62 FOREST INFLUENCES 



of forest types. Forest ecology deals with the influence and interre- 

 lation of the environmental factors of soil, climate, light, and such 

 contributing factors as other plant life, animal life, topography, site, 

 and atmospheric phenomena. These influences and factors are not 

 the same for any two species or groups of species because of different 

 responses and reactions to the factors of environment. There is con- 

 stant competition among the various species for the light above and 

 for food in the soil. A site which may be most favorable for one 

 species may be a mediocre or poor location for another species. Soil 

 conditions are constantly in a state of improvement or deterioration. 

 Artificial or natural conditions may modify these soil factors. All 

 these environmental influences affecting the quality of the site are 

 known as the site factors. The very range and distribution of forest 

 trees may be changing. Forest conditions are practically never in a 

 state of equilibrium or stabilization. The study of silvics and forest 

 ecology constitutes one of the most important fundamental fields in 

 the broad subject of forestry. 



3. INFLUENCE OF FOREST ON ITS ENVIRONMENT 



The two major effects of forest on environment are in relation to 

 climate and soil. Forests also exert a great influence on the social 

 and economic life of mankind which is discussed elsewhere. 



1. Climate. It has not yet been definitely determined whether the 

 forest has an influence on the climate over large areas or regions. 

 However, it has been proved that the forest has a pronounced effect 

 on the local climate. The temperature within the forest is subject 

 to much less fluctuation than that in the open. The greatest influence 

 of the forest on air temperature is experienced in warm regions. 

 Hardwood forests exert a greater influence in the summer than soft- 

 woods and have far less effect in the winter on account of the dense 

 foliage in the summer and their leafless condition in the winter. Forests 

 or rows of trees break the effect of the wind. The influence of a 

 shelterbelt may, according to Bates, be appreciable to the windward 

 for a distance equal to five times the height of the windbreak and 

 to the leeward for a distance of fifteen to twenty times the height. 

 Windbreaks may in extreme cases save 70% of the moisture lost by 

 evaporation. 



2. Soil. The soil temperature is affected by the forest in the same 

 way as the air temperature but to a greater degree. Much less frost 

 heaving is found inside the forest than outside. Forest vegetation 

 reduces the surface runoff and increases the amount of water that 



