types met on a ridge. The south slope was seeded with seed 

 from species found on the north slope. Seedlings of hemlock and cedar 

 and larch were found germinating along with those of Douglas fir and 

 yellow pine in the spring of the year, but in the fall only some of the seed- 

 lings of yeUow pine and Douglas fir were left. The seedlings of the other 

 species were unable to live through the dry period of the summer, due to 

 their small roots and their inability to reach the moist layer of soil below 

 the dry surface before they perished. These conditions are repeated year 

 after year, and yet the type remains unchanged. It is very noticeable 



