BIOTIC FACTORS 283 



The duration of the experiments probably was too brief for an 

 ultimate pronouncement upon the effect of fires. 



The influence of fires upon the forests of Finland has been studied 

 in detail by Heikinheimo (1915) and Kujala (1926). 



Cutting. — The utilization of the forest by cutting it down began 

 when man of the later stone age gave up his caves to build huts. The 

 natural forest landscape was subjected to a marked change only by 

 the complete cutting down of large areas of forest. Not only does 

 such cutting considerably change the lighting conditions, but wind and 

 precipitation are given freer rein, and the soil structure itself is changed 

 markedly. While the power for nitrification increases, air capacity 

 and permeability of the soil are reduced. In this way complete clear- 

 ing impoverishes the soil, even where floods and erosion are not to be 

 feared. Burger's (1922) experiments on the influence of clearing upon 

 forest soil are very instructive. 



On a clearing near Zurich the air capacity of the top-soil layer was 

 reduced by one-third. The permeability of good beech-wood soil near 

 Bienne was thirty times that of soil which had lain waste for ten years 

 and had been used part of that time as a logging camp. 



The vegetational changes which take place upon cleared land have 

 been dealt with by Sendtner (1854) and Kerner (1863) and by more 

 recent authors. The changed light conditions bring about a regular 

 series of changes in vegetation. Dziubaltowski (1918) gives exact 

 data on the fioristic phases of development upon cut-over Quercus 

 pedunculata-Carpinns hetulus woodlands in Poland. A phase with 

 many "accidental" pioneers of a ruderal character is followed by a 

 sod carpet of Agrostis alba; and Salix caprea and some shrubs such as 

 Genista germanica, G. tindoria, and Cytisus nigricans appear. From 

 the eighth year onward, herbs and shrubs decrease continually because 

 of the increasing shade of trees, until the equilibrium of the shady high 

 forest is reached. However, we have few precise data on the simul- 

 taneous ecological changes, and the subject must be recommended for 

 further study. As a preHminary, the studies of Salisbury (1924) on 

 the regeneration of British low woods are to be mentioned. 



The vegetational changes in the regular operation of selective 

 cutting are less apparent than in bare cutting. The chosen trees are 

 taken out at intervals, and no important changes of light and soil 

 conditions are caused. In the composition of the forest, however, 

 a change due to preference and protection of certain tree species may 

 take place. Repeated cutting is followed by the dominance of the 

 most vigorously growing timber varieties. In this way almost pure 

 dwarf woods of Q. pedunculata and Carpinus hetulus are formed in the 



