80 Forests and Trees 



a vigorous growth which would not stand the exposure of 

 the hilltop. A northern slope, because it retains the snow 

 longer in the spring and is therefore more moist, is fre- 

 quently well wooded, while the southern exposure is parched 

 and naked. 



When it has been decided that certain trees are adapted to 

 the soil and climate, modified as these may be by local con- 

 ditions of drainage or exposure, then other characteristics 

 may be considered. The persistence or length of life of a 

 species and its rate of growth should be taken into account. 

 Trees which grow rapidly will give results sooner than those 

 of slower growth, but cannot be depended upon to last 

 as long. In establishing a forest the rapid growers, like 

 the poplars and birches, often take possession of the ground 

 and form a cover which protects the slower types until 

 they get a start. These in turn take possession and form 

 a more permanent growth. For ornamental planting the 

 same result can be secured by mixing the poplars and willows 

 with conifers and hardwoods. The one type gives quick 

 results and the other insures permanency. 



The liability of a species to insect attacks or fungous 

 diseases also affects its value, either as a shade or a forest 

 tree. In considering this, however, it is well to remember 

 that a pure stand of any species is likely to promote the 

 increase of any pest or disease to which it is at all subject. 

 In Manitoba, of recent years, the Manitoba maple has been 

 much injured by repeated attacks of the green aphis. This 

 is no doubt partly due to the fact that this tree is subject 

 to that pest, but it is also due to the trees being so plentiful 



