THE LIFE PROCESSES OF TREES 25 



The action of the factors of site on tree life is very com- 

 plex and it is difiScult to determine often just the amount of 

 each separate factor that is needed by a tree. Nearly all trees 

 will grow well under favorable conditions; Only a few will 

 live where the conditions are not favorable. It must be re- 

 membered that a site favorable to one tree may be an un- 

 favorable site for another tree. The trees found growing 

 naturally in the same locality and on the same site do not 

 differ widely in their demands. For this reason forests usually 

 are made up of a mixture of trees. 



Heat. — Trees cover the greater portion of the earth from 

 the equator to the limit of tree growth in the polar regions. 

 Many trees grow luxuriantly in the hot climate of the Torrid 

 Zone. Provided there is enough moisture present, the extreme 

 heat of the sun in that region is not too great for certain kinds 

 of trees. Some of the largest known trees of the world are 

 found in the temperate climate, such as the redwood trees of 

 California. But as we pass into the colder regions of the 

 north trees which farther south grow to great height become 

 smaller and smaller until they are shrublike and finally disap- 

 pear. There is a line in the colder regions called the timber 

 line beyond which there is no tree growth. The same gradual 

 dwindling in size and the final disappearance of trees is 

 noticed in ascending a mountain. Trees which grow naturally 

 in a warm region will not grow in the colder regions of the 

 north. Trees that grow in the temperate climate will wither 

 and. die under the hot sun of the warmer regions. There is an 

 optimum region for each tree — a certain portion of the country 

 where it will grow best. Much farther north or south it 

 will not thrive and will finally be crowded out of the forest 

 by other trees better suited to the conditions. The trees found 

 growing naturally in any portion of the country are probably 

 better suited to its climate than trees that grow elsewhere. 

 There is danger in planting a tree not native to a certain region. 

 Unless the climate where the trees are planted is similar to the 



