82 General Care of Trees 



with rubble or broken stone before putting down earth and 

 sod, keeping the space around the bole open and in addition 

 introducing through the filled ground aerating pipes which 

 connect the air chamber with the outer air. 



In any case, trees so treated should be at the same time 

 severely pruned, since some of the feeding roots are bound 

 to die and it is necessary to establish an equihbrium between 

 root system and crown system. Otherwise nature will do 

 the pruning and perhaps not in the manner desired. 



Different species vary in their sensitiveness to the with- 

 drawal of air and, as we have seen before, trees which have 

 grown up in swampy conditions make pro\ision in other 

 ways to meet their requirements for air, and may therefore 

 be covered to a considerable extent with impunity. In 

 general it will be found that the trees naturally growing in the 

 swamp will stand filling best. 



The depth and character of the soil used for cover will 

 naturally also ha\"e to be considered, since with a loose soil 

 and a moderate depth the respiration may not be materially 

 interfered with, while a thinner layer of clay would be 

 detrimental. 



In most cases, however, the safer way is either to avoid 

 the grading or to replace with new trees. 



Whenever it becomes necessary in grading, making of 

 streets, laying of conduits, etc., to cut into and destroy part 

 of the root system, a corresponding amount of the crown 

 system should be removed by proper pruning, keeping in 

 mind the fact that the side on which the roots are destroyed 

 is the one that needs to be most severely pruned. 



The aeration and watering of street trees is a most difficult 

 question, especially where macadam or other solid pavement 

 is used. It is, therefore, necessary in the first place to choose 

 species which are frugal in that respect. Here again, species 



