SHADE TREES. 



209 



One of the common effects of drought on trees is the premature yellow- 

 ing and falling of the leaves. Quite often, however, as in the case of the 

 elm, the leaves fall off in large quantities without turning yellow, and the 

 not unusual habit of this tree of shedding its terminal branches may be 

 associated with drought. During dry periods the leaves of rock maples 

 often sun scorch, particularly when strong winds are blowing; and what 

 is known as bronzing of the foliage is associated with a lack of water. 



Drought in summer interferes with the development of the tissue, 

 thereby affecting the growth of trees. In times of unusual rainfall a 

 renewed activity often takes 

 place in the fall, when many 

 shrubs will begin to blossom 

 again and throw out new 

 leaves. The result, especially 

 in very cold winters, is a 

 susceptibility to winterkilling 

 on the part of the tissue. 



Drought is responsible for 

 many pathological conditions 

 in trees. Many of them, 

 such as the rock maple, the 

 European cut-leaf birch, the 

 white ash and others, become 

 weakened and therefore more 

 susceptible to attacks from 

 borers and in some instances 

 to scale insects, as a result of 

 which many trees die. When 

 plants enter the winter resting 

 period after a drought in the 

 fall they are very likely to 

 become victims of winter- 

 killing. 



Severe drought affects the 

 roots of trees, which are un- 

 able to thrive with so little soil moisture for any length of time, especially 

 when the soil is dry as powder to a considerable depth. During the past 

 three years the root systems of numerous maples, elms and other trees 

 have been severely affected by drought, as shown by the cases of staghead 

 and the unusually large number of trees that have died during this period. 

 Trees affected by severe drought sometimes die suddenly, but more often 

 they linger in a dying condition for a few years. The wood of trees like 

 the elm, when dying from drought, is invariably quite brittle, owing to 

 the fact that the decreased water supply from the roots causes a trans- 

 formation of the sapwood into hard wood. 



Shade trees growing in dry situations may be greatly helped over periods 



FIG. 82. Showing maple with staghead effect. 



