24 FARM FORESTRY 



some trees like the willow. If a branch is broken off from the 

 tree by the wind or by the weight of snow and ice, it will start 

 to grow if a portion of the branch becomes covered with earth. 

 Many trees like the willow and poplar can be reproduced by 

 making cuttings of the new twigs and planting them in the 

 ground. On high mountains or in the far north where the 

 summer season is too short to mature seeds, some trees will- 

 reproduce themselves by layering. This takes place when a 

 branch of the tree touches the ground and produces roots. 



NATURAL FACTORS THAT INFLUENCE THE LIFE OF TREES 



Trees cannot live without heat, light, moisture and food. 

 These are often spoken of as factors of site. Site means the 

 location or situation, as bottomland, side hill, upper slope or 

 ridge. Trees vary in the amount of each of these factors de- 

 manded for their best growth. The kind of trees found in 

 any situation is determined by the presence or absence of 

 these factors in varying amounts. Some trees require more 

 heat than other trees. On a moist situation are not found the 

 same trees that grow on dry situations. Trees are seen grow- 

 ing beneath the shade of other trees, while others demand the 

 full sunlight. Some trees will not grow well unless they have 

 a fertile soil, while other trees will grow on poor soil. Trees 

 are often found growing where one or more of these factors 

 are not favorable to them. But they do not grow well under 

 such conditions. They will have very slow growth and be 

 more or less stunted and of poor form. The amount of each 

 of these different factors needed is not definite. There is a 

 maximum and a minimum amount of each which a certain 

 species of tree will stand. If the limit is exceeded either way 

 the tree will die. There is also an optimum amount for each 

 factor under which a tree will do its best. The factors often 

 supplement each other. On a deep, rich soil less moisture is 

 required than on a thin, poor soil. Less light is required where 

 the soil and moisture conditions are very favorable. 



