/m DIVISION OF 



FORESTRY 



Correspondence Courses fft- L Agrteulture 



UNIVERSITY OF CAUFORNIA 



Farm Forestry 

 LESSON II 



Factors Influencing the Development of Trees 



Tree growth, like all plant development, is dependent upon cer- 

 tain factors of habitat; namely, temperature, light, moisture, soil 

 salts. A certain degree of each factor, varying for each species, 

 is necessary for best growth. In fact, the character of tree growth 

 in any situation is determined largely by the degree to which each 

 of these factors is present. 



Temperature. For each species of tree there is a minimum, opti- 

 mum and maximum degree of heat. Below fifty and above 115 

 degrees Fahrenheit, growth practically ceases and trees take on a 

 dwarfed and shrubby form. Towards the equator the vegetation 

 becomes more luxuriant, yet the development of trees is not so 

 vigorous as in the temperate regions. Temperature is affected 

 similarly by altitude and latitude. Consequently, tree growth is 

 stunted whether it occurs at the summits of high mountains or in 

 the arctic zone. 



A temperature favorable to the development of any one species 

 may vary between wide limits. The range of White Pine, for in- 

 stance, extends from southeastern Canada to northern Georgia, and 

 Quaking Aspen occurs in practically all parts of North America. 

 However, if a study be made of a species throughout its natural 

 range, one portion of the range will show a maximum development 

 and the temperature of this situation may be said to be the optimum 

 for the species. In a higher or lower temperature than this the 

 development of the species is not so vigorous. In any one section, 

 however, the differences in temperature are so slight as to be 

 negligible, and temperature as a factor of plant growth possess little 

 importance. 



Light. So important is sunlight in the growth of trees that it 

 is said to be the purpose of trees to convert sunlight into wood. 

 Practically all trees thrive in full sunlight, but there are a few 

 which have adapted themselves to conditions of less light. Trees 

 which require full sunlight for development are designated as 

 "light demanding" (intolerant), while those which thrive with less 

 than full sunlight are classed as "shade enduring" (tolerant). Most 

 trees are more tolerant when young than in later life. This prop- 

 erty enables seedlings to become established in the shade of 

 mother trees. 



Of the more tolerant trees, Hemlock and Dogwood are good 

 examples, while Yellow Poplar and "Old Field" Pine represent 



